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THE ESSENTIALS OF HISTOLOGY.
Descriptive and Practical. By Sir E. A.
SCHAFER, M.D.,Sc.D., F.R.S., Professor
of Physiology in the University of Edin-
burgh. With 645 Illustrations, many of
which are coloured. 8vo, Ios. 6d. net.
THE ESSENTIALS OF MORBID
HISTOLOGY, for the Use of Students.
By ALBERT S. GRUNBAUM, M.A., M.D.,
F.R.C.P., D.P.H., Professor of Patho-
logy, University of Leeds. With 151
Illustrations, 22 in Colour. 8vo, 7s. 6d.
net.
THE ESSENTIALS OF CHEMICAL
PHYSIOLOGY. By W. D. HALLI-
BURTON, M.D., F.R.S., F.R.C.P., Pro-
fessor of Physiology in King’s College,
London. With Illustrations. 8vo, 5s. net.
LONGMANS, GREEN, AND CO.,
Lonpon, New York, BomsBay, CaALtcuTTa, AND MADRAS
bien
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ESSENTIALS OF
PHYSIOLOGY(2
; BY
nant s
Fy" At BAINBRIDGE
M.A., M.D. (CANTAB.), D.Sc. (LOND.), F.R.C.P.
PROFESSOR OF PHYSIOLOGY, UNIVERSITY OF DURHAM
a AND
Jێ ACWORTH MENZIES
' M.D. (EDIN.)
LECTURER ON PHYSIOLOGY, UNIVERSITY OF DURHAM COLLEGE OF MEDICINE
WITH 1384 ILLUSTRATIONS
LONGMANS, GREEN, AND CO.
39 PATERNOSTER ROW, LONDON
FOURTH AVENUE & 30TH STREET, NEW YORK
BOMBAY, CALCUTTA, AND MADRAS —
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All Rights Reserved
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PREFACE ©
Our object in writing this book has been to bring together in a concise
form the fundamental facts and principles of Physiology, primarily
with the object of meeting the requirements of the medical student
preparing for a pass examination in the subject of Physiology. Con-
siderations of space have led us to exclude as far as possible histological
details and descriptions of chemical and experimental methods which
form part of each student’s laboratory course, and for which separate
text-books are used. We have also omitted, for the same reason, all
matter of purely historical interest.
In view of the transitional state of anatomical nomenclature, we
have, after much consideration, retained the terminology hitherto used
in this country, and have inserted the Basle nomenclature in brackets.
While it is impossible to mention all the sources upon which we
have drawn, we wish to acknowledge our especial indebtedness to
Professor Starling, not only for permission to use many figures from
his Principles of Physiology, but also for advice and information on
many points. Our thanks for permission to use figures, which are
as far as possible separately acknowledged in the text, are also due to
Professor Sir E. A, Schafer (Quain’s Anatomy and Essentials of Hist-
ology), Professor J. N. Langley (Jowrnal of Physiology), Dr M. S.
Pembrey (Practical Physiology), J. Barcroft, Esq. (Respiratory Function
of the Blood), Dr A. Hertz, Dr Homans, Dr W. E. Hume, Professor R,
Howden (Gray’s Anatomy), and the Council of the Royal Society. We
‘must also thank the Publishers and others who have kindly supplied us
with blocks, namely, Messrs J. & A. Churchill, Hodder & Stoughton,
Macmillan & Co., Ltd., Mr Edwin Arnold, the Cambridge University
Press, Messrs Baird & Tatlock, Ltd., and Messrs Hawksley.
Finally, we are indebted to Miss F. H. Miller for her unwearying
efforts in the production of the original illustrations, and to Messrs
Longmans, Green & Co. for the great pains they have taken in the
reproduction of the figures.
F, A. BAINBRIDGE.
J. ACWORTH MENZIES.
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CONTENTS
OHAP. PAGE
I. INTRODUCTORY . . : kta oh ; ; - “ ; 1
II. CHEMISTRY OF THE Bopy. . 5 : . . oe
IlI. MuscLe. ; , Be Ws kg ; } ; ; ‘ 18
IV. NERVE FIBRES . noe ; : ; ; > ; ‘ 33
VY. THE CENTRAL Nervous System 5 . : : ; ; , 39°
Section 1. The Neuron . ; ; " : ; y 39
,, 2. Structure of Spinal Cord . ‘ . : 47
;, 8 Conduction in Spinal Cord. ; : : : ; 50
», 4 Reflex Action . ; ‘ ; ; , 58
» 9. The Brain Stem. . ‘ ; . : 66
;, 6. Cerebellum . Bary. ; ; ; 71
no me Mid Brain =; : ‘ 76
,, 8 Fore Brain. ; : ; : 79
», 9. Autonomic System . ; ' . ; en a 99
VI. THe ORGANS OF SENSE. : : ; : ? 3 OS
Section 1. Cutaneous ; 2 , ; ; UR cc gl ee.
5 i Seema and Snell es ae in? RR een =. eee RS
,, 9% The Sense of Sight . : ; ¢ oat
» 4 Hearing . F ; ; : . , } prs 141
,, 5. Proprio-ceptive Senses. ; 4 ; eee
VII. Bioop : ‘ : ; h ; ; i : 3 ; . 155
VIII. Tar Vascunar MEcHANISM. : Sgt hee ; ; oo Nee
Section 1, General Features of the Circulation . ‘ ‘ 198
,, 2. Phenomena of the Normal Heart Beat. el sh
» 9% The Pulse : ; . : ‘ : : . 204
; 4, Causation of the Heart Beat . ; : . 209
,, 5. Regulation of the Vascular Mechanism : . . .~ 219
», 6 Lymph . ; «+ 989
vii
s*
~
_
viii ESSENTIALS OF PHYSIOLOGY.
CHAP.
IX. THE RESPIRATORY SYSTEM
Section 1. Respiratory Movements .
3, 2. Chemistry of Respiration
3. Regulation of Respiratory Movements.
4, Tissue Respiration .
», 5, Changes in Character of the Air Breathed
6
7
. Muscular Exercise .
X. THE DicEsTIvVE SysTEM . : :
Section 1. The Nature of Digestion .
», 2, Changes in the Food in the Mouth .
,, 3 Gastric Digestion
,, 4. Intestinal Digestion
», 5, Absorption
» 6. Large Intestine
XI. METABOLISM :
Section 1. Methods of Inv aviation
,, 2 Metabolism of Fat .
,, 8 Metabolism of Carbohydrate
,, 4. Metabolism of Protein
5, 5. Total Metabolism
», 6. The Liver
XII. SKIN AND ANIMAL HEat .
XIII. Foop ann Ditrtr
XIV. THe UrInARkY SYSTEM
Section 1. Structure of the Kidney and Composition of Urine .
», 2 Formation of Urine .
» 9 Micturition
XV. Ducriess GLANDS
XVI. REPRODUCTION
INDEX ,
. Effect of Respiratory Movements on the Circulation
ESSENTIALS OF PHYSIOLOGY.
CHAPTER I.
INTRODUCTORY.
Every living structure is derived, so far as our present knowledge
goes, from another living structure and exhibits certain well-marked
features. It takes up non-living material and builds it up in a more
or less modified form into its own framework, it has the power of
growing up to a certain limit, and it is capable of giving rise to other
living organisms like itself. Further, it has the property of irritability,
that is, it may be affected by a change in its immediate surroundings
which is called a stimulus, and to which it responds by some change
in itself, usually movement or secretion. These manifestations of life
form the subject-matter of the science of Physiology, which naturally
falls into two divisions, Vegetable and Animal. With the former of
these, however, we are not here concerned.
_ The functions of animal life may be studied in their most primitive
condition in a unicellular organism, such as ameeba. This minute
creature may be observed under the microscope to enfold particles of
food material, to assimilate what is useful in these particles, and to
reject what is useless; it may be seen to respond to chemical or
mechanical stimuli by movement, sometimes contracting into the
smallest bulk by becoming spherical, at other times protruding part
of its substance and transferring itself, as it were, into the protruded
part, and so changing its position. It may also be observed at a
certain stage in its life to divide into two, and each of the young
amcebze so produced grows until it too gives rise in its turn to another
generation.
In the higher animals the body is composed of a multitude of cells,
and this complexity of structure is accompanied, as in a community
of persons, by a specialisation of function whereby certain cells are
I I
2 ESSENTIALS OF PHYSIOLOGY.
modified to subserve movement, others to produce secretions, and so
on; in this way the efficiency of the organism as a whole is increased.
Obviously in such a community of cells it is of the utmost importance
that the various groups should work in harmony, and to ensure this
they must be linked up by some controlling mechanism.
Two such mechanisms are found in the body: (1) a system of
chemical messengers, or hormones, and (2) the_nervous system. The
former is the more primitive of the two methods. A hormone
is produced in one organ and is carried by the blood to another,
exciting or restraining its activity. For example, as the acid contents
of the stomach pass into the bowel they lead to the production of a
hormone in the intestinal wall. The newly formed substance is taken —
up by the blood and carried to the pancreas, which it stimulates to
secrete the juice required for the next stage of the digestive process.
Such a method of communication is comparatively slow, and where
rapidity of transmission is important the messages are conveyed by
the nervous system. The latter, in fact, bears much the same relation
to the blood current as the telegraphic system bears to ordinary letter
post. Thus if a foreign body touches the surface of the eyeball, infor-
mation of the fact is sent along certain nerve fibres to the brain, and
impulses return to the muscles of the eyelids, causing the lids to close,
within a small fraction of a second. bs
In any living organism the unit of structure is a minute, jelly-like
mass known as a ce//. The simplest organisms consist of a single cell,
those which are higher in the scale of life being composed of many
cells. Each cell is composed of a semi-fluid material, known as proto-
plasm, containing a denser circumscribed structure, the nucleus. In
some cases a well-defined cell-envelope exists, notably in vegetable cells
and in the mammalian ovum, but in many animal cells no definite
envelope can be demonstrated, and in these the boundary is probably
determined by the condensation of molecules which is known to take
place on the surface of colloid solutions, and which gives rise to the
physical condition known as surface tension.
Protoplasm is semi-transparent, and may be homogeneous in
appearance, or may show traces of structure in the form of a network
containing hyaline fluid in its meshes. It is the working part of the
cell, and often contains granules which represent the products of its
activity. Such granules are especially seen in secreting cells, and
occupy corresponding spaces in the cell protoplasm. Protoplasm itself
varies greatly in composition, but it always contains a large proportion
of albuminous substances or proteins.
The nucleus is essential to the life of the cell. When a cell is
INTRODUCTORY. 3
divided into a part which contains the nucleus and a part which does
not, an experiment which may be performed with the larger unicellular
organisms, the part separated from the nucleus becomes inactive and
dies. Further, when cell-division occurs, the nuclear changes which
lead to the formation of two daughter nuclei precede the division of the
protoplasm. The nucleus also differs in chemical composition and in
staining reactions from the rest of the cell. It contains a substance
called nuclein, which is a compound of nucleic acid with protein.
Nucleic acid is distinguished by containing a considerable proportion
of phosphorus in its molecule.
In the human body all the tissues and organs subserve, directly or
indirectly, the production of movement, whether that takes the form
of locomotion, work, speech, or writing. The nervous and muscular
tissues are the master tissues of the body, the remaining tissues and
organs being designed for their protection and nutrition. The bony
skeleton forms a framework which is necessary for the carrying out of
movements, and which shields the brain and other important structures
from injury. The skin is also protective in function, and, with the eye,
ear, and other sense organs, it receives impressions from the outer
world which are of service in determining the bodily activities. The
digestive tract converts insoluble food substances into soluble bodies
which are then absorbed into the blood. The circulatory system
conveys the blood to all the cells and tissues of the body so that they
receive nourishment. The blood also reeeives from the cells and tissues
the waste products formed by their activity and carries these to the
lungs and kidneys, by which they are excreted. Besides excreting one
of the waste products, carbonic acid, the lungs take up oxygen from the
air and convey it to the blood, from which it passes to the cells and
tissues. Further, in addition to the glands which secrete the digestive
and other juices, there are in the body certain glands whose function it
is to produce various hormones. All these subsidiary systems are only
of importance in that they sérve to sustain the muscular and nervous
structures. The muscular system, in its turn, is to be looked upon as
the organ of expression of the nerve centres. The life of the body
therefore, consists ultimately in its nervous-activities.—
CHAPTER II.
THE CHEMISTRY OF THE BODY.
Tue body of an animal is composed of water, organic compounds,
and inorganic salts. If the body, or any part of it, be dried at a_
temperature of 105° C., the loss of weight indicates the amount of water
present. If the dried solids be exposed to a high temperature in the
presence of oxygen, the organic compounds are all oxidised, and the
residue consists of the inorganic matter.
THE INORGANIC SALTS.
The chief salts which are found in the body are the chlorides,
phosphates, sulphates, and carbonates of sodium, potassium, calcium,
and magnesium. Iodine, fluorine, and a few other chemical elements
are also present in small amount; iron enters into the composition of
the coloured corpuscles of the blood. Generally speaking, sodium is the
base most largely present in the body fluids, such ‘as the plasma of the
blood, and potassium is that most abundant in the cells and tissues ;
while the bones owe their rigidity to the large proportion of calcium
phosphate and carbonate which they contain.
The functions of the inorganic salts are various, and are not yet
completely understood. The waste carbonic acid from the tissues is
conveyed to the lungs partly in the form of carbonates and bicarbonates.
Sulphates and phosphates are to some extent waste products derived
from the breaking down of organic compounds, Phosphates also serve
a useful purpose in maintaining the balance between acids and bases in
the body by undergoing change from mono- to di-hydrogen phosphate,
or the reverse, as occasion requires. The salts as a whole, but especially
the chlorides, have, moreover, important functions depending upon their
physical properties (p. 14).
THE ORGANIC COMPOUNDS.
The organic constituents of the body fall naturally into two main
groups, non-nitrogenous and nitrogenous. The substances comprised
in these two groups may be looked upon as fragments of the protoplasm
4
THE CHEMISTRY OF THE BODY. 5
or living material of the cell, for analysis necessarily involves the death
of the living structure. It is clear, however, that protoplasm itself is
largely composed of nitrogenous material, though it derives much of
the energy necessary for its activities from the combustion of the non-
nitrogenous compounds.
THE NON-NITROGENOUS SUBSTANCES.
These again fall into two groups: (1) those in which the combined
oxygen is sufficient to oxidise all the hydrogen of the molecule, and (2)
those in which the oxygen is insufficient to combine with the hydrogen
of the molecule.
(I.) The former group consists of the Carbohydrates, and the members
of it which occur normally in the body are either hexoses, that is, each
contains six carbon atoms in its molecule, or are formed by a combina-
tion of two or more hexose molecules. Pentoses, with five carbon atoms
each, also occur ; an example is xylose, which enters into the formation
of the molecule of the nucleic acid derived from the pancreas. The
carbohydrates found in the body are dextrose, levulose (fructose),
galactose, lactose, and glycogen. Others occur as constituents of
food, eg. cane-sugar and starch. The first three have the formula -
C,H,,0,, and belong to the group of mono-saccharides. Lactose is a
disaccharide, that is, it belongs to a group of substances formed by the
condensation of two monosaccharide molecules with the abstraction of
a molecule of water. ;
2C,H),0, - HO =C,,H,0)).
Glycogen is a polysaccharide, and is formed by the condensation of a
large number of monosaccharide molecules, as in the formula
nCgH,0, — nH,0 = (CgH1095)n.
The symbol ‘‘n” may have a very high value. Thus starch is supposed
to have the formula 200(C,H,,0,). :
The Monosaccharides.— Dextrose may be looked upon as the current
carbohydrate coin of the body. It is a soluble crystalline substance,
having the formula
CH,OH
(CHOH),. °
CHO
It is an aldehyde, and, like other aldehydes, when heated with an
alkaline solution of a cupric salt, it reduces the latter with the forma-
tion of yellow cuprous oxide. This property of dextrose and other
“reducing sugars” forms the basis of the tests of Trommer, Fehling,
ri ESSENTIALS OF PHYSIOLOGY.
and Benedict. Dextrose has also the power of reducing acid solutions
of cupric salts, differing in this respect from. reducing sugars which
belong to the class of disaccharides. On being heated with phenyl-
hydrazine and acetic acid, dextrose forms a compound, phenylglucosazone,
which crystallises in yellow needles, generally arranged in loose sheaves.
Solutions of dextrose are decomposed by the action of yeast into carbonic
acid and alcohol. Further, dextrose is dextro-rotatory, that is, its
solutions rotate the plane of polarised light to the right.
Lzvulose (fructose) and galactose occur in the body in smaller
quantity. The former is levo-, the latter dexto-rotatory. Like dextrose,
they have the property of reducing alkaline solutions of cupric salts,
and they also form osazones.
Dextrose and galactose are aldehydes and are known as aldoses, —
while leevulose is a ketone and belongs to the group of ketoses.
CHO CH,OH CHO
eee bo eet
ete | ar 10.0.4
coe eH HO.C.H
4.008 ae HGH
da. on On,0H bit on
Dextrose Leevulose Galactose
The only disaccharide which occurs in the body is lactose, the sugar
of milk. Two other disaccharides, cane sugar and maltose, are of
physiological importance, the former being an important food-stuff,
the latter an intermediate stage in the digestion of starch. Lactose
and maltose are reducing sugars and form characteristic osazones.
Each of the three, when boiled with dilute mineral acid, undergoes
hydrolysis, the molecule taking up water and being split into two
molecules of a monosaccharide.
C,.H5.0,, + H,O = 2C,H,,0,.
The polysaccharide, glycogen or animal starch, occurs chiefly in the
liver and muscles as a storage product. Its solution in water differs
from that of vegetable starch, which is also a polysaccharide (1) in
that it is more markedly opalescent than that of the latter, and (2)
in giving a reddish brown colour with iodine, whereas vegetable starch
gives a blue colour. The polysaccharides do not reduce alkaline solu-
tions of cupric salts. They undergo hydrolysis when boiled with dilute
mineral acid or as the result of ferment action, yielding first poly-
THE CHEMISTRY OF THE BODY, 7
saccharides of smaller molecule than the starches, called dextrins, and
a disaccharide, maltose, and giving as the final product the mono-
saccharide, dextrose,
The first-formed dextrin products of the hydrolysis of the wiarehes
are called erythro-dextrins, because they give a red colour with iodine ;
the later-formed substances are called achroo-déxtrins because they
give no colour with iodine, and consist of smaller molecules than the
erythro-dextrins. The process of hydrolysis of a polysaccharide may be
summarised in the equation
(C,H, )05)n + 0H,O = nC,H,,0,.
Inosite.—Inosite is a substance found in muscle and formerly
called muscle-sugar. It has the formula C,H,,0,+2H,O, but does not
belong to the carbohydrate group, being a benzene derivative. It
does not reduce an alkaline solution of cupric sulphate, does not rotate
polarised light, and is non-fermentable.
(II.) The second group of non-nitrogenous substances, those in
which the oxygen of the molecule is insufficient to combine with the
hydrogen, consists of the Fats. The chief fats found in the body are
tristearin, C,H,(C,,H,,0,),, tripalmitin, C,H,(C,,H,,0,),, and triolein, ”
C,H, (C,,H,,0,),. They are compounds of the corresponding fatty
hada stearic, palmitic, and oleic, with the trivalent alcohol, glycerol.
Stearic and palmitic acids are saturated compounds, and the former
and its esters, being higher in the series than palmitic acid and its
esters, have a higher melting point. Oleic acid is unsaturated, and
both the acid itself and triolein have a relatively low melting point,
being fluid at room temperature.
Fats are insoluble in water, but are soluble in ether or in warm
alcohol. They are decomposed on heating with alkalies, the fatty
acid uniting with the alkaline base to form a soap and the glycerol
being set free (saponification). Ifa neutral fat, such as pure olive oil,
is shaken up with water, the fat becomes broken up into fine globules
which run together again when the shaking ceases. If, however, some
soap is present, each globule becomes coated with a layer of soap
molecules, which so reduce the surface tension between the fat and
the water that the globules remain apart. Such a suspension of fat
globules is called an emulsion. A fine emulsion of this kind occurs in
the lymph during the absorption of fat, the place of the soap: PONE.
probably taken by the proteins of the lymph itself.
Lipoids.—The term lipoids includes a number of substances which
resemble fats in being soluble in ether. The commonest of these are
lecithin and cholesterol, which are constantly associated in the body,
ay ESSENTIALS OF PHYSIOLOGY.
occurring especially in the nervous tissues, eee and the plasma and
corpuscles of the blood.
Lecithin is a complex fat, and when boiled with baryta water yields
two fatty acids, glycerophosphoric acid, and choline, which belongs to
the group of amines. Lecithin, in virtue of its nitrogen content, has
also affinities with proteins. It belongs to the group of phospholipines,
which also includes cephaline and sphingo-myeline. Some other lipoid
substances found in the brain resemble lecithin in containing nitrogen,
but contain no phosphorus.
Cholesterol, C,,H,,OH, is a complex monatomic alcohol, and is
included in the group of lipoids simply on account of its solubilities.
It forms colourless, square, flat crystals, often notched at one corner,
and gives a red colour with strong sulphuric acid. 3
Lipoid substances enter into the composition of cell protoplasm,
occurring especially in the superficial layer, or. ‘plasma skin.” This
surface layer is less permeable to salts and other substances in watery
solution than to water. Substances such as alcohol and alkaloids,
which are soluble in oily media, can penetrate the cell. —
THE NITROGENOUS SUBSTANCES.
The nitrogenous substances contained in the body are (1) proteins,
which form the greater proportion of the solid constituents of the cells,
tissues, and body fluids, and (2) derivatives of proteins.
The Proteins.
The physical properties of proteins are those of colloid substances
(p. 16). Some proteins, however, may be obtained in the crystalline
form, for example egg-albumin, and hemoglobin, the pigment of blood.
Others, for example peptone (which is not, however, a constituent of
the body), are capable of diffusing through an animal membrane.
The molecule of protein is very large; it contains the elements
C, H, N, O, and § in the following proportions :—
C 50°6 — 54°5 per cent.
BiG Bie FSO
N 150-176 ,,
O 215-235 ,,
8 LOB OP ~
Proteins have the power of combining either with acids or alkalies, and
this property is of service in maintaining the reaction of the cells and
fluids of the body at its normal level.
When a solution of a protein, such as egg-albumin, is warmed
with dilute acid or alkali, it is converted into metaprotein and shows
Ne Se ee Se UL
THE CHEMISTRY OF THE BODY. 9
changes in its characters. The egg-albumin solution is neutral, and is
coagulated on heating if salts are present. The metaprotein solution
does not coagulate on heating, and gives a precipitate of metaprotein
on neutralisation, the precipitate being soluble in excess of either acid
or alkali.
If a solution of egg-albumin is ch joted to the action of superheated
steam, or is boiled for « long time with mineral acid, or is subjected to
the action of gastric juice or of pancreatic juice, the albumin takes up
water and the molecule is finally split up into small molecules, the
process being known as hydrolysis. The splitting up occurs in stages,
the molecules becoming progressively smaller. After the substance has
passed through a metaprotein stage, a series of hydrated proteins are
formed, the first formed products being called proteoses and the later
ones peptones, The hydrated proteins are soluble in water and are not
coagulated on boiling. Proteoses are distinguished from peptones in
that they are precipitated if their solution is saturated with ammonium
sulphate, whereas peptones are not precipitated in this way. If the
hydrolysis is continued, the peptones are further split into substances
called polypeptides, which do not show protein characteristics, and which
consist of groupings of amino-acids. By still further hydrolysis these are
split into their constituent amino-acids.
All proteins give certain colour reactions by which their presence
in solutions may be recognised. The most useful of these are the
following :—
(1) The Xanthoproteic reaction.—Nitric acid is added to the solution
and it is boiled. A yellow colour is produced, which changes to orange
on cooling and adding ammonia.
(2) Millon’s reaction.—A solution of mercuric and mercurous nitrates
is added to the protein solution. A precipitate is formed and becomes
red on heating.
(3) Piotrowski’s reaction.—With dilute copper sulphate and excess of
caustic alkali, most proteins give a violet colour, but in the case of
proteose or peptone the colour is pink.
(4) Hopkins’ reaction.—Glyoxylic acid is added to the protein
solution, and then strong sulphuric acid is poured down the side
of the tube so as to form a layer at the bottom. A violet colour is
produced at the junction of the two fluids.
Millon’s reaction depends upon the presence of tyrosine in the
protein molecule; Hopkins’ reaction depends upon the presence of
tryptophane. Gelatin, which does not contain either tyrosine or
tryptophane, gives neither of these reactions.
The.chief proteins found in the body are (1) protamines, (2) histones,
10.2 ESSENTIALS OF PHYSIOLOGY.
(3) albumins, (4) globulins, (5) phosphoproteins (6) scleroproteins, and
(7) conjugated proteins,
(1) Protamines are basic in character and only occur in combination,
They are chiefly found, combined with nucleic acid, in the: Spornneenee
of certain fishes. .
(2) Histones are also basic in character, and occur in the combined
form. An example is globin, the protein constituent of hemoglobin.
(3 and 4) Albumin and globulin occur in all cells and in many of
the body fluids, and are distinguished from each other by their
solubilities, Albumin is soluble in water or weak salt solution, and its
molecules are aggregated to form a precipitate in a saturated solution
of ammonium sulphate. Globulin is insoluble in water, soluble in
weak salt solution, and is precipitated in a half-saturated solution of
ammonium sulphate. Albumin or globulin in solution, on being
heated, undergoes first of all a change which is probably chemical
in nature, and is known as denaturation. A physical change follows,
and consists in the aggregation .of the molecules to form a coagu-
lum, The presence of inorganic salts is favourable to aggregation,
but is unfavourable to denaturation. Acids and alkalies, on the
other hand, favour denaturation but hinder aggregation. Thus, if
the protein is heated with more than the merest trace of acid or
alkali, an acid or alkaline solution of metaprotein is obtained which
will not coagulate on heating, but yields a precipitate of metaprotein
on neutralisation. A trace of acid favours coagulation of an albumin
or globulin solution, because the acid combines with the protein to
form a salt.
The effect of salts in favouring the coagulation or precipitation of
proteins is due to the ionisation of the salt. A solution of protein is
really a suspension, and the suspended particles carry an electric
charge, which is positive in an acid solution, negative in an alkaline
solution. As all the particles carry a similar charge, they will tend to
repel one another, but if the charge be reduced, precipitation or co-
agulation will take place. Hence, in the case of an ionised salt, the
effective ion which brings about coagulation is that which carries a
charge opposite in sign to that of the protein particles. It is found
that the effectiveness of an ion is determined by its valency. In the
case of acid solutions the trivalent anion of potassium citrate has a
greater coagulating power than the divalent SO,, and the latter again is
more effective than the monovalent Cl. So also in alkaline solutions,
barium chloride with a divalent kation is more effective than sodium
chloride with a monovalent Na ion.
(5) Caseinogen, the chief protein of milk, is a phosphoprotein, that
NN ae
se Ss eS
THE CHEMISTRY OF THE BODY. II
is, it contains phosphorus combined in its molecule in addition to the
five elements common to proteins generally. It is insoluble in water,
but soluble in weak alkalies, and is precipitated from its alkaline solution.
by acetic acid, the precipitate being soluble in excess of the acid. The
alkaline solution is not coagulated by heat.
(6) The Scleroproteins are distinguished by their relative insolu-
bility. They form the chief constituents of the fibrous and horny
structures of the body. Thus, white fibres are mainly composed of
collagen, yellow fibres of elastin, and hair, horn, and hoofs of keratin.
When collagen is boiled with water it yields gelatin, a substance —
which is soluble in boiling water, the solution setting to a jelly on
cooling. Elastin and keratin are both insoluble in hot or cold water,
dilute acids or alkalies, or in alcohol or ether. Keratin is remarkable
for the amount of sulphur contained in its molecule.
(7) The Conjugated Proteins. — These are nucleoprotein, gluco-
protein, and chromoprotein, and each consists of a protein combined with
another body called the prosthetic group. Nuwcleoproteins are a constant
constituent of cell nuclei. They are soluble in water, weak salt solution,
or dilute alkalies. They possess acid characters, hence the affinity of
nuclear chromatin, which contains nucleoprotein, for basic dyes. A
nucleoprotein is a compound of a protein with nuclein, and the latter
consists of protein combined with nucleic acid, an organic acid contain-
ing phosphorus. If a nucleoprotein is subjected to digestion by gastric
juice, an insoluble brownish residue is obtained. This residue consists
of nuclein. If the nucleic acid obtained from nuclein is hydrolysed, the
two substances, adenine and guanine, are constantly found among the
products of disintegration. These two bodies belong to the purin
group, that is, they may be regarded as derivatives of purin, which has
the formula
N=CH
|
HC C—NH
NCH
(Tai 83
NO
Adenine has the formula C,;H,;N,, while guanine is C;H;N,O. Both
these substances, when oxidised, yield uric acid, C;H,N,O,.
In glucoprotein the combined body is a carbohydrate radical, often
glucosamine (C,H,,NO;), and therefore containing nitrogen; the
glucosamine is split off when the glucoprotein is boiled with mineral
acid. It reduces alkaline solutions of cupric salts. Most glucoproteins
belong to the group of mucins. These are soluble in weak alkalies,
they are not coagulated by heat, and they are precipitated by acetic
12° ESSENTIALS OF PHYSIOLOGY.
acid, the precipitate being insoluble in excess of the acid. The mucoids,
for example ovomucoid, are also glucoproteins.
The best example of a chromoprotem is the blood pigment, heemo-
globin, in which a protein, globin, is combined with an iron-containing
body, hematin. Hemoglobin crystallises with comparative ease, and
is freely soluble in water.
The Derivatives of Proteins.
When a protein molecule is broken up, either by prolonged boiling
with a mineral acid, or by the action of certain enzymes, the resulting
products are found to belong to the class of amino-acids ; that is, they are
fatty acids in which one or more atoms of hydrogen have been replaced
by NH, groups. Some of these acids are combined with the benzene
ring, e.g. tyrosine and tryptophane ; two, proline and oxyproline, contain
the pyrrol ring; one, histidine, the iminazol ring; and one, cystine,
contains sulphur, that is, it is a thioamino-acid. Whereas the fats of
_ the body belong to the upper end of the fatty acid series, it is to be
noted that the amino-acids belong to the lower end of the same series.
The simplest amino-acid found in the body is glycine, or amino-
acetic acid, CH,.NH,.COOH. This is one of the most abundant deriva-
tives of gelatin. ‘The next in the series, alanine or amino-propionic
acid, occurs in abundance, both in its simple form, CH,.CH.NH,.COOH,
and also combined, with phenyl as phenylalanine, with oxyphenyl as
tyrosine (C,H,.OH.CH,CH.NH,COOH), with indol as tryptophane,
with iminazol as histidine, and with sulphur as cystine. Although
analysis of any one protein will yield all or most of the amino-acids
which have so far been isolated, nevertheless the relative proportions
of amino-acids contained in different proteins vary widely, and to this
variation the proteins probably owe their distinctive characteristics.
Gelatin is remarkable in containing no tyrosine, tryptophane, or cystine.
The following tables from Starling’s Principles of Human Physiology
give (1) a list of amino-acids, and (2) the proportion of the different
amino-acids contained in various proteins.
Mono-amino-acids.
Glycine (amino-acetic acid)
Alanine (amino-propionic acid)
TS aa Lean. Sieh eit eee Mhaaiee nae ok
fatty series.
Amino-valerianic acid y
Leucine (amino- isobutylacetic acid)
Isoleucine (amino-caproic acid)
Aspartic acid : \ sire es
Glutamic acid : ’ .p Dibasic acids.
THE CHEMISTRY OF THE BODY.
Phenylalanine
Tyrosine (oxyphenylalanine) .
Proline (pyrrolidine carboxylic acid) _
Oxyproline (oxypyrrolidine
id
acl
carboxylic
Tryptophane (indolamino-propionie acid)
13
; Benzene derivatives.
Heterocyclic
pounds.
Diamino-acids and their Compounds.
_Lysine (diamino-caproic acid)
Arginine (guanidinamino-valerianic acid)
Histidine (iminazolalanine) .
Diamino-tryoxydodecoic acid (derived from a 12- carbon acid).
Cystine (derived from amino- thiopropionic acid).
com--
The ‘‘ hexone bases.”
Argigine is not a simple amino-acid, but is a compound of amino-
valerianic acid with guanidine.
A similar compound of guanidine occurs
in muscle as creatine, or methylguanidine acetic acid. When creatine
is boiled with baryta water it is split into urea and sarcosine (methyl-
glycine), thus giving rise to an amino-acid.
;
ity
7
4
>
= ; a
5 = oo : > : : : ; | &.8
= =} Ss |} 8 30 a = A et tere
< = |asi/3B/2)] 8 |2/8/8 }e¢
Bo = D s + o | 6a
ge Pes Baal) Sat Boner ah ona lager
2 = <3) ==)
‘Glycine 0°0 0°0 3°8 0°9 | 0°0 0°0 0°0 165 | 4°7
Alanine 2°7 81 3°6 2°7 |1°5 4°2 a 08 | 1°5
Serine 0°6 ° 0°33 | 0°12) 0°5 06 | 7°8 04 | 0°6
Amino-valerianic
acid ve ... |present| 0°38 | 7‘2 aes 43)... |.1°0 | 0°9
Leucine 20°0 71 20°9 6:0 | 9°35 | 29°0 OO. vase heed oe
Proline 1°0 2°25 ey 2°4-) O70". 293: ALOE. | SSF B4
Oxyproline = oe eae ooo | O28) 10 Se AG ee He Se
Glutamic acid . ee 8°0 6°3 | 36°5 |15°55| 1:7 «- | 0°88] 3°7
Aspartic acid a Raa i Set 4°5 1°3.|1°39| 474 ... | 0°56] 0°3
Phenylalanine 3°1 4°4 2°4 | 2°6 | 3°2 4°2 we | 0°4 | 0°0
Tyrosine 2°1 1‘1 2°1 2°4 | 4°5 1°5 .. | 070 | 3:2
Tryptophane present| present| present} 1°0 | 1°50 | presen re Hn AY
Cystine 2°3 0°2 0°25 | 0°45| 2 03 we |. Jover 10
Lysine aS. 2°15 | 1°0 0°0 |5°95| 4°3 | 0:0 |12°0| 2°75} 1:1
Arginine ety 2°14 | 11°7 8°4 13°81] 5°4 |87°4 | 58°2) 7°62) 4°5
Histidine . cas 11 Leet 26" ad 0°0 | 12°9 | 0°4 0°6
The second of these tables shows that the special characteristics
of the separate proteins are associated with differences in the relative
proportions of their constituent amino-acids, and it has also been shown
that there are differences in the way in which the amino-acids are
I4* ESSENTIALS OF PHYSIOLOGY.
grouped to form the protein molecule. Moreover, when a protein is
hydrolysed by boiling with a mineral acid, the proportion of nitrogen
which is recovered in the form of ammonia, of monoamino-, and of
diamino-nitrogen varies considerably according to the protein examined.
It is especially noticeable that, whereas monoamino-acids are most
abundant in albumin and globulin, protamines, for example salmine,
derived from the spermatozoa of the salmon, are especially rich in basic
(diamino) nitrogen.
PHYSICAL PROCESSES WHICH OCCUR IN
. THE BODY.
Ions.—When sodium chloride is dissolved in water its molecules —
undergo dissociation into sodium zons, which are charged with positive
electricity, and chlorine ions, which are charged with negative electricity.
Ions are not necessarily the same as atoms, since a solution of sulphuric
acid in water contains hydrogen ions and SO, ions. Substances which
undergo dissociation when in solution are called electrolytes, since an
electrical current passed through such a solution is conducted by the
movement of the ions; and owing to the presence of electrolytes in
- the tissues of the body, these are able to conduct electrical currents.
Many substances, however, such as sugar, when dissolved in water
do not undergo dissociation, and the dissolved molecules carry no
electrical charge.
Diffusion and Osmosis.—When a substance such as sodium chloride
is dissolved in water, the dissolved molecules behave like the molecules
of a gas; they are in constant movement and exert pressure upon the
walls of the vessel containing them. If, for example, a vessel is divided
into two compartments by a vertical membrane, and if one compartment
is filled with water and the other with 1 per cent. salt solution, the
molecules of salt in their movements will beat upon the membrane ;
and if the latter is permeable to molecules of salt, they will pass through
it into the distilled water until the amount of sodium chloride on the
two sides of the membrane becomes equal. This process is known as
diffusion, and the rate at which it occurs varies with the percentage of
sodium chloride originally present in the solution.
If the membrane allows water but not sodium chloride to pass
through it, it is said to be semd-permeable, and the pressure exerted by
the molecules of salt upon the membrane is called osmotic pressure and
can be measured in the following manner. A semi-permeable mem-
brane is made by filling the pores of an earthenware cell with silicic
acid or copper ferrocyanide ; the cell is filled with 1 per cent. sodium
PTO agen ee Oe a, ee eee
’ “ *
Corr a ‘ tal — ri it °
a) (2 5 $1 fw Th Pork "
aw POE Wet Cra ty
THE CHEMISTRY OF THE BODY. 15
chloride solution, closed by a cork through which passes a tube attached
to a mercurial manometer, and immersed in distilled water (fig. 1).
Since the molecules of salt cannot pass through the membrane, they
exert pressure on the wall of the cell and the surface of the mercury ;
water passes through the membrane into the cell, and the mercury is
forced downwards in one limb of the manometer and upwards in the
other until the difference of height in the two limbs is 5000 mm. Hg.
This pressure balances the osmotic pressure exerted by the molecules
of salt; and in raising the column
of mercury the salt solution does |
work. If a 2 per cent. salt solu- bk Mercatial Manonitter:
tion is used, the osmotic pressure :
is twice as great.
Similar experiments with other
substances show that the osmotic
pressure of any substance in solution
depends, not upon its nature, but
solely on the number of its mole-
cules in solution, and is proportional
therefore to the concentration of the | 2
solution. A gram-molecule of F—= cena Oe rhage
any substance is its molecular Distilled Water:
weight in grams. The molecular
weight of dextrose is 180, and
a gram-molecule of dextrose is
180 grams. <A _ gram - molecular
solution of dextrose contains 180 Los en oe pchiere ma
grams in 1 litre, whereas a Fic. 1.—Osmometer. (Starling’s
similar solution of sodium chloride Elements of Physiology. )
contains 58°5 grams per litre. If
the sodium chloride did not dissociate, the two solutions would contain
the same number of molecules and their osmotic pressure would be
the same. But since sodium chloride does dissociate and each of its
ions behaves like a molecule as regards osmotic pressure, the solution
of sodium chloride, if completely dissociated, will exert twice the osmotic
pressure of the solution of dextrose. |
It is difficult to measure osmotic pressure in the manner just
described, since the membranes are apt to give way and leak ; and
indirect methods are usually employed, of which the best is the
determination of the freezing-point of a solution. When a substance
is dissolved in water, the freezing-point of the water is lowered, the
lowering being proportional to the concentration and osmotic pressure
Ql COU
BELG. (S771 (942879 S>
16% ~ \'\ BSSENTIALS OF PHYSIOLOGY.
of the dissolved substance. The lowering of the freezing-point below
0° C. is expressed by the letter A.
When a gram-molecule of any substance is dissolved in 1 litre
of water the freezing-point is lowered by 1:87° C., and its osmotic
pressure = 17,000 mm. Hg. The osmotic pressure of a substance in
solution can be calculated from the formula
Osmotic pressure =.
A
para * 17,000 mm. Hg.
Solutions which have the same osmotic pressure are.said to be
zsotonic, and the tissues in mammals are isotonic with a solution con-
taining 0-9 per cent. sodium chloride in water; this is known as
normal saline solution. When: the tissues are immersed in stronger,
1.e. hypertonic, salt solution, water passes from the tissue into the salt
solution by osmosis; when the salt solution has a lower osmotic
pressure than the tissues, it is hypotonic, and the tissues take up water
from the solution. In an isotonic solution, the tissues neither take up
nor lose water.
Both osmosis and dercion take place in the body, but the
membranes are not completely impermeable to substances such as
~ sugar or salt, so that osmosis is soon brought to an end by the passage
of these substances through the membrane.
Colloids.—The term colloid was originally applied to all substances,
such as starch and proteins, which would not form crystals or pass
through an animal membrane, in contra-distinction to easily crystallis-
able bodies, such as sugar, which diffuse rapidly through animal
membranes and are called crystalloids. .
It is now known that the colloidal form is a state in which sub-
stances exhibit certain characteristic features, and that a very large
number of substances, including metals, may exist in the colloidal form.
Further, some colloids, such as hemoglobin, are crystallisable.
Most colloids consist of very large molecules or aggregates of
molecules, and their characters depend largely on the fact that they
do not form true solutions in water or other solvents, but that their
pseudo-solutions consist of particles, suspended in a very dilute
solution of the colloid. Owing to the size of the particles, colloidal
suspensions do not follow the laws of true solution, and they exert only
a very small osmotic pressure.
If an electrolyte, such as sodium chloride, is added to a colloidal
suspension, the salt concentrates at the surface of each colloidal
particle, this being called adsorption. Its occurrence can be readily
demonstrated by dipping strips of blotting paper, which is colloidal,
THE CHEMISTRY OF THE BODY. 17
into a solution of adye. The dye accumulates on the particles in the
paper, and the latter becomes more deeply ee than the solution
into which it is dipped.
The amount of adsorption is relatively much greater in dilute than
in strong solutions of the electrolyte. Its importance lies in the fact
that the velocity with which a chemical change takes place varies
with the concentration of the interacting substances. If two. electro-
lytes, which can interact, are added to a colloidal solution, they
become concentrated on the surface of the colloidal particles, and at
these surfaces the reaction between them will pence more rapidly
than if the colloid were absent.
Solutions of colloids are called sols ; in certain circumstances the
particles may aggregate into larger masses, forming a precipitate, or
the solution may change to a jelly, called a gel. An example of this
process is the coagulation of protein by heat. |
bo
CHAPTER IIL.
MUSCLE.
THREE varieties of muscular fibres are found in the body, namely, those |
forming skeletal muscle, those found in the walls of the blood-vessels,
digestive tract, uterus, and other organs, and those in cardiac muscle.
The structure of cardiac muscle will be considered later (page 177).
The skeletal muscles, which are under the control of the will, are called
voluntary muscles, the other kinds of muscle being termed involuntary,
since, although they are under the control of the central nervous
system, they are independent of the will.
VOLUNTARY MUSCLE.
A skeletal muscle consists of fibres bound together by connective
tissue. The fibres have an average diameter of 50u and vary in length,
some being as long as 3 to 4cm. Each fibre is enclosed in a delicate
elastic sheath (sarcolemma), and shows alternating light and dark
bands crossing it transversely ; owing to the presence of these stripes,
this form of muscle is often called striped or striated muscle. Each
fibre contains a number of oval nuclei; in mammals these lie immedi-
ately under the sarcolemma, but in frogs they are scattered throughout
the substance of the fibre. In a stretched fibre a narrow clear line,
known as Hensen’s line, can sometimes be seen running transversely
in the middle of each dark band. Frequently, too, a dotted line is
visible in the middle of each light band; it is termed Krause’s
membrane.
The fibres consist of fibrils, or. sarcostyles, which run longitudinally
and are imbedded in a material known as sarcoplasm. It is probable
that each sarcostyle is made up of a number of short segments called
sarcomeres, but the exact structure of the sarcostyles and the signifi-
cance of Krause’s membrane and Hensen’s line in different animals is
still uncertain. Schafer has shown that the sarcostyles in the rapidly
contracting wing muscles of insects are divided by the membranes of
Krause into sarcomeres. He considers that the dark central part (or
18
_—
MUSCLE. | 19
sarcous element) of each sarcomere is divided into two parts by Hensen’s
line, and pervaded with longitudinal canals which are open towards
Krause’s membrane; and when the muscle contracts, the clear sub-
stance at either end of the sarcomere’ passes into the pores of the
-sarcous element, so that the sarcomere becomes shorter and thicker.
The shortening of the whole muscle is thus regarded as the result of
the shortening of its sarcomeres.
When a living muscle is examined with polarised light the dim
segments are seen to be doubly refracting (anisotropous), while the
clear segments are singly refracting (isotropous).
In certain animals, such as the rabbit, some of the muscles are pale
and others are red in colour. The pale muscles have the structure
just described, whereas the red muscle fibres contain more sarcoplasm
than the pale ones, and their nuclei are scattered throughout the
substance of the fibres; the capillaries also show numerous small
saccular dilatations. The red colour is due to the presence of hemo-
globin in the fibres. These muscles contract more slowly than the
_ pale muscles, but their contraction is more prolonged. In many
animals these two varieties of fibre are found together in the same
muscle. All muscle fibres are supplied with nerve fibres, some of
which are motor and end in the muscle fibres in end-plates, while
others are sensory and convey impulses from the muscle to the central
nervous system.
- Chemical and Physical Characters of Muscle.—Muscle contains
about 75 per cent. of water and 25 per cent. of solid substances, of
which proteins form 18 to 20 per cent. The other constituents are a
small amount of fat, glycogen (4 to 1 per cent.), inosite, and a number
of nitrogenous extractives including creatine, xanthine, and hypo-
xanthine ; the most important of these is creatine, which forms 0°2 to
0-4 per cent. of the muscle.
At a variable period after death the proteins coagulate, the product
being called myosin, and the muscles become rigid and opaque; this
condition is known as rigor mortis.
The nature of the proteins in fresh muscle can be studied if
coagulation is delayed by cooling the muscle. The living muscle is
minced at a temperature of 0° C., and is then extracted with ice-cold
saline solution (0°9 per cent. NaCl) and filtered ; the filtrate contains
two proteins, namely paramyosinogen and myosinogen. The former is a
globulin which coagulates at 47° to 50° C., and constitutes about 20 per
cent. of the total protein in muscle. The remaining four-fifths consist
of myosinogen, which has the characters of an albumin, though it
coagulates at the low temperature of 56° to 60° C. When the solution
20 ESSENTIALS OF PHYSIOLOGY.
is warmed it clots, the proteins being converted into myosin, though the
myosinogen passes through a transition stage as soluble myosin, which
clots at 40° C.; in the frog’s muscle soluble myosin is present as such
in the living muscle. The coagulation of muscle plasma can be —
prevented by the removal! of calcium salts, but there is no evidence
that it is brought about by a ferment.
Resting muscle is very extensible and can be stretched by applying
a weight to it; it is also feebly but perfectly elastic, and returns
completely and rapidly to its original length when the weight is
removed. | |
THE CONTRACTION OF VOLUNTARY MUSCLE.
In response to a stimulus a living muscle alters its form, becoming
shorter and thicker; this change of form constitutes muscular
contraction, and can be very easily studied in the muscles of the
frog. For this purpose the gastrocnemius muscle and the sciatic
nerve which supplies it are generally used, and are known as a muscle-
nerve preparation. The- fibres in the gastrocnemius do not run
regularly from end to end of the muscle, and when it is desirable to use
a muscle the fibres of which run approximately parallel to one another,
the sartorius may be chosen. .The muscle may be made to contract
either by a mechanical stimulus such as a pinch, or by a chemical
stimulus, for instance the application of acid or ammonia, or by an
electrical current. On account of the ease with which its strength can
be graduated, the electrical current is the most convenient of these
artificial stimuli, and it may be applied either as a constant current or
in the form of single or repeated induction shocks. The normal
stimulus to muscular contraction during life is an impulse passing from
the central nervous system along the nerve which ends in the muscle
fibres. In a musele-nerve preparation, the muscle contracts either
when it is stimulated directly or when the stimulus is applied to the
nerve,
It was formerly supposed that even when the stimulus was applied
to the muscle itself the latter did not respond directly to the stimulus,
but contracted because this acted upon the nerve fibres running within
the muscle. There is no doubt, however, that muscle can be excited
to contract independently of impulses reaching it along nerve fibres
(independent irritability of muscle). Curare paralyses the endings of
nerve fibres in muscle, and when it is injected into an animal stimula-
tion of a motor nerve has no effect upon the muscle, whereas direct
stimulation: of the muscle causes it to contract.
The changes which take place in a contracting muscle are (1) a
MUSCLE. 21
change of form and physical condition, (2) an electrical change,
(3) chemical changes, and (4) evolution of heat.
Changes in Form and Physical Condition.—These may be studied
in the muscle-nerve preparation of a frog by fixing the upper attach-
ment of the gastrocnemius muscle and attaching the tendo Achillis by
a thread to the short arm of a lever; the long arm carries a writing
point, and a weight can be attached to it if so desired. The muscle,
when it contracts, pulls on the lever, and the movement of the writing
point can be recorded on a smoked moving drum.
When a single induction shock of suitable strength is applied to the
muscle, it responds by a single contraction or twitch. A graphic record
of such a contraction is shown in fig. 2.
/ \
| /
/ \
/
/ \
’ /
Fi \
j /
| / \
J \
Pe? Ev ar ea SONY ease
be |
SVP AP DDD LD DD PDP DPS LD PDD DDS.
Fic. 2.—Simple muscle twitch. ©
The vertical line marks the moment at which the stimulus was applied. Below
is a time-record, each double vibration representing ,;}, second.
The contraction lasts about one-tenth of a second, and the tracing
shows three parts, namely, (1) a short latent period, during which the
muscle shows no visible change ; (2) a period of shortening of the muscle,
whereby the lever is raised ; and (3) a period of relaxation, during which
the muscle returns to its former length. The length of the latent
period is due partly to the inertia of the lever and recording apparatus.
This source of error can be avoided by interposing a muscle between a
source of light and a rapidly moving photographic plate ; the thickening
of the muscle, when it contracts, is photographed, and the interval
between the application of the stimulus and the beginning of contrac-
tion is measured. This interval, which is only 0°0025 second, repre-
sents the true latent period. The relaxation is not an active process,
but is due to the weight of the lever pulling the muscle back to its
former length; if an isolated muscle lying on mercury, and_ there-
fore not subject to any tension, is made to contract, it relaxes very
imperfectly.
22 ESSENTIALS OF PHYSIOLOGY.
A certain strength of stimulus.is necessary in order to produce any
visible shortening of a muscle ; a further increase in the strength of the
stimulus, beyond this point, causes the muscle to contract more strongly,
until finally the contraction becomes maximal. The varying degree of
the response of the muscle to varying strength of stimulus is due to the
fact that a weak stimulus affects only a few fibres, whereas a strong
stimulus throws into contraction a large number of fibres; when all the
fibres are stimulated, the shortening is maximal. Each fibre, however,
if it contracts at all, gives the maximal contraction of which it is capable
for the conditions under which it is placed, whatever the strength of the
stimulus, This is known as the “all or none law,” and holds good
whether the muscle is stimulated directly or through its nerve.
When a muscle contracts, the contraction travels from the point of
stimulation in the form of a wave at the rate of 5 to 6 metres per second
in mammals and 3 to 4 metres in cold-blooded animals. This rate can be
measured by resting two levers on a muscle, one at the middle, the other
at one end, and applying a stimulus to the opposite end of the muscle.
The lever nearer the stimulated point will rise earlier than the one at
the opposite end of the muscle; and if this interval is measured and
the length of muscle between the two levers is known, the rate at
which the wave travels can be calculated. The length of the wave is
measured by multiplying the rate at which it travels by its duration at
any one point ; it varies in frog’s muscle from 150 to 300 millimetres.
A rise of temperature quickens, and a fall of temperature delays,
BES phase of the contraction. If the muscle is lifting a small load,
the lever often rises higher at a high than at a low temperature, since
the sudden jerk given to the lever by the rapid contraction imparts to
it a greater momentum than when the pull on the lever takes place
more gradually. On the contrary, when a heavy weight is attached to
the lever, a slowly contracting cooled muscle may be more effective in
raising the lever than the rapidly contracting warmed muscle. These
differences are of purely mechanical origin, and the actual force of the
contraction for a given load and for the same strength of stimulus
remains unchanged between 5° C. and 20° C. Cooling increases the
excitability of muscle, and maximal stimuli should therefore be used in
studying the effect of temperature on the height of contraction.
Prolonged exposure to a temperature of 0° C. destroys the vitality of
muscle.
If a muscle is repeatedly stimulated, the height of the contractions
diminishes, and all phases of the contraction, including the latent
period, are prolonged; finally, the muscle may fail to contract in
response to a stimulus. This condition constitutes fatigue.
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t
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f
Lf
MUSCLE. | 23
When a weight is attached to the lever the height of the contrac-
tion diminishes as the weight is increased, until, finally, the muscle
fails to raise the weight at all. The product of the weight raised,
and the height through which it is lifted, represents the work done by
the muscle ; up to a certain point, an increase in the weight raised
increases the work done by the muscle.
Contracting muscle differs from resting muscle in being more
extensible and more elastic, that is, it returns to its original length
more rapidly than resting muscle when the force stretching it is
removed.
Lsotonic and Isometric Curves.—When a muscle lifts a weight
attached to a lever, the weight follows the movement of the lever,
and the pull or tension which it exerts upon the muscle remains un-
changed during the contraction; the curve thus obtained is called
wsotonic. The muscle may be made, however, to pull against a strong
spring so that its length remains almost unaltered during its contrac-
tion, although it exerts a varying tension upon the spring ; the minute
movements of the spring are magnified and recorded photographically.
In this case the contractile stress set up in the muscle varies
throughout the contraction, though the length of the muscle undergoes
no appreciable change. Curves thus obtained are called isometric, and
resemble isotonic curves in their general form.
Tetanus.—lf a. second stimulus is sent into a muscle before the
shortening caused by the first stimulus is at an end, the muscle
shortens still further, this being called summation of effects. When
the stimuli are rapidly repeated (fifty or more per second), the muscle
contracts very strongly, and remains contracted so long as the stimuli
are continued ; the prolonged contraction is known as tetanus.
Constant Current.—When a constant current is passed through a
muscle, it enters at one point called the anode, and, after traversing
the muscle, leaves it at the kathode. When the circuit is completed,
v,e. at the make of the current, the muscle gives a single twitch, and
there is another twitch when the current is broken; during the
passage of the current the muscle remains relaxed. The contraction
taking place when the current is made starts at the kathode, whereas
the contraction occurring at the break of the current starts at the
anode. This can be observed by clamping a skeletal muscle in the
middle and attaching a lever to each end; one electrode is placed on
each half of the muscle. When the current is made, the portion of
the muscle connected with the kathode contracts first, and the lever
attached to this half of the muscle rises before the other lever. The
same phenomenon is still more readily seen, and can be directly
fe
24 ESSENTIALS OF PHYSIOLOGY.
observed, in slowly contracting muscles, such as the cardiac muscle
of the frog. }
Electrical Changes in Muscle.—When a constant current is passed
into a muscle through metallic electrodes, electrolysis takes place in
the muscle; the electrolytes accumulate near the electrodes, and give
rise to a current which passes in the opposite direction to that in
which the constant current is passing, and tends to neutralise the
latter. This phenomenon is known as polarisation, and in recording
the electrical currents occurring in muscle it is BOUPEREEY.* to use non-
polarisable electrodes.
The electrical currents in muscle may be observed. by means of.
either the capillary electrometer or the string galvanometer. The
\*
a : :
WY cy Mane dE p\q—5
: Y
a eh
CAMERA
Fie. 3.—Scheme of string galvanometer.
a, bis the quartz thread; N and S are the electro-magnets; E is a microscope.
The magnified i image falls on the slit H, and is photographed. (Hume.)
capillary electrometer consists of a glass tube drawn out at one end
to a capillary, and partly filled with mercury; the capillary tube
opens into another tube containing 10 per cent. sulphuric acid. Two
platinum wires pass, one into the sulphuric acid, the other into the
mercury. When the electrometer is connected with two points of
different potential, the mercury moves up or down the capillary tube.
If the point connected with the acid is negative as compared with that
connected with the mercury, the latter moves down towards the acid ;
if the point connected with the acid is positive, the mercury moves
away from the acid. The movements are rapid and are proportional
to the difference of potential ‘between the two points under observation ;
they may be directly observed under the microscope, or may be recorded
photographically.
The string galvanometer consists of a very delicate quartz thread,
silvered over and hanging between two strong electro-magnets through
which a current is passing; by means of terminals passing from each,
end of it, the thread can be connected with a muscle or other structure.
Ee
/
MUSCLE. | 25
When a current passes through the thread, it is pulled towards one
or the other magnet according to the direction of the current. The
thread is illuminated by a strong light, and a magnified image of its
movements can be thrown on a screen and observed directly ; and if
the screen is replaced by a moving photographic plate, these movements
can be recorded. .
When a perfectly uninjured muscle, ¢.g. the sartorius of a frog, is
connected by means of non-polarisable electrodes at B and, C (fig. 4),
with the string galvanometer, no electrical current can be detected,
and the muscle is said to be <soelectric. The application of a single
induction shock at the point A causes a contraction travelling as a wave
Fic. 4.—Diagram to show the method of investigating electrical changes in muscle.
D, Eis the thread ; F and G are the electro-magnets of the string galvanometer.
from A to C, and at the same time an electrical change is recorded by
the galvanometer.
The stimulus produces. a chemical change, which travels along
the muscle at the same rate as, and which precedes, the wave of
contraction. As this wave of excitation passes along the muscle, it
gives rise to an electrical change of such a kind that the excited part
of the muscle is negative to the resting part. The electrical changes
also precede the mechanical shortening of thg muscle ; they take place
chiefly during the latent period, and mre tontgieed long before the
contraction is over.
Since excited muscle is ES to resting muscle, the electrical
change is diphasic, as is shown diagrammatically in fig. 5. The first
movement, of the thread (represented by the ascending part of the
curve in the figure) occurs when the muscle at B is excited, and becomes
negative to the resting muscle at C; the current flows through the
26 ESSENTIALS OF PHYSIOLOGY.
galvanometer from C to B. When the excitatory process reaches C,°
and is still present at B, there is no electrical current and the thread
swings back to its original position. This period is extremely short,
and almost immediately the excitatory process passes off at B, though it
is still present at C; the point C is now negative to B and the thread
swings once more, this time in the opposite direction. Finally the whole
muscle ceases to be excited and the thread returns to its resting posi-
tion. This diphasic current is known as the cwrrent of action.
If the muscle is injured at the point C, a current flows through the
galvanometer from B to C.even when the muscle is resting, and is
called the current of rest
or injury. If the muscle .
is thrown into contraction,
the point B becomes elec-
trically negative as com-
pared with its resting state,
and then returns to its
former condition; the in-
jured muscle at C does not
contract and its electrical
condition remains un-
changed. The record of
the galvanometer therefore
shows a single (monophasic)
Fic. 5.—Diphasic electrical change in muscle cetoeeee ; Rae a“ a:
(diagrammatic). rent of action in this case
The horizontal line represents the position of the thread flows in the opposite direc-
when at rest. . , etl
tion to that of the injury
current, it is often spoken of as the negative variation of the injury
current.
Thus it is evident that both injured and contracting muscle are
negative to normal resting muscle. These changes are abolished by
the death of the muscle, and are thus bound up with the chemical
changes taking place in a contracting or injured muscle. They are
independent, however, of the mechanical shortening, and still occur*in
an excited muscle even when its power to contract is abolished by
steeping it for a short time in distilled water.
Although it is customary to speak of excited muscle as “negative ”
to resting muscle, it must be remembered that it is really electro-
positive to the resting muscle; in the same way, zinc is the electro-
positive element in a battery. The term “negative” simply means
that the current passes through the galvanometer towards the excited
EES ee
¥
MUSCLE. 7 -
tissue, and has no reference to the actual electrical condition of this
tissue.
The electrical changes in muscle can also be demonstrated without
the aid of a galvanometer. Two muscle nerve preparations are made,
and the nerve of one preparation A is brought into contact at two
points with the muscle of the other preparation B. When the muscle
of B contracts, the current of action set up in it acts as a stimulus to
the nerve of A; this stimulus is conducted to the muscle of A which is
thrown into contraction. :
Chemical Changes during Contraction.—Living muscle is constantly
taking up oxygen from the blood and giving off carbonic acid. During
contraction it takes up more oxygen and gives off more carbonic acid,
and at the same time heat is evolved and lactic acid is formed. The
production of acid during muscular contraction can be readily demon-
strated in a frog’s muscle-nerve preparation ; if the muscle is made to
contract vigorously for a few minutes it becomes acid to litmus paper.
Another method of illustrating the same fact consists in injecting into
the dorsal lymph sac of.a frog a solution of acid fuchsin, which is
colourless in neutral and red in acid solution. When one gastro-
cnemius muscle is made to contract an hour or two after the injection,
the contracting muscle becomes red, whereas the resting muscles are
not coloured.
It can be shown that the acid formed is lactic acid by means of
Hopkins’ test.
A few drops of an alcoholic extract of muscle, two or three drops of a
saturated solution of copper sulphaté, and 5 c.c. of strong sulphuric acid are
mixed in a test tube and placed in boiling water for a minute or two. The
fluid is cooled and a few drops of alcoholic thiophene solution are added ; on
warming the solution, a cherry red colour develops if lactic acid is present.
In all probability, when a muscle contracts the first chemical change
taking place is the breaking down of a compound of dextrose with some
other substance the nature of which is unknown, the decomposition
resulting in the formation of lactic acid; in the presence of an
adequate supply of oxygen, the lactic acid can be subsequently
oxidised to carbonic acid and water according to the following
equation :— ,
C,H,O, + 30, = 3CO, + 3H,0.
lactic acid
This is shown by the observation that an isolated frog’s muscle,
when made to contract in an atmosphere free from oxygen, gives off
less carbonic acid and contains more lactic acid than_when it: contracts
in an atmosphere containing oxygen. Further, if a muscle which
ig
28 ESSENTIALS OF PHYSIOLOGY.
contains lactic acid is exposed for some hours in pure oxygen, the
amount of lactic acid in it decreases. Evidently the muscles possess
the power of destroying lactic acid. |
This process also takes place in the living animal, but if the supply
of oxygen to the muscles is deficient, e.g. during asphyxia, or if the
formation of lactic acid is very rapid, for example in actively contract-
ing muscles, the acid is not completely oxidised and passes into the
blood and may be excreted in the urine. In man the excretion of
lactic acid in the urine is 3 to 4 mgr. per hour during rest ; and this may
be raised by severe exercise to 400 mgr. or more hourly. Hence the
appearance of lactic acid in increased amount in the blood and urine
may be taken as evidence that the supply of oxygen to the muscles
is either absolutely or relatively deficient. Owing to the passage of
lactic acid into the circulating blood, the muscles themselves do not
become acid in reaction even after severe exercise, though it is said
that the muscles of animals hunted to death are acid.
It is probable that part of the lactic acid is not oxidised, but is
synthesised again by the muscles into the carbohydrate compound by
the decomposition of which it was originally formed.
The chemical changes taking place in contracting muscle are not
confined to the process just described, and the oxidation of carbo-
hydrate, fat, and protein is also increased during muscular activity.
Heat Production in Muscle.—During muscular contraction heat
is produced, and the contraction of a large number of muscles, such
as occurs during muscular exercise, may be sufficient to raise the
temperature of the whole body one or two degrees. The heat produced
in a small isolated muscle during a single contraction cannot be
measured by a thermometer, but is usually determined by means of
a thermopile, which consists of a junction between two metals, the |
metals being connected with a galvanometer. When the junction is
heated, an electrical current is set.up and passes through the
galvanometer. In the more recent forms of thermopile the metals
used are copper and an alloy called constantan, and the thermopile
may consist of a large number of such junctions, which are connected
with a string galvanometer. A muscle, such as the frog’s sartorius, is
placed in contact with these junctions, and can be made to contract
by sending a current through electrodes placed one at each end of the
muscle. The muscle is fixed at one end and attached at the other,
either to a recording lever or to a spring, whereby the tension on the
muscle can be varied or the changes in contractile stress occurring
during its isometric contraction can be studied. Any production of
- heat in the muscle gives rise to a current through the thermopile, and
te Pr
MUSCLE. ; 29
to deflection of the thread of the galvanometer; the amount of
deflection produced by a unit of heat is previously determined.
Experiments made with this apparatus show that the production
of heat in muscle occurs both during and for some time after its
contraction.
(1) When a muscle contracts, a certain amount of potential energy
is liberated as free energy, which can be used in the carrying out of
work, or can be evolved as heat, or may be manifested partly as work
and partly as heat. The amount of the energy set free and available.
for this purpose depends upon the initial length of the muscle fibre.
If the muscle is stretched by a spring or weight, the energy set
free and the contractile stress, that is, the force with which the fibres
tend to contract, are greater than when it is not stretched. The
proportion of the energy thus set free, which appears as work or as
heat respectively, depends upon the mechanical conditions under which
the muscle is placed. If the muscle is allowed to lift a weight, it does
work which is measured by the product of the weight raised and the
height through which it is lifted ; and if the mechanical conditions are
favourable, the greater part of the energy set free during the contrac-
tion may appear as work. If the muscle is not allowed to contract and
to do work, the whole of the energy set free during the contraction
appears as heat. |
(2) After the contraction is over, a further evolution. of heat takes
place, which is due partly to the oxidation of lactic acid, and partly
also to the rebuilding of the precursor of lactic acid, the energy needed
for this process being supplied by the oxidation of other substances in
the muscle with the liberation of heat and of carbonic acid.
The efficiency of muscular contraction is the proportion of the total
energy set free during and just. after the contraction, which appears as
work. The mechanical conditions under which muscular contraction
takes place in the body are usually such that the efficiency is com-_
paratively constant; in man, from 20 to 28 per cent. of the total energy
set free during muscular contraction appears as work.
Fatigue and Rigor Mortis.— When a muscle passes into rigor mortis
it shortens slightly, becomes rigid and opaque, and loses its elasticity
and extensibility. During its passage into rigor the muscle becomes
acid, the acid formed being lactic acid, and heat and carbonic acid are
evolved.
These changes are all due to the breaking down of the carbohydrate
precursor of lactic acid. In this process heat is evolved, and the lactic
acid reacts with the sodium carbonate in muscle, setting free carbonic
acid, which is given off. When the amount of lactic acid formed in the
30 ESSENTIALS OF PHYSIOLOGY.
muscle attains a certain level, the proteins are coagulated and the
physical characters of the muscle are altered. It is for this reason
hat the muscles in fatigued animals pass into rigor mortis more rapidly
after death than the muscles of resting animals, since at the time of
death they already contain some lactic acid. That the accumulation
of lactic acid in muscle can cause fatigue is shown by the fact that the
contraction characteristic of fatigued muscle can be induced in the fresh
muscle by perfusing it with blood containing lactic acid. Fatigue is
thus brought about in part by the same cause as that which ultimately
leads to rigor mortis; but the larger amount of acid produced in the
latter brings about an irreversible change in the proteins, causing the
death of the muscle, whereas the acid is gradually removed from a
fatigued muscle, which is thus restored to its normal condition.
When a muscle is repeatedly stimulated through its nerve, it
gradually becomes fatigued and finally fails to contract. When this
point is reached the muscle will still contract, if directly stimulated,
so that its failure to contract when stimulated through its nerve must
be due to fatigue of either the nerve trunk or the nerve endings. It
can readily be shown that it is the nerve endings which become
fatigued in such an experiment. |
Two muscle nerve preparations are made, and the nerves are
continuously stimulated with a tetanising current, the passage of the
stimulus to one muscle being prevented by cooling a small portion of
the nerve near the muscle to 0° C.; in this way both nerves are
stimulated, but only one muscle is thrown into contraction. After a
short time the muscle which is contracting becomes fatigued and fails
to contract. The cooled nerve is then warmed, the stimulation being
continued, and the muscle of this preparation at once contracts
vigorously. Since both nerve trunks have been equally stimulated, it
is evident that the fatigue does not lie in the nerve trunks and must
therefore have its seat in the nerve endings.
Experiments of this kind have shown that both medullated and
non-medullated nerves are practically incapable of fatigue, and even
after six hours of continuous excitation the nerves are just as excitable
as at the outset.
VOLUNTARY CONTRACTION.
The contraction of a muscle under the influence of the will is much
longer than a single twitch and represents a short tetanus, since even
the quickest movement which an individual can carry out voluntarily
lasts at least one-tenth of a second, and usually longer. The nature of
voluntary movement has recently been demonstrated by recording the
MUSCLE. gt
electrical changes occurring during a voluntary contraction. Using ?
the string galvanometer, about 50 electrical variations per second can
be observed in the contracting muscle; and if the electrical variations _
of a motor nerve during a voluntary or reflex contraction of the muscle
which it supplies are similarly recorded, it is found that about 50
impulses per second are passing along the nerve. When a skeletal
muscle is stimulated 50 times a second it passes into tetanus ; and it may
be concluded that voluntary muscular contractions are almost always
tetanic in character, and are brought about by the discharge of rapidly
repeated impulses from the cells of the central nervous system.
UNSTRIATED MUSCLE.
The muscle fibres which occur in the walls of the digestive tract,
blood-vessels, and other organs show no transverse striation and are
called plain or involuntary muscle fibres. Each fibre is spindle-shaped
and has an oval nucleus ; its cell substance frequently shows a delicate
longitudinal striation. The fibres are united to one another by a
cement substance which can be stained with silver nitrate.
The changes taking place in smooth muscle, when it contracts,
differ in many respects from those occurring in skeletal muscle. In the
first place, the duration of the contraction is very prolonged ; the
latent period may be from 0°2 to 0°5 second, and the contraction may
last for two or three minutes. Secondly, the muscle is much more
easily excited by a constant current than by induction shocks, and
frequently fails to give any response to a single induction shock.
Owing to the slowness of the process, the origin of the contraction at
the kathode when a constant current is made, and at the anode when
the current is broken, can be observed with great ease.
Thirdly, smooth muscle shows a great tendency to contract rhyth-
mically, the rhythm being most easily evoked when the muscle is
stretched. The effect of tension is often well seen in the hollow organs
whose walls are partly composed of smooth muscle. If such an organ
is rapidly distended by the injection of fluid, the sudden tension
placed on the muscle fibres in its wall causes them to contract forcibly,
the contraction being sometimes continued in a rhythmic manner for
a short time. Smooth muscle is also susceptible to chemical stimuli
-
a
ri
ad |
“e
fo
’
1
J
™
hs
ie
AYy
7
7
a ky
3 and can be thrown into contraction by substances such as barium salts.
rt One of the most characteristic features of smooth muscle is its
a power to remain in a state of partial contraction or tone, even after
a it is cut off from any connection with the central nervous. system.
FP Stimulation of the nerves to the muscle may bring about either an
=s increase or a decrease of this tone; and most unstriated muscles have
ESSENTIALS OF PHYSIOLOGY.
a double nerve supply, one set of nerves when stimulated causing con-
traction, and the other relaxation of the muscle.
CILIARY MOVEMENT.
Cilia are delicate filaments projecting from the border of columnar
epithelial cells, and are found in the greater part of the respiratory
passages, in the generative organs, and elsewhere.
Their movement consists in a rapid bending of each cilium, followed
by a slow return to the erect position. Ali the cilia on a ciliated
surface bend in the same direction, the movement travelling as a wave
over the surface. The movements are repeated ten to twelve times a
second, and serve to carry forward solid particles of dust or mucus in.
the respiratory passages ; in the Fallopian tubes they assist the passage
of the ovum.
Ciliary movement is entirely independent of the central nervous
system, and its mode of production is unknown. .
wie? i.
hae
-
CHAPTER IV.
NERVE FIBRES.
Tue nerve fibres form the medium by which the various structures of
the body are brought into communication with the central nervous
system, and thus indirectly with each other. The fibres for the most
part leave the central nervous system in bundles which are bound
together to form nerve trunks. The nerve trunks give off branches in
their course, the ultimate ramifications consisting of individual fibres,
Nerve fibres are of two kinds, medullated and non-medullated. A
medullated nerve fibre is a cylindrical structure and consists of the
nerve fibre proper, known as the axis cylinder, surrounded by two
sheaths. The outer sheath is a transparent structureless membrane
called the neurolemma. The inner sheath is thicker, and is ce baposed
of a refractive material of a fatty nature, called myelin. The. ye.
sheath is interrupted at regular intervals, the interrupti s bei
known as the nodes of Ranvier. About midway between ea
nodes of Ranvier a nucleus is found, lying under the neurolemma and
surrounded by a small quantity of protoplasm.
A non-medullated nerve fibre consists of axis cylinder rae neuro-
lemma. with nuclei, and has no myelin sheath.
The axis cylinder of a nerve fibre is a process of a nerve cell,
usually the process known as the axon. It consists of an aggregation ©
of fine fibrils, imbedded in an interfibrillar fluid material.
In the nerve trunks the fibres are bound together in cylindrical
cords or funiculi, each of which is enclosed in a sheath of dense
connective tissue, the perineurium. The funiculi are held together
in the nerve trunk by looser connective tissue, the epineurium. Both
medullated and non-medullated nerve fibres are usually present in a
nerve trunk, such as is found in the limbs. © ;
When a nerve fibre is traced to its peripheral termination it is
found to end either in free fibrils or in a special end-organ. Free
fibrillar endings occur in the epithelium of the skin, around special
sensory cells, such as those of the taste buds and of the organ of
33 3
.
34 ESSENTIALS OF PHYSIOLOGY.
hearing, around gland cells and in other situations. Terminations
in special end-organs are found in the touch corpuscles of the skin
and in the end-plates in striped muscle fibres. |
The chief chemical constituents of nerve fibres are protein, nucleo-
protein, lecithin, and cholesterol. The myelin sheath of medullated
fibres is mainly formed of lecithin.
THE FUNCTION OF NERVE FIBRES.
Nerve fibres have one function only, that of the conduction of
impulses. Generally speaking, any one nerve fibre conducts in one
direction only, and the fibres are classified as efferent and afferent,
according as they conduct impulses away from or towards the central.
nervous system. The direction in which an impulse normally passes
along a nerve depends upon the connections of the fibres and not upon
a property of the fibres themselves, these being in reality capable of
conducting in either direction. Thus in a motor nerve fibre the
excitatory process originates in the central nervous system. and is
propagated towards the muscle, and in a sensory nerve the excitatory
process is set up in the nerve-ending, for example in the skin, and is
propagated towards the nerve centres.
The function of nerve fibres has been studied chiefly by means of
artificial stimuli, mechanical, thermal, electrical, and chemical, motor
nerve fibres being generally used for experiment because the propaga-
tion of an impulse in a motor nerve ‘is easily demonstrated by the
resulting muscular contraction. Two other phenomena accompany or
follow the propagation of a disturbance along a nerve: (1) an electric
response, and (2) the occurrence of a refractory period. (1) The
electric response is detected by the use of a galvanometer, and consists
in a brief wave of negativity which accompanies the disturbance, the
excited part of the nerve being negative to the resting part. (2) The
refractory period follows immediately the passage of the disturbance,
and lasts about one-thousandth of a second. During that period a
second stimulus applied to a motor nerve fails to excite a muscular
contraction.
In the study of the function of nerve fibres two points have to
be considered, the excitatory process and the propagation of the
resulting disturbance.
The Excitatory Process.—Nothing is known as to the physico-
chemical nature of the normal excitatory process. A similar condition
may, however, be set up artificially in various ways. For example,
cutting or pinching a motor nerve, or the sudden application to it
of heat, or dipping it in Strong salt solution or in glycerol, or the
ar. aa <_— ee
tl i a
; 1 ;
NERVE FIBRES. — 35
application to it of electrical stimuli, will be followed by contraction
of the muscle with which it is connected: . The’ most convenient form
of stimulus for experimental purposes is the electric current.
Induced Current.—Induced electricity is generally used for purposes
of stimulation, because its strength can be easily controlled and also
because of the brief duration-of the current. When a single induction
shock of sufficient strength is applied to the nerve of a muscle-nerve
preparation, (1) an excitatory process is set up in the nerve at the point
stimulated, (2) a disturbance arising at that point is propagated alone
the nerve to the muscle, and (3) the muscle contracts.
Constant Current,—lf a length of nerve is introduced into the
circuit of a constant current, an excitatory process is only set up in the
nerve, as in muscles, when the circuit is made or broken, and not
during the passage of the current. The passage of the current is
nevertheless accompanied by changes in the excitability, conductivity,
and electromotivity of the nerve, which together constitute the condi-
tion known as electrotonus. The changes in excitability may be
shown in the nerve of a muscle-nerve preparation from rom the frog. The
nerve is laid upon a pair of non-polarisable electrodes connected with
a galvanic cell, a reverser being introduced into the circuit. The
electrodes from an induction coil are applied to the nerve between the
non-polarisable electrodes and the muscle. The position of the induc-
tion coil which gives a minimal effective stimulus is found before the
constant circuit is made, then the key of the latter is closed, and
the stimulus from the induction coil is repeated. If the positive pole
or anode is the nearer to the stimulating electrodes, the stimulus will
now be ineffective, because the stimulated area of the nerve is in a
condition of anelectrotonus or reduced excitability. If the direction of
the constant current be reversed, repetition of the stimulus will give
a maximal contraction of the muscle, because the stimulated area is in
a condition of kathelectrotonus or increased excitability. Associated
with this electrotonic variation in excitability is the fact that, as already
pointed out in connection with the stimulation of muscle, the excitatory
process set up by the closing of a constant circuit occurs at the kathode,
and that set up by breaking the circuit at the anode. In other words,
the excitatory process is produced by the setting up of Kashslectistonnal
and by the resolution of anelectrotonus.
The Propagated Disturbance.—The propagation of an impulse along
a nerve may be demonstrated by the effect produced in an organ, such
as muscle, or by recording the electric response by means of a capillary
electrometer or string galvanometer. For the latter purpose the nerve
is included in the galvanometer circuit, non-polarisable electrodes
o
36 ESSENTIALS OF PHYSIOLOGY.
being used. A single induction shock is applied to the nerve at a
distance from the non-polarisable electrodes, and the movement of the
mercury in the capillary electrometer or of the string of the string
galvanometer is photographed on a rapidly moving plate. In this way
it is shown that the part of the nerve which is excited at any intent is
negative to all other parts of the nerve.
By recording the electric response, it can be shown that when any
part of a nerve is stimulated the disturbance is propagated in both
directions. If the middle of a length of nerve is stimulated by a single
induction shock, each end of the nerve being in circuit with a galvano-
meter by means of non-polarisable electrodes, both galvanometers record
the passage of a current of action. Apart from the electric response, .
the changes produced in a nerve by the passage of an impulse are very
slight. No appreciable production of heat can be detected by the most
- delicate thermo-electric methods. Chemical changes must take place,
because the nerve fibres lose their irritability when completely deprived
of oxygen, and the loss of irritability occurs more rapidly if the nerve
is stimulated ; but the true nature of the chemical changes is unknown.
Possibly the changes which occur during the propagation of the dis-
_ turbance are largely physical, and depend upon changes of surface
tension. °
‘The Velocity of a Nervous Impulse.—The rate at which an impulse
is transmitted in the nerve of a frog may be measured by using a
muscle-nerve preparation and recording the contraction of the muscle
on a rapidly moving drum or pendulum myograph, (1) when the stimulus
is applied as far as possible from the muscle, and (2) when it is applied
close to the muscle. The two tracings are recorded on the same
abscissa, the point of stimulation being at the same point of the tracing
for both.. The difference in latent period is measured by means of a
time tracing taken from a tuning-fork giving 250 double vibrations
per second. This difference gives the time taken by the impulse to
travel along the length of nerve between the two points stimulated.
In the case of frog’s nerve the rate of transmission is found to be about
28 metres per second. In human nerves it is estimated to be about
four times this rate.
Conditions which affect Excitability and the Propagation of the
Disturbance in Nerve.—It has already been pointed out (p. 30) that
nerve fibres cannot be fatigued. Their excitability and conductivity
may, however, be affected by temperature, drugs, and the passage of a
constant current, as well as by the passage of a pea impulse along
the nerve, and by injury.
Generally speaking, the irritability of nerve for induction shocks is
:
i a! io le
ital a
-
x
NERVE FIBRES. 37
"increased by a rise of temperature, and diminished by cooling. With
currents of longer duration the converse effect is obtained, within limits,
the excitability being increased by cooling the nerve because by the
fall of temperature the subsidence of the excitatory process is delayed,
although at the same time the initiation of a propagated disturbance is
hindered.
The effect of volatile drugs on nerve is tested by edebociete a- length
of the nerve to a muscle in a glass tube which is made air-tight, and
filling the tube by a side connection with the vapour of the drug to be
tested. Carbonic acid gas and ether abolish both the excitability and
conductivity of nerve, the excitability being the earlier to disappear.
If the gas or vapour be replaced by pure air, the nerve returns to its
normal condition, conductivity returning before excitability. Chloro-
‘form acts on nerve in the same way as ether but more powerfully,
recovery being incomplete or not occurring at all when the drug is
replaced by air.
The effect of the passage of a constant current on the excitability
_of nerve has already been described. The effect on conductivity is
fairly parallel with that on excitability.
The refractory period which féllows the stimulation of a nerve is a
result of the passage of the propagated disturbance, and not merely a
local condition of depressed excitability following the excitatory process.
This is proved by sending the second stimulus into the nerve at a
different point from the first, in which case the refractory condition is
found to be the same as when the second stimulus is applied at the same
spot as the first. The refractory state does not end abruptly, but
passes off gradually, so that there is a gradual fall in the strength of
the stimulus required to produce the second response.
Electrotonic Currents in Nerve.—Reference has already been aie
to the fact that the electromotivity of nerve is altered by the passage of a
constant current. Uninjured nerve is isoelectric. If, however, a nerve
is injured, as, for example, by division of its fibres, and is connected
with a galvanometer by means of non-polarisable electrodes, one of
which is applied to the injured area, and the other to the uninjured
surface, a current, the so-called demarcation current or current of
injury, will flow through the galvanometer from the uninjured surface
to the injured part; that is, within the nerve itself the injured
part is electro-positive to the uninjured. If a constant current
(“polarising current”) be passed through a part of the nerve, non- |
polarisable electrodes being used, the demarcation current will be
inereased at the anode, where the polarising current enters the nerve,
and diminished at the kathode. Electrotonic currents are set up in
38 ESSENTIALS OF PHYSIOLOGY.
the nerve which have the same direction as the polarising current.
These electrotonic currents are due to ionisation of the electrolytes
which occur in solution between the electrode and the axis cylinder,
Negatively charged ions are attracted to the anode, with a resulting
concentration of ions which are positively charged on the axis cylinder
near the anode. In the same way, negatively charged ions accumulate
on the axis cylinder close to the kathode. The electrotonic currents
are caused by the differences of potential thus produced.
Electrotonic currents in a nerve can produce the excitatory process
in another nerve in contact with the first. If the nerve of a muscle-
nerve preparation be laid on a second nerve which is stimulated by
single induction shocks, each stimulus sets up electrotonic currents in
the stimulated nerve, and these, setting up an excitatory process in the |
other nerve, lead to contraction of the muscle.
CHAPTER V.
THE CENTRAL NERVOUS SYSTEM.
SECTION I.
Tue essential characteristic of life is the power of reaction to a
stimulus, and in the higher animals this reaction is effected through
the central nervous system, and constitutes reflex action. The life of
the individual is to a large extent made up of a long series of reflex
acts, varying in complexity, and carried out in response to stimuli
arising in the outer world or within his own body.
In response to external stimul, the animal acts as a whole and
carries out movements directed to attack or defence, to the procuring
of food, and the like; in response to internal stimuli, the activities
of the different organs of the body are co-ordinated in such a way
that the individual behaves as such and not as a group of independent
organs.
Instances of this nervous control will be referred to in connection
with the work of the heart, the function of respiration, the production
of the digestive juices, and the other functions which are concerned
with animal life. i
But the activities of the central nervous system are not limited
to the management of these vital processes, In addition, the central
nervous system is the seat of those processes which are concerned with
conscious existence. Impressions are conveyed to the brain from the
outer world, and give rise to sensations of smell, taste, hearing, sight,
and touch, and these in their turn call forth emotions, such as pleasure
or pain. The activities of the central nervous system find expression
in muscular movement, resulting in locomotion, speech, writing, or
gesture. In the study of the various nervous functions, the science
of Physiology has to deal with the mechanism by means of which
afferent impressions are received and conducted to the nerve centres,
with that by which they are associated in these centres, and with the
further mechanism by means of which efferent impulses are transmitted
39
40 A TIALS OF PHYSIOLOGY.
to the active tissues of the body. The consideration of the mental
processes and the emotions is the province of the sister science of
Psychology. :
The nervous system consists of the brain and spinal cord (spinal
medulla), with the nerves by means of which these central organs are
connected with the other structures of the body. Microscopic examina-
tion of the brain and spinal cord shows that they are built up of
nerve cells and herve fibres, supported by a special form of con-
nective tissue called neuroglia. :
The nerves contain nerve fibres only, held together by ordinary
7 connective tissue. Every nerve fibre, however, is a process of a nerve
~~ cell; and if a nerve fibre is divided, the part which is no longer in.
connection with the nerve cell undergoes degeneration. The histo-
logical unit of the nervous system is therefore the nerve cell with
its processes, and this unit is known as a neuron.
The Neuron.—Every neuron consists of a nerve cell, known as a
cyton, and its processes. In the simplest form the cyton is bipolar,
with one process connected with each pole. Examples of this type
are found in the spinal ganglia of fishes and in the ganglion on the
auditory nerve in man. A modification of this form occurs in the
human spinal ganglia, the two processes becoming fused together for
a short distance in the course of development, so that the cell is
histologically unipolar, though physiologically it is still a bipolar cell.
A third form is multipolar, there being more than two processes
connected with each cell. One of these processes is unbranched, and is
known as the axon; the others are branched, and are called dendrons.
The neurons which enter into the structure of the brain and spinal
cord belong to the multipolar type, varying however in the shape of
the cyton, the number and character of the dendrons, and the length
of the axon in different regions,
The cyton, although differing in shape and size in different regions,
is always distinguished by certain definite characteristics. It is usually
large as compared with other cells, varying in diameter from 20 to 100 p.
It possesses a large, spherical nucleus which contains little chromatin,
and within the nucleus a well-marked nucleolus, The cell substance
is distinguished by the presence of certain bodies known as Nissl
spindles and also by having delicate fibrils running through it. The
Nissl spindles are granular aggregations in the form of minute spindles,
as the name implies, are arranged more or less concentrically in relation
to the nucleus, and stain well with methylene blue or toluidin blue.
They appear to consist mainly of nucleoprotein, and also contain some’
iron. They are found in all parts of the cyton and in the basal parts
THE CENTRAL NERVOUS SYSTEM. 41
of the dendrons, but not in the axon hillock, the part of the cell from
which the axon takes origin. The fibrils run through the cell from —
dendrons to axon, and, with some interfibrillar material, form the
substance of the latter structure. Many nerve cells have a well-marked
lymph capsule surrounding them.
The axon is, as has been said, a fibrillar structure, and usually
becomes the axis cylinder of a nerve fibre, acquiring a myelin sheath
soon after leaving the cyton. Axons are of variable length, the longest
being those which extend from the lumbar region of the spinal cord to
the foot. They do not branch as the dendrons do, but those which
run a course in the central nervous system give off delicate twigs
at right angles to their course, known as collaterals. Each axon ends
by a terminal arborisation, either in relation with the cyton or dendrons
of another neuron or in relation with muscle fibres or gland cells. The
termination in connection with striped muscle frequently enters into
the formation of a special form of nerve-ending known as an end-plate.
The axons of certain cells in the central nervous system, belonging to
what is known as Golgi’s second type, are very short, do not acquire
a myelin sheath, and form their terminal arborisation in relation with
the cell -body of a neighbouring neuroti.
Dendrons are found in their most typical form in the neurons of the
brain and spinal cord. They branch in a tree-like manner, and the
branches frequently exhibit minute enlargements or projections. The
dendrons are short as compared with the axon, and never extend any
distance from the cyton.
The processes of nerve cells do not anastomose, but come into
relationship by the more or less intimate mingling of the terminal
arborisation of the axon of one neuron with the dendrovs or cyton
of another neuron. Such a communication, in which there is contact
without continuity, is called a synapse. The contact is possibly not
direct, the transmission of impulses from one neuron to the other being
effected through an intermediate layer of some substance which does
not form part of either neuron.
In addition to the fibrils which have been described as occurring in
the substance of each nerve cell, an extracellular network of fibrils has
been described, but it has not been established that there is continuity
between the two networks. A third fibrillar network las been
demonstrated, chiefly in invertebrate animals, which is said to be
continuous from neuron to neuron, and, on the basis of this description,
a theory has been put forward that nerve impulses are transmitted
through the central nervous system by means of a continuous network
of fibrils and not by a series of synapses. The evidénce upon which
a ae
42 ESSENTIALS ME PHYSIOLOGY.
this theory of continuity rests, however, is at present insufficient to
justify its adoption. ——
The Function of the Neuron.—The type of neuron which is
most easily studied in the mammal is the unipolar form found in the
ganglia on the posterior roots of the spinal nerves. As has already
been pointed out, these cells are functionally bipolar, the single pro-
cess resulting from the fusion of the two poles, and the two processes
separate at some little distance from the cell, one passing towards the
spinal cord and the other towards the periphery. Each becomes the
axis cylinder of a medullated nerve
fibre, but that which has a centrifugal
course is functionally homologous with
the dendrons of the neurons of the
central nervous system. If either of
the two processes is cut off from the
cyton, it degenerates, while the por-
tion left in connection with the cell
body undergoes no obvious change.
\lt may therefore be assumed that the
cyton governs the nutrition of all parts
of the neuron. Moreover, when one
of its processes has been divided in
this way, so that the normal function
of the neuron is interfered with,
changes occur in the substance of the
cyton itself. The Nissl spindles under-
Fic. 6.—Two motor nerve-cells from 20 disintegration, so that the cell stains
the dog. (Photographed from diffusely with methylene blue (fig. 6).
dies alannah * : Sua a This change is known as chromatolysis,
Histology. and it indicates that the Nissl bodies
a, normal; 0, eee of prolonged gre concerned in some way with the func-
tional activity of the neuron. Further
evidence in support of this conclusion is afforded by the fact that the
Nissl bodies of the cells of the central nervous system diminish in number
after an animal has been in active exercise. Chromatolysis also occurs
as a result of the action of certain poisons, in fevers, and in asphyxia.
Conduction of an impulse in a neuron takes place in one direction
only, In the case of the fibres of the posterior spinal nerve roots, the
conduction is from the periphery to the central nervous system. The
anterior root fibres, on the other hand, conduct from centre to
periphery. They are the axons of multipolar nerve cells which lie in
the grey matter of the spinal cord. If the posterior root be divided
a es a a en. fe
*
THE CENTRAL NERVOUS SYSTEM. 43
between the spinal cord and the ganglion, stimulation of the peripheral
portion will have no result, either as regards sensation or muscular or
glandular activity. Stimulation of the central portion will, however,
be followed by sensation, and may result in reflex muscular movements.
If the anterior root be divided, stimulation of the peripheral portion
will be followed by muscular movements, while stimulation of the
central end will give rise neither to sensation nor reflexes. In the
production of a simple reflex by stimulation of the central end of the
divided posterior root, the impulse passes from the terminal arborisa-
tion of the fibre or of its collaterals in the grey matter of the spinal
cord across a synapse to the dendrons of a second neuron, the axon of
that neuron passes it on to the dendrons of one or more of the multipolar
cells the axons of which constitute the anterior nerve roots, and the
impulse, altered in character, is thus transmitted to the responding
muscles. There is thus a law of conduction, called the “law of forward
direction,” according to which an impulse will pass across a synapse
from the axon of one neuron to the dendrons of another, but not in
the reverse direction.
The Function of the Cell.—In a reflex action the afferent impulse
is usually greatly modified in its passage through the central
nervous system. For example, the comparatively slight stimulus of
a crumb in the larynx may be followed by violent coughing, accom-
panied by contraction of other muscles besides those concerned with
expiration. Again, when a reflex movement is excited by stimulation
of an afferent nerve, the impulses travelling along the efferent nerve
have a rhythm which is independent of that of the exciting stimulus.
It was formerly thought that these and other modifications of the
impulses in the nervous system were brought about by the nerve cell.
It has been shown,. however, in certain invertebrates that reflex action
can still take place for a short time when the cells associated with the
fibres forming the reflex arc have been destroyed. This and other
Similar observations indicate that the characteristic features of reflex
action must be attributed not to the nerve cell but to the synapse.
The function of the nerve-cell is purely nutritive.
’ The Function of the Axon.—The function of the axon is most
conveniently studied in the spinal nerve trunks. These contain true
axons, which arise from the multipolar cells in the grey matter of
the spinal cord, as well as afferent fibres in connection with the cells of
the ganglia of the posterior roots of the spinal nerves, the latter not
being axons in the restricted sense of the term, but showing no
difference in function from the axons proper except as regards the
direction in which they normally conduct impulses.
. *
44 ESSENTIALS OF PHYSIOLOGY.
The only function of these nerve fibres. is to conduct impulses.
During the propagation of an impulse an electrical current is produced
in the fibre, and a current is also induced in it by injury. The excit-
ability, conductivity, and electromotivity of the fibre are modified by
the passage through it of a constant current. These phenomena have
been already described in the chapter on Nerve, and need not be
further alluded to in this connection. |
It has already been pointed out that when an axon or other process
is separated from the cyton to which it belongs, the severed part
undergoes degeneration (fig. 7), usually described as Wallerian
degeneration. This may be followed by a growth of fibres from the
undegenerated portion into the distal
part of a divided nerve trunk, the
new growth being spoken of as _ re-
generation.
Degeneration and Regeneration of
Nerves.—-Most nerve fibres contained
in a nerve trunk possess two sheaths,
(1) a covering of lipoid material known
as the myelin sheath, lying next to
the fibre and interrupted at regular
intervals, and (2) an outer, structure-
Fic. 7.—Diagram showing effects of
section of spinal nerve roots. less covering, the neurolemma. Lying
Degenerated portions black. 6, sectionef ynder the latter. about the middle of
anterior root ; a, section of posterior ; 4 x
Beth toote poriehera! Salat of each myelin segment, is a nucleus sur-
. rounded by a little protoplasm.
When a nerve is divided the part which is cut off from the cell soon
dies and undergoes degenerative changes. It shows a temporary rise
in irritability followed by a gradual loss of excitability, and after two to
five days the nerve is no longer excitable and will not conduct impulses.
The degenerative changes take place simultaneously along. the entire
length of the part of the nerve cut off from the cell, and are visible
within twenty-four hours in a warm-blooded animal. The histological
changes in a medullated nerve fibre consist first in a thickening of the
neurolemma, with enlargement of the nuclei and increase of their
surrounding protoplasm, so that the myelin sheath is broken into
segments. A few days later the nuclei are seen to have multiplied and
become more numerous; the myelin is in scattered droplets, and the
axis cylinder is broken up into short lengths. The myelin and axis
cylinder are gradually absorbed by the action of phagocytes, and after
three or four weeks nothing is left but the neurolemma containing
protoplasmic material in which are embedded many nuclei (fig. 8).
weg thn, hth "7S Se ee
AED SURE Sevtinatiniatiieec—-<-nooee
con
THE CENTREL-NERVOUS"SYSTEM. a as
The chemical changes are equally marked. The complex lipoids
which compose the myelin are broken down with the formation of
simpler substances. Lecithin is split up into a nitrogenous base,
choline, a fatty acid, which is usually oleic acid, phosphoric acid, and
glycerol. In consequence of these chemical changes it is possible at
Fie. 8.—Degeneration and regeneration of nerve fibres in the rabbit. (Ranvier. )
From Schafer’s Zssentials of Histology.
A, part of a nerve fibre in which degeneration has commenced in consequence of the section,
fifty hours previously, of the trunk of the nerve higher up ; my, medullary sheath becoming
broken up into drops of myelin; p, granular protoplasmic substance which is replacing the
myelin ; n, nucleus ; g, neurolemma. B, another fibre in which degeneration is-proceeding,
the nerve having been cut four days previously ; cy, axis-cylinder partly broken up. C, more
advanced stage of degeneration. D, commencing regeneration of a nerve-fibre. Several
small fibres, t’, ¢’, have sprouted from the somewhat bulbous cut end, b, of the original
fibre, t; a, ‘an’ axis- cylinder which has not yet ia ye ‘its medullary sheath ; 8, s’, neuro-
lemma of the original fibre.
this pba to distinguish degenerating from normal fibres by means
of Marchi’s fluid, which is a mixture of osmic acid with potassium
bichromate. This fluid stains degenerating myelin black, but leaves
normal nerve fibres unaffected. When all the myelin has been
absorbed, the completely degenerated fibre no longer stains with
Marchi’s fluid; hence this method of identifying degenerating nerve
46 _ ESSENTIALS OF PHYSIOLOGY.
fibres is only available during the first three or four weeks after the
fibres have been cut off from the nerve cells.
When the nerve fibres are completely degenerated, they can be
distinguished from normal fibres by a special method of staining with
hematoxylin called the Weigert-Pal method. The hematoxylin stains
the myelin bluish black, and fibres from which the myelin has dis-
appeared are left unstained. |
When a nerve is divided the portion still connected with the cell
does not degenerate, though changes take place in the nerve cell.
Within one or two days the cell swells and the Nissl granules disappear ;
and after a time the cell shrinks. Later, regeneration usually occurs,
and within three months the cell may return to a normal condition ; in
other cases complete atrophy of the cell takes place.
Regeneration,—After a time regeneration takes place and the nerve
may be restored to a normal condition ; this occurs more rapidly if the
cut ends of a divided nerve are sutured together. Regeneration is
brought about solely by the outgrowth of the part of the axon which
is still in connection with the nerve cell, though the process is
hastened by the presence of the neurolemma of the degenerated fibre,
which seems to assist the outgrowth of the new fibre. The part played
by the nerve cell is shown by the fact that extirpation of the nerve cells
in a portion of the spinal cord prevents regeneration in motor fibres
arising from that part of the spinal cord. Further, if the cut end of
the peripheral portion of a divided nerve is covered by a rubber cap so as
to prevent the growth of new fibres into it, no regeneration takes place.
Histological observations show that in the course of regeneration
small fibrils with bulbous ends extend from the axis cylinders of the
central portion of the divided nerve, and pass into the neurolemmal
sheaths of the degenerated distal portion. The mode of growth seems
to be similar to that which has been observed to occur in the embryonic —
nerve tissue of the frog. If fragments of the primitive nerve tube of a
frog embryo are kept in lymph, the fibres can be seen under the micro-
scope to grow out-from the nerve cell. }
It has been held by some observers that regeneration of the peri-
pheral part of a divided nerve can occur apart from any outgrowth
from the central end of the nerve, the process being called autogenetic
regeneration. The experiments which have just been described show
that this is not the case.
The time taken for a nerve to regenerate and to become functionally
connected with the motor or sensory structures to which it was formerly
attached varies with the distance to be traversed by the outgrowing
fibres, and may be several months; «-; “a~<
THE CENTRAL NERVOUS SYSTEM. 47
Regeneration still takes place if two nerves are divided and the
central end of one is connected with the peripheral end of the other,
provided that both nerves are either afferent or efferent. Thus, if the —
vagus nerve and the cervical sympathetic nerve are divided and the
central portion of the vagus is sutured to the peripheral portion of the
cervical sympathetic, regeneration will occur, and stimulation of the
vagal portion of the united nerve will produce the effects formerly
resulting from stimulation of the sympathetic nerve. This experiment
shows that the effects of stimulation of a nerve are really due to changes
in the nerve ending, and not to any specific change in the nerve itself.
When nerve fibres are divided in their course in the brain or spinal
cord they undergo degeneration, but regeneration never occurs.
SECTION II.
THE SPINAL CORD (SPINAL MEDULLA).
The spinal cord consists of white matter (substance) and grey matter
(substance), the former lying superficially, the latter deeply. The
white matter is composed of medullated nerve fibres, running in a
longitudinal direction, and having no neurolemma. The grey matter
contains numerous nerve cells, arranged for the most part in two horns
(columns), an anterior and a posterior, in each of which the cells form
groups corresponding with the primitive segments of the body,
Numerous fine medullated nerve fibres run into the grey matter, and
terminate by forming arborisations in relation with the nerve cells,
These fine fibres are called collaterals, and they arise at right angles
from the medullated fibres of the white substance. The axons of each
segmental group of cells in the anterior column of grey matter form
an anterior root of a spinal nerve. The corresponding posterior nerve
roots are formed of the axons of the cells in the respective spinal
_ ganglia, and enter the spinal cord in the neighbourhood of the posterior
horn (column) of grey matter. The course taken by these fibres in
the spinal cord will be described later.
The spinal cord consists of two symmetrical halves, separated by the
anterior median fissure in front and by a septum of pia mater called
the posterior median fissure (septum) behind. The grey matter forms
a crescent in each half in a transverse section, the convexity of each
crescent being towards the middle line and being connected with the
convexity of the crescent in the other half of the spinal cord by a
commissure of grey matter. The central canal, containing cerebro-
spinal fluid and lined by ciliated epithelium, lies in this commissure.
48 ESSENTIALS OF PHYSIOLOGY.
In front of the grey commissure, and uniting the white matter of the
two halves of the spinal cord, is the white commissure.
The white matter is subdivided by the posterior horn of grey
matter into an antero-lateral and posterior column (funiculus), The
former is again roughly subdivided by the bundles which form the
anterior nerve roots into an anterior and a lateral column.
THE REGIONS OF THE SPINAL CORD.
Three regions are distinguished in the spinal cord, the cervical,
thoracic, and lumbar, each of which possesses definite structural
characteristics. The cervical and lumbar regions exhibit enlargements
corresponding with the outflow of nerves to the arm and leg respectively.
The differences in structure between the three regions are best seen
by a comparison of transverse sections. The cervical region is oval in
section, the long axis of the oval lying transversely, its anterior median
fissure is relatively shallow, and the central canal is in front of the true
centre of the cord. The white matter is large in amount, the anterior
horn (column) of— grey matter is broad, and there is a well-marked
septum subdividing the posterior column : bees. of white matter
into medial and lateral portions. The region is cylindrical, its
anterior median fissure is deeper, and the central canal is centrally
placed. The anterior and posterior horns of grey matter are both
narrow, and each grey crescent shows a projection in its concavity,
known as the lateral horn. At the base of each posterior horn of grey
matter, towards its medial aspect, is a special column of nerve cells,
ealled Clarke’s column (the dorsal nucleus), The lumbar region re-
sembles the thoracic region in shape, depth of anterior median fissure,
and position of central canal, but its white matter is absolutely and
relatively smaller in amount, and both anterior and posterior horns of
grey matter are broad in section. Generally speaking, the white matter
diminishes progressively in amount from above downwards, and the
grey matter is most abundant in the regions from which the outflow of
the nerves to the limbs takes place.
THE NERVE CELLS OF THE SPINAL CORD.
The nerve cells jn the grey matter are for the most part irregular
in shape, but those in Clarke’s column are somewhat fusiform, with
their long axes in the long axis of the spinal cord. The cells in the
anterior horn are larger in size than those in the posterior horn. The
axons of the former emerge from the anterior surface of the spinal cord
in several groups, which unite to form the anterior root of a spinal |
nerve. The axons of the cells of the posterior horn fall into two groups :
THE CENTRAL NERVOUS SYSTEM. 49
(1) those which run a short course in the grey matter and form
terminal arborisations in relation with other nerve cells in the grey
matter (cells of Golgi’s second type), and (2) those which acquire a
myelin sheath, run into the white matter, divide into a short descend-
ing and a longer ascending branch, and enter into the formation of the
tracts of the white matter.
Certain well-defined groups of nerve cells may be recognised; three
in the anterior horn, one in the lateral horn, the cells constituting
Clarke’s column, and those of the posterior horn. The anterior horn
cells are very numerous in the cervical and lumbar regions, from which
arise the nerves to the limbs. |
The Nerve Roots.—-The anterior and posterior roots meet a short
distance from the lateral aspect of the spinal cord, and unite to form a
spinal nerve. Just before it joins the anterior root the posterior root
exhibits a swelling, the spinal ganglion; the unipolar cells of the
ganglion are situated for the most part around the periphery of the
structure, and the single process of each divides in a T-shaped manner
into two fibres, one of which has a peripheral and the other a central
direction.
The exit of the anterior root has already been described. The fibres
of the posterior root, consisting of the central divisions of the processes
of the ganglion cells, enter the spinal cord in the neighbourhood of the
posterior horn of grey matter. Each divides into a short descending
and a longer ascending branch. The longest ascending divisions run
upwards in the posterior column to reach the medulla oblongata. The
others turn into the grey matter at varying distances above their point
of entry to terminate by arborisations around nerve cells. (The descend-
ing fibres end in the same manner. Both ascending and descending
fibres give off fine medullated collateral branches at intervals, these also
entering the grey matter to form terminal arborisations in relation with
nerve cells,
The Collateral Fibres. — Collaterals enter the grey matter from
white fibres in all parts of the anterior, lateral, and posterior columns.
Those from the long tracts of the white matter may be looked upon as
associational in character, serving either to distribute the impulses con-
veyed by the main fibres and so to promote co-ordination, or to form
part of a system of relays by which impulses are conveyed by short
tracts from segment to segment of the spinal cord. The collaterals
from, and terminations of, posterior root fibres, on the other hand, or
many of them, are concerned with the formation of reflex arcs in the
Spinal cord itself ; in addition to the fibres of the posterior root which
have been described as running to the medulla oblongata i in the posterior
4
50 ESSENTIALS OF PHYSIOLOGY.
column, there are four main groups of collaterals, including the termina-
tions of main fibres, which run into the grey matter. These are (1)
fibres to the anterior horn of the grey matter, (2) fibres to the posterior
horn, (3) fibres to Clarke’s column, and (4) fibres to the grey matter
of the opposite half of the spinal cord, running across in the grey
commissure. All these fibres end by forming arborisations in relation
with nerve cells.
THE FUNCTIONS OF THE SPINAL CORD.
The functions of the spinal cord are two: (1) it is a centre or series
of centres for reflex actions, and (2) it conducts impulses between the
higher centres in the brain and the spinal nerves which transmit these
impulses to or from the active or receptive tissues of the body. It is
possible that the spinal cord may possess a low degree of automatic
activity under certain conditions. Graham Brown has recently shown
that in an animal from which the entire brain has been removed, and
in which the afferent nerves from the limbs have been divided, there
may occur under a certain degree of anesthesia alternate flexion and
extension of the limbs. No definite conclusions can, however, be based
at present on this experiment.
SECTION III.
CONDUCTION IN THE SPINAL CORD.
The spinal cord forms a pathway for impulses which originate in the
brain, and are distributed to all parts of the body through the anterior.
spinal nerve roots and the spinal nerves. It also conducts impulses
which are set up by stimulation of afferent nerve endings, chiefly in the
skin and muscles, and which reach it by the spinal nerves and posterior
spinal nerve roots to pass in an upward direction to the brain.
_ There are two methods of conduction: (1) by long tracts of nerve
fibres situated in the white matter, and (2) by short association tracts
or relays. Most of the long tracts are found in the peripheral part of
the white matter. The short tracts lie more deeply, their fibres run-
ning a little way in the white matter, then turning into the grey matter,
to terminate by arborisations in relation with nerve cells, the axons of
which in their turn form other short tracts in the white matter ; so that
impulses are conducted by a series of relays.
The tracts have been mapped out by two methods: (1) by observing
the time during development at which the fibres in the various
areas of the white matter of the spinal cord acquire a myelin sheath,
Ble ee EET Ie te ee Gt =
—e hed SR e OAES
hata f- C gigs
4 | 7
THE CENTRAL NERVOUS SYSTEM. : 51
and (2) by studying the degeneration which follows various lesions,
either pathological or experimentally produced.
The Myelination Method.—This method depends upon the fact
that in the course of development the various conducting tracts acquire
their myelin sheaths at different periods, so that by examining embryos
at different stages of development it is possible to determine the limits
of each particular tract. The longer fibres in the spinal cord become
myelinated later than those fibres which run a shorter course ; thus the
pyramidal tracts do not acquire their myelin till after birth (fig. 9).
The appearance of function coincides
with the period of acquisition of
myelin.
The Degeneration Method.—The
degeneration method is based upon the
fact that when a medullated nerve fibre
is divided, the portion which is cut off
from the nerve cell of which it forms
a part undergoes degeneration. If the
spinal cord is cut across in an animal,
certain tracts degenerate in the part
above the section, and other tracts de-
generate in the part below the section.
The former are said to have under-
ray eocondiag “eh oreapienatiber sae: thee Fic. 9.—Section through the cervical
fibres are axons of cells which lie below spinal cord of a new-born child,
the point of section. The latter are stained by Weigert’s method to
show absence of medullation in
said to have undergone descending de- pyramidal tract. (Bechterew.
generation, and their fibres are axons From Starling’s Principles of
hich li , Physiology. )
of cells whic ie above the point of ca, anterior commissure; Fp, crossed
section. The extent of the degenera- Pyramidal twas oeroot fibres
tion varies in different parts of the
white substance. In the case of the’ short tracts it extends for a
limited distance from the section; in the case of the long tracts it
may extend from the section to the upper or lower extremity of
the spinal cord. From what has been said of the function of the
neuron, it will be evident that the tracts which show descending de-
generation are those which convey descending impulses, while those
which show ascending degeneration are those which are concerned with
conducting aseending impulses. |
The chief conducting tracts of the spinal cord are shown in
fig. 10,
re a
Re tate ee Re RR a lO
yon
FSO PUGb sh «
t .
52 ‘ESSENTIALS OF PHYSIOLOGY.
THE DESCENDING TRACTS (OR FASCICULI).
The principal long descending tracts are the direct and crossed
pyramidal tracts (the anterior and lateral cerebro-spinal fasciculi), both
of which take origin in the cerebral hemisphere. The pre-central con-
volution on each side contains certain large pyramidal cells, known as
Betz cells, the axons of which unite to form a tract which runs through
Posterior median sulcus,
Cornu-commissural fasciculus.
Funiculus gracilis.
Comma tract,
Funiculus cuneatus.
Posterior
Lissauer’s nerve roots.
bundle.
Crossed pyra-
midal tract.
_ Direct cere
bellar tract.
Relay fibres
Superficial antero- lateral tract.
Olivo-spinal tract.
Relay fibres.
Direct pyramidal tract.
Anterior median fissure.
Anterior
nerve roots.
Fic. 10.—Diagram of the principal tracts in the spinal cord. (Gray’s Anatomy.)
the mid-brain and pons to the medulla oblongata. In the latter
structure the majority of the fibres in each tract cross to the opposite
side to become the crossed pyramidal tract (lateral cerebro-spinal
fasciculus) of the spinal cord, while the remaining fibres run down with-
out crossing to become the direct pyramidal tract (anterior cerebro-
spinal fasciculus) of the same side. Some of the uncrossed fibres, how-
ever, join the crossed pyramidal tract which has crossed from the
opposite ‘side.
The fibres of the crossed pyramidal tract terminate by running into
ee
eS ——- =— <<
,
THE CENTRAL NERVOUS SYSTEM. 53
the grey matter at the base of the posterior horn of grey matter, where
they form arborisations around nerve cells. By means of intermediate
neurons these cells are brought into relationship with the cells of the
anterior horn of grey matter, the axons of the latter forming the nerve
fibres of the anterior nerve roots.
The fibres of the direct pyramidal tract cross in the spinal cord itself
in the anterior white commissure to terminate in relation with the nerve
cells of the grey matter of the opposite side.
Both pyramidal tracts convey motor impulses from one cerebral
hemisphere to the opposite side of the body, a number of fibres
terminating in each segment of the spinal cord, so that the tracts
become progressively smaller as they descend. The fibres of the
direct pyramidal tract all cross in the cervical and thoracic regions,
Less is known of the function of the other tracts and fibres which
undergo descending degeneration. The best marked of these is the
rubro-spinal tract, which lies in the lateral column, immediately in
front of the crossed pyramidal tract, and consists of the axons of cells
forming the red nucleus of the mid-brain. These fibres cross in the
mid-brain close to their place of origim —
The olivo-spinal tract lies close.to the surface opposite the anterior
horn of the grey matter. As its name indicates, it is made up chiefly
of fibres which are the axons of cells in the olivary nucleus of the
medulla oblongata.
Vestibulo-spinal jibres, probably derived from the cells of Deiters’
nucleus, are found in the antero-lateral column.
The cerebello-spinal tract of Lowenthal, also lying in the antero-lateral
column, consists of scattered fibres derived from cells in the cerebellum.
The comma tract, lying in the posterior column, consists of the short
descending branches of fibres which enter the spinal cord from the
- posterior nerve roots.
THE ASCENDING TRACTS.
The ascending tracts may be classified as exogenous or endogenous,
according as they originate from cells in the ganglia of the posterior
roots of the spinal nerves, or from cells in the grey matter of the
spinal cord itself,
The exogenous tracts are the funiculus (fasciculus) gracilis (Goll’s
column), the funiculus (fasciculus) cuneatus (Burdach’s column), and
the bundle of Lissauer. The first and second of these together form
the greater part of the posterior column, while the third lies close to
_ the tip of the posterior horn of grey matter.
The funiculus gracilis occupies the mesial portion of the posterior
eer
54 ESSENTIALS OF PHYSIOLOGY.
column. It consists of long ascending fibres derived from the posterior
nerve roots ; these terminate in the medulla oblongata by arborisation
round the cells of the nucleus gracilis. The fibres of this tract are
Situated first in the funiculus cuneatus; as they ascend in the spinal
cord they come to lie nearer the middle line and more posteriorly, so
that in the cervical region those associated with the lower limb occupy
a position mesial and dorsal to those which have entered the spinal
cord at higher levels.
The fibres of the funiculus cwneatus are also ascending, and are
derived from the posterior spinal nerve roots. Many of them terminate
by arborisation in relation with the cells of the nucleus cuneatus of the
medulla oblongata. Some, however, enter the grey matter of the
spinal cord itself.
The bundle of Lissauer likewise consists of the ascending divisions of
posterior root fibres. These have a short course and terminate by
running into the grey matter of the spinal cord. Some authorities
consider that certain fibres of the tract of Lissauer are intersegmental
in character, because they acquire their myelin sheaths at a late stage,
and also because they do not degenerate under conditions which lead
to the degeneration of the other exogenous tracts, for example, in
locomotor ataxia.
The chief endogenous ascending tracts are the direct cerebellar and
the antero-lateral ascending tracts, which lie in the peripheral part of
the antero-lateral column.
The direct cerebellar tract (dorsal spino-cerebellar fasciculus) consists
of fibres which are the axons of cells in Clarke’s column, and it is found
only in the thoracic and cervical regions. It runs through the medulla
oblongata and forms part of the restiform body, ie nf finally in
the vermis of the cerebellum.
The antero-lateral ascending tract (ventral spino-cerebellar fasciculus)
is found in the lumbar as well as in the thoracic and cervical regions.
Its fibres are derived from the cells of Clarke’s column of the opposite
side, and it runs through the medulla oblongata and pons to the mid-
brain, where it turns round to form part of the superior cerebellar
peduncle, and ends in the vermis of the cerebellum.
Associated with the ventral spino-cerebellar tract are two other
groups of fibres: (1) the spuno-thalamic, which ascend through the
medulla oblongata, pons, and mid-brain to terminate in the thalamus,
and (2) the spino-tectal, which terminate in the corpora quadrigemina.
The ventral spino-cerebellar, spino-thalamic, and spino-tectal fasciculi
together form what was formerly known as Gowers’ tract.
ee ob
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BOTS SS Ee as a, ee) oe ee ee
THE CENTRAL NERVOUS SYSTEM. 55
THE PHYSIOLOGICAL PATHS IN THE SPINAL CORD.
Division of the posterior spinal nerve roots of the nerves supplying ~
a limb results in the immediate loss of all sensation in the limb. There
is also loss of muscle tone, aud the limb is paralysed owing to the ab-
sence of afferent impressions from it. Later, the afferent fibres undergo
degeneration centrally to the lesion, if the latter is between the root
ganglion and the spinal cord. .
Complete section of the spinal cord in the lower thoracic region is
followed by immediate loss of movement and sensation in the hind limbs.
There is also loss of vascular to: SrOrTaSe Meearal the blood-
vessels (passing off in twenty-four hours), and the hind limbs become poiki-
lothermic, that is, their temperature varies with that of the surrounding
medium. Further, the reflex visceral centres in the lumbar region are
cut off from the inhibitory impulses from the higher centres, so that mic-
turition and defecation become simple reflexes. Later, the motor fibres
below the point of section and the afferent fibres above the section, which
have entered by posterior roots below the lesion, undergo degeneration.
Hemisection of the spinal cord in the lower thoracic region results
in motor paralysis of the homolateral hind limb, accompanied by loss /
of muscle sense and tactile discrimination on the same side. There is,
also loss of taetile localisation, and of the senses of heat, cold, and pain|
in the contralateral hind limb, the fibres for these senses having |
crossed in the spinal cord shortly after their entrance by the posterior
roots. Immediately below the lesion, however, this sensory paralysis’
occurs in a narrow zone on the side of the section, depending on fibres
which have been divided Hetore"their crossing. The remote effects of
hemisection consist in in ascending and descending degeneration of the
divided fibres above and below the lesion respectively.
Transection at various levels shows that the motor paths and the paths
for muscle sense and for tactile discrimination cross thé middle line above
the Tevel of-the- spinal cord, whereas the paths for tactile localisation,
the senses of heat and cold, and the sense of pain cross in the spinal
~ cord itse @ vicinity of the posterior roots conveying thése impulses
from the Batty Further, in the disease known as syringomyelia,
in which the central canal of the spinal’ cord is dilated” and there
is. pressure on the adjacent structures and interference with their
functions, it is found that the sense of pain may be lost while those of
temperature and tactile localisation are unimpaired, or the temperature
sense may be lost while the other two senses are intact. There are,
therefore, separate bundles of fibres for the transmission of the different
impulses which give rise to these various sensations, *
2
56 ESSENTIALS OF PHYSIOLOGY.
In man, the motor impulses are conveyed from the brain to the
neurons of the different anterior nerve roots almost entirely by the
pyramidal (cerebro-spinal) tracts, but in the lower animals other tracts
are also used. As has already been stated, the greater part of the
motor path from the brain crosses in the medulla oblongata to form
the crossed pyramidal tract, and the fibres of this tract terminate in
the grey matter of the spinal cord in relation with the neurons con-
nected with the anterior nerve roots of the same side. Hence a lesion
involving one of these tracts results in motor paralysis on the same
side of the body. There will be a certain degree of weakness of muscles
on the opposite side of the body after a unilateral lesion of the human
spinal cord, because of the interference with the direct pyramidal tract,
the fibres of which cross immediately before their termination; but
this is relatively insignificant, and, as the fibres of the direct pyramidal
tract have all crossed in the cervical and thoracic regions, a unilateral
lesion in the lower thoracic region will result in motor paralysis of the
homolateral hind limb only. |
| The localisation of the paths for the various sensory impulses (fig. 11)
has been ascertained by the study of diseased conditions in man, as well
as by observing the results of experimental localised lesions in animals.
By such methods it has been shown that the funiculus gracilis and the
funiculus cuneatus are concerned with the transmission of those
kinesthetic (muscle sense) impulses which pass to the cerebral hemi-
spheres and cerebellum, the paths crossing in the medulla oblongata
just above the decussation of the motor tracts ; a cell station occurs on
these paths below their decussation.
- The direct cerebellar and the antero-lateral ascending (Gowers’)
tracts also convey kinesthetic impulses, but these differ from those
conveyed to the cerebral hemispheres in that they are not connected
with conscious sensation. Both these tracts are uncrossed in their
course to the cerebellum, and in neither is there a cell station on
the path. |
The fact that the four long ascending tracts convey impulses of
muscle sense is to be associated with the importance of the function of
equilibration, a function which requires for its performance delicate
muscular adjustments.
Impulses of pain, heat, and cold are conveyed into the posterior
horn of grey matter by fibres of the posterior roots; They then pass
by a second neuron to the spino-thalamic fibres of the opposite side,
and thus reach a cell station in the optic thalamus, from which they
are passed on to the cerebral cortex. Tactile impulses pass up the
posterior column of the same side for four or five segments before
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58 ESSENTIALS OF PHYSIOLOGY.
forming a cell station in the grey matter, and being conveyed by other
neurons to the anterior column of the opposite side. They also ulti-
mately reach the optic thalamus, and pass to the cerebral cortex by a
fresh relay (fig. 28).
SECTION IV.
REFLEX ACTION.
In the course of evolution survival has depended, to a large extent,
upon the rapidity and efficiency with which reflex movements are
carried out ; and the essential characters of a reflex action are (1) that
it should be rapid, and (2) that it should be co-ordinate, that is, that
the muscles concerned in the reflex act should contract together so
as best to attain the end for which the reflex exists. Further, the
response to a stimulus must be limited in its extent, and must not
involve the whole muscular system. Finally, evolution is made
possible by the capacity of the central nervous system to form new
reflexes, and this capacity is the basis of habit and of educability. It is
in this respect that the nervous system of man has become so much
more highly differentiated and complex than that of the lower animals,
In man the reflex functions of the spinal cord have become to a
large extent subordinate to those of the brain; and the spinal cord,
when separated from the brain as a result of injury, displays a compara-
tively feeble reflex power. In the lower animals the spinal reflexes are
more pronounced, and can be readily studied either in the pithed frog
or in a spinal mammal, 7.e. an animal which is allowed to recover after
transection of the spinal cord, this being usually made in the upper
thoracic region.
The Reflex Arc.—When a stimulus falls upon a sensory surface or
sense organ, which is cailed a receptor, it gives rise to an impulse which
is conveyed by an afferent nerve to the spinal cord. Here it travels
through a number of neurons, where its character is modified, and
finally the impulse leaves the spinal cord along an efferent nerve to
reach the muscle or gland which responds to the stimulus. This path
is called a reflex arc, and consists of (1) a receptor, (2) afferent nerve,
(3) neurons in the spinal cord, (4) efferent nerve, and (5) muscle or
gland (fig. 12).
Interruption of the are at any point abolishes the reflex action.
The time occupied by an impulse in travelling from the receptor
through the central nervous system to the muscle or gland (effector
organ) is called the total reflex time. A part of this time is occupied
in the transmission of the impulse along the afferent and efferent
THE CENTRAL NERVOUS SYSTEM. 59
nerves and by the latent period of the effector organ. When this is
deducted from the total reflex time, a period remains, known as reduced ©
reflex.time, which is occupied by the passage of the impulse through
the neurons in the central nervous system. Its duration varies with
the complexity of the reflex action, and is taken up mainly in the
passage of the impulse across the synapse between the axon of one
neuron and the dendrites of the next neuron, the synapse offering a
certain resistance to this passage.
It has been found in the frog, for example, that when one leg moves
in response to a stimulus applied to that leg, the reduced reflex time is
rather less than one-hundredth of a second. The reduced reflex time
Fie, 12.—Diagram of a reflex arc.
a, receptor ; b, intermediate neuron ; c, muscle fibre.
for reflex winking of the eye in man has been estimated at about
one-twentieth of a second. |
The time required RI cg Nocera with
increasing strength of the stimulus, and becomes longer when the
spinal cord is fatigued or is under the influence of drugs, such as
chloroform. Increase in the strength of the stimulus may also increase
the extent of the reflex response. If the strength of stimulus necessary
to elicit a particular reflex is determined, and the experiment is
_ repeated with a stronger stimulus, there may be (1) an increase in the
strength of the original response, and (2) additional Muscles may also
be thrown into contraction. Ii other words, with a stronger stimulus
there is a spread of the excitation in the grey matter of the spinal cord,
whereby additional reflex arcs become involved ; this spreading of the
excitation is known as irradzation. |
When, for example, a harmful (nociceptive). stimulus, such as a
sharp prick, is applied to the sole of the foot in the spinal dog, the leg
is ‘flexed and withdrawn from the stimulus. When the strength of
60 ESSENTIALS OF PHYSIOLOGY.
the stimulus is increased, the muscular response may extend to the
opposite leg and to other parts of the body. This experiment shows
that an impulse reaching the spinal cord finds its way most easily
along a certain path, and that its spread up or down the cord is
normally prevented by the resistance offered by neighbouring synapses.
After the injection of strychnine-this resistance disappears, and an
impulse reaching the cord at any level evokes generalised muscular
movements which are inco-ordinate. The normal limitation of the
: reflex response is thus
LS \ g
~ an important part of
L.4 \ the means by which the
ce object of the reflex is
L.5. \l ae ee obtained.
S| Inhibition. — Re-
a oak.
| flexes may be restrained
$2. % or completely inhibited
by impulses, voluntary
or involuntary, from the —
higher nerve centres. If
a reflex is elicited in a
frog from which the
cerebral hemispheres
have been removed, and
then a crystal of com-’
mon salt is applied to
Fic, 18.—Diagram to show muscles and nerves con- the optic lobes and the
cerned in Sherrington’s experiment on the 4: ulus is repeated, the
reciprocal innervation of antagonistic muscles.
(Starling’s Principles of Physiology. ) reflex movement may
L.3, L.4, L.5, 3rd, 4th, and 5th lumbar nerve roots. S.1,5.2: pot occur owing to in-
ist and 2nd sacral roots. ?
SCIAT.N:
hibitory impulses result-
ing from the stimulation caused by the salt. A reflex act, which is
in progress, may also be checked or inhibited by the advent of another
stimulus to the central nervous system/ Further, inhibition forms a
constituent of many reflex actions, the movement of certain muscles
being accompanied by the simultaneous inhibition of others.
Reciprocal Innervation.— When firm pressure is applied to the
sole of the foot in a spinal dog, it responds by rapid extension of the
leg, due partly to contraction of the extensor muscles of the thigh.
It is evident that this extension is only possible if the flexor muscles
(hamstrings) are at the same time relaxed, and their relaxation is not
a passive but an active process and forms an essential part of the ex-
tensor reflex. This can be shown by separating the extensor”and flexor
Eis > < a ¢
. % et
.THE CENTRAL NERVOUS SYSTEM. 61
muscles of the thigh from their lower attachments and connecting them
with recording levers. It is then found that the application of firm
pressure to the foot produces simultaneously contraction of the extensor —
and inhibition of the flexor muscles.
This precess, which holds good for the action of antagonistic muscles
in. general, is spoken of as reciprocal inneryation; and its importance
for the efficiency of a reflex action is demonstrated by the effects
which are observed when it no longer takes place. The scratch reflex,
for example, in the dog consists of rhythmic movements of flexion and
extension of the leg, each flexion recurring about four times a second ;
in the normal animal the movements are directed to the removal of
an irritant. The same reflex can be evoked in a spinal dog by the
application of a weak electrical stimulus to the skin of the ‘“‘scratch
area,” which occupies the thoracic region; with each flexion of the
thigh the flexor muscles contract and the extensors are inhibited ; with
each extension of the thigh the converse takes place. After the injec-
tion of strychnine into the animal, the application of the stimulus
produces contraction of both flexor and extensor muscles, and, since the
extensors are the more powerful, the limb becomes rigidly extended and
the reflex can no longer’be carried out.
Since the muscles in the body are limited in number, whereas the
impulses which may reach the spinal cord or brain are almost infinite
in variety, it is clear that the same muscle must at times be used in
response to different kinds of stimuli. For example, the flexor muscles
of the leg contract during the scratch reflex, and also in response to a
painful stimulus; in each case the impulse travels down the efferent
nerve to the muscle. The motor side of the reflex arc is, therefore, to
some extent identical for both the scratch reflex and the response
to the painful stimulus; it is therefore spoken of as a final common
path. | ;
Further, we find, as might be expected, that the final common path
can only be traversed by one set of impulses at the same time. If, for
example, the scratch reflex is evoked in a spinal animal, and while it is
taking place a strong nociceptive stimulus is applied to the sole of
the foot, the scratch reflex immediately stops and is replaced by
flexion of the leg (fig. 14). Conversely, if the flexion reflex is in pro-
gress, the application of the scratch stimulus, if sufficiently strong,
may inhibit the flexion reflex and produce the scratch reflex.
The two reflexes cannot co-exist; one or the other must prevail,
and the one which prevails (prepotent reflex) is usually that which is
most important for the well-being of the body. The fact tha
opposed reflexes such as those just mentioned cannot co-exist is of
62 ESSENTIALS OF PHYSIOLOGY.
great importance, since, if they were taking place simultaneously,
neither would be carried out effectively.
Facilitation.— When a stimulus which is insufficient to produce
a reflex response is repeated at short intervals, the reflex is often
ultimately evoked (fig. 15). Evidently the preceding stimuli, though
causing no visible response, bring
about some change in the neurons of
the reflex are whereby the stimulus
finally becomes effective. This process
is known as facilitation, and forms the
basis of habit. Each time a reflex
action takes place it becomes easier for
it to be brought about on a subse-
quent occasion. This is well seen in
the case of many skilled movements,
which, in the first instance, are learnt
by voluntary effort; in time the ad-
justment of the impulses concerned in
the carrying out of these movements
becomes so exact that, in response to
a suitable stimulus, they take place
without voluntary effort and almost
independently of consciousness.
By these means reflex actions are
so adjusted as to bring about a definite
movement as rapidly as possible in
response to a suitable stimulus. For
this purpose the co-existence of con-
sciousness is not necessary, indeed it
Fie. 14.—Scratch reflex temporarily 8 oe | Bineee oe, any ih
inhibited by application of a flexes, particularly ‘those occurring in
painful stimulus to foot. (Star- ¢gonnection with the visceral system,
ling’s Principles of Physiology.) d : ‘
Siew “A. sunt: Sot eka take place without affecting conseious-
Signal B, stimulation of paw by strong nessatall. Other reflexes, for instance
induction shock. : :
the closing of the eye when an object
approaches it suddenly, are associated with consciousness, though the
reflex act precedes and takes place independently of consciousness. A
third group of reflexes, for example micturition, can to some extent
be modified by voluntary impulses.
The Knee Jerk.—Reflex action may be brought about not only by
stimuli falling upon the surface of the body, but by stimuli arising
within the body itself, for instance in the joints and muscles. Keflexes
THE CENTRAL NERVOUS SYSTEM. 63
which are brought about by impulses originating in the deeper tissues
of the body are often called ‘‘deep” reflexes. One of the most
important is the knee jerk, which consists in contraction of the extensor
Fic, 15.—Reflexes produced by summation ot weak stimuli. (Sherrington.
Starling’s Principles of Physiology.)
A, Reflex contraction of flexor muscles of knee. |B, Reflex inhibition of extensor
muscle. In each case the effect follows the sixth stimulus, the stimuli being applied
to the central end of the internal saphenous nerve.
_ muscles of the thigh in response to a sharp tap on the patellar tendon.
The stretching of the extensor muscles by this tap gives rise to a
stimulus in sensory ,structures (muscle-spindles) within the muscle
itself ; from these structures the impulse passes to the spinal cord
along the afferent nerve fibres of the muscle.
\
~ “64 ESSENTIALS OF PHYSIOLOGY.
The reflex character of the knee jerk has been denied on the ground
that it is a single twitch, whereas in genuine reflexes there is a rhythmic
discharge oF impulses to the muscle. This argument is discounted by
the fact that the sudden extension of the leg which is produced by
applying pressure to the foot of a spinal animal is undoubtedly a reflex
action and is also a single twitch,
The knee jerk takes place very rapidly, the reduced reflex time
being only 0:002-second ; and though it must be regarded as a true
reflex, the impulse passes, in all probability, through only. one synapse
in the spinal cord.
In man the part of the spinal cord concerned with the knee jerk is
the third and fourth lumbar segments ; and the knee jerk is abolished
either by destruction of-this—part of the spinal cord or by division of
the afferent or efferent nerves of the extensor muscles of the thigh.
The knee jerk is absent in locomotor ataxia, in which the posterior
lumbar nerve roots are diseased and the afferent. path of the reflex arc
is interrupted. Exaggeration of the knee jerk may be brought about
by section of the hamstring muscles or of the afferent nerves from these
muscles.
The knee jerk can also be increased by impulses from the higher
parts of the central nervous system. If, for example, the fists are
firmly clenched at the moment at which the knee jerk is elicited, the
jerk is more marked, this being called reinforcement of the knee jerk.
Further, it is often exaggerated in disease of the cerebral cortex or of
the pyramidal tracts, probably owing to the cutting off of restraining
impulses which normally pass to the spinal cord from the brain.
The knee jerk and other similar reflexes serve to protect joints and
ligaments from injury when a sudden strain occurs, which tends to
separate the joint surfaces or to stretch ligaments; and the extreme
rapidity of the reflex is no doubt associated with this protective function.
The production of the knee jerk and other tendon reflexes is
dependent on the existence of muscle tone. The skeletal muscles are
in a constant condition of slight tonic contraction, which is due to the
continuous discharge of impulses from the spinal cord to the muscles,
as is shown by the following observation. When the brain of a frog is
pithed and the animal is’ suspended by its head, the muscles do not
become flaccid and the limbs remain very slightly flexed. When the
spinal cord is destroyed, or the posterior roots of the spinal nerves are
divided, the limbs become fully extended and the muscles lose their
tone.
In mammals, muscular tone is lost when the skeletal muscles are
cut off from the central nervous system, or during deep anesthesia.
“a
a 0
THE CENTRAL NERVOUS SYSTEM. | 65
The normal maintenance of this tone is really a reflex action and
is dependent on afferent impulses, which in the mammal originate
chiefly in the muscles themselves. It is abolished, therefore, not
only by section of motor nerves, but also by section of the posterior _
nerve roots containing the afferent fibres coming from the muscles.
_ Impulses passing down the spinal cord from the brain may also control
and modify muscular tone; and this is sometimes greatly increased,
when the passage of these impulses is prevented by injury to the spinal
cord in animals, or by disease of the pyramidal tract in man.
The afferent impulses passing from muscles to the spinal cord may
not only bring about reflex actions, but play an important part in the
co-ordination of reflex actions brought about by external stimuli, If the
afferent nerves from the muscles of a limb are divided, the movements
of that limb are inco-ordinate (ataxic), even though cutaneous sensation
is not interfered with. On the contrary, division of the cutaneous
nerves has but little effect upon the co-ordination of muscular move-
ments, provided the afferent muscular fibres are intact. For instance, a
cat, even after the division of all the cutaneous nerves to its four paws,
is still able to balance itself almost as accurately as a normal animal.
The Visceral Spinal Reflexes.—Local centres exist in the spinal cord
associated with reflexes connected with the blood-vessels and sweat glands.
Centres also exist in the lumbo-sacral region of the spinal cord for
_ the functions of micturition, defeecation, erection, and parturition. All
these centres are normally controlled to a greater or less degree by the
higher centres, but the reflex function of each can be carried out when
all connection with the higher centres has been severed.
SPINAL SHOCK.
_ This is a condition, following transverse section of the spinal cord,
in which the reflex functions of the cord are abolished in the part
posterior to the lesion. It is seen in its simplest form in the
frog. If the spinal cord of a frog is divided just posterior to the
medulla, the muscles of the limbs become flaccid and remain: in this
condition for about half an hour. During that time no reflex can be
elicited in the limbs. As the shock passes off, muscular tone returns,
the animal assumes a fairly normal Position, and reflex muscular con-
tractions ¢an once more be evoked, In mammals the condition of
spinal. shock is more prolonged. | of the spinal cord below the
origin of the phrenic nerves results in loss of muscle tone and of
vascular tone in the body posterior to the lesion. The sphincters of
the anus and bladder are relaxed, the reflexes for defecation and
micturition are abolished, and muscular reflexes cannot be elicited.
5
66 ESSENTIALS OF PHYSIOLOGY.
The condition lasts for a few days, at the end of which time the blood-
vessels and muscles have regained their tone, the sphincters of the anus
and bladder are contracted, defecation and micturition occur reflexly,
and it is possible to evoke muscular reflexes.
The condition of spinal shock is not due to the effect of the operation,
nor to the fall of blood pressure, for it only affects the region of the cord
posterior to the lesion. It is believed to result from the cutting off of im-
pulses which are conti ching the ¢ the higher centres.
SECTION YV.
THE BRAIN.
There are three main divisions of the brain, named respectively the
fore-brain, mid-brain, and hind-brain. The fore-brain consists of the
cerebral hemispheres and most of the structures bounding the third
ventricle, the mid-brain of the
corpora quadrigemina and the
cerebral peduncles, and the hind-
brain of the cerebellum, pons,
and medulla oblongata.
THE MEDULLA OBLON-
GATA.
The medulla oblongata may
be regarded structurally as an
upward continuation of the spinal
; cord, in which certain conducting
fe aa Rape gers ipiepeaeticlye ran tracts decussate, and the struc-
pyramids. (Testut.) From Gray’s ture of which is further compli-
ssa ) cated by the appearance of cell
1, Anterior median fissure; 2, posterior median q
’ “sulcus; 3, motor roots; 4, sensory roots; 5, Stations on some of these tracts.
base of the anterior horn, from which the a .
head (5’) has been detached by the crossed Owing to the decussations and
Ast pomeriir ‘nore Gaeta rer in also to other modifications, the
saat upper part of the medulla oblon-
gata differs greatly in structure from the lower part. In the upper
part, the pyramidal tracts occupy a position anteriorly close to the
anterior median fissure, each on the side of the cerebral hemisphere
from which it is derived. The tracts are here known as the pyramids.
In the lower part of the medulla oblongata, the greater part of each
pyramid crosses to take up the position which it occupies in the
spinal cord as the crossed pyramidal tract (fig. 16). "The decussating
fibres separate the grey matter continuous with the anterior horn of
the spinal cord into two parts. One, continuous with the head of the
THE CENTRAL NERVOUS SYSTEM. hae 4
horn, is pushed towards the lateral aspect of the medulla, and is con-
tinued upwards as the nucleus ambiguus, which is the nucleus of origin
of the cerebral fibres of the spinal accessory nerve and of the motor |
fibres of the vagus, glossopharyngeal, facial, and trigeminal nerves. The
portion of grey matter which is continuous with the base of the anterior
horn lies behind the decussating pyramids, and in the upper part of
the medulla oblongata lies close to the floor of the fourth ventricle,
where it forms the nuclei of the hypoglossal nerves. The further
upward continuation of this part
forms the nuclei of the sixth,
fourth, and third nerves in the
mid-brain.
The grey matter which is
continuous with the posterior
horn of the spinal cord lies
nearly transversely in a section
of the lower part of the medulla
oblongata, and two outgrowths
appear on its dorsal aspect, one
projecting into the funiculus
gracilis, the other into the funi-
culus cuneatus. These out-
8 i to
Fie. 17. — Transverse section passing
growths form the nucleus gracilis
and the nucleus cuneatus respec-
tively, and in them the fibres of
the corresponding funiculi ter-
minate by arborisation. Most of
through the sensory decussation.
(Schematic.) (Testut,) From Gray’s
Anatomy.
1, Anterior median fissure ; 2, posterior median
sulcus ; 3, 3’, head and base of anterior horn
(in red); 4, hypoglossal nerve; 5, bases of
posterior columns; 6, gracile nucleus; 7,
cuneate nucleus; 8, 8, lemniscus ; 9, sensory
decussation ; 10, pyramid.
the axons of the cells of the
two nuclei pass forward, and cross the middle line as internal
arcuate fibres to form the lemniscus or fillet of the opposite side. This
decussation takes place just above that of the pyramids (fig. 17). The
fillet rans upwards dorsal to the pyramid to terminate in a cell station
in the thalamus. It is joined in the medulla oblongata by the spino-
thalamic fibres, which have already decussated in the spinal cord.
The sensory decussation separates the base from the apex of the
posterior horn of grey matter. The base forms a column of grey
Matter in which are found the sensory nuclei of the vagus and
glossopharyngeal nerves, and which is connected with the nuclei of
_ the vestibular nerve and of the sensory root of the facial nerve. The
head forms the spinal nucleus of the fifth nerve.
The upper portion of the medulla oblongata is characterised mainly
(1) by the fact that the central canal comes to the suface posteriorly
ee
es;
°
68 ESSENTIALS OF PHYSIOLOGY.
and opens out into the fourth ventricle, and (2) by the appearance of
a lateral projection behind the pyramid, known as the olive, and con-
taining a folded sheet of grey substance internally, the olivary nucleus.
In the floor of the fourth ventricle are the nuclei of the hypoglossal
and of the vagus and glossopharyngeal nerves. Lateral to these nuclei
are the nucleus gracilis and nucleus cuneatus. Near the nucleus
cuneatus is the spinal (descending) root of the fifth nerve, and ventral
to the hypoglossal nucleus is the nucleus ambiguus. The interior of
3 this part of the medulla ob-
fg longata is occupied chiefly
Ae by the olivary nucleus and
the formatio reticularis.
The latter consists of nerve
e
'
{
J
t
'
t
{
1
Qe
fibres, some running trans-
versely and some, includ-
PEE OR f ing those of the fillet, run-
ning longitudinally. It also
oe wy Zi contains some scattered
) nerve cells. Ventral to the
formatio reticularis is the
Bese Powakses % i pyramid, and lateral to the
: nucleus cuneatus and the
Fic. 18. — Diagram of upper part of medulla spinal root of the fifth
oblongata, :
a, Pyramid; 06, fibre from olivary nucleus; c, spino- Begs tee — ee
’ ‘thalamic fibres and Gowers’ tract; d, restiform body; fibres, the restiform body
é, nucleus cuneatus; /, nucleus gracilis; g, nucleus of : é
vagus ; h, nucleus of hypoglossal nerve. or inferior peduncle of the
cerebellum. The hypo-
glossal nerve crosses the formatio reticularis from its nucleus to emerge
in front of the olive, and the vagus and, at a higher level, the glosso-
pharyngeal nerve take a more lateral course through the formatio
reticularis to reach the surface behind the olive. Some of the internal
arcuate fibres, arising from nerve cells in the nucleus gracilis, nucleus
cuneatus, and olivary nucleus, cross the middle line to form part of the
restiform body. The external arcuate fibres are derived from the
gracile and cuneate nuclei and pass forward to the anterior median
fissure, where they sweep backward over the pyramid and olive of the
’ opposite side to join the restiform body (fig. 18).
THE FUNCTIONS OF THE MEDULLA OBLONGATA.
Like the spinal cord, the medulla oblongata acts as a reflex centre
or series of centres, and it also serves as a conducting path for impulses
passing between the brain and spinal cord.
THE CENTRAL NERVOUS SYSTEM. 69
The reflexes which are carried out through the medulla oblongata
are those concerned with the secretion of saliva and of the gastric and
pancreatic juices; with the movements of the cesophagus, stomach,
and intestine, including those involved in vomiting ; with the regula-
tion of the heart and blood-vessels; and with the Dia Mage “of
respiration.
The physiological conducting paths in the medulla oblongata are
(1) the motor path formed by the pyramid ; (2) the rubro-spinal tract,
lying dorsal to (4); (3) the chief sensory path, consisting of (a) the
funiculus gracilis and funiculus cuneatus continued upwards from the
spinal cord, (0) the cell stations in the nucleus gracilis and nucleus
cuneatus, (c) the sensory decussation, and (d) the fillet, in which are
also included the spino-thalamic fibres which have already crossed in
the spinal cord ; (4) the antero-lateral ascending tract, running upwards
just behind the olivary nucleus ; (5) the direct cerebellar tract running
into the restiform body of its own side; (6) the vestibulo-spinal path,
or posterior longitudinal bundle (medial longitudinal fasciculus), which
lies in the formatio reticularis behind the fillet, and is concerned with
the function of equilibration, connecting Deiters’ nucleus and the nuclei
of the third, fourth, and sixth cranial nerves with the spinal cord ;
(7) the spino-tectal fibres, which run upwards, forming part of the
antero-lateral ascending tract.
THE PONS.
The outstanding feature of the structure of the pons is the
presence of a large number of decussating nerve fibres, which pass back-
wards on each side to form the middle peduncles of the cerebellum,
These transverse fibres occupy the ventral part of the pons, and
split up the pyramid into a number of separate bundles (fig. 19).
Behind them are the fillet and the formatio reticularis with the con-
ducting paths described in connection with the medulla oblongata,
these paths occupying much the same relative positions as they do in
the medulla.
Lying posteriorly in the upper part of the fourth ventricle, and in
or near the floor of the ventricle, are found the nuclei of the fifth, sixth, .
seventh, and eighth nerves. The upward continuation of the nucleus
ambiguus forms the motor nuclei of the fifth and seventh nerves. The
sixth nucleus, also motor, lies close to the floor of the fourth ventricle
and near the motor nucleus of the facial. The sensory nucleus of the
fifth nerve, lying laterally to the motor nucleus, receives some of the
sensory fibres of this nerve. The other sensory fibres of the fifth
70 ESSENTIALS OF PHYSIOLOGY.
end in the spinal root. The cochlear division of the eighth nerve ends
in two nuclei, which lie close to the restiform body, the accessory nucleus
on its ventral aspect, and the tuberculum acusticewm on its dorso-lateral
aspect. The vestibular division of the eighth nerve ends partly in the
chief vestibular nucleus in the floor of the fourth ventricle, and partly
in the nucleus of Deiters, which lies laterally to the chief nucleus and
is distinguished by the large size of its nerve cells.
= ®
3 8 oo
Fic. 19.—Coronal section of the pons, at its upper part. (Testut. )
From Gray’s Anatomy.
1, Fourth ventricle; 2, anterior medullary velum; 3, mesencephalic root of trigeminal ;
4, nerve-cells associated with this root; 5, posterior longitudinal bundle; 6, formatio
reticularis; 7, lateral sulcus; 8, section of superior peduncle; 9, medial lemniscus ;
9, lateral lemniscus; 10, 10, transverse fibres of pons; 11, 11, pyramid; 12, raphe; V,
exit of Vth nerve.
Groups of nerve cells, the nucler pontis, lie among the transverse
fibres in the ventral portion of the pons. These nuclei form a cell-
station on the path of certain tracts which connect the cerebral hemi-
spheres with the cerebellum. Axons from cells in the cortex of each
cerebral hemisphere descend to the pons, where they arborise in relation
with the nuclei pontis. The axons of the cells of the nuclei pontis
become the transverse fibres of the pons, and cross the middle line to
pass backwards and become the middle peduncles of the cerebellum.
THE CENTRAL NERVOUS SYSTEM. 71
THE FUNCTIONS OF THE PONS.
The nucleus of the sixth nerve and the motor nuclei of the fifth and
seventh nerves receive efferent fibres from the cortex of the cerebral
hemisphere, which have descended in the pyramid and crossed the
middle line in the pons itself. Hence a unilateral lesion in the pons —
may be characterised by paralysis of the external rectus muscle of the
eye and of the muscles of the face on the side of the lesion, along with
paralysis of the arm and leg on the side opposite to the lesion.
The pons also forms the crossing place for the path, described above,
of the fibres which connect one cerebral hemisphere with the cerebellar
hemisphere of the opposite side.
The motor paths and the rubro-spinal tract pass through the pons,
the former giving off fibres to the pontine motor nuclei in the manner
already described.
The ascending tracts already described in connection with the
medulla oblongata, with the exception of those which form the restiform
body, pass through the pons unchanged. The fillet receives in its
course additional fibres from the nuclei of the cochlear and fifth nerves.
* The chief vestibular nucleus and the nucleus of Deiters are concerned
with the function of equilibration. The axons of these nuclei divide into
two groups, one group passing to the cerebellum in the restiform body,
the other joining the posterior longitudinal bundle which links the
oculo-motor and vestibular nuclei with the spinal cord.
SECTION VI.
THE CEREBELLUM,
The cerebellum is connected with the medulla oblongata and with
the vestibular nuclei of the pons by the restiform body or inferior
cerebellar peduncle on each side, with the pons by the two brachia
pontis or middle cerebellar peduncles, and with the mid-brain by the
two brachia conjunctiva or superior cerebellar peduncles. It consists
of a middle lobe or vermis and two lateral lobes. The surface of the
cerebellum is thrown into numerous folds, and the superficial layer,
consisting of grey matter, is thus much increased in extent. The
interior of the cerebellum is mainly composed of white matter,
but it also contains some masses of grey matter, known as the
‘nucleus dentatus, nucleus emboliformis, nucleus globosus, and nucleus
fastigii.
The cortical substance consists of two layers. with, a single row of
large nerve cells lying between them (fig. 20). These large cells are
72
known as the cells of Purkinje. They are flask-shaped, the axon
coming from the base and the dendrons from the apex of each cell.
The dendrons branch extensively, and the branches all lie in a plane
COC CON Sete ses oe aw
ESSENTIALS OF PHYSIOLOGY.
Cell of Purkinje.
Molecular
layer.
Golgi cell,
Nuclear layer.
Axons of
granule cells
cut trans-
versely.
Granule cells.
ST ee
te,
ee il alee ne a -
- - 5 Ce
oT gre PPO ii a
* #, . a]
~ ° aoe 2 spi. .
. . S . be
Medes oe ey ies et.
“ P «fe > ie
Le ale ay Ut es
PIE We ist . ae
Pig! yaw ee m.5e +.
an a ose EPO
eo" . . i pe
- ad fee Pore” & Peg
F office. Pe a4.
= hae eete * tito:
+ *.4 . oa
s «Ue Dis weet .
Sg ais "6
othe . 0 ag . .
> . “ag f ae
. 4 . hie ae Le?
. - * : .
: . ey ey Car
. . .
* . A fe
ile e eteofete fee,
. - are Ae a
{Small cell
of molecular
layer.
Basket cell.
Ls : ; “Axon of cell of Purkinje.
Neuroglia cell. yee
7 ; Tendril fibre.
Moss fibre,
(Diagrammatic, after Cajal and
Fic. 20.—Transverse section of a cerebellar folium.
Kolliker.) From Gray’s Anatomy.
across the direction of the fold or leaflet, so as to resemble a tree
trained against a wall.
The inner (granular) layer contains a large number of small, rust-
coloured nerve cells. The dendrites of these are short, and the axons
pass into the outer layer, where they divide in a T-shaped manner, the
THE CENTRAL NERVOUS SYSTEM. I>"
divisions running in the direction of the surface fold and thus crossing
the dendrons of the cells of Purkinje at right angles. Afferent fibres
from the white matter arborise round the cells of the granular layer,
and are called moss-fibres because of the appearance of their
terminations.
The outer layer of grey matter, or molecular layer, contains the
dendrons of Purkinje’s cells, the axons of the small cells of the internal
layer, scattered nerve cells, and the terminations of afferent fibres from
the white matter. The nerve cells of this layer are called basket cells,
from the fact that their axons and the collaterals from the axons
terminate round the cytons of the cells of Purkinje in a basket-like
fashion. The afferent nerve fibres which reach the molecular layer
arborise in apposition with the dendrons of the cells of Purkinje, and are
hence known as climbing or tendril fibres. The Purkinje cells receive,
therefore, three sets of afferent impulses, namely, (1) from the axons of
the cells of the granular layer, (2) from the basket cells, and (3) from
the tendril fibres.
The axons of the cells of Purkinje end in the nucleus dentatus,
from which the impulses they transmit are passed on by other neurons.
The superior peduncles (brachia conjunctiva) pass to the mid-brain
and run under the inferior colliculi. There the fibres of each cross the
middle line and divide into ascending and descending branches. The
ascending divisions end in the thalamus, the red nucleus, and the nuclei
of the oculo-motor nerves. The descending divisions are believed to
reach the anterior columns of the spinal cord. The brachia conjunctiva
also convey the fibres of the antero-lateral ascending tract of the same
side to the vermis of the cerebellum.
The brachia pontis form part of the connecting path between the
cerebral hemispheres and the hemispheres of the cerebellum, the
connection being a crossed one.
The restiform bodies contain both efferent and afferent fibres. The
efferent fibres run from the nucleus fastigii and dentate nucleus: to the
nucleus of Deiters and the medulla oblongata. The afferent fibres
are (1) the direct cerebellar tract from the same side of the spinal
cord to the vermis; (2) fibres from the medulla oblongata, (a) from
the nucleus gracilis and nucleus cuneatus of the same and the opposite
sides, (6) from the olivary nucleus of the opposite side, and (c) from
the formatio reticularis of both sides; (3) fibres from the chief vesti-
bular nucleus and from Deiters’ nucleus. .
The cerebellum thus receives afferent fibres from the spinal cord,
some direct through the restiform body, others by way of the superior
peduncles, and other fibres which convey impulses from the spinal cord
74 ESSENTIALS OF PHYSIOLOGY.
but themselves arise in cell-stations in the medulla oblongata. It also
receives afferent paths from the cerebral hemispheres, but whereas
the connection with the cerebral hemispheres is a crossed one, that
with the spinal cord is for the most part uncrossed.
The efferent fibres from the cerebellum are (1) those which run in
the superior peduncles (brachia conjunctiva) to the mid-brain and
thalamus, (2) fibres in the middle peduncles to the nuclei pontis, and
(3) those which run in the restiform bodies to the vestibular nuclei and
the medulla oblongata.
THE FUNCTIONS OF THE CEREBELLUM.
A knowledge of the cerebellar functions has been obtained partly by
observation of the results of disease in man and partly by experiments
on animals. When the normal impulses from the cerebellum are want-
ing, there is ctiye co-ordination of muscular movements as well
as defectiye muscle tone. Asa result, the power of maintaining the
equilibrium of the body is. impaired and the gait is staggering, though:
consciousness and volition are not affected. Further, observation of
the results of localised lesions shows that different areas of the cerebellar
cortex are associated with different groups of muscles, although no
histological difference can be detected in the various parts of the
cerebellar cortex. The cerebellum is, therefore, a reflex centre for the
ee ee and for the co-ordination of muscular
movements. It is especially concerned with the maintenance of the
position of the body in relation to gravity, and with co-ordination of
the movements of the body as a whole.
The effects of removal of the cerebellum, or of any part of it, vary
according to the length of time that has elapsed since the operation was —
performed. The immediate effect of complete removal is chiefly a con-
dition of ataxia or inco-ordination. If a pigeon, for example, shortly -
after removal of its cerebellum, attempts to fly, it only succeeds in
making exaggerated and inco-ordinated movements of its wings. The
attempts of a dog to walk under the same circumstances are equally
futile. After some weeks or months, the animal regains the power of
co-ordinated movement to a certain degree. The pigeon is able to fly
and the dog to walk, but the power of movement is still greatly
. . bin 16 . . . ~ .
impaired. The condition is described as one of asthenia, atonia, and
astasia, that is, there is loss of strength, loss of muscle tone, and un-
steadiness of movement due to a condition of tremor accompanying
attempts at muscular contraction. A dog is no longer able to walk
with the normal diagonal movements of the limbs, but progresses by
means of a series of jumps. The explanation of the partial recovery
a a tac ay
THE CENTRAL NERVOUS SYSTEM. 7 5.
is found in the fact that the cerebral hemispheres consciously direct
the movements which, when the cerebellum is intact, are unconsciously.
co-ordinated. In this way, the cerebral mechanism compensates to
some extent for the loss of cerebellar function. It will be remembered
that two of the long tracts concerned with conveying afferent impulses
from the muscles on each side terminate in the cerebellum, while the
remaining two end in the thalamus, and are brought into relation with
the cortex of the cerebral hemisphere by other neurons.
If one lateral half only of the cerebellum is removed, the weakness
and tremors are limited to that side of the body. For two or three
weeks the animal is unable to stand, and lies on the affected side with
its head and trunk turned in the same direction (fig. 21). Later, it
succeeds in standing and walking by abducting the limbs of the weak
side or by availing itself of the support of a wall.
Fig. 21. ae with right half of its cerebellum removed.
(From Schifer’s Text-book of Physiology. )
Stimulation of the cortex of the cerebellum gives rise to movements
of the body, but stronger stimuli are required than in the case of
stimulation of the cortex of the cerebral hemispheres. Weak stimuli
applied to the central nuclei of the cerebellum are effective in producing
movements, especially of the eyes and head. Stimulation of Deiters’
nucleus in the pons, on the other hand, is followed by movements of
the trunk and limbs.
Observation of the effects produced by localised lesions, experimental
or pathological, shows that there is localisation of function in the
cerebellar cortex. The immediate effect of such a partial injury is the
occurrence of muscular contractions, giving rise to so-called ‘‘ forced
movements.” Injury to the anterior part of the vermis is followed by
movements of the head, injury of the anterior part of a lateral lobe by
movements of the fore-limb of the same side, and injury of the posterior
part of the lateral lobe by movements of the hind limb of the same side.
An additional fact which has some bearing on the function of the
cerebellum is that in an animal from which the cerebral hemispheres —
have been removed, the cerebellum being intact, the muscles frequently
pass into a condition of tonic contraction leading to what is known as
decerebrate_ rigidity; this condition disappears on, division of the
posterior nerve roots.
76 ESSENTIALS OF PHYSIOLOGY.
Summary.—The cerebellum receives afferent impulses (1) by way
of the vestibular nerve from the labyrinth of the ear, the end-organs
in which are affected by changes in the position of the head, and (2) by
the afferent nerves from the muscles, from the end-organs which are con-
nected with muscle sense. Both these sets of impulses are known as
proprio-ceptive to distinguish them from the extero-ceptive impulses
conveying information as to the environment of the body. The
cerebellum discharges efferent impulses which maintain muscle tone
during rest, and which co-ordinate muscular movements during activity.
These functions are especially of importance in the maintenance of
equilibrium. The absence of the cerebellum is therefore attended by
loss of tone and power in the muscles, as well as by a condition of
inco-ordination or ataxia. Cerebellar ataxia is thus associated with
muscular weakness, whereas spinal ataxia is associated with exaggerated
muscular movements. -
SECTION VII.
THE MID-BRAIN.
The mid-brain consists of the cerebral peduncles on its ventral
aspect and the corpora quadrigemina on its dorsal aspect. The cerebral
aqueduct runs through its substance and connects the third with the
fourth ventricle. The cerebral peduncles contain the portions of the
motor and sensory tracts which intervene between the fore-brain and
the pons. The corpora quadrigemina are two pairs of prominences,
the superior and inferior colliculi, seen on the dorsal surface.
If a coronal section is made through the mid-brain, it is seen that
the cerebral peduncle is divided into a dorsal and a ventral portion
by a layer of grey substance containing pigmented nerve cells and
known as the substantia nigra (fig. 22). The portion of the peduncle
ventral to the substantia nigra is called the pes (base), and it consists
entirely of nerve fibres running longitudinally. The dorsal portion
is known as the tegmentum. The fibres which form the middle three-
fifths of the pes are the pyramidal fibres, and extend from the cortex
of the cerebral hemisphere of the same side to become the pyramid of
the medulla and the pyramidal tracts of the spinal cord. Some of the
fibres, as has already been stated, cross in the hind-brain to the nuclei
of the cerebral motor nerves. The fibres of the medial fifth of the
base are the fronto-pontine fibres, and those of the lateral fifth are
the temporo-pontine fibres, forming connections between the frontal
and temporal cerebral lobes respectively and the nuclei pontis, and
thus with the contralateral lobe of the cerebellum. The fibres which
ee tet
THE CENTRAL NERVOUS SYSTEM. ri |
lie dorsally to the substantia nigra form the upward continuation of
the fillet, which is here bent at a right angle so that it is divisible into
a medial and a lateral portion. The medial part is continued upwards
to the thalamus, but the larger part of the lateral portion ends in the
inferior colliculus of the corpora quadrigemina. The other tracts of
fibres found in the tegmentum are the posterior longitudinal bundle
(medial longitudinal fasciculus) and the superior peduncle. The
tegmentum also contains a certain amount of grey matter, the greater
part of which forms a well-defined group of nerve cells, known as the
Inferior colliculi,
Cerebral aqueduct.
‘ »\ — Nucleus of oculomotor
at nerve.
Lateral fillet.
Posterior longi-
tudinal bundle.
Medial fillet.
Raphe.
Fic. 22.—Transverse section of mid-brain at level of inferior colliculi.
(Gray’s Anatomy. ) 4
red nucleus, lying near the middle line, The red nucleus gives origin
to the fibres of the rubro-spinal tract, and receives most of the fibres
of the superior peduncle.
The corpora quadrigemina are composed of grey matter, but the
superior colliculi are covered by a layer of white fibres derived from
the optic tract. Most of the fibres of the lateral fillet end in the
inferior colliculus, the majority in that of the same side, some, however,
crossing to the opposite side. The inferior colliculus is also connected
by a bundle of fibres, the inferior brachium, with the internal (medial)
geniculate body, Other fibres connect it, by way of the tegmentum,
with the thalamus and the temporal lobe of the cerebral hemisphere.
These various connections form part of the auditory tract. The lateral
: fillet conveys impulses from the nuclei of the cochlear nerve. These
78 ESSENTIALS OF PHYSIOLOGY.
are distributed to the inferior colliculus and internal geniculate body,
and thence to the temporal lobe of the cerebral hemisphere.
The superior colliculus, on the other hand, is concerned with the
function of vision. It receives fibres from the optic tract, which
arborise round its nerve cells, and it gives origin to fibres which pass
to the nuclei of the third (oculo-motor) nerve. It is also connected by
Beane Optic nerve.
Optic chiasma.
_- Thalamus.
fad wi External geni-
culate body.
----- Superior
colliculus.
SY ees | = | Rete rales Oculo-motor
nuclei.
- a --+--- Optic radiation.
_.----- Visuo-sensory
area,
Fic. 23.—Diagram showing the path of the visual impulses,
The oculo-motor nuclei are connected by commissural fibres (not shown in figure).
a bundle of fibres, the superior brachium, with the external (lateral)
geniculate body. The optic tract ends in cell stations in the superior
colliculus, the external (lateral) geniculate body, and the thalamus
(fig. 23). From the two latter of these stations, other neurons carry
impulses to the cerebral cortex. The superior colliculus is also the
place of origin of the tecto-spinal fibres, which are found in the antero-
lateral column of the spinal cord. These cross in the mid-brain by the
fountain decussation of Meynert, and descend in the formatio reticularis
of mid-brain, pons, and medulla oblongata.
THE CENTRAL NERVOUS SYSTEM. | 79
The upward prolongation of the nucleus of the fifth nerve lies in
the grey matter lateral to the cerebral aqueduct. The nuclei of the
third and fourth nerves are found in the grey matter of the floor
of the aqueduct. The posterior longitudinal bundle has a position
immediately ventral to the latter nuclei. Some fibres of this bundle
arise in a nucleus, the nucleus of the posterior longitudinal bundle, which
lies at the upper part of the mid-brain, immediately under the thalamus.
The bundle also receives a number of fibres from the superior colliculus.
Its other connections have already been described.
The mid-brain, pons, and medulla oblongata together form the brain-
stem, which conveys the. conducting paths between the fore-brain and
the spinal cord. In addition to its conducting function, the mid-brain
forms a cell-station on the optic and auditory paths, and it also serves
as a reflex centre for contraction of the pupil through the oculo-motor
nucleus. It has also been suggested that the red nucleus may form a
cell-station on an indirect motor path from the cerebral hemisphere to
the spinal cord, the route being by way of the cerebro-cerebellar path
through the pons, then by the superior peduncle to the red nucleus,
and by the rubro-spinal tract from the latter to the spinal cord.
SECTION VIII.
THE FORE-BRAIN.
The fore-brain consists of the two cerebral hemispheres, together
with certain masses of grey matter and other structures situated around
the third ventricle, and comprising the thalami, the corpora geniculata,
the hypophysis or pituitary body, and the pineal gland. The thalami
and geniculate bodies are composed of grey matter. Each thalamus
forms the lateral boundary of the third ventricle on its own side.
The internal (medial) and external (lateral) geniculate bodies lie
ventral to the thalamus on each side, and are in close relation with
the superior colliculi.
The pineal gland lies immediately above the superior colliculi. It
contains no nerve structure, being composed of alveoli with earthy
phosphates in their interior, and it has not, so far as is known, any
function. It is supposed to be the homologue of the pineal eye of
the lizards.
THE CEREBRAL HEMISPHERES.
The cerebral hemispheres constitute the largest part of the brain.
Each consists of an external layer of grey matter, thrown into folds
or convolutions, with white matter internally. A mass of grey matter,
80 ESSENTIALS OF PHYSIOLOGY.
known as the corpus striatum, lies in the interior of each hemisphere,
lateral to the thalamus and separated from it by a sheet of white
matter, the internal capsule. Each hemisphere also contains in its
interior a lateral ventricle, which is in communication with the third
ventricle.
1. Outer fibre lamina.
2. Outer cell lamina.
8. Middle cell lamina.
4. Inner fibre lamina.
5. Inner cell lamina,
=
pace
Bre a
Fie, 24.—Structure of cortex of motor leg area.
(Starling’s Principles of Physiology. )
The grey matter consists of nerve cells and nerve fibres, arranged
in layers. The axons of the nerve cells become either projection
fibres or association fibres. Other fibres terminate by arborisation in
the grey matter ; sof¥e of these are projection fibres, most of which
are axons of cells in the thalamus, and others are association fibres,
proceeding from cells in other parts of the cortical layer.
The structure of the grey matter varies in different regions of
Z Ae
THE CENTRAL NERVOUS SYSTEM. 8I
the cerebral hemisphere, but, whatever the local modifications, the
general plan is the same in all parts, and shows an arrangement in ~
five layers (fig. 24). These layers are named as follows :—
(1) The outer fibre lamina, or molecular layer.
(2) The outer cell lamina. |
(3) The middle cell lamina, or granule layer.
(4) The inner fibre lamina.
(5) The inner cell lamina, or layer of polymorphic cells.
A convenient modification of this description is to speak of the
layers in their relation to the middle cell lamina or layer of granules.
This arrangement gives (1) a supragranular layer, consisting of a fibre
lamina and a cell lamina; (2) the granule layer itself; and (3) an
infragranular layer, consisting of a fibre lamina and a cell lamina.
The outer fibre lamina contains medullated nerve fibres running
horizontally, a few scattered nerve cells, and the dendrons of many of
the cells of the next layer.
The outer cell lamina contains pyramidal nerve cells, and may be
subdivided. into layers of small, medium, and large pyramids, the
small pyramids being most superficial, and the large pyramids most
deeply situated. i |
The middle cell lamina contains pyramidal cells, but is especially
characterised by the presence of a large number of stellate cells, some
of which are large and others small.
The inner fibre lamina consists of medullated nerve fibres, running
horizontally, but in the motor area it contains also large and often
solitary pyramidal nerve cells, called Betz cells, the apical dendrons of
which may extend to the outer fibre lamina.
The inner cell lamina contains a large number of irregular or poly-
morphic cells, as well as some pyramidal cells, the cells of Martinotti,
the apices of which point centrally, while the axons pass towards the
surface.
The Functions of the Cell Layers.—Information may be obtained
as to the function of the different cell layers of the grey matter in
three ways: (1) by observing the order of their development in the
child ; (2) by a comparison of their relative proportion in man and
in the lower animals ; and (3) by a comparison of the different regions
in the human adult cerebrum, and by observations of the differences in
persons suffering from amentia or dementia.
(1) The study of development of the cortex Sows that the inner
cell lamina is the first to appear, and that it has attained three-fourths
of the adult depth at the sixth month of foetal life. It is followed by
the middle cell lamina, which, however, has only one-half of the adult
6
.
82 ESSENTIALS OF PHYSIOLOGY.
depth at the sixth month of foetal life. The outer cell lamina is the
last to appear, and develops slowly after birth. The outer fibre lamina
is well developed at birth, and is associated in its further growth with
that of the outer cell lamina. The inner fibre lamina is well developed
at birth, attaining its adult depth almost at once.
(2) The inner cell lamina is the first to appear in the evolution of
the cerebral cortex, and is well developed in the lower mammalia; in
the mole, for example, it forms the greater part of the depth of the
cortex. It is followed by the middle cell lamina, the outer being the last
to appear, and attaining a low degree of development in all animals below
man. ‘The outer cell lamina, however, increases progressively in depth
from the insectivora through the rodents and ungulates to the carnivora.
(3) The least developed portion of the human cerebral cortex is
the grey matter of the hippocampus, in which the only cell lamin
represented are the middle and inner. The ascending frontal convolu-
tion (motor area) is characterised by the presence of Betz cells in
the inner: fibre lamina. The visuo-sensory area, situated in the
occipital lobe, is distinguished by an increase in depth of the middle
cell lamina. The outer cell lamina is most highly developed in what
are known as the association areas. These are three in number: the
posterior, occupying the posterior part of the parieto-temporal region ;
the middle, in the island of Reil; and the anterior, which lies in that
part of the frontal lobe known as the pre-frontal region. The pre-frontal
region is the highest zone of association, and in it the outer cell lamina
undergoes the greatest development, varying considerably, however, in
different individuals. The outer cell lamina in this region is more or
less pronounced according as the mental development is of greater or
less degree. It is imperfectly developed in idiots and imbeciles, and
its cells are atrophied in cases of dementia,
On the basis of these facts, J. S. Bolton ascribes different functions
to the three cell laminz. The polymorphic layer subserves the in-
stinctive activities concerned with obtaining food and shelter, with
seeking protection from danger, and with the functions connected with
sex. The middle cell lamina is concerned with the reception. and
transformation of afferent impulses. The outer cell lamina is psychic or
associational, and has to do with the mental processes, especially with
those included under the terms ‘ voluntary attention,” and “ inhibition.”
THE TRACTS OF THE CEREBRAL HEMISPHERES,
The nerve fibres of the white matter of the cerebral hemispheres
are either projection fibres or they are associational in character,
including under the latter term the commissural fibres which connect
- oe we,
nm
at i aN A a
—
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er ee ep) are
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THE CENTRAL NERVOUS SYSTEM. 83
the two hemispheres. The projection fibres are those which connect
the cerebral cortex with the lower parts of the central nervous system,
and they are either afferent or efferent. The associational fibres link
up different parts of the cortex. The fact that nerve fibres acquire
their myelin sheaths at the time at which they become functionally
active has been made use of by Flechsig in studying the nerve tracts
of the cerebral hemispheres, and, from his observations, he has come to
conclusions as to the functions of the different areas of the cortex
which are as a whole identical with those based upon the study of the
structure of the grey matter in different regions of the adult cortex. The
first fibres to become myelinated in the cerebral hemispheres are those
afferent projection fibres which are distributed to the sensory areas of
the brain. The last to acquire a myelin sheath are those which are
connected with the higher centres of association.
The Projection Fibres.—The efferent projection fibres are the
pyramidal (cerebro-spinal) and the cerebro-cerebellar. The pyramidal
jibres are the axons of the Betz cells in the pre-central convolution or
motor area. ‘They converge towards the base of the hemisphere, and
occupy the genu and the anterior two-thirds of the posterior limb of
- the internal capsule (fig. 27).. From this .situation they descend in
the pes of the cerebral peduncle and through the pons and medulla
oblongata to the spinal cord in the manner already described.
The cerebro-cerebellar fibres are the fronto-pontine from the frontal
lobe and the temporo-pontine from the temporal lobe. These descend,
the former in the anterior limb of the internal capsule, the latter in
the posterior third of its posterior limb, to the base of the cerebral
peduncle, after passing through which they reach the pons to arborise
round the cells of the nuclei pontis.
The afferent projection fibres are the thalamo-cortical, the optic
radiation, the auditory radiation, and some fibres of the superior
cerebellar peduncle.
The thalamo-cortical fibres form the final relay on the path of
afferent impulses from the lower centres to the cortex of the cerebral
hemisphere, and they are distributed to all parts of the cortex. The
thalamo-frontal fibres run towards the frontal lobe in the anterior limb
of the internal capsule. The thalamo-parietal fibres pass through the
posterior limb of the internal capsule to reach their destination in the
parietal lobe. Those to the island of Reil run under the lenticular
nucleus. Those to the temporal lobe run in the posterior end of the
posterior limb of the internal capsule, and are joined by other fibres
from the inferior colliculus of the corpora quadrigemina and the medial
geniculate body to form the auditory radiation. Those to the occipital
84 ESSENTIALS OF PHYSIOLOGY.
lobe run in the internal capsule behind the auditory radiation, and are
joined by other fibres from the external geniculate body to form the
optic radiation.
Some of the fibres of the superior cerebellar snctate: have cell
stations in the thalamus, but others are believed to pass through the
posterior end of the internal capsule without interruption to end in
the cortex in the neighbourhood of the central sulcus or fissure of
Rolando. :
The Association Fibres.—The short association fibres lie immediately
under the cortex and connect the grey matter of adjacent convolutions.
The long association fibres form tracts which unite areas of the cortex
which are at some distance from each other. One tract, the superior
longitudinal fasciculus, runs between the frontal and occipital lobes ;
another, the inferior longitudinal fasciculus, connects the temporal and
occipital lobes; a third, the uncinate fasciculus, connects the frontal
and temporal lobes ; a fourth connects the parietal and occipital lobes ;
while a fifth runs from the anterior perforated space (substance) over
the corpus callosum to the hippocampus.
The commissural fibres connect the two cerebral hemispheres and
are grouped in the corpus callosum, and the anterior, posterior, and
hippocampal commissures. The corpus callosum contains fibres from
all parts of each cerebral hemisphere except the olfactory bulb and
parts of the temporal lobe. The olfactory lobes are connected by the
anterior and hippocampal commissures, the anterior commissure also
containing fibres which connect the two temporal lobes. The relation-
ships of the posterior commissure are unknown.
- The association and commissural fibres are either axons of cells in
the grey matter of the cortex or collaterals from axons ; they terminate
by arborisation in relation with other nerve cells.
THE FUNCTIONS OF THE CEREBRAL HEMISPHERES.
The ascent of the animal scale is marked by a progressive increase
in the size and development of the cerebral hemispheres, and in man
these structures are absolutely and relatively larger than in any of the
lower animals. This increase in size is associated with a corresponding
increase in functional importance, the higher centres acquiring a more
marked control over those in the spinal cord. This fact becomes more
apparent when the results of removal of the cerebral hemispheres in
different animals are compared. In the case of the frog, when the
animal has recovered from the shock of the operation, there is at first
sight little difference from the normal condition. The posture - is
normal, equilibrium is maintained, and is regained if the frog is placed
LLL ALLL LIL LOA A CELL AS Vi
eee
t
SP a A ee
erat oxas
THE CENTRAL NERVOUS SYSTEM. 85
on its back. When the animal is placed in water, it swims to the
margin and crawls out ; if it is placed on an inclined plane, it crawls to —
the top and balances itself there. If, however, it is not stimulated in
any way, it will remain in the same attitude until it dies. The com-
plicated reactions known as volitional impulses are wanting, and the
frog shows no spontaneous movements.
_ Similar phenomena may be observed in a pigeon from which the
cerebral hemispheres have been removed. There is the same mainten-
ance of posture unless the animal is disturbed; and the power of
equilibration is not affected. The pigeon flies in a normal manner if
it is thrown in the air, but it soon alights and resumes its ‘resting
attitude. It pecks at the ground if it is hungry, but does not feed
itself.
The removal of the cerebral hemispheres in mammals is astiediy
followed by a fatal result, but Goltz succeeded in performing the
operation in a dog by carrying it out in successive stages; and” he
afterwards kept the animal alive for a year and a half. Temporary
paralysis followed, but was recovered from, and thereafter, in marked
contrast with the frog and pigeon under similar circumstances, the dog
showed a tendency to be in continual restless movement. It even
learned to feed itself when food was placed near its nose. It responded
to stimuli, if painful, by growling or barking and turning its head
towards the stimulated spot, though it showed no sign of recognition
of the persons who fed it, and gave no indication of fear when
threatened or of pleasure when caressed.
The absence of the cerebral hemispheres, therefore, in the frog,
pigeon, and dog is associated with a condition in which the animal
respohds- to stimuli in a more direct and simple fashion than is the
case when the brain is intact. In the normal animal, the effect of a
stimulus is modified by impulses arising out of the memory of previous
experiences. When the cerebral hemispheres have been removed, the
memory records are absent, and the response to the stimulus is
simplified; in other words, there is an absence of intelligence, of —
volition, and of emotion. But the machinery for the carrying out of
muscular movements in a co-ordinate manner still remains, and can be
set in action by a suitable stimulus. |
The function of the cerebral hemispheres is therefore associative,
combining the effects of immediate with those of past stimuli, and
giving out efferent impulses based on such combinations. The grey
matter of the cortex is not only excited by stimuli, but the stimuli
produce a permanent record in its cells, known as memory, which
exercises an important influence on all subsequent actions.
86 ESSENTIALS OF PHYSIOLOGY.
THE LOCALISATION OF FUNCTION IN THE
CEREBRAL HEMISPHERES.
It has already been pointed out that histological and embryological
researches have indicated that different areas of the cortex subserve
different functions, and both observation of the results of disease in
man and experimental studies in connection with animals have con-
firmed and extended these conclusions. Injuries and tumours of
different parts of the human cortex give rise to muscular paralysis,
blindness, deafness, aphasia, or mental deficiency, according to the site
of the lesion. Experiments carried out on animals, either of the nature
of stimulation of various parts of the surface of the cerebral hemi-
spheres or of removal of localised portions, have given results parallel
with those derived from the study of diseases and injury in human
beings; and as a consequence it has been possible to map out the
surface of the hemispheres into areas, each of which possesses a definite
function. The pre-central convolution is motor in function, the post-
central is sensory and is especially concerned with the reception: of
kinzesthetic impulses, that is, impulses from muscles, tendons, and
joints, and with tactile discrimination. The mesial aspect of the
occipital lobe, or that part of it which lies on the borders of the
calcarine fissure, is visuo-sensory, while the convolutions immediately
adjacent to the visuo-sensory area are visuo-psychic. The audito-
sensory area and the audito-psychic areas are situated in the superior
temporal convolution. The area for taste and smell is in the hippo-
campal convolution. No special area has been discovered for the senses
of heat, cold, pain, and tactile localisation. The parieto-temporal region,
the island of Reil, and the pre-frontal region form the three special
association areas (fig. 25).
The Motor Area.—Stimulation of etbhioe the grey matter or of the
underlying white matter of the pre-central convolution gives rise to
muscular movements on the opposite side of the body. The latent
period is longer for stimulation of the grey matter than when the white
fibres are excited, but a stronger stimulus is required to elicit movement
from the white matter than from the grey. In either case the resulting
movements are co-ordinated, groups of muscles being affected, and
contraction of a particular group being accompanied by reciprocal
relaxation of the corresponding antagonistic group. In other words,
“movements, not muscles” are represented in the cortex. Further,
stimulation of a particular point in the pre-central convolution is invari-
ably followed by the same movement, so that, for example, excitation
of one point will result in extension of the thigh, of another in flexion
THE CENTRAL NERVOUS SYSTEM. 87
on
3
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Intermediate
pre-central.
Post-central.
Intermediate
post-central
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-—-—— —
| Fic. 25.—Diagrams (orthogonal) of cortical areas as determined by the distribution and
arrangement of fibres and cells. (A. W. Campbell.) From Quain’s 4natomy.
I., lateral surface; II., mesial surface.
88 , ESSENTIALS OF PHYSIOLOGY.
of the thigh, of another in flexion of the leg, and so on. The repre-
sentation of the movements of the leg, trunk, arm, and face is in that
order on the convolution from above downwards, and each of these
sub-areas may be further subdivided into points for the movements of
particular groups of muscles, |
Correlated with the development of the cerebral cortex is the
increased size of the pyramidal tract, which in monkeys, and still more
in man, is the principal path for the rapid conduction of voluntary
impulses to the motor neurons of the spinal cord. Other motor paths
from the cerebral cortex probably exist in the human nervous system,
but have fallen into disuse, and it is for this reason that the effects of
lesions of the cortex or of the pyramidal tracts in any part of their
course are so much more severe and permanent in man than in the
lower animals.
It must not be supposed that the motor area of the cerebral cortex
is the actual seat of voluntary impulses. On the contrary, its activity
is aroused by impulses from other parts of the brain, and represents
only a small fraction of the total process of which it forms a part.
Generally speaking, the movements produced are on the opposite side
of the body to that stimulated, but in some cases muscles on both sides
of the body may be simultaneously affected ; for example, stimulation
of a point concerned with the movements of the eyes will result in both
eyes being turned towards the opposite side. If the right side be
stimulated, the eyes are turned to the left by the contraction of the
right internal rectus and the left external rectus, with simultaneous
relaxation of the right external and left internal recti. Similarly the
areas for the trunk and neck govern movements of both sides of the
body. In all cases in which a movement is carried out by the combined
action of muscles on the two sides of the body, the muscles of both sides
are bilaterally represented in the cortex.
If a stronger stimulus is applied to a motor point on the cortex than
is necessary to elicit the movement peculiar to that point, the excitation
will spread to adjacent areas, just as irradiation occurs in the spinal
cord. By increasing the strength of the stimulus it is possible to throw
all the muscles of the body into convulsive contractions. A similar
phenomenon is exhibited in Jacksonian epilepsy, in which a localised «
irritation of a motor area, such as that due to the pressure of a spicule
of bone, causes a general convulsion beginning in the part of the body
represented at the site of the lesion.
The motor area has been stimulated in conscious human beings ; the
stimuli elicited movements without any sensation other than a conscious-
ness of the movements which took place. There is therefore no reason
aI i pane pa Pa By © tts
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THE CENTRAL NERVOUS sysTEM. © °!' 1% 89 UN,
to ascribe any sensory function to the pre-central convolution, and it
must be considered as purely motor.
The effect of removal of the motor area varies in different animals, —
In all cases the immediate effect is paralysis of the muscles on the
opposite side of the body. In the dog recovery takes place, and the
power of movement becomes almost as complete as it was before the
operation. In the monkey recovery is less complete, and a certain degree
of weakness may remain as a permanent result. When the motor area or
any part of it is destroyed by disease in man, recovery is still less com-
plete. In the ascent of the animal scale the functions of the nervous
system become more and more transferred*to the higher centres ; hence
injury of these centres in the higher animals, and especially in man,
is productive of more serious interference with the neuro-muscular
mechanism than is the case in animals lower in the scale.
The Area for Skin and Muscle Sense.—It is obviously a matter of
considerable difficulty to locate the area for tactile and muscular sensi-
bility, inasmuch as stimulation of a sensory area in animals is followed
by no objective phenomena, and opportunities rarely arise for observa-
tion of the results of stimulation in conscious human beings. There
are, however, recorded cases of stimulation of the post-central convolu-
tion in conscious individuals, and in these sensations of numbness and
touch were evoked. Apart from stimulation, the localisation of the
sensory areas rests upon the effects of removal, the distribution of the
thalamo-cortical fibres which represent the upward continuation of the
fillet, and histological and clinical observations.
Stimulation of the post-central convolution in animals is followed
.by muscular movements, but a stronger stimulus is required to elicit
these than is necessary if the pre-central convolution is stimulated, and
the latent period is longer in the case of the post-central, indicating that
the impulse has to traverse a larger number of neurons. This result
is what might be anticipated, as it is probable that the post-central and
pre-central convolutions are connected by short association fibres.
Removal of the post-central convolution in monkeys is said to result
in ataxia of the muscles of the opposite side of the body without
paralysis. On the other hand, Graham Brown has excised a part of
the post-central convolution opposite the arm area of the pre-central
in a young chimpanzee, and records that after a short period of weak-
ness of the opposite fore-limb there was no appreciable permanent motor
defect.
The thalamo-cortical fibres are distributed not only to the post-
central convolution but also to the temporal, frontal, and occipital
lobes, and therefore sensory impulses are distributed to a much wider
CTUCHO? ESSENTIALS OF PHYSIOLOGY.
area of the cortex than that from which motor impulses arise. The
chief tract of thalamo-cortical fibres, however, terminates in the post-
central gyrus. |
The available evidence indicates, therefore, that the post-central
gyrus is a sensory area specially connected with the tactile and
muscular senses, and that the senses of heat, cold, and pain have a
wider distribution on the cortex of the cerebral hemisphere, corresponding
with their wide distribution on the surface of the body.
The Visual Area.—The visuo-sensory area occupies the greater part
of the mesial aspect of the occipital lobe, the visuo-psychic area sur-
rounding it and extending on to the lateral aspect of the lobe.
Extirpation of both occipital lobes results in total blindness, extirpa-
tion of one lobe leading to blindness of the homolateral half of each retina.
. Stimulation of the visuo-sensory area is followed by movements of the
teyes. From the direction in which the eyes are turned in response to
stimuli applied to various parts of the area, it may be inferred that
the retinal impulses are projected on to the cortex according to a
definite plan. Thus stimulation of the upper part of the occipital
lobe is followed by a downward movement of the eyes, while stimula-
tion of a lower point of, say, the right lobe leads to a deviation of
both eyes towards the left. In the former case the movement is that
which would normally follow the excitation of the upper part of the
retina, in the latter it is that which would occur from excitation of the
right side of either retina. The fovea centralis, or part of the retina
concerned with distinct vision, is represented bilaterally.
Impulses are transmitted from the retinee to the occipital lobes
by the optic nerves, optic tracts, and optic radiations. Each optic
nerve, consisting of the axons of nerve cells in the retina, divides at
the chiasma, the fibres from the mesial side of the retina crossing to
the opposite side to take part in the formation of the optic tract of
that side. Thus each optic tract is made up of fibres from its own side
of each retina. The tract fibres terminate by arborisation in the
pulvinar of the thalamus, the external geniculate body and the
superior colliculus of the corpora quadrigemina. The fibres which
enter into the optic radiation arise in the thalamus and external
geniculate body, and are distributed to the cortex of the occipital
lobe (fig. 23). The relay fibres from the superior colliculus are dis- ©
tributed to the oculo-motor nuclei, and appear to be concerned with
the function of equilibration and with reflex contraction of the pupil.
The Auditory Area.—The localisation of the auditory area is less
definite than that of the visual area, largely owing to the difficulty of
ascertaining the degree of deafness produced in animals by experimental
a | _ & eo" — |
Peril Coe ee ee ee eS ae
- « 7
THE CENTRAL NERVOUS SYSTEM. gI
lesions. A partial decussation of the auditory tracts occurs, similar to
that of the optic nerves, so that each cerebral hemisphere receives
impulses from both ears.
The audito-sensory area is located in the temporal lobe, and has
been supposed to be limited to the middle region of the superior
temporal convolution, the audito-psychic area being adjacent to it.
But whereas, so far as can be judged, extirpation of one temporal lobe
is followed by partial deafness, and removal of both lobes by complete
deafness, Schafer has shown that removal of thé superior temporal
convolutions alone from both sides in monkeys does not result in
complete deafness. The superior temporal convolution is probably the
chief centre for hearing, but there may be subsidiary areas outside it.
ce
Fic. 26.—Diagram showing the paths for auditory impulses in the pons.
a, Accessory nucleus ; 6, tuberculum acusticum ; c, trapezoid nucleus ; d, lateral fillet.
Stimulation of the superior temporal convolution in monkeys is followed
by pricking of the opposite ear and turning of the head towards the
opposite side. |
Impulses are conveyed from the cochlea to the auditory centre by
the cochlear division of the eighth nerve, the auditory tract, and the
auditory radiation. The fibres of the cochlear nerve are derived from
the spiral ganglion, and terminate in the tuberculum acusticum and
the accessory nucleus. The axons from the tuberculum acusticum turn
over the restiform body to become the série acustice: (striae medullares)
in the floor of the fourth ventricle. These dip into the substance of
the pons at the middle line, some passing to the trapezoid nucleus and
superior olivary nucleus of the same side, others to the corresponding
nuclei of the opposite side. Some fibres terminate in these nuclei on
each side, others are continued directly into the lateral fillet. The fibres
from the accessory nucleus constitute the trapezium, and also enter
into the formation of the lateral fillet (fig. 26). Many of the trapezoid
we 4
Q2 ESSENTIALS OF PHYSIOLOGY.
fibres have cell stations in the trapezoid and superior olivary nuclei.
The lateral fillet terminates in the inferior colliculus and internal.
geniculate body ; from the latter new fibres arise to be distributed to
-the superior temporal convolution as the auditory radiation.
The Area for Smell and Taste.—The area for smell and taste is
located in the hippocampal convolution and the neighbouring structures.
Extirpation of this area has not yielded definite results. Stimulation
causes movements of the lip and nostril on the same side, such as
would be caused by a disagreeable or irritating odour applied to the
nostril. Further information is derived from a comparative study .of
the degree of development of these parts of the brain in animals which
have, and others which have not, a highly developed sense of smell.
The olfactory nerve fibres are non-medullated processes of cells in
the olfactory mucous membrane, and terminate in structures known as
glomeruli in the olfactory lobe in relation with the dendrites of certain
“mitral” cells, the axons of which convey the impulses transmitted to
them by the olfactory fibres to the hippocampal region of the same or
the opposite side.
The nerves of taste are the chorda tympani to the anterior two-
thirds and the glossopharyngeal to the posterior third of the tongue.
The taste fibres of both nerves terminate in a column of cells in the
pons, which also receives afferent fibres from the fifth nerve. The con-
ducting tract from this nucleus to the cortical area for taste has not
been traced,
The Association Areas.—There are three great association areas:
the posterior in the parieto-temporal region, the middle in the island
of Reil, and the anterior or pre-frontal. Stimulation of these areas
gives rise to no obvious motor response, but disease or imperfect develop-
ment in man is accompanied by various forms of mental deficiency.
' The pre-frontal area is the highest associational centre ; it is the last to
- develope and the first to retrograde. In it the outer cell lamina attains
its greatest depth, and atrophy of the cells of this lamina is found in
cases of dementia. The posterior association area, lying between the
' visuo-psychic and the audito-psychic areas, is concerned with mental
images, especially with the processes involved in the perception of
spoken and written language. Lesions of this area are accompanied
by interference with the appreciation of words, the different forms of
sensory aphasia.
The human brain is characterised by the great development of the
association areas. They represent the material basis for the memory
of past stimuli, and for the comparison of one set of stimuli with
another. In other words, they are the anatomical structures concerned
Se ee
THE CENTRAL NERVOUS SYSTEM. 93
with knowledge, intelligence, and, still further, with the faculties of
inhibition and voluntary attention, which find their highest development —
in man.
SPEECH AND APHASIA.
The pre-eminence of man is intimately related with the power of
speech or the use of words ‘as symbols to awaken the memory of past
stimuli. The production of spoken or written words is of course only
a specialised use of the muscular system; but for the appreciation of
language the existence of word-hearing and word-seeing centres has
sometimes been assumed. It is found that aphasia or loss of the
power of speech may take several forms. Thus there may be motor
aphasia, in which the power of forming words is Jost, or sensory
aphasia, in which there is inability to comprehend spoken or written
language. |
Motor aphasia was formerly ascribed to a lesion of the third left
frontal (Broca’s) convolution, but it has been ascertained that the
actual cause of the aphasia in such a case is destruction of the sub-
cortical fibres in the neighbourhood of the lenticular nucleus, along
with some degree of sensory aphasia. The term anarthria is applied
to the loss of power of articulate speech due to a lesion of the sub-
cortical motor fibres. Sensory aphasia may take the form of word-
blindness or word-deafness. In word-blindness, vision may be perfect
and the words on a printed page may be distinctly seen, but they are as
meaningless as if they were in an unknown language, and there is no
power of associating the written symbol with past stimuli. Similarly
in word-deafness, hearing may be perfect, but spoken words are un-
intelligible sounds. Anarthria may occur without any loss of intelli-
gence, but sensory aphasia is always accompanied by some degree of
mental deficiency, especially in the case of word-deafness.
THE THALAMUS AND INTERNAL CAPSULE.
The thalamus forms the lateral boundary of the third ventricle.
It is a large ganglionic mass and receives the terminations of the fibres
of the fillet. The outgoing fibres from the thalamus are distributed to
all parts of the cortex of the cerebral hemispheres, and cortico-thalamic
fibres also run from the cortex to the thalamus. On the lateral aspect
of the thalamus is the internal capsule, to and from which fibres
radiate from and to all parts of the cortex. In a horizontal section, the
internal capsule is seen to consist of a short anterior limb pointing
outwards and forwards, and a longer posterior limb pointing outwards
and backwards ; the junction of the two limbs is termed the genu.
+
94 ESSENTIALS OF PHYSIOLOGY,
The lateral aspect of the internal capsule is bounded by the
lenticular nucleus of the cerebral hemisphere. The anterior limb is
bounded on its mesial aspect by the head of the caudate nucleus of
the hemisphere. The anterior limb of the internal capsule contains
the fronto-pontine fibres. The genu and the anterior two-thirds of the
posterior limb contain the Pyrat fibres for the head, arm, trunk, and
before backwards. Behind the
pyramidal fibres are in order the
fillet, the auditory radiation, the
temporo-pontine fibres, and the
optic radiation (fig. 27),
THE PATHS BETWEEN THE
CEREBRAL HEMISPHERES
AND THE SPINAL CORD.
Afferent impulses enter the
spinal cord by the posterior nerve
roots. Those for muscle sense
and tactile discrimination travel
by the posterior column to the
nucleus gracilis and nucleus
cuneatus in the medulla oblon-
gata. From these nuclei fibres
arise and cross to the opposite
- side, where they enter into the
formation of the fillet. The fillet
Gas ee passes through the pons and
Fie. 27.—Diagrammatie répresentation of — . ; P
the internal capsule, as seen in hori- ™id-brain and internal capsule
zontal section. (Cunningham.) From to terminate in the thalamus,
PORE Er incite of aperaiayy: from which the impulses are con-
veyed to the cortex of the cerebral hemisphere by the thalamo-cortical
fibres (fig. 28). The paths for tactile localisation, pain, heat, and cold
cross in the spinal cord very shortly above the entrance of the posterior
roots by which they were conveyed. They travel up by the spino-
thalamic tract, or possibly by short segmental tracts, to the medulla
oblongata, and there join the fillet, their subsequent course being that
just described.
The pyramidal (cerebro-spinal) fibres take origin as the axons of.
the Betz cells in the motor area, pass through\the white matter of the
cerebral. hemisphere, through the internal capyule, the crusta of the
mid-brain, and the pons to become the pyramid of the medulla
AUDITORY RADY
TEMPORO-PONTINE
+4 OPTIC
6) t RAD,
\.
leg in the order mentioned from
THE CENTRAL NERVOUS SYSTEM. 95
oblongata. Most of these fibres cross in the pyramidal decussation
to form the crossed pyramidal tract, and terminate by turning into
the grey matter of the spinal cord, where they pass their impulses on,
probably through an intermediate neuron, to the cells of the anterior
horn, the axons of which become the motor fibres of the nerves to the
Fie. 28.—Diagram showing paths of Fic, 29.—Diagram showing the paths
sensory impulses. ! for motor impulses.
a, Skin ; b, muscle ; c, funfculus gracilis ; d,
spino-thalamic tract; e, nucleus gracilis ;
J, thalamus. :
x
a, Cellin anterior horn of grey matter; b, cell in
posterior horn; c, crossed pyramidal tract;
d, direct: pyramidal tract; e, decussation of
pyramids ; f, motor area in cortex.
skeletal muscles (fig. 29). The pyramidal fibres which do not cross in
the medulla oblongata form the direct pyramidal tract of the spinal
cord, and cross by degrees to the grey matter of the opposite side to
come into relation with the cells of the anterior horn. Some of the
uncrossed fibres join the crossed pyramidal tract of the same side, and
their further course is unknown. Generally speaking, however, the
ee
*
96 ESSENTIALS OF PHYSIOLOGY,
pyramidal fibres from each cerebral hemisphere convey impulses to
the contralateral side of the body.
PATIGUE.
The seat of fatigue in a muscle-nerve preparation is the end-plates.
The causation of fatigue, however, in the intact animal is a very complex
process, and other factors than the end-plates are concerned, the most
important being the central nervous system. Even the fatigue brought
about by muscular exercise probably has its origin to a larger extent in
the nervous system than in the muscles themselves, the part of the
nervous system which becomes fatigued being, in all probability the
synapses. The changes in the nerve cells which are brought about by
prolonged exercise also indicate that fatigue is partly nervous in origin.
It must be remembered also that the sense of fatigue does not correspond
exactly with the degree of fatigue as measured by the capacity of the
muscles to do work ; and the effect of psychical influences in lessening
or abolishing the sense of fatigue is well known. i
SLEEP.
Every active tissue of the body has alternating periods of activity
and rest. Thus the ventricles of the heart have a period of contraction
of three-tenths of a second followed by a period of relaxation of five-tenths
of a second. In the case of the other muscular tissues and of the
' glands, the periods are longer and are often irregular, but the same
general principle holds good. The active phase of the cells in the
cerebral cortex which subserve consciousness coincides with the waking
period, while the resting phase is the period of sleep. During sleep
consciousness is in abeyance and the activity of all the vital processes
is lowered ; respiration is slower, the heart beats more slowly, glandular
secretions are reduced in quantity, metabolic. changes generally are
diminished, and the temperature falls. Histological observations on the
nerve cells in sparrows show that certain spindle-shaped, clear bodies
(Nissl spindles) disappear from the cells during the activity of the day
and are restored during the night’s rest. It may be concluded that
katabolic changes exceed anabolic during the waking hours, and that
the reverse is the case during sleep.
The cause of sleep has been much discussed, and it is generally
agreed that it is associated with a diminished supply of oxygen. It has
been shown experimentally that if oxygen is withheld the subject of
experiment may become unconscious before he is aware of any un-
pleasant symptom. The cells of the cerebral cortex are therefore
peculiarly susceptible to a deficiency of oxygen. It has been suggested
Mot <n oneal Geflwes| e ~
Oe Rtg mete, eae
THE CENTRAL NERVOUS SYSTEM. 97
that these cells accumulate an excess of oxygen during sleep and
gradually use it up during the day. However that may be, there is —
evidence to show that the blood-flow is side-tracked during sleep, so that
the brain receives a smaller supply than it does in the waking hours.
If a limb be enclosed in a plethysmograph, it is found that the volume of
the limb increases during sleep owing to dilatation of the blood-vessels.
As a result, the supply of blood to the brain is diminished. Howell
suggests that the dilatation of peripheral blood-vessels may be due
to fatigue of the vaso-motor centre, but the ultimate cause is still
uncertain. Probably the lessened blood supply is secondary to the
diminution in functional activity.
THE CEREBRO-SPINAL FLUID.
The cerebro-spinal fluid forms a water cushion by which the brain-
and spinal cord are protected from jarring shocks during any sudden
movement of the body ; it may also contain substances which influence
the functional activity of the nervous tissues. It occupies the space
between the membranes of the brain and spinal cord, and fills the ventri-
cular cavities of the brain and the central canal of the cord. The fluid
lying in the ventricular cavities communicates with that which fills the
space between the membranes at the foramen of Majendie and at two
other foramina, one of which lies at each side of the recess of the fourth
ventricle.
The cerebro-spinal fluid resembles lymph, but is much less con-
centrated than that fluid. It is clear and limpid, and has a specific
gravity of about 1007. It contains traces of proteins, and its inorganic
constituents correspond with those of blood plasma.
If the dura mater is punctured, the cerebro-spinal fluid escapes from
the opening, showing that it is under a certain degree of pressure,
which can be measured by inserting a.cannula between two vertebree
and connecting it with a manometer. It is found that the pressure
corresponds roughly with the venous blood pressure, and that it varies
to a slight extent with variations in arterial and venous pressure.
If the cerebro-spinal fluid is allowed to escape, it is rapidly replaced.
This is shown in cases where the escape continues for some time, either
in conditions experimentally produced in animals, or in such accidental
circumstances as fracture of the base of the skull in man. A loss of
100 ¢.c. or more per hour has been known to continue for weeks in a
human being.
The cerebro-spinal fluid is derived from the choroid plexus, and
Dixon and Halliburton have recently shown that it is formed by a
process of secretion. The normal stimulus to secretion is a hormone
; 7
98 ESSENTIALS OF PHYSIOLOGY,
which can be extracted either from the choroid plexus itself or from
brain tissue by means of normal saline or other fluids ; injection of
this extract into the blood stream is followed by an increased flow of
cerebro-spinal fluid, which may be collected by means of a cannula. The
hormone contained in the extract is not destroyed by boiling and is
soluble in alcohol. It is not a protein, but must have a relatively large
molecule, since it will not pass through a Chamberland filter.
That the fluid is not formed by filtration is shown by experiments in
which a fall of blood pressure is accompanied by an increased produc-
tion of the fluid. For example, if one vagus nerve is divided and the
peripheral portion stimulated, the blood pressure may fall to zero both
in the carotid artery and in the cranial veins, while at the same time
the pressure of the cerebro-spinal fluid shows a very marked rise. The
explanation of this rise is found in the diminished supply of oxygen
to the brain and the local accumulation of carbonic acid. The pressure
exhibits a considerable rise if the air breathed is either deficient in
oxygen or contains an excess of carbonic acid—for example, if it contains
5 per cent. of the latter gas. In this connection it is interesting to notice
that the cerebro-spinal fluid may contain in normal circumstances 53 to
61 volumes per cent, of carbonic acid, 40 volumes of which are in firm
combination, whereas it only contains 0°1 to 0:3 volume per cent. of
oxygen.
The production of the fluid is also increased, but to a less extent,
by injections of cholesterol, pilocarpine, or atropine, and by the adminis-
tration of anesthetics. It is diminished by increased ventilation of
the lungs, or by an increased amount of oxygen in the air breathed. If
there is a sufficient supply of oxygenated blood, the pressure of the
cerebro-spinal fluid varies with the blood pressure; it rises therefore
with the administration of adrenalin, though this substance has no effect
on the amount of the secretion.
Changes in the pressure of the cerebro-spinal fluid affect the respira-
tory, vaso-motor, and cardio-inhibitory centres, a moderate rise of
pressure having a stimulating effect and a greater rise paralysing the
centres. When the pressure is raised to 300-400 mm. Hg, respiration
ceases in a few seconds, a little later the cardio-inhibitory centre is
paralysed, and later still the vaso-motor centre. When the pressure
is relieved, the vaso-motor centre recovers first and the respiratory _
centre last.
THE CENTRAL NERVOUS SYSTEM, 99
SECTION IX.
THE AUTONOMIC SYSTEM.
In contradistinction to skeletal muscle, the unstriped muscle which
is found in the walls of the arterioles, digestive tract, uterus, bladder,
and elsewhere, is not under the control of the will, though its contrac-
tion is regulated by impulses arising in the central nervous system.
The nerves which supply these structures, and also those to the secretory
glands, form the autonomic system.
This consists of (1) branches of some of the cranial nerves, including
the vagus and the chorda tympani, and of fibres issuing from the
anterior roots of the second and third sacral nerves and known as the
nervi erigentes; and (2) the sympathetic system, the pre-ganglionic
fibres of which leave the spinal
cord in the anterior roots of
all the spinal nerves from the
first thoracic to the fourth
lumbar.
The pre-ganglionic nerve
fibres are medullated and
small, varying in diameter
Fie, 30.—Diagram to show relation between
‘ pre-ganglionic and post-ganglionic fibres.
from 2 to Ap ; they are not E, Spinal cord; A, pre-ganglionic fibre; B, cell
: . : station; C, post-ganglionic fibre; D, unstriped
distributed directly to the ptaceeata Tint ,
tissue which they supply, but
every fibre ends round a nerve cell which lies in a sympathetic
or other ganglion; from this cell a fresh fibre, which is called post-
_ ganglionic and is usually non-medullated, starts and passes to the
peripheral tissue (fig. 30). The point at which the pre-ganglionic
fibre comes into contact with the dendrites of a ganglion cell
forms a cell station or synapse; and the nervous impulse, issuing
from the central nervous system along a pre-ganglionic fibre, normally
passes across the synapse to the ganglion cell and then along the
post-ganglionic fibre to the peripheral tissue. |
Although the entire autonomic system is built up on this general
plan, namely (1) pre-ganglionic fibre, (2) cell station, (3) post-ganglionic
fibre, and (4) nerve ending, the actual anatomical distribution of the
fibres and the situation of the cell stations are very varied. The fibres
issuing from the brain and from the sacral region of the spinal cord
have their cell station close to or actually within the organ which they
supply. The fibres of the sympathetic system take a different course.
Lying along each sidelof the vertebral column is a chain of ganglia which
forms the lateral sympathetic chain (fig. 31). As arule there is one
TOO
© oO
fdieos
Th
- wo Pw -
ESSENTIALS OF PHYSIOLOGY.
Superior cervical ganglion.
oe Cervical sympathetic nerve.
_--: Inferior cervical ganglion.
‘ aD _- Subclavian artery.
4" Kc
wae D5)
, Splanchnic nerve,
, .
/ Solar plexus,
7
Nervi erigentes.
Fie, 31.—Diagram of autonomic nervous system (excluding cranial nerves).
Pre-canclionic fibres. red: post-gvanglionic. black.
THE CENTRAL NERVOUS SYSTEM. IOI
ganglion corresponding with each spinal nerve root, but the upper four
thoracic ganglia are fused into one larger mass, the stellate ganglion. In
the cervical region there are in most animals only two ganglia, inferior
and superior, united by the cervical sympathetic nerve. The pre-gang-
lionic sympathetic fibres leave the anterior root in the thoracic and upper
lumbar region as a series of small nerves, the white rami communicantes,
each of which enters the corresponding ganglion of the lateral sym-
pathetic chain. Some of these fibres, including those which carry
impulses to the blood-vessels of the skeletal muscles and skin, and to
the hairs and sweat glands, have their cell stations in one or other of
the sympathetic ganglia; and the post-ganglionic fibres form small
nerves (called grey rami), one of which joins each of the spinal nerves.
The sympathetic fibres which supply the blood-vessels and other
structures of the head leave the spinal cord by the first to the fourth
or fifth thoracic white rami, and pass through the stellate ganglion up
the cervical sympathetic nerve to the superior cervical ganglion ; their
cell stations lie in this ganglion, and the post-ganglionic fibres leaving
it are distributed to the blood-vessels, salivary glands, and other
structures in the head.
The fibres which supply the heart leave the spinal cord by the second
and third thoracic white rami, and have their cell station in the stellate
ganglion, from which post-ganglionic fibres pass directly to the heart.
The fibres which are distributed to the abdominal viscera issue from
_ the spinal cord by the lower six thoracic and the first lumbar white rami,
pass through the corresponding ganglia of the sympathetic chain without
forming a cell station, and are gathered up into two large nerves, one on
each side, known as the splanchnic nerves. These enter two large ganglia,
the semilunar ganglia or solar plexus, in which lie thecell stations of almost:
all the fibres running in the splanchnic nerves. Some of the post-gang-
lionic fibres leaving these ganglia are distributed to the blood-vessels of
the abdominal viscera, while others supply the walls of the digestive tract.
Fibres also pass along the white rami of the upper lumbar nerves to
the inferior mesenteric ganglia, from which post-ganglionic fibres run
in the hypogastric nerves to be distributed to the pelvic organs.
Nicotine, in small doses, first stimulates and then paralyses the
synapses between the pre-ganglionic fibres and the nerve cells in the
autonomic ganglia, thereby preventing the passage of an impulse
through the cell stations ; it does not affect the nerve fibres themselves.
By means of nicotine, the course taken by the autonomic fibres and the
situation of their cell stations have been determined. The drug is
painted on a ganglion, and the fibres passing to and from it are
stimulated ; if stimulation of the fibres passing to the ganglion is
102 ESSENTIALS OF PHYSIOLOGY,
ineffective, it is clear that they have their cell station in that ganglion.
For example, when nicotine has been painted on the superior cervical
ganglion, stimulation of the cervical sympathetic nerve produces none
of the effects which are observed in the normal animal, - whereas
stimulation of the fibres leaving the ganglion produces the same effect
after the application of nicotine as before. The experiment shows that
the fibres running in the cervical sympathetic nerve have their cell
stations in the superior cervical ganglion.
The fibres of the autonomic system supply not only the blood-
vessels but other structures, including the walls of the digestive tract
and pelvic viscera, the heart, sweat glands, and hairs. Their course
and function will be fully considered in subsequent chapters, but may
be summarised here.
I. Cranial autonomic fibres.
Third nerve.—The autonomic fibres pass to the ciliary ganglion,
where they have their cell station, and supply the intrinsic muscles
of the eye. ; |
Seventh and ninth nerves.—The autonomic fibres supply vaso-
dilator fibres to the tongue, and secretory fibres to the salivary glands.
The vagus sends inhibitory fibres to the heart, motor fibres to the
cesophagus, stomach, small intestine, and bronchioles, and secretory
fibres to the stomach and pancreas; the cell stations probably lie in
the walls of the structures supplied by the different fibres.
Il. Sacral autonomic jfibres.—These supply dilator fibres to the
blood-vessels of the penis, and motor fibres to the muscles of the
rectum and bladder.
Ill. Sympathetic fibres.
(1) The fibres to the head leave the spinal cord in the first five
thoracic white rami, and run in the cervical sympathetic nerve ; this
contains vaso-constrictor fibres for the blood-vessels, secretory fibres
for the salivary glands, and dilator fibres for the pupil.
(2) The fibres to the heart have their cell station in the stellate
ganglia, and convey accelerator impulses.
(3) The fibres to the abdominal viscera leave the spinal cord in
the lower six thoracic and the first lumbar white rami. Most of them
have their cell stations in the semi-lunar and superior mesenteric —
ganglia, from which they are distributed. They convey constrictor
impulses to the blood-vessels of the stomach, small intestine, kidneys,
and spleen, inhibitory impulses to the muscular walls of the stomach
and small intestine, and motor impulses to the ileo-colic sphincter.
(4) The pelvic viscera are supplied from the white rami of the last
thoracic and upper lumbar nerves, the cell station being in the
ee ee
4
P
|
|
|
Nei : 4
.
THE CENTRAL NERVOUS SYSTEM. 103
inferior mesenteric ganglia. The nerves convey constrictor impulses
to the blood-vessels of the pelvic organs and inhibitory impulses
to the muscular coats of the colon, uterus, and bladder.
(5) Fibres are also contained in all the white rami which have
their cell station in the lateral chain of ganglia, the post-ganglionic
fibres passing usually, but not always, into the corresponding spinal
nerve to be distributed to the blood-vessels of the muscles and -skin,
and to the sweat glands and hairs in the area supplied by that nerve.
It will be noticed that many organs are supplied by two sets of
fibres having opposite functions.
FUNCTIONS OF THE GANGLIA.
The cells of the ganglia serve as distributing centres, and each
pre-ganglionic fibre arborises round a number of cells, so that the post-
Sp.cord
Fic, 82.—Diagram showing the structures concerned in the ‘‘axon-reflex”’
referred to in the text. (Starling’s Principles of Phystology.)
ganglionic fibres are much more numerous than the pre-ganglionic
fibres.
. At one time various reflex actions were attributed to the sympa-
thetic ganglia, but these have been proved to be not true reflexes, but
pseudo- or axon-reflexes. For example, when the nerves connected
with the inferior mesenteric ganglion are divided, with the exception of
the right hypogastric nerve to the bladder, stimulation of the central
end of the left hypogastric nerve causes contraction of the right half of
the bladder. This is due to the fact that some of the fibres leaving the
104 ESSENTIALS OF PHYSIOLOGY.
' spinal cord branch in the ganglion, one branch passing down the left
nerve, the other entering into the formation of a cell station connected
with the right hypogastric nerve (fig. 32). Stimulation of the central
end of the left nerve causes an impulse to pass up to the point of junction
of the two branches, and then down the other branch through the cell
station to the bladder. This effect is called an axon reflex, and depends
upon the fact that nerve fibres may conduct impulses in either direction.
The autonomic system also contains afferent fibres, though these
are less numerous than the efferent fibres; in the splanchnic and
hypogastric nerves about one-tenth of the fibres are afferent. |The
stimulation of these fibres by abnormal processes in the abdominal
organs may give rise to pain. The: pain is usually referred, however, ©
not to the organ itself, but to the surface of the body ; for instance,
afferent impulses from the stomach may give rise to pain which is
referred to an area of skin at the lower border of the ribs, and this
area may actually be tender to touch. The position of the referred
pain and of tender cutaneous areas has proved of value in man as a
means of localising disease of the internal organs.
-
CHAPTER VI.
THE ORGANS OF SENSE.
SECTION I.
THE organs of sense, with their nerves, form the medium by which
afferent impressions are conveyed to the cortex of the cerebral hemi-
spheres. If the sensory mechanism is concerned with impulses excited
by stimuli from without, it is described as exter ive ; if the stimuli
arise in the viscera, the mechanism is called enteroceptive ; if they arise
in the muscles or sense organs affected by the position of the body,
the mechanism is propri ive. Thus the exteroceptive system
includes the structures which have to do with sensations of pressure,
taste, smell, sight, and hearing ; the enteroceptive system includes the
mechanisms for hunger and thirst ; and the proprioceptive system has
to do with sensations of position of the head and limbs and of the
degree of muscular contraction.
The structures concerned in the production of sensation are: (1)
an end-organ, (2) a chain of neurons which transmits the impulse, and
(3) the sensory, psychic, and association areas in the cortex to which
the impulse is transmitted. The end-organs for each sense are
structurally adapted to receive the stimulus for that particular sense,
this being known as the adequate stimulus. Thus the rods and cones of
the retina are stimulated by waves of light, but not by waves of sound,
while the hair cells in the organ of Corti are excited by sound waves
but not by those of light. The nerve fibres which transmit the im-
pulses, on the other hand, appear to be able to transmit any variety of
stimulus, and they all give the same type of electrical variation when
stimulated by an electric current.
Although the various sense organs differ widely in structure, certain
general principles can be formulated which are applicable to
them all. . eh
(1) Stimulation of the end-organs of any particular sense gives
rise to the sensation peculiar to that sense, and to that sensation only ;
105
106 ESSENTIALS OF PHYSIOLOGY.
in other words, each sense has its own specific quality or modality.
This characteristic was stated by Miiller in the form of a law, which
he called the ‘‘law of specific nerve energy.” It is better, however, in
view of the restricted modern use of the word energy, to speak of the
law of specific irritability. The quality of the sensation aroused might
_ be determined by the nature of the receptive end organ, by the con-
ducting apparatus, or by the area of the cortex to which the impulse
is transmitted. Direct stimulation of the central portion of a divided
sensory nerve gives rise to the specific sensation, so that the modality
is not determined by the end-organ. For example, pressure on the
ulnar nerve trunk excites a pricking sensation referred to the distribu-
tion of the nerve, and section of the optic nerve in a conscious patient
is accompanied by the sensation of flashes of light. Again, there is
no reason to believe that the conducting nerve fibres have any more
influence on the nature of the impulse they convey than an electric
wire has in determining the nature of a telegraphic message. It is,
therefore, in the cortex of the cerebral hemisphere that the ex-
planation of the specific character of the sensation is to be sought,
and this conclusion is supported by the fact that sensations may be
aroused in the absence of any stimulation of the end-organs, e.g. in
dreams or hallucinations.
(2) In the case of each sense a certain minimal strength of stimulus,
known as the threshold stimulus, is necessary to evoke a sensation.
The exact strength of the threshold stimulus for any particular sense
varies in different individuals, and also in the same individual at
different times. A succession of subliminal stimuli, that is, of stimuli
each of which is below the threshold value, may excite a sensation by
a summation effect, just as the summation of subminimal stimuli may
excite a reflex action. The threshold value will vary with the condition
of the sense organ. For example, the mechanism may be fatigued, and
will then be less responsive to stimulation. This is well illustrated
in the case of smell. The air of a clased room, which is occupied,
becomes disagreeable, but the occupants of the room do not notice the
unpleasant smell, which is at once apparent to anyone coming in from
the outer air. The threshold value also varies with the state of
adaptation of the sense organ. Thus an eye which has been exposed
to light is said to be light-adapted, while one that has been in
darkness for a time is dark-adapted. The threshold value of the
light stimulus for the dark-adapted eye is much lower than, actually
about one-fiftieth of, that required to produce a sensation in the light-
adapted eye.
(3) The increase of stimulus necessary to cause a difference in the
be a
9 EE ee ee a ies ae as ee, ee ee Lee ae eo
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THE ORGANS OF SENSE. 107
degree of sensation bears a constant proportion to the strength of the
original stimulus. If the eye is being stimulated by the light of one
hundred candles, the extra stimulation required to produce the sensation
of more light can be derived from one more candle. If, however, the
original stimulus came from one thousand candles, then ten more candles
would be required for a difference to be detected.
- Similarly, if a weight of 30 grams is held in the hand, one- more
gram must be added to excite a sensation_of increased weight, and a
smaller increase would not be noticed. if ‘the original weight is 60
grams, 2 grams must be added for the difference to be perceptible. So
also in the case of sound, the stimulus must be increased by one-seventh
to lead to the perception of increased sound volume. In all cases this
law, which is known as Weber’s law, only holds good within certain
definite limits, the limits for the pressure sense as tested by weights
lying between 50 and 1000 grams.
CUTANEOUS SENSATIONS.
The sensations aroused by the application of different varieties of
stimuli to the skin are those of pressure, including tactile localisation
and tactile discrimination, heat, cold, and pain. - These various sensations
-are independent and do not result from different forms of stimulation
applied to the same set of nerve endings. The evidence that tactile
localisation, tactile discrimination, heat, cold, and pain are distinct senses
is (1) histological, (2) the existence of independent spots in the skin,
stimulation of each of which gives rise to one variety of sensation only,
and (3) the fact that interference with the conducting paths may
result in blocking of one set of impulses, for example, those giving rise
to pain, while the other senses are unaffected.
Histological. The End-organs in the Skin.—The peripheral fibre
from a cell in the posterior root ganglion or the homologous ganglion
on a cerebral nerve terminates in the skin in various ways. The
termination may be free, or it may be protected. ree nerve endings
are found in the anterior epithelium of the cornea ; the axis cylinder of
the nerve fibre loses its myelin sheath at the periphery of the cornea,
and, after forming a plexus in the corneal substance, the fibre terminates
between the epithelial cells in the form of fine varicose fibrils. Similar
terminations are found in the epidermis, and fibrillar nerve endings
occur also around the hair follicles. The protected nerve endings are all
formed on the same general plan. There is a central soft core
surrounded by a variable amount of fibrous tissue, arranged sometimes
irregularly, and sometimes, as in the Pacinian corpuscles, in lamine.
The nerve fibre loses its myelin sheath and runs. into the core of the
108 ESSENTIALS OF PHYSIOLOGY.
end-organ, where it ends in one or more knob-like extremities. Such
endings are the end-bulbs found in the conjunctiva, penis, clitoris, and
in the synovial membrane of some joints: the towch corpuscles of Meissner
in the connective-tissue papille of those parts of the skin which are
most sensitive to pressure, that is, in the hand and front of the forearm,
the lips, the foot, and the mammary papilla: the Pacinian corpuscles
found in the subcutaneous tissue of the hand, foot, and genital organs,
and in the mesentery and some organs, such as the pancreas: the
corpuscles of Golgi and Mazzoni and the corpuscles of Ruffint, found in
the subcutaneous tissue of the fingers.
The Sensory Spots in the Skin.—If a cooled metal point is drawn
gently along the skin of the forearm or back of the hand, a sensation of
cold is produced at certain definite spots, which may be marked out
with coloured ink. If the point is heated and the experiment repeated,
it is found that other spots, quite distinct from those for cold, respond
and give the sensation of heat. Similarly, by using the prick of a
needle as the stimulus, another series of spots may be marked out
which give rise to pain. Lastly, by testing the pressure sense
with a bristle a fourth set of spots may be located. The most numerous
spots in the skin are those for pain; next in order are the pressure
spots, those for cold being less numerous, and the heat spots fewest of
all. The specific character of the different nerve endings is confirmed
by the effect of other stimuli. The application of menthol to the skin,
for example, stimulates especially the nerve endings for cold and gives
rise to a sensation of coolness. If the arm be held in a jar of carbonic
acid gas, on the other hand, there is a sensation of warmth, the nerve
endings for heat being especially affected. Electrical stimulation of
the various spots excites a specific sensation, namely, warmth in a
heat spot, pain in a pain spot, and so on. Additional evidence as to
the independence of the sensations is derived from the effect of cocaine
on the surface of the eyeball. Under normal conditions suitable
stimulation of the conjunctiva excites the sensations of heat, cold, or
pain, whereas stimulation of the cornea gives rise to the sensation of
pain only. The application of cocaine to the eye paralyses the nerve
endings for pain, but does not affect those for heat or cold.
The Conduction of Cutaneous Impulses.—The nerve fibres which
convey the cutaneous impulses are bound together in common trunks.
Prolonged pressure on these trunks acts as a block to the conduction of,
impulses, and occasionally cases occur in which, by the pressure of a
bony outgrowth or otherwise, a partial block is produced, whereby the
pressure and temperature senses are lost, while that of pain is retained,
or vice versa. A similar partial block occurs at times in the disease
—
Ta ltt Vee ee eee
THE ORGANS OF SENSE. 109
known as syringomyelia, in which there is pressure on the conducting
paths in the spinal cord owing to distension of the central canal, In~
these cases also there may be loss, for example, of the senses of pain
and temperature, while that of pressure is retained.
The cutaneous senses are therefore independent and are subserved
by different nerve fibres, but the further question as to whether each
sense has a specific nerve ending cannot be so decisively answered.
Special functions can, however, be ascribed to some of the nerve endings
in the skin with a reasonable degree of probability.
The fact that any stimulus applied to the cornea gives rise to_pain
indicates that interepithelial fibrillar nerve endings are associated with
the pain sense. The skin areas most sensitive to pressure are those,
like the palmar surface of the fingers, where the touch corpuscles of
Meissner are most abundant. Where hair is present the pressure spots
immediately surround the point of emergence of the hair. It may
therefore be concluded that the touch corpuscles in hairless regions and
the fibrillar nerve endings around the hair follicles are the peripheral
terminations of the pressure sense fibres. Finally, the position of the
Pacinian corpuscles and of those of Golgi and Mazzoni and of Ruffini
makes it clear that these structures can only be affected by deep
pressure. |
THE DISTRIBUTION OF THE CUTANEOUS SENSORY
. NERVE ENDINGS.
Reference has already been made to the fact that the cornea is
richly supplied with nerves for pain, and that the skin of the palmar
surface of the fingers is markedly endowed with the pressure sense.
The distribution of the various sensory nerves is therefore unequal, and
investigations have been made, especially in connection with the pressure
sense, to determine (1) the degree of pressure which can be detected at
different parts of the skin surface, and (2) the relative acuteness in
different areas of tactile discrimination, that is, of the power of appreci-
ating two separate pressure stimuli applied simultaneously.
(1) Von Frey’s method of estimating the degree of pressure on the
skin which can be appreciated is to use hairs of different thickness, the
pressure required to cause each to bend being known. In this way he
found that the skin of the nose and lips and the mucous membrane of
the tongue are most sensitive to pressure, while the skin of the region
of the loins has a very low degree of sensitivity. The minimum
stimulus which could be detected in different skin areas is*shown in
the following table :— .
Ss
IIO ESSENTIALS OF PHYSIOLOGY.
Area, Grams per sq. mm,
bo
Tongue and nose .
Lips ; 2°5
Finger-tip and terikisad: 3-
Palm, arm, thigh 7
Forearm 8
Back of band ‘ 12
Abdomen, outside of thigh 26
Back of forearm . 33
Loins . ; : : . 48
(2) Tactile discrimination is measured by means of the ssthesio-
meter, an instrument somewhat resembling a pair of compasses, one
limb of which can be moved along a scale on which the distance between
the two points can be read off. When the two points, armed with small
pieces of cork, are applied to the skin a sufficient distance apart, the
resulting sensation is of two separate stimuli. When the two points
are approximated, the degree of approximation varying for different
parts of the skin surface, the double stimulus is productive of a single
sensation. The minimal distance apart at which the points give rise to
_separate sensations is shown in the following table :—
Skin region, : Distance in mm.
Tip of tongue. : THOT 1-1
Volar surface of Anaantic ; 2°3
Palm of hand. ; : es
Back of hand. ; x: wb ae
Middle of back, upper arm, éfieh ; ck ae ik
These figures have no reference to the distances which separate the
actual pressure spots, but include on an average about ten such spots
in each case between the limbs of the esthesiometer.
LOCAL SIGN,
Just as a localised stimulus of a definite kind applied to the skin
invariably evokes the same reflex, so also in a conscious individual a
stimulus applied to a particular skin area is referred to the stimulated
spot. There must therefore be a specific quality in the impulse
conveyed to the association centres according to the ae area in which
it originates, and this quality is called ‘local sign.” It has been
supposed by some authorities that local sign is due to a recognition of
the muscular reflexes which are or may be evoked as a result of the
stimulus, but it is more probable that it is developed as the result of
experience, If the index and middle fingers be crossed and a pea be
a i eS ee a ee
THE ORGANS OF SENSE. III
placed between them in this position (Aristotle’s experiment), the
sensation produced will be of two peas, because the individual has no
previous experience of a single stimulus applied in this way.
PROTOPATHIC AND EPICRITIC SENSIBILITY.
The investigations of Head have made it possible to subdivide
cutaneous sensations into two main groups, protopathic and epicritic,
This classification is based upon a study of the order of return of the
cutaneous sensations during regeneration after division of nerves.
Head observed the recovery of sensation in patients in whom nerves
had been accidentally divided, and also had one of his own cutaneous
nerves divided in order to study the matter subjectively. He found
that after section of a cutaneous nerve all cutaneous sensations were
lost, but that there remained the sensations of deep pressure and of
pain due to deep pressure, which depend on stimulation of. the afferent
fibres in the muscles. During regeneration of the divided nerve, the
first sensations to return are those of pain, of heat for temperatures
above 38° C., and of cold for temperatures below 24° C. These are the
protopathic sensations, and they return from seven to twenty-six weeks
after the division of the nerve. Tactile localisation and discrimination,
the accurate localisation of pain in the skin, and the sense of heat and
. cold between 37° C. and 25° C., are recovered in from one to two years
after the nerve section, and constitute the epicritic sensations.
Protopathic sensations are ‘“ ‘affective ” in character, the sensa-
tion being intense, prolonged, and usually disagreeable; they are
obviously associated with the defensive) mechanism of the body.
The seat of the protopathic sensations is believed to be located in the
thalamus. LEpicritic sensation, on the other hand, is on a higher
- plane, and is essential for the development of manual dexterity. It
may be looked upon as the latest and highest evolution of the tactile
sensory mechanism, and the seat of sensation is in the cortex of the
cerebral hemispheres.
SECTION II.
THE SENSES OF TASTE AND SMELL.
The sense of taste is localised in the mucous membrane of the
mouth and fauces, especially in that covering the tongue, and the end-
organs concerned are widely distributed in the mucous membrane.
The principal distribution of the nerves of taste, however, is in connec-
tion with the vallate papillae which are found at the base of the tongue,
Structures called taste bulbs lie in the stratified squamous epithelium
I12 ESSENTIALS OF PHYSIOLOGY.
on both sides of the vallum which surrounds each papilla. A taste
bulb is an oval body bounded externally by spindle-shaped cells, and
containing in its interior other cells which are fusiform; each of the
latter terminates at its peripheral extremity by a hair-like process, which
projects into the vallum through an aperture called the gustatory
pore. The terminations of the fibres of the nerves of taste penetrate
the other pole of the taste bulb, and end by arborisation round the
fusiform cells, which are the end-organs for the sense of taste. Serous
glands lie in the connective tissue subjacent to the vallate papillae, and
their ducts open into the lower part of each vallum. . 7
The substances which act as stimuli for the sense of taste must
be in solution. Ordinary foodstuffs possess both taste and flavour,
the appreciation of the latter depending on the sense of smell, so that,
if the nose be firmly held so as to prevent air currents reaching the
olfactory membrane while food is in the mouth, flavours are not appre-
_ ciated. There are only four true taste sensations—sweet, bitter, salt,
and sour. Any other sensation excited in the mouth, for example,
astringency; is due to stimulation of the nerves of common sensibility.
As in the case of the skin, so in the case of taste, different nerves
are concerned in the different sensations. This is shown (1) by the fact
that some areas of the tongue are more sensitive than others to the
different sensations, and (2) by the effect of drugs. Thus (1) the tip—
of the tongue is most sensitive to substances giving rise to the sensation
of sweetness, the back to those whith "aodub' a? Bitter Sein the
sides and upper surface to appreciation of Sourness, while a salt taste
may be excited-over the surface generally. There are, moreover,
substances which give different taste sensations according to the part
of the tongue on which they are placed. Parabrom-benzoic sulphinid,
for example, excites a sweet sensation if placed on the tip of the tongue,
but only a bitter sensation if placed on the posterior part. Further,
the individual papille have been tested, with the result that some are
found to be more sensitive to substances which give rise to a sweet
- gensation, others to those which excite the other taste qualities, while
most are sensitive to more than one quality. (2) Further evidence for
the existence of specific nerves for the various taste sensations is derived
from the effect of drugs. Cocaine applied to the papille has no effect
on the production of salt sensations, but it abolishes the other three
qualities in a definite order, bitter being the first and sour the last to
disappear. Gymnenic acid, from the leaves of Gymmena sylvestre, on
the other hand, abolishes only the production of sweet and bitter
sensations, the former going first.
The nerves of taste are the chorda tympani to the anterior two-
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THE ORGANS OF SENSE. “113
thirds and the glossopharyngeal to the posterior third of the tongue.
The taste fibres have their central termination in each case in the
column of grey matter which forms the sensory nucleus of the fifth
nerve, the nervus intermedius and the glossopharyngeal nerve.
The Sense of Smell.—The end-organs for the sense of smell are
limited in their distribution to the upper part of the nasal cavities.
The membrane lining this region is yellow in colour, and consists of a
charaeteristic epithelium lying on a connective-tissue layer. The
epithelial layer is formed of a superficial layer of columnar, supporting
cells and several layers of nerve cells; the latter are elongated in shape,
and each possesses a nucleus. The prolongation of the cell peripheral
to the nucleus lies between the columnar cells, and terminates at the
surface in six to eight hair-like processes ; the central prolongation is
- continued as a non-medullated nerve fibre to the olfactory lobe, where
it arborises in the manner already described (p. 92). The connective-
tissue layer contains small alveolar glands, known as Bowman’s glands,
the secretion of which moistens the surface of the membrane.
The adequate stimulus for the sense of smell must bein the gaseous
- form, or in the condition of excessively minute particles. The gases or
particles are conveyed to the lower or respiratory portion of the nasal
cavities by the air currents due to the respiratory movements. From
the respiratory passage the gases reach the olfactory region by diffusion.
No satisfactory classification of odours has been arrived at, and it is
usual to describe them as pleasant or unpleasant. Other sensations
excited in the nasal mucous membrane, such as the irritation produced
by ammonia, are due to stimulation of the endings of the fifth nerve.
The chief characteristics of the sense of smell are (1) its extreme
delicacy, and (2) the ease with which it may be fatigued. (1) The
delicacy of the sense of smell is indicated by the dilution of a substance
which can still be perceived. Musk can be detected in a dilution in
air of one in eight millions; mercaptan in a dilution of one in twenty-
five billions. (2) The fatiguability of the olfactory sense is shown by
the insensibility of a person sitting in a closed room to the fact that
the air has become vitiated, and also by absence of sensation from a
particular perfume after it has been inhaled for a short time.
Certain odours are antagonistic, that is, one will neutralise the
effect of another. Thus the odour of iodoform is annulled by mixing
it with balsam of Peru, and carbolic acid neutralises to some extent
the odour of putrefaction. |
The estimation of the threshold stimulus for the sense of smell is
made by means of the olfactometer of Zwaardemaker. This instrument
consists of a porous cylinder which is impregnated with the odorous
8
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e
Il4 ESSENTIALS OF PHYSIOLOGY.
substance. A tube is inserted into the cylinder for varying distances,
so that a greater or less part of the cylinder is exposed to the air which
passes through the tube. The end of the tube outside the cylinder is
placed in a nostril, and the smallest amount of exposed cylinder surface
which will give a sensation indicates the threshold stimulus for the
substance tested.
The olfactory sense is developed to a varying extent in different
animals, Generally speaking, it is present to a greater degree in many
of the lower animals than in man; in the dog, for example, it is
highly developed.
SECTION III.
THE SENSE OF SIGHT.
The end-organs of the sense of sight are situated in the eyeball,
which is protected from injury by its situation in the orbital cavity and
also by the eyelids. The surface of the eyeball is kept moist by the
tears, which are secreted by the lachrymal gland. Loss of moisture
occurs through evaporation from the surface of the eyeball, and super-
fluous tears are drained away through the puncta lachrymalia at the
inner end of the eyelids into the lachrymal sac, and thence by the nasal
duct into the nasal cavity. The lachrymal secretion is slightly alkaline
and contains sodium chloride.
THE EYEBALL.
The eyeball consists of three coats surrounding the transparent
-media which constitute the dioptric apparatus. It is covered in front
by the conjunctiva, a connective-tissue membrane with a superficial
layer of stratified squamous epithelium. The conjunctiva is reflected
over the posterior surfaces of the eyelids, and is represented on the
cornea only by the layer of stratified epithelium. The outer coat of
the eyeball consists of the sclera and cornea. The sclera forms five- —
sixths of the coat, and is protective in function. It is opaque, and is
made up of dense fibrous tissue with some elastic fibres and flattened
cells, some of which are pigmented. The cornea forms one-sixth of the
outer coat, and has a somewhat greater convexity than the sclera. It
is transparent, and is made up of parallel lamelle of white fibrous
tissue with spaces between, in which lie flattened branched cells, the
corneal corpuscles. It is covered in front by stratified squamous
epithelium, which rests on a homogeneous-looking membrane composed
of closely woven fibrils, and known as the anterior elastic lamina. The
posterior surface of the cornea is covered by a single layer of flattened
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THE ORGANS OF SENSE. 115
cells, resting on an elastic, homogeneous membrane, called the posterior
elastic lamina. In the inner part of the sclera, close to its junction
with the cornea, is a vein, the canal of Schlemm (sinws venosus scleree),
which encircles this part of the eyeball. Behind the sinus is a pro-
jecting ridge of the sclera, the scleral spur, which forms the point of
origin of the radial fibres of the ciliary muscle. At the margin of the |
cornea the posterior lamina breaks up into trabeculee, some of which
are attached to the anterior surface of the scleral spur, while the others
form the ligamentum pectinatum tridis, which is continuous with the
substance of the iris. The angle between cornea and iris is known as
the filtration angle, because here the aqueous humor drains between
the fibres of the ligamentum pectinatum iridis into the canal of
Schlemm. .
The middle coat consists of the choroid, ciliary processes, and iris,
together forming the vascular tunic of the eyeball. The choroid is
separated from the sclera by a lymph space, which, however, is traversed
by strands of non-vascular fibrous tissue, constituting the lamina
suprachorotdea, and forming the outer layer of the choroid itself.
Internal to the lamina suprachoroidea is a layer containing the larger
blood-vessels, and internal to that is another layer, the chorio-capillaris,
which contains a network of capillaries; the capillary layer is bounded
internally by a structureless membrane, the /amina basalis. Scattered
throughout the choroidal tissue are numerous pigment cells.
‘The choroid lines the sclera to within a short distance of the sclero-
corneal junction, and is continued forward from that point as the czlzary
processes, about seventy in number, which from behind appear as a circle
of radially arranged vascular projections. - The posterior surface of the
ciliary processes is covered by a double layer of cubical pigment cells,
forming the forward prolongation of the retina, and called the pars
ciliaris retine. The substance of the ciliary processes consists of
connective tissue with pigment cells and blood-vessels, together with
the ciliary muscle. The latter is composed of smooth muscle fibres, and
consists of a radial and a circular portion. The radial fibres arise from
the scleral spur and pass backwards to be inserted into the ciliary
processes and choroid; the circular fibres form a bundle which lies
internally to the radial portion.
The zrvs is continued forward from the ciliary processes and is incom-
plete in front, leaving a circular aperture, the pupil. The iris is
composed of connective tissue, with a variable number of pigment cells
and numerous blood-vessels. In dark eyes the pigment cells are
numerous ; in blue eyes they are fewer in number. The fibres of the
ligamentum pectinatum iridis connect the iris with the posterior lamina
116 ESSENTIALS OF PHYSIOLOGY.
of the cornea. The anterior surface of the iris is covered by a single
layer of flattened cells, continuous over the ligamentum pectinatum
with that of the posterior surface of the cornea. The posterior surface
is covered by a double layer of cubical pigment cells continuous with
the pars ciliaris retin. A ring of circularly arranged smooth muscle
fibres lies close to the margin of the pupil, forming the sphincter
pupillz, and radially arranged smooth muscle fibres, the dilator pupille,
lie close to the posterior surface of the iris.
The internal coat of the eye consists of the retina, a delicate, semi-
transparent membrane lining the posterior three-fourths of the eyeball,
and ending abruptly just behind the ciliary processes in a jagged
margin, the ora serrata. The retina consists from without inwards of
the following layers (fig. 33) :—
. The layer of pigment cells.
. The layer of rods and cones.
The outer nuclear layer,
The outer molecular layer (outer plexiform layer). ©
. The inner nuclear layer.
. The inner molecular layer (inner plexiform layer).
. The layer of ganglionic nerve cells.
. The layer of nerve fibres (stratum opticum).
The structures forming these layers are supported by the fibres of
Miller, which extend from the level of the bases of the rods and cones
to the inner surface of the retina. The ends of Miiller’s fibres are
expanded and fused together to form the outer and inner limiting
membranes, the former lying between the layer of rods and cones and
the outer nuclear layer, and the latter bounding the retina internally.
Each fibre has a nucleus at the level of the inner nuclear layer.
(1) The cells of the pigment layer are hexagonal on surface view,
and when seen from the side they exhibit an outer non-pigmented
portion containing a nucleus, and an inner pigmented part from which
delicate processes run between the rods and cones.
(2) and (3) The layer of rods and cones and the outer nuclear layer
together form one layer of neurons, Each rod consists of (@) an outer
cylindrical segment which is transversely striated, and which, in a dark-
adapted retina, contains visual purple or rhodopsin, and (4) an inner
fusiform segment, vertically striated in its outer fourth and granular
in the remaining three-fourths. Each rod is prolonged into the outer
nuclear layer as a varicose fibril, in the course of which is a nucleus.
The nucleus in some animals is transversely striated. ach fibril ends
in a knob in the outer molecular layer, the knobs of several adjacent
rod neurons coming into relationship with the dendritic arborisation of
THE ORGANS OF SENSE. 117
a single rod bipolar of the inner nuclear layer. The cones are shorter
and thicker than the rods, and each consists of an outer and an inner
segment. The outer segment is conical and is transversely striated.
The inner segment is vertically striated in its outer two-thirds and
granular in its inner third. The continuation of the cone in the outer
nuclear layer is a comparatively thick fibre which contains a nucleus
y immediately under the outer limiting membrane. The cone fibre
Membrana
limitans interna. ---
' Stratum opticum."*~ Se
—_s>
|
Ganglionic layer.*--
Inner molecular.
layer.
Centrifugal fibre. -”
Boia: Diffuse amacrine
cell.
<q
Inner nuclear...
layer. "*Amacrine cells.
Fibre of Miiller.
--« Horizontal cell.
Oute r molecular- |
layer.
~d--<-<
Outer nuclear---
layer. : 5
Membrana rites Cone granules.
limitans externa.
Nd |
Layer of rods..-
and cones.
lolololololofeyaye
Fic. 33.—Plan of retinal neurons. (After Cajal.) From Gray’s Anatomy.
"t 5 {> Pigmented layer.
terminates in the outer molecular layer by a branched extremity, which
is in relationship with the dendritic arborisation of a single cone bipolar
of the inner nuclear layer.
(4) and (5) Most of the nuclei of the inner nuclear layer belong to
bipolar nerve cells, one fine process from the nucleus passing into the
outer molecular layer, the other towards the inner surface of the retina.
The outer process ends in a dendritic arborisation in relation. either
with the knobs of several rod neurons (rod bipolars), or with the
terminal branching of a cone neuron (cone bipolars). The inner process
of a rod bipolar arborises about the cyton of a cell in the ganglionic
118 ESSENTIALS OF PHYSIOLOGY.
layer ; that of a cone bipolar arborises in the inner molecular layer in
relation with the dendrons of a ganglionic cell. In the outer part of
the inner nuclear layer are horizontal cells, the dendrons and axons of
_ which arborise in the outer molecular layer, so that these cells have an
associational function. In the inner part of the layer are cells which
have no axons and are therefore called amacrine cells. The dendrons
of these cells arborise in the inner molecular layer in relation with the
dendrons of the ganglionic cells.
(6), (7), and (8) The cells of the ganglionic layer are arranged in a
single row in most parts of the retina, Each has branched dendrons
which extend into the inner molecular layer, where they terminate at
different levels in relation with the terminal arborisations of the axons
of the cone bipolars. The axons of the ganglionic cells are continued
as non-medullated nerve fibres in the stratum opticum. Some of the
fibres in the stratum opticum have a centrifugal course and run into
the retina to terminate by arborisation in the inner nuclear layer.
Three parts of the retina require a special description—the macula
lutea, the place of exit of the optic nerve, and the ora serrata.
The macula lutea is the part of the retina concerned with distinct
vision. When an object is “looked at” its image is formed on the
macula, or more particularly on the fovea centralis, a small depression
in the centre of the macula. At the fovea there are no rods, and the
cones are longer than in the remainder of the retina. The cones and
their nuclei are the only retinal structures present in the fovea, and the
cone fibres are inclined away from the fovea towards the inner nuclear
layer. In the peripheral region of the macula the ganglionic layer is
several cells deep, and cones are more numerous than rods. The
proportion of cones, diminishes from the macula to the periphery of the
retina, and near the ora serrata very few cones are present. The
macula is situated slightly to the lateral side of the posterior pole of the
eyeball.
The fibres of the stratum opticum converge to a point about
3 mm. to the nasal side of the macula lutea to form the optic nerve,
which passes back through a gap in the choroid coat and a perforated
part of the sclera known as the lamina cribrosa, The place of exit of
the nerve as seen from the interior of the eyeball is a sharply defined
pale area, nearly circular in outline, and is called the optee disc. At
the disc all the retinal layers are absent except the stratum opticum,
the fibres of which acquire a myelin sheath as they emerge from the
eyeball.
The layers of the retina cease abruptly at the ora serrata, and are
represented in the ciliary region by two layers of cells, the deeper,
et ee eet 4
a el
THE ORGANS OF SENSE. 119
pigmented layer being a continuation forwards of the pigment layer of
the retina, and the superficial layer consisting of columnar cells.
THE CONTENTS OF THE EYEBALL.
The cavity of the eyeball.is divided into two unequal portions by the
crystalline lens and its suspensory ligament. The larger, posterior
space is occupied by the vitreous humor (body); the smaller, anterior
space by the aqueous humor.
The crystalline lens is a biconvex, transparent structure, and lies
immediately behind the pupil. It is composed of concentrically
arranged fibrous lamine, made up of prismatic fibres with serrated
edges. The central portion of the lens is firmer and denser than the
peripheral portion, which is more jelly-like. The posterior surface has
a higher degree of convexity than: the anterior surface, and rests in the
hyaloid fossa of the vitreous body. The lens is enclosed in a structure-
_ less capsule, which, towards the equator, is continuous on the anterior
surface with the suspensory ligament of the lens. The latter is
prolonged backwards as the zonula ciliaris, to be attached to the ciliary
processes.
The vitreous humor is transparent and resembles a thin jelly, contain-
ing a few scattered delicate fibres. It is enclosed in a delicate capsule,
the hyaloid membrane, which in the neighbourhood of the ciliary
processes is thickened to form the zonula ciliaris. The latter splits in
front into two layers, the anterior being the suspensory ligament of the
lens, and the posterior lining the concavity in the vitreous body, the
hyaloid fossa, in which the lens rests. A canal runs through the
vitreous humor from the optic disc to the posterior pole of the lens, and
is lined by a continuation of the hyaloid membrane. This canal
contains the hyaloid artery in the embryo.
The space in front of the lens is divided into an anterior and a
posterior chamber by the iris, and contains the aqueous humor, which
is a watery fluid, alkaline in reaction, and containing salts, chiefly
sodium chloride, with a trace of protein.
THE NUTRITION OF THE EYEBALL.
The vascular tunic of the eyeball receives the long and short
posterior ciliary arteries and the anterior ciliary arteries. The outer
layers of the retina receive nourishment by means of lymph derived
from the blood-supply to the choroid, The inner layers of the retina
have a direct blood-supply through the distribution of the central artery
of the retina, which enters the eyeball with the optic nerve. The
L
120 ESSENTIALS OF PHYSIOLOGY. |
cornea is supplied with lymph from the blood-vessels which surround
its margin. The veins of the choroid converge to form four or five
main trunks, the vene vorticose. The blood from the retinal artery is
returned by the corresponding retinal vein.
INTRA-OCULAR TENSION.
By inserting a cannula connected with a manometer into the anterior
chamber of the eyeball, it can be shown that the contents of the globe
exert a pressure on the walls equal to about 25 mm. of mercury. If the
intra-ocular pressure be recorded on a revolving drum simultaneously
with that of the carotid artery, it will be seen that the two tracings
run a parallel course. Obstruction of the descending aorta, for example,
causes an immediate rise in both curves, and these remain at the new
level till the obstruction is removed, when both fall simultaneously.
The chief source of fluid in the eyeball is the vessels of the ciliary
processes, and as the pressure of the intra-ocular fluid varies with the
blood-pressure, it may be assumed that it is derived from the blood-
vessels by a process of filtration. Normally, the addition of new fluid
is balanced by the draining away of an equal amount, mainly through
the filtration angle and the canal of Schlemm, but also. to a very small
extent by the posterior lymphatics of the eyeball. In certain diseased
conditions this drainage is interfered with, and fluid accumulates in the
eyeball, causing a rise of pressure. This condition is known as glaucoma,
and if it is not promptly relieved, it results in atrophy of the retina from
pressure, and therefore leads to blindness.
The mechanism of transudation of fluid from the ciliary processes
and escape of an equal quantity by the canal of Schlemm and posterior
lymphatics of the eyeball is not only of importance for the nutrition
of the non-vascular contents of the eyeball, but the state of tension
which is thereby maintained gives the eyeball the degree of rigidity
which is necessary if it is to serve any useful purpose as an optical
instrument.
THE FUNCTION OF SIGHT.
The function of sight, in the commonly accepted sense of the word,
involves (1) the formation of a real image of external objects in the
retina, (2) changes in the retinal end-organs, (3) the transmission of
the stimulus due to the retinal changes to the cortex of the occipital
lobe, (4) the changes in the cortex, visuo-sensory and visuo-psychic
areas, which result in a visual sensation, and (5) the associational pro-
cesses of comparison of the sensation with previous sensations by which
visual judgments are formed.
ye ee eee
THE ORGANS OF SENSE. 121
THE FORMATION OF AN IMAGE IN THE RETINA.
The eyeball may be compared with a photographic camera, the
cornea, aqueous humor, lens, and vitreous body forming a system of
lenses, the choroid coat being comparable with the dark lining of the -
camera, and the retina acting as the sensitive plate. If a segment of
the sclera with the choroid be excised from the back of the eye of a
recently killed ox, and the eye be held in front of an electric lamp;
an inverted image of the lamp will be seen upon the retina, similar
to the image which may be observed on the ground-glass screen of
a camera.
In the camera the image is usually. formed by means of a single
biconvex glass lens. The optical axis of such a lens is a line drawn
through its optical centre, and entering and leaving it in a direction
perpendicular to the plane of the lens. A ray of light passing along
Fie. 34.—Diagram of the course of parallel rays through a bicon-—
vex lens (L), by which they are converged to the principal
focus (F). A, axial ray. (Starling’s Principles of Physiology.)
this axis enters and leaves the lens with its direction unchanged, but
rays falling upon the lens outside the axial ray and parallel with it are
refracted, both on entering and leaving the lens, towards the axial ray,
so that they meet it or come to a focus at a point which is known as
the principal focus of the lens (fig. 34). A pencil of divergent rays
falling upon such a lens will, if the lens have a sufficient degree of
convexity, be brought to a focus at a point behind the principal focus.
If an object, such as an arrow, be placed at some distance from a
biconvex lens, the pencil of rays from the tip of the arrow will be
brought to a focus behind the lens, and the same will hold for rays
from the butt and every other point on the arrow. The axial ray of
each pencil will pass through the optical centre (nodal point) of the
lens with its direction unchanged. The result will be the formation of
a real, inverted image of the arrow in the plane in which the rays are
focussed.
The formation of an image in the eye is more complicated in that
the cornea, aqueous humor, lens, and vitreous body form, not a single
lens, but a system of lenses differing in refractive index, so that altera-
122 ESSENTIALS OF PHYSIOLOGY.
tion in the direction of rays of light takes place at the anterior surface
of the cornea, anterior surface of the lens, and posterior surface of the
lens. It has been calculated, however, that the net result of the
refraction in the eye is the same as that which would occur ina uniform °
medium the anterior surface of which is 2°3 mm. behind the anterior
surface of the cornea, and the nodal point 0°47 mm. in front of the
posterior surface of the lens. Such a theoretical arrangement is known
as the reduced or schematic eye (fig. 35).
By means of the schematic eye the size of an image on the retina
can be ascertained. In the normal or emmetropic eye at rest parallel
rays are brought to a focus on the retina, and for practical purposes
rays proceeding from a point 6 metres or more distant from the eye
may be regarded as parallel. If, therefore, a diagram of the reduced
eye and of an object at 6 metres distance be drawn to scale, and if
lines be drawn from the
‘periphery of the object
through the nodal point
to the retina, the size
of the retinal image can
be measured. The size
of the image of an
object nearer than 6
metres can also be cal-
culated, it being as-
sumed that the eye is
accommodated for the object. The angle subtended by the object, and
therefore by its image in the retina, is spoken of as the visual angle.
As the distance of.the nodal point from the retina in an emmetropic
. Fic. 35.—Diagram showing formation of an image
in reduced eye.
eye is known (15:5 mm.), the size of the retinal image of an object
is easily ascertained if the visual angle is measured.
The limit of retinal discrimination corresponds with a visual angle
of sixty seconds, that is, in order that two points of light may be
distinguished as separate points they must subtend an angle at the
nodal point of not less than sixty seconds. A visual angle of this size
corresponds with the diameter of a single cone in the fovea centralis ;
it subtends a base of 4:38 on the retina, and the cones in the fovea
vary in diameter between 2 and 5p.
SIGHT-TESTING.
The acuteness of vision is tested either by means of groups of dots
of varying sizes or by means of special test types. The test card is
usually placed at a distance of 6 metres from the eye to be tested, and
SIE Coe eae! TOM Tie? ae NTT ee ET ee
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THE ORGANS OF SENSE. 123
at this range the distance between adjacent dots or the segments of the
‘ various letters, as the case may be, subtends visual angles of known ~
size. The visual acuity is expressed as a fraction, of which the
numerator is the distance in metres at which the test is made, and
the denominator is the distance at which the smallest type read
should be distinguished by an emmetropic eye, as, for example, 6/6
(normal), 6/9, 6/12, and so on.
ACCOMMODATION.
An emmetropic eye at rest is in focus for parallel rays, that is, for
rays coming from a distance of 6 metres or more. The images of
objects within this distance are brought to a focus on the retina by
an effort of accommodation. The essential part of the act of accom-
modation is that the crystalline lens becomes more convex, so that
divergent rays are brought toa
focus on the retina. On looking
obliquely into an eye which is
being accommodated for a near
object, the alteration in shape of
the lens can be observed. The
iris can be seen to move forward
because of the increasing con-
a b ¢ a b is)
1 2
Fic, 36.—Sanson’s images, (1) with eye
vexity of the lens. The altera-
tion in shape is limited to the
anterior surface, and this can
at rest, (2) during accommodation,
(Starling’s Principles of Physiology.)
a, images from anterior surface of cornea: 3},
from anterior surface of lens; c, from
osterior surface of lens.
be demonstrated by means of j
Sanson’s images, which are most conveniently observed with the help of
an instrument known as the phakoscope. This consists of a triangular
box with truncated angles. At one angle is an aperture for the eye
of the observer, at another an opening for the observed eye, at the
third two triangular openings for the admission of light. Opposite
the observed eye is another opening in which a wire is placed. The
person who is being observed first relaxes his accommodation by looking
through the aperture opposite to him at an imaginary distant object.
The source of light is seen to be reflected from his eye at three positions,
namely, the anterior surface of the cornea, and the anterior and posterior
surfaces of the lens (Sanson’s images). The images reflected from the
anterior surfaces of the cornea and lens are erect; that from the
posterior surface of the lens is small and inverted. If the observed eye
be accommodated for the wire opposite it, the middle image only is
altered, coming nearer the anterior image and becoming smaller in size
(fig. 36). This shows that the anterior surface of the lens moves forward
124 ESSENTIALS OF PHYSIOLOGY.
in accommodation, and also that it becomes more convex. The absence
of movement of the other images indicates that the cornea and posterior
surface of the lens remain stationary during accommodation.
The Mechanism of Accommodation.—The lens is an elastic structure,
and, as has already been stated, it is enclosed in a capsule which is
connected with the ciliary processes by the suspensory ligament. The
contents of the eyeball exert a pressure or tension on the coats of the
eye, amounting normally to the equivalent of 25 mm. of mercury.
In consequence of this tension the suspensory ligament exerts a pull
on the lens capsule, and the convexity of the anterior surface of the
lens is in this way diminished. When the lens is removed from the
eye it assumes a more convex shape in virtue of its elasticity. The
same change of shape takes place with the lens in position, when the
FAR NEAR
Fic, 37.—Diagram showing mechanism of accommodation. :
_ (After Helmholtz and Foster. )
a, suspensory ligament; }, ciliary muscle.
tension of the suspensory ligament is diminished during accommodation
by the action of the ciliary muscle. Contraction of the radial fibres
of this muscle pulls forward the posterior part of the ciliary processes
with the attached suspensory ligament, and in this way the latter is
relaxed and the lens becomes more convex anteriorly in virtue of its
elasticity (fig. 37). The circular fibres of the ciliary muscle also take
part in accommodation, approximating the ciliary process to the lens
by their contraction. The effect of contraction of the ciliary muscle
may be demonstrated by two experiments: (1) by the action of eserine,
and (2) by the movement of a needle inserted into the ciliary processes.
(1) When eserine is instilled into the human eye, it causes the ciliary
muscle and sphincter of the iris to contract, and the slackening of the
_ suspensory ligament can be shown by oscillations of the lens which
take place when the head is quickly. moved.. (2) When the point of a
needle is inserted into the ciliary processes in an animal, and the
ciliary muscle is stimulated to contract, the end of the needle outside
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THE ORGANS OF SENSE, 125
the eye moves backwards, showing that the point in the ciliary processes
has been pulled forward.
When the ciliary muscle contracts in accommodation, there occur
at the same time contraction of the pupil and convergence of the eyes.
The contraction of the pupil is effected by the sphincter pupille, and
is of service in sharpening the definition of the image formed on the
retina, just as the definition of an image in the photographic camera
is improved by the use of a small diaphragm. The convergence of
the eyes is effected by the contraction of the internal recti, and results
in the image of the object looked at being formed on the fovea of
each eye.
In the emmetropic eye the far point (punctum remotum) of distinct
vision is at infinite distance, while the near point (punctwm proximum)
varies with age. The elasticity of the lens, and consequently the range
of accommodation, diminish steadily as age advances, and the near point
therefore gradually recedes. This is shown in the following table :—
Range of
Age. Accommodation | Near Point.
in Dioptres.
10 14 7 cm,
20 10 10- >,
30. 7 if een
40 4°D ee ae
50 2°5 40 ,,
60 1 100 ,,
70 0°25 400 ,,
In civilised life the power of accommodation is called into play more
for reading than for any other purpose, and it will be seen from the
table that between the ages of forty and fifty the near point recedes to
a greater distance than it is convenient to hold a book. Moreover, it ‘is
found that the prolonged effort of accommodation required for reading
cannot be kept up if more than three-fourths of the total power of
accommodation is being utilised. It is therefore necessary, usually about
“the age of forty-five, to supplement the mechanism of accommodation for
reading or other near work by the use of convex lenses of such strength
as to bring the near point to a range of about 25 cm. or ten inches.
The term presbyopia is used to indicate the failure of accommodation
which occurs about the age of forty-five.
Accommodation is a voluntary act, and is peculiar in that respect
in that the ciliary muscle and the sphincter pupille are composed of
smooth muscle. Both these muscles, as well as the internal recti, are
126 ESSENTIALS OF PHYSIOLOGY.
supplied by the third cerebral nerve. Definite groups of nerve cells
can be localised in the nucleus. of the third nerve for each muscle
supplied by it, stimulation of particular areas in the nucleus being
followed by contraction of particular muscles. The groups of cells for
the ciliary muscle, sphincter pupille, and internal recti lie close
together in the anterior part of the nucleus; and the centre for the
internal rectus on each side is connected with that of the sixth nerve
of the same side, so that when the internal rectus contracts there is
reciprocal relaxation of the external rectus of the same eye.
AMETROPIA.
The condition of the normal or standard eye is called emmetropia,
and any departure from the standard is known as ametropia. When
the antero-posterior diameter of the
eyeball is too short, so that parallel
rays are focussed behind the retina,
the condition is called hypermetro-
pia ; when the antero-posterior dia-
meter is unduly long, so that parallel
rays are focussed in front of the
retina, the term myopia is used to
indicate the defect (fig. 38); when -
the rays of light entering the eye in
one meridian are refracted to a
greater or less degree than those
which enter in another meridian, the
condition is spoken of as astigmatism
(fig. 39).
A moderate degree of hyper-
metropia does not necessarily involve
Fic. 38.—-Diagrams of course taken by
parallel rays on entering the eye.
(Starling’s Principles of Physiology.) is usually compensated by increased
A, an emmetropic; B, a hypermetropic; and. power of the ciliary muscle, so that
C, a myopic eye.
defective vision, because the defect.
rays of light are focussed on the
retina by an increased use of the accommodative mechanism already
described. The extra strain involved in this compensatory effort,
however, often leads to unpleasant symptoms, and it is advisable
in many cases of hypermetropia to correct the error by the use of
convex spectacles.
Myopia cannot be overcome by any accommodative act, and
distinct vision of distant objects can only be obtained in a myopic eye
by the use of concave spectacles, which cause the rays of light entering
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THE ORGANS OF SENSE. 127
the eye to diverge in such a way that they are focussed on the
retina.
Astigmatism is usually due eb unequal Saenbais of the cornea, the
commonest form showing a greater convexity in the vertical than in
the horizontal meridian. In other words, the cornea is not spherical,
but resembles the back of a spoon. Consequently the rays entering the
eye in the meridian of greater curvature are brought to a focus in front
of those which enter in the meridian of lesser curvature (fig. 39), and
as a result the eye cannot focus both bars of a cross simultaneously.
The defect is counteracted by the wearing of cylindrical lenses, that
is, lenses which resemble a vertical section of the superficial part of a
cylindrical glass rod.
Fic. 39.—Diagram to show the course of rays in an astigmatic eye. (Waller.)
From Starling’s Principles of Fhysiology. :
The rays (from P) in the vertical meridian, vv, come to a focus sooner than those in the
orizontal meridian, hh.
The unit of measurement for degrees of refractive error and. for the
strength of compensating lenses is the dioptre. This term indicates a
lens of such a strength that by it parallel rays are brought to a focus
at one metre distance. A 2 dioptre (2D) lens has a focal distance of
half a metre, ‘a 3D lens of a third of a metre, a $D lens of two. metres,
and so on.
SPHERICAL AND CHROMATIC ABERRATION.
The crystalline lens, like other lenses, has greater refractive power
at its periphery than towards its centre, and consequently rays which
pass through it near its margin are brought to a focus sooner than
those which pass through its centre, and give rise to a certain degree of
blurring of the image on the retina. The unequal refraction which
produces this result is known as spherical aberration, and it is corrected
in the eye (1) by the centre of the lens being denser and more highly
refractive than the peripheral portion, and (2) by the iris acting as a
diaphragm and cutting off the peripheral rays.
By chromatic aberration is meant a fault common to all lenses and
shared by the crystalline lens, whereby each sector of the lens acts as a
-
128 ESSENTIALS OF PHYSIOLOGY.
prism, dispersing the coloured rays which are combined in white light.
The rays at the violet end of the spectrum are of short wave-length
and are more refrangible, and therefore come to a focus sooner than
the rays of greater wave-length towards the red end of the spectrum.
Consequently the images formed on the retina are surrounded by violet
and red halos ; but these do not arouse any sensation, for two reasons :
(1) because the rays of medium refrangibility, which are brought to a
focus on the retina, are the most luminous, and the effect of the
stimulation excited by these is to depress the sensitivity of the
adjacent parts of the retina by contrast, and (2) because the visual
apparatus is relatively insensitive to the rays at the extreme ends of
the spectrum. |
THE FUNCTIONS OF THE IRIS.
The pupil varies in size with the degree of light entering the eye
and with other conditions, becoming smaller when the sphincter pupille
contracts and wider on contraction of the dilator pupille: We have
seen that the pupil contracts with accommodation, and that the result
is improved definition of the image on the retina. The improvement
is due to the cutting off of the peripheral rays, with the consequent
correction of spherical aberration. ‘Secondly, the iris regulates the
amount of light entering the eye, and so protects the retina from over-
stimulation. If the intensity of the light is gradually increased, the
pupil does not contract, but if the increase is sudden, the pupil becomes
smaller, and afterwards slowly returns to its former size as the retina
becomes adapted to the increased stimulus. On the other hand, in a
person in a dark room the pupils are widely dilated and remain in this
condition until the eyes are once more exposed to light. The alteration
of the pupil with varying degrees of light is due in mammals to a
reflex nervous mechanism, the optic nerve conveying the afferent
impulses, and the third nerve and sympathetic fibres the efferent
impulses, to the sphincter and dilator of the pupil respectively. The
fibres to the sphincter travel by the ciliary ganglion and short ciliary
nerves, and stimulation of these in any part of their course is followed
by constriction of the pupil, while section of the third nerve is followed
by dilatation. The dilator fibres emerge from the spinal cord by the
first two thoracic anterior roots, and run up in the cervical sympathetic
to the superior cervical ganglion, from which post-ganglionic fibres run
along the internal carotid artery to the Gasserian ganglion, where they
join the ophthalmic division of the fifth nerve and travel to the dilator
fibres of the iris by the nasal branch and the long ciliary nerves
(fig. 40). Section of the cervical sympathetic nerve is followed by con-
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THE ORGANS OF SENSE. 129
traction of the pupil, and stimulation of the distal end leads to dilata-
tion. As section of the third nerve is followed by dilatation and section
of the sympathetic by contraction of the pupil, it is obvious that tonic
impulses are constantly passing along both nerves. The fact that
dilatation of the pupil is due to active contraction of the radial muscle
fibres of the iris, and not merely to relaxation of the sphincter, is proved
by two experiments. (1) Localised stimulation of the periphery of the
iris leads to contraction of that part :
of the iris only, and (2) if a sector
of the iris be separated by two radial
cuts it will contract, either on local
stimulation or on stimulation of the
sympathetic nerve in the neck, The
sympathetic root of the ciliary gang-
lion contains vaso-constrictor fibres
for the vessels of the eye.
In man, and in other animals in
which there is a partial decussation of
the optic nerves, the reflex contraction
of the pupil to light is bilateral, that
is, light falling on one eye leads to
contraction of both pupils. This is
due to the fact that by means of the
decussation each optic nerve forms
connections with both superior corpora
quadrigemina, and thus with the
nuclei of both third nerves. If one
optic nerve is atrophied, the pupil
of that eye will contract when light
falls on the other eye, but not when
Fie. 40.—Scheme of the nerves
the eye (after Foster).
light falls on the affected eye. giles ayant rig aoe a, third nerve;
5 5 . , Sympathetic root of ciliary ganglion ;
Contraction of the pupil occurs c, nasal branch of fifth nerve: d, ciliary
not only as a result of the light Shoe clliare fs S00 PEATE: RAFTER dh
reflex, but also from other causes.
The various factors which bring about diminution in the size of the
pupil are : |
1. Light falling on the retina, giving the reflex effect already described.
2. (a) Accommodation, (6) Sleep. In both of these conditions the
contraction of the pupil is an associated condition. In accommodation,
it occurs simultaneously with the contraction of other muscles supplied
by the same nerve; in sleep the eyes are rotated upwards and inwards,
and the pupils contract in association with the convergence.
ca
130 ESSENTIALS OF PHYSIOLOGY.
3. Drugs. Morphia taken internally, and eserine or pilocarpine,
either taken internally or applied directly to the eye, cause the pupil to
contract. Contraction also takes place in the third stage of chloroform
or ether anzesthesia.
Dilatation of the pupil occurs :
(1) On the removal of the light stimulus from the eye.
(2) As a result of stimulation of sensory nerves.
(3) In emotional conditions, such as fear.
(4) From the action of drugs, for example, from the internal admini-
stration or local application of atropine, or from the local application
or intravenous injection of adrenalin. The pupil is also dilated in the
early stages of chloroform or ether anesthesia.
THE RETINAL IMAGE.
When light falls upon the eye it excites chemical, histological, and
electrical changes in the retina.
The Chemical Changes.—We have seen that the outer segments of
the rods contain a substance called rhodopsin or visual purple.
Rhodopsin can be dissolved out of the retina by a solution of bile salts,
and it is rapidly decolorised on exposure to light. If an animal be
kept in the dark for some time and then killed and its retina examined,
the latter will be of a deep red colour, which soon fades on exposure to
light. If, on the other hand, the eyes have been exposed to bright light
the retina is pale. The effect of light in bleaching rhodopsin is in pro-
portion to its intensity, so that if a rabbit is kept in the dark for a
time and then its eye is exposed opposite a window, a picture of the
window, called an optogram, is formed on the retina; in the optogram
the window pane areas are bleached, while the rhodopsin is not
decolorised in the shaded parts corresponding with the bars. As
visual purple is not present in the cones, the fovea does not show this
chemical change. |
The Histological Changes.—In an eye which has been exposed to light
fine processes of the pigment cells are found to extend between the
rods and cones, the processes themselves being laden with pigment
granules, whereas if the animal has been kept in darkness before the
examination of its eye, the cells of the pigment layer are flat, the pro-
cesses being retracted. Further, in some animals, for example the frog,
the cones are retracted on exposure to: light and extended when the
animal is kept in darkness (fig. 41). The cells of the pigment layer
have the power of restoring the visual purple, for when a retina, which
has been bleached by exposure to light, is laid upon the pigment layer,
rhodopsin again appears in the rods.
Vie! le aca! ge ied I ©
oe
/
a = I ae
THE ORGANS OF SENSE. 131
The Electrical Changes.—When an excised eye is placed in circuit
with a string galvanometer, it is found that a current passes through |
the eye from the posterior to the anterior pole. When light is allowed
to fall upon the retina, there is first a small negative variation of
this current, followed by a marked positive variation.
THE FUNCTION OF THE RODS AND CONES.
The layer of rods and cones is the part of the retina in which the
impulses are excited which give rise to visual sensations. This is
proved by three facts. (1) In the fovea, which is the area for most
se, seal i ib au 23
Fic, 41.—Sections of the frog’s retina. (From Starling’s
Principles of Physiology. )
A, after exposure to light. B, kept in the dark. (Engelmann.)
distinct vision, cones only are present, the other layers of the retina
being absent. (2) No sensation is excited when light falls on the
optic disc, where the rods and cones are absent and nerve fibres only
are present. The optic disc is therefore called the blind spot, and it
is to be noted that there is no sensation of darkness arising from it,
but merely the absence of any sensation at all. The existence of the
blind spot can be demonstrated in the following way. If the left eye
be élosed and the right eye gaze steadily at the cross in fig. 42, and
if the book be moved to and fro, it will be found that at a distance of
about eight inches from the eye the white circle will disappear. The
black background will appear to be continuous, showing that the gap
is: unconsciously filled up. (3) The level of the layer which is stimu-
lated by light can be determined by means of Purkinje’s images. If,
in a darkened room, a strong beam of light is focussed on. the sclera
just external to the cornea, images of the retinal vessels will be seen.
132 ESSENTIALS OF PHYSIOLOGY.
If the position of the light is altered, that of the images will also change.
The degree of displacement of the light and that of the displacement
of the images, as well as the distance of the latter from the eye,
being known, the distance between the retinal vessels and the layer
of the retina which is stimulated by the shadows can be calculated ;
and it is found in this way that the structures which are stimulated
correspond in position with the layer of rods and cones. The
appearance of Purkinje’s images is due to the fact that the light,
falling from an unusual direction, casts the shadow of the vessels on
a part of the retina unaccustomed to such a stimulus. |
Inasmuch as the rods and cones are of different structure, it is to
be expected that their functions also differ, and there is evidence that
Fie. 42,—(Starling’s Principles of Physiology.)
this is the case. The cones are most abundant in the central area of
the retina, and the rods are in greater proportion in the peripheral
part. Associated with this distribution is the fact that central vision
is more distinct in ordinary light and peripheral vision more distinct
in dim lights. Thus a star which is seen by indirect, or peripheral,
vision, may be invisible when the eye is directed towards it. This
fact suggests that the cones are adapted for vision in good light and
the rods for use in dim light, an assumption which is supported by a
comparative study of the retina in animals. In most birds, which
go to roost when twilight falls, or even when the sky is obscured by
heavy clouds, cones only are present. In owls and bats, on the other
hand, which are nocturnal in their habits, there are only rods.
The study of the field of vision for colour shows that whereas the
central area of the retina is responsive to all coloured rays, the
periphery is colour-blind. Moreover, all parts of the retina are colour-
blind in dim light.
It is justifiable, therefore, to believe that the cones are functional.
in good light, and are responsive to stimulation by white and coloured
a . . . .
rays, whereas the rods come into play in dim lights and are colour-
he,
ts
5
|
THE ORGANS OF SENSE. 133
blind. It may further be assumed that the function of rhodopsin ig,
to sensitise the sods and go mike them morecxsitablento thease’
stimuli for which they..are-.adepted.
Adaptation.—If two persons enter a room in which there is a |
moderate degree of light, one from bright daylight, the other after
being for some time in a dark room, the former will experience a
sensation of comparative darkness, whereas the latter may be dazzled.
The eyes in the one case are light-adapted, in the other dark-adapted,
and the condition of adaptation determines the degree of sensation
produced by the stimulus. In the same way, if one goes out of doors
froma lighted room at night, at first one must grope one’s way, but
objects gradually become more distinct as the eyes become adapted to the
darkness. After ten minutes the retina is twenty-five times as sensitive
as it was on first leaving the bright light. Further, the dark-adapted
eye is colour-blind, and must become light-adapted once again before
colours can be recognised.
VISUAL SENSATIONS.
Although the sensation of light can be excited by various forms
of stimulus applied to the eye, for example by a blow, the adequate
stimulus consists. of the waves in the ether which emanate’ from
luminous bodies. The sensation of white light results from a com-
pound stimulation, for white light can be dispersed into a series of
rays of differing wave-length by passing it through a prism, each
particular wave-length giving rise to a different quality of sensation
known as colour, The dispersed rays constitute the spectrum of white
light, and only some of these act as an adequate stimulus to the retina.
The visible rays range from those which give rise to the sensation of
red, with a wave-length of 760 millionths of a millimetre, through
orange, yellow, green, blue, indigo to violet, with a wave-length of
397 millionths of a millimetre. The ultra-red and ultra-violet rays
do not excite any visual sensation.
The images formed upon the retina are merely records of light,
shade, and possibly colour. Light and shade are due to varying
intensity of white light. The fact that certain objects reflect only
particular coloured rays depends upon their property of absorbing
the rays which they do not reflect. Thus grass absorbs the rays from
both ends of the spectrum and reflects those of the middle, while a
scarlet poppy absorbs all the rays of short wave-length, reflecting only
the longer waves of the red end of the spectrum.
The invisibility of the ultra-violet rays, or at least of a certain
number of them, is due to the fact that they are absorbed by the
*
134 ESSENTIALS OF PHYSIOLOGY.
refractive media of the eye, especially by the lens. After removal of
the lens for cataract, visual sensations may be excited by rays of as
short wave-length as 313 millionths of a millimetre. The ultra-violet
rays are described as actinic, because they can be detected by their
effect on silver salts, for example in a photographic plate.
The ultra-red rays, on the other hand, are not absorbed by the
refractive media of the eye, but are invisible because they do not
form an adequate stimulus for the end-organs of the retina. They
are heat rays, and can be detected by means of a thermometer.
The impulses excited in the retina pass to the occipital lobes of
the cerebral hemispheres by the tracts already described (p. 90),
those from the left half of each retina reaching the left occipital lobe,
and those from the right halves passing to the right lobe. Each tract
has cell stations in the thalamus and the external geniculate body.
The fovea of the retina is represented bilaterally in the brain.
The impulses conveyed by the optic tracts reach the visuo-sensory
area of the cortex, and by means of association fibres are transmitted
to the visuo-psychic area. They are then passed on to the great
association areas, and, with the aid of the memory of previous visual,
tactile, and other sensations, visual judgments are formed, as regards
size and distance, shape, depth, and other properties of the objects
seen. In connection with the production of visual sensations we have
to consider the time required to excite a sensation, the duration of
the effect of a stimulus, the result of variations in the degree of
stimulation, the effect of adaptation, contrast and fatigue, and, sinh
the nature of the sensation.
A certain interval must elapse between the application of a stimulus
to the rods and cones and the production of sensation. This latent
period has not been measured, but it has been ascertained that the
reaction time for sight is rather longer than that for hearing or for
stimulation of the skin. By reaction time is meant the time taken
to make a voluntary movement in response to a given stimulus. This
includes time taken in the receptor organ, in the afferent nerve fibres
with their intermediate cell stations, in the sensory and psychic areas
of the cortex, in the association area and the motor area, and, finally,
in the efferent path and in the effector organ. The reaction time for
sight is usually about one-fifth of a second, for hearing about one-
seventh, and for skin stimulation rather less.
More definite information is available as to the dwration of the
effect of a stimulus. If a bright electric light is looked at for a few
seconds and then the eyes are closed, the image of the light will persist
for a short time and then fade away. This is known as a positive
THE ORGANS OF SENSE. 135
after-image, and is best seen on waking from sleep. The same per-
sistence of sensation is the cause of the solid appearance of the spokes
in a rapidly revolving wheel. The normal duration of the visual
sensation can be measured by means of revolving discs on which are
black and white sectors. If the gaze be directed to such a disc while
it is revolving slowly, the separate sectors can be distinguished. With
an increased speed of revolution the disc appears to be of a uniform
shade of grey, at first producing a sensation of ‘‘flicker,” and later,
as the rate of rotation increases, and fusion is complete, showing a
uniform and steady grey appearance. If the rate of rotation at which
| complete fusion occurs and the size of the sectors are known, it can
: be calculated that the duration of each impression on the retina after
the withdrawal of the stimulus is about one-fiftieth of a second. The
fusion of sensations is comparable with the fusion of single muscular
contractions to produce tetanus.
. x COLOUR VISION.
The various colour sensations are due to stimulation of the retina
by rays of different wave-length, and it has long been a subject of
discussion as to whether the various qualities of visual sensation are
associated with stimulation of different end-organs or different
chemical substances in the retina, Many theories have been put
forward, but none of these accounts for all the facts, and only two
need be mentioned here.
The Young-Helmholtz. theory postulates the presence in the retina
of three photo-chemical substances, one of which is susceptible to
stimulation by the spectral red rays, and to a diminishing extent by
the other rays from the orange to the violet; a second, which is affected
chiefly by the green rays and to a less extent by those toward either
end of the spectrum; and a third, which is mostly affected by the
violet rays, and to a diminishing extent by the remainder of the
spectrum from indigo to red. Stimulation of all three substances to
: an equal extent excites the sensation of white. Stimulation mainly
| confined to the red substance gives the sensation of red, whereas equal
stimulation of the red and green substances with slight affection of the
violet substance gives rise to the sensation of yellow. The other
| colour sensations are excited in the same way by varying degrees of
: stimulation of the three substances.
On this theory there are three primary sensations, and the
hypothesis finds its chief support in the facts connected with colour
: blindness. Total colour blindness is rare, but about 4 per cent. of
European males are partially colour blind, the commonest form being
m6 * ESSENTIALS OF PHYSIOLOGY.
an inability to distinguish between red and green. This defect is
associated in some cases with an absence of sensation from the rays of
the red end of the spectrum, while in others there is no inability to
\ distinguish the spectral red. Cases of the former type are red-blind,
of the latter green-blind, and on the Young-Helmholtz theory they are
accounted for by the absence “ the red and green elements respectively
from the retina,
According to Hering’s theory there are four instead of thes primary
colour sensations—red, green, blue, and yellow. These are arranged in
pairs, the two colours in each pair being complementary or antagonistic.
If a circular disc, coloured one half red and the other half green, is
rotated rapidly, the sensations of red and green will be fused and the
resulting sensation will be grey, or absence of colour. The same fact
holds for blue and yellow. Red and green on the one hand and blue
and yellow on the other are therefore antagonistic colours. On
Hering’s theory there are three photo-chemical substances in the
retina, each of which arouses a different sensation according as it is
undergoing assimilation or dissimilation. One of these substances is
broken down when stimulated by red rays and built up when green
rays fall upon it; the second undergoes dissimilation or katabolism
under the influence of yellow, and assimilation or anabolism under the
influence of blue rays; the third is broken down by white light and
built up again in the absence of light.
Hering’s theory does not account for the shortening of the red end
of the spectrum in those colour-blind persons who cannot distinguish
red from green, but it is supported by the effects of fatigue of the
visual apparatus and by the facts of simultaneous and successive
contrast. — .
Fatigue of the mechanism connected with visual sensations can be
demonstrated by looking fixedly at a bright object for a short time and
then transferring the gaze to a white surface, when a dark spot or
negative after-image will be seen, corresponding with the position
on the retina of the image of the bright object. Owing to the fatigue
of the part of the apparatus already stimulated, the white paper does
not excite the same sensation in it as in the remainder of the retina.
It may be supposed that the white-black substance undergoes active
katabolic change as the result of the excessive stimulus, and that
this is succeeded by anabolism when the intensity of the stimulus is
reduced.
The effect of stemultaneous contrast may be shown by placing a disc
or cross of grey paper on a coloured sheet, and covering the whole with
tissue paper. The grey will appear green on a red background, red on
THE ORGANS OF SENSE. 137
a green, blue on a yellow, and yellow on a blue, the grey strip in each
case assuming the colour complementary to that of the background.
This phenomenon is supposed by Hering to be due to dissimilation of
any one of the three photo-chemical substances in part of the retina,
this being accompanied by assimilatory changes in the same substance
in the adjacent parts of the retina (or the converse), a process known
‘‘retinal induction.”
> ihioeesiéi contrast is seen if one gazes at a coloured disc, for
- example red, for half a minute or a minute, and then transfers the gaze
to a white sheet. The complementary colour will be seen on the sheet :
it is green if the original disc be red, blue if it be yellow, and so on, and
it constitutes one form of negative after-image. On Hering’s theory the
after-image is due to anabolic changes following katabolic, or vice versa.
On another theory, however, the phenomenon is ascribed to changes in
the cortex of the cerebral hemisphere.
THE MOVEMENTS OF THE EYEBALL.
In a state of rest the two eyeballs lie in the orbital cavities with
their optic axes projecting horizontally forwards and parallel with each
other. Conjugate movements of the two eyes take place, either
upwards or downwards, or to the right or left; or certain of these
movements may be combined or may be accompanied by rotation of
the eyeballs. Further, during accommodation there is a movement
_of convergence of the eyeballs.
These various movements take ‘place about the three principal
axes of each eyeball, the antero-posterior, vertical, and horizontal
axes, and are effected by the six extra-ocular muscles of each eye.
The cornea is rotated upwards and inwards by the superior rectus,
upwards and outwards by the inferior oblique, and directly upwards
by the combined action of these two muscles. The inferior rectus
rotates the cornea downwards and inwards, the superior oblique turns
it downwards and outwards, and both these muscles together turn it
directly downwards ; the internal rectus rotates the eyeball inwards,
the external rectus turns it outwards. The cornea may be rotated
into intermediate positions by the combined action of two or more
muscles acting together. When any one musclé contracts there is
reciprocal relaxation of its antagonist ; thus contraction of the internal
rectus is accompanied by relaxation of the external rectus of the
same eye.
The external rectus is supplied by the sixth, the superior oblique
by the fourth, and the remaining muscles, together with the levator
palpebree superioris, by the third cerebral nerve.
138 ESSENTIALS OF PHYSIOLOGY,
In man vision is binocular, and the eyeballs always move together
in such a way that the image of the object looked at falls on the fovea
of each eye. If the object is a distant one, the visual axes are parallel ;
if the object is a near one, there is convergence of the visual axes. In
either case objects which are not in the direct line of vision fall on
“corresponding points” of the two retine. Ii the mechanism for
combined movements fails for any reason, the images of external
- objects are not formed on See points, and double vision, or
diplopia, results.
THE FIELD OF VISION.
When an object is looked at its image is formed on the fovea,
and it is seen distinctly. This is known as “direct vision.” At the
| same time, surrounding
objects are focussed on
the retina outside the
fovea and are seen less
distinctly. This is
known as “indirect
vision.” The extent of
the outer world in-
cluded in both direct
and indirect vision con-
stitutes the visual field,
and is measured by
means of an instrument
called the perimeter. <A
simple form of this con-
sists of a graduated arc
Fic. 43,—Perimetric chart for right eye, showing fields Which can be moved
for white, blue (and yellow), red, and green. ; as d
(After Howell. ) into any meridian, an
which is provided with
a white spot at its axis. The subject closes one eye, and with the other
gazes steadily at the white spot. A white or coloured object is then
moved from the extreme end of the arc until it comes into the field of
vision, when its position is recorded on a chart. This is repeated for
other meridians, and then the recorded points are connected on the
chart by lines, thus giving a map of the field of vision. The field for a
white object is larger than that for a coloured object, and, of the
primary colours, blue and.yellow have the largest field and green the
smallest, while red is intermediate (fig. 43).
The field for white extends to 90° on the temporal side, about 80°
ie) SC a an
Pa
THE ORGANS OF SENSE. 139
downwards, 65° to the nasal side, and 50° upwards. The field on the
nasal side is obstructed by the bridge of the nose, but the area of the -
retina on which the obstructed rays would fall is insensitive. When
both eyes are in use, the. fields of vision overlap, so that the combined
field extends to 65° on either side of the central point, that is, the
point which is focussed on the fovea.
VISUAL JUDGMENTS.
The flat picture formed on the retina gives rise to sensations of
light, colour, and shade. These sensations are conveyed to the associa-
tion areas of the brain, where the interpretation of the picture, or visual
judgment, takes place. If, for the sake of simplicity, the image of a
single object be considered, judgments are formed as to its position in
space, its distance from the eye, its size, form, and solidity. These
judgments are based partly on the visual sensation, partly on previous
experience derived not only from vision but also from the other senses.
A new-born infant is unable to interpret its visual sensations, but it
gradually learns to correlate these with tactile and other impressions,
until finally the visual sensation alone conveys impressions which at
first were dependent on other senses as well as that of sight. |
An object can be localised as the result of experience that an image
on a given part of the retina corresponds with a definite position in
space. The image on the retina is inverted, but the object is seen in
the upright position because the interpretation of the image is again
the result of experience. The retina, in fact, acquires “local sign.”
Experience also enters largely into judgments of size and distance,
and the latter are closely related to each other. If the size of an
object is known, its distance is estimated by the visual angle which it
subtends ; in other words, by the size of its image on the retina. On
the other hand, if the distance of an unfamiliar object is known, its
size can be judged in the same way. Other factors, however, enter into
judgments of distance. If the object is close at hand, the degree of
convergence of the eyes and of accommodation required to see it distinctly
are of assistance. The importance of convergence can be shown by
holding a pencil vertically about 40 cm. from the face and attempting
to touch it from one side with another pencil, first with one eye closed,
and then with both eyes open. If the object is distant, its outline is
more or less indistinct, owing to the fact that the atmosphere is never
perfectly transparent, and the degree of blurring varies with the distance.
The estimation of the size and distance of an unfamiliar object at an
unknown distance is assisted by comparison with other objects which
140 ESSENTIALS OF PHYSIOLOGY.
are more familiar, and the relation in space of. which to the unfamiliar
object can be determined.
The judgment of solidity is ionetiiated mainly on binocular vision.
A solid object not too far away from the eyes gives rise to a slightly
different image in each eye, and the fusion of these images in conscious-
ness results in the idea of solidity. The same fact is made use of in
connection with the stereoscope. Two pictures taken from slightly
different points of view are fused by means of prisms, and in this way
give the impression of depth which cannot be obtained from a single
flat picture.
Binocular vision is thus of the greatest importance in assisting the
formation of judgments of the solidity of objects, and still more in
estimating accurately the position
‘ * i U7 of those which are close at hand.
It follows, therefore, that whereas
the permanent loss of an eye
involves a certain diminution of
the field of vision, it also involves
the much greater disadvantage
of increasing the difficulty of
making the visual judgments on
which depend the performance of
accurate mechanical work.
oN OPTICAL ILLUSIONS,
aw
Tne. 44.—Z@llner’ ee (Starling’ It follows from what has
| a of Phys I been said as to the interpreta-
tion of visual sensations that the
judgments based upon these are exceedingly fallible, and that this
is so is a matter of everyday knowledge. Judgments based upon
experience are biassed by that experience, as is well shown by the
accompanying illustration (fig. 44), in which parallel lines appear
to be alternately convergent and divergent because of the short
oblique cross lines. Another simple illustration may be made by taking
two straight lines of equal length and drawing divergent lines away
from the ends of the one and centralwards from the ends of the other,
when the former will appear the longer of the two.
a ?
A
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i,
a eee
Site, OP ae ey oT Cae, Peas re ia LS as wal otal
THE ORGANS OF SENSE I4I
SECTION IV.
THE SENSE OF HEARING.
The ear consists of three parts, the outer, middle, and inner ear.
The outer ear consists of the pinna and the external auditory meatus.
The pinna is functionless in man, but in some of the lower animals it
serves to collect the sound waves and conduct them to the meatus,
along which they are transmitted to the- membrana tympani. The
meatus is about 2°5 cm. in length, and is directed inwards and forwards.
It is slightly curved in its course, the-convexity of the curve being
upwards ; in consequence of the curve itis difficult for foreign bodies
to reach the membrana tympani, which stretches a across the inner end
of the meatus. f
THE MIDDLE EAR.
The middle ear or tympanic cavity consists: of a chamber in the
temporal bone containing a chain of ossicles by which the sound waves
are transmitted to the interna (fig. 45). The cavity is bounded
laterally by the membrana tympani) its medial, superior, inferior, and
posterior walls are bony, and anteriorly it exhibits two openings, that
of the Eustachian tubé (auditory tube) below, and the canal for the
tensor tympani muscle above.
The membrana tympani, which separates the external from the
middle ear, lies obliquely, and is shaped like a shallow funnel with the
concavity outwards, the central depression being called the umbo. The
membrane is semi-tra is composed of three layers, an outer
cutaneous layer continuous with the skin lining the ‘meatus, an inner
mucous layer formed of the mucous membrane lining the tympanic
cavity, and a middle fibrous layer pampered of radial. and circular
fibres. .
Two openings are present in the troilind wall of the tympanic cavity,
both bridged across by membrane in the fresh condition. - One, the
fenestra ovalis (fenestra vestibuli), is oval in shape; the other, the
fenestra rotunda (fenestra cochlez), lies below and behind the fenestra
ovalis,
The ossicles of the middle ear are three in number, the malleus, incus,
and stapes. The malleus, or hammer bone, consists of a head and two
processes, the longer of which (the handle) is attached to the tympanic
membrane, its tip reaching to the umbo, while the shorter process, the
processus gracilis, projects anteriorly. The posterior surface of the head
of the malleus articulates with the body of the incus by a saddle-
142 ESSENTIALS OF PHYSIOLOGY.
shaped joint of such a nature that, when the head of the malleus moves
outwards, a cog-like process upon it is locked in a corresponding depres-
sion on the incus. If, however, the head of the malleus moves inwards,
the joint surfaces separate, and in this way traction on the membrane j
enclosing the internal ear is avoided. The incus, or anvil bone, consists |
of a body and two processes (crura). The body articulates with the
malleus, and the longer of the two processes with the stapes. The
stapes, or stirrup bone, articulates by its head with the long crus of
the incus, and its base is attached to the membrane which closes the
eather tepid nti hahaha ialiial
ee ee eee ee ee ete 2 ae oe ae ee ee
D _ Chain of 7
Semicircular canals ossicles. 1
opening into" ~~ ~ ~~~
utricle.
ee ee Membrana ~
tympani.
Cochlea.
| Fic, 45.—Scheme of ear. (After Landois. )
fenestra ovalis. The head of the malleus is attached by an anterior,
a superior, and a lateral ligament to the wall of the tympanic cavity, )
and the short process of the incus is attached by a ligament to the !
posterior wall of the cavity. As the result of these attachments the
malleus and incus can be rotated only around an axis which passes
through the processus gracilis of the malleus and the short process of
the incus. This movement of rotation takes place when the handle
of the malleus moves inwards with the membrana tympani, the head of 4
the malleus and body of the incus moving outwards, and. the long
process of the incus moving inwards and exerting pressure through the
stapes on the membrane in the fenestra ovalis. The movements of the
THE ORGANS OF SENSE. 143
ossicles are controlled to some extent by the tensor tympani, which is
inserted near the root of the handle of the malleus, and the stapedius, —
which is inserted into the neck of the stapes.
The Eustachian tube connects the cavity of the tympanum with
that of the pharynx. It is generally closed, but is opened each time —
swallowing occurs. When it opens, the air pressure in the middle ear is
adjusted to that of the atmosphere, and in this way the tympanic
structures are protected from the injurious effects of too small or too
great a pressure on the membrana tympani.
THE INTERNAL EAR,
The internal ear consists of a series of cavities in the temporal bone,
forming the osseous labyrinth, within which is a corresponding series
of membranous structures, the membranous labyrinth. The osseous
labyrinth contains a clear fluid, the perilymph; the membranous
labyrinth is filled with a similar fluid, the endolymph.
The anterior portion of the labyrinth, or cochlea, contains the end-
organs of hearing; the posterior part is concerned with the sense of
position, and will be described later.
The Cochlea.—The cochlea consists of a tube coiled in a spiral
fashion round a central bony modiolus and making altogether two and
a half turns round the latter. A bony ridge projects from the modiolus.
into the tube and is known as the osseous spiral lamina; attached to
this is a membrane, the basilar membrane, which_ extends. to the outer
wall of the tube, where it meets a fibrous projection, the spiral ] ligament.
A relatively thick layer of connective tissue, the lambus lamine spiralis,
rests on the osseous spiral lamina, and ends abruptly near the basilar
membrane by an overhanging border. A delicate membrane, the
vestibular membrane or membrane of Reissner, is attached to the
upper surface of the limbus and to the wall of the tube in such a way
as to cut off a portion, triangular in area, known as the canal of the
cochlea (ductus cochlearis). The bony tube is thus divided by the
membrane of Reissner and the osseous spiral lamina with the basilar
membrane into three divisions, the scala vestibuli above Reissner’s
membrane, the canal of the cochlea already described, and the scala
tympani. The scala vestibuli and scala tympani form part of the
bony labyrinth and communicate at the apex of the cochlea by the
helicotrema. The scala tympani is closed at its lower end by the
membrane in the fenestra rotunda. The scala vestibuli and scala
tympani contain perilymph. The canal of the cochlea forms part of
the membranous labyrinth and contains endolymph. “It communicates
°
144 ESSENTIALS OF PHYSIOLOGY.
with the saccule of the posterior part of the membranous labyrinth by
a fine tube, the canalis rewnrens. . .
The Organ of Corti.—The end-organ for hearing lies in the canal
of the cochlea and is called the organ of Corti. It consists of a
_ specialised epithelium resting on the basilar membrane (fig. 46). On
section the epithelium is seen to be arranged in relation with two rows
of rod-like cells, the rods of Corti, which are inclined towards each other
in such a way as to form a tunnel. The basal ends of the rods are
expanded, and in the angle which each forms with the basilar membrane
is a small nucleated mass of protoplasm. The free end of each outer
rod is shaped like the head of a swan, the back of it fitting into the
end of the inner rod, which resembles the proximal end of the ulna.
as ———---~-~o
|
e
Fic. 46.—Structure of organ of Corti (diagrammatic).
a, membrana tectoria; b, hair cell; c, basilar membrane; d, rods of
Corti; e, cochlear nerve fibres and (on the left) spiral ganglion.
It is estimated that there are about 6000 inner, and about 4000 outer,
rods. The heads of the outer rods are continued outwards as
phalangeal processes which unite with corresponding processes on the
outer supporting cells to form the reticular membrane.
On either side of the rods of Corti are hair cells, a single row on
the inner side, four or five rows on the outer side; the free ends of
the outer cells occupy the apertures in the reticular membrane. The
hair cells are supported by the cells of Deiters. Beyond the hair cells
the supporting cells become shorter, and the epithelium is continuous
with the flattened cells lining the canal of the cochlea.
A thick membrane, the membrana tectoria, extends from the limbus
laminee spiralis so as to rest upon the organ of Corti.
The fibres of the cochlear nerve occupy the centre of the modiolus
and are distributed along the spiral lamina. They are the central
processes of the bipolar cells of the spiral ganglion, which lies in the
THE ORGANS OF SENSE. 145
spiral lamina. The peripheral processes of these cells emerge from the
spiral lamina to be distributed as fibrils over the hair cells of the —
organ of Corti.
THE MECHANISM OF HEARING.
When sound waves fall upon the tympanic membrane they cause
the latter to vibrate. The membrane has no periodicity of .its. own,
partly because of its peculiarities of structure, and partly because its
vibrations are damped by the attached handle of the malleus. With
each inward movement of the membrane the handle of the malleus and
the long process of the incus also move inwards, the latter carrying the
stapes with it. The malleus and incus together form a lever, the
fulcrum of which is the axis. of movement described above, the handle
of the malleus being the power arm and the long process of the incus
the load arm. The length of ‘the handle of the malleus is to that of
the process of the ineus as 3 to 2,.and it therefore follows that’ the
movement of the tympanic membrane is diminished ‘in amplitude in
the proportion of 3-to 2, while at the same time it is increased in force
by one half, or in the proportion of 2 to 3, - Further, as.the membrana
tympani is twenty times the size of the membrane in the fenestra oyalis
on which the vibrations are directed, it follows that the pressure of a
sound wave on the membrana tympani is increased. to thirty times in’
its passage across the middle ear (3/2 x 20 = 30). nat
The vibrations of the membrana tympani,: transmitted by the
chain of ossicles to the fenestra_ovalis, set up corresponding vibrations
in the perilymph; these- travel up the scala vestibuli and down
the scala tympani. The wave in the perilymph is communicated.
through the delicate membrane of Reissner to the endolymph of the
canal of the cochlea, and thus the stimulus is conveyed to the organ of
Corti. The increase of pressure in the canal of the cochlea is passed
on to the scala tympani, causing the membrane which re the fenestra
rotunda to bulge towards the middle ear.
The adequate stimulus for the organ of Corti is the wave set up in
the endolymph as the result of sound waves in the air. Sound waves
consist in an alternate condensation and rarefaction of the gases of
the atmosphere, and they travel through the air at a rate of about
350 metres a second. They give rise to two kinds of sensation, one
that of noise, when the sound waves follow each other irregularly, the |
other that of a musical note, when the waves follow one Bnouher with
a certain rhythm.
Musical sounds vary in pitch, in intensity, and in timbre or quality.
(1) Pitch depends on the rapidity of the vibrations constituting the
IO
146 | ESSENTIALS OF PHYSIOLOGY.
note, and the more rapid the vibrations, the higher the pitch. The highest
note which can be appreciated by the human auditory apparatus has a
frequency of about 40,000 vibrations per second, but some animals can
detect sounds of higher pitch than this. The lowest note used in
music, that of the sixty-four foot organ pipe, has a frequency of sixteen
vibrations per second, and gives an impression rather of vibration than
of sound. :
(2) Intensity or loudness depends on the amplitude of the vibrations
giving rise to the note. This can be shown by recording the vibrations
of a tuning fork on a moving drum, when it is seen that the more
extensive the movement of the fork, the louder is the note produced
by it. 3
(3) Quality or tembre is due to the form of the wave. In the case
of the tuning fork the wave is simple, and the note is a pure one
uncombined with any secondary vibrations. The notes produced by
musical instruments, on the other hand, owe their distinctive quality to
the production of overtones, which combine with the fundamental note
and produce a compound wave. A violin string, for example, not only
vibrates as a whole, giving the fundamental note, but also vibrates in
segments, producing the overtones which are due to the vibration of
halves, thirds, fourths, and still smaller segments of the whole string.
The particular quality of the tones produced by any instrument depends
upon the number and degree of prominence of the particular overtones.
When two notes are sounded together, the result may be, on the one
hand, consonance or harmony, or, on the other hand, dissonance or dis-
cord. Discord is due to the fact that the two notes have nearly the
same vibratory period, with the result that at certain intervals the
summit of one wave occurs at the same instant as the trough of the
other, so that the two neutralise each other, causing a momentary
silence. Later, the summits of the two waves will correspond, and the
degree of sound will be momentarily increased. The rapidly alternating
increase and diminution in volume of the sound wave constitutes what
is called a beat, and gives rise to the jarring sensation known as discord.
When two notes sounded together give a sensation of harmony, there
are no such beats, the two waves being combined to form a compound
wave of regular rhythm.
The auditory mechanism is not only capable of appreciating sounds,
but also of distinguishing differences of pitch within limits, and even,
in the case of trained musicians, of analysing a combination of notes,
sounded together, into its constituents. Various theories have been held
as to the part played by the different structures in the cochlea and by
the cerebral cortex itself in the discrimination of pitch, but it will only
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THE ORGANS OF SENSE. 147
be necessary here to state the known facts and the view, based upon
these, which is most generally held. 3
(1) It is a well-known physical fact that a string which vibrates
with the rhythm of, say, middle C, will be thrown into vibration if that
note is sounded near it. Similarly, if a vibrating tuning fork be held
over the mouth of a tall glass jar and water be slowly poured into the
jar, when the water is at a certain depth the sound of the tuning fork
will be intensified by the resonance of the column of air in the jar.
(2) From the distribution of the cochlear nerve fibres around the
hair cells, it may be assumed that the latter are the end-organs for
hearing.
(3) The rods of Corti are not present in the cochlea of birds, and
are therefore not an essential part.of the auditory mechanism.
(4) The basilar membrane is composed of about 24,000 radial fibres,
and it increases in width from the base to the apex of the cochlea, the
shortest fibres being 0°041 mm. and the longest 0°495 mm. in length.
(5) Experimental destruction of the base of the cochlea in dogs
made the animals deaf to high notes, whereas destruction of the apex
resulted in deafness to low notes. Similar results have followed disease
in man. ;
These facts suggest that different parts of the basilar membrane
resonate to notes of different pitch, the longer fibres responding to low
notes and the shorter fibres to high notes. The vibrations of the basilar
membrane set up waves in the endolymph, by which the hair cells are
stimulated, after-vibrations being damped by the tectorial membrane.
On this theory, the analysis of sound takes place in the cochlea, each note
causing definite fibres of the basilar membrane to resonate, and thus
acting as a stimulus to the hair cells opposite that part of the membrane,
If this hypothesis is correct, a note of any particular pitch will always
excite an impulse in the same nerve fibres.
The auditory impulses reach the cerebral cortex of the temporal lobe
by the auditory tract (p. 91), arriving first at the audito-sensory area,
and being transferred to the audito-psychic area. The further convey-
ance of the impulses to the association areas enables judgments to be
arrived at as to the nature of the sounds, as, for example, the rumbling
of thunder or the meaning of spoken language.
In some of the lower animals the judgment of the direction from
which a sound proceeds is aided by movement of the external ears. In
man the projection of sound is more difficult, especially if its source is
in line with the mesial plane of the body. Some assistance is obtained
by moving the head and noting to which ear the sound is more distinct
in each position. Asa rule, man relies largely on the co-operation of sight
148 ESSENTIALS OF PHYSIOLOGY.
to localise the direction of a sound, and a blindfolded person has great
difficulty in forming a judgment as to the source of a brief sound
produced in line with the mesial plane of his body.
VOICE AND SPEECH.
Voice is a musical note produced by the vibration of the vocal cords
of the larynx and modified in character by the resonating chambers
formed by the upper respiratory passages, the mouth,. the accessory
sinuses of the nose, and the chest... The vocal cords are thrown into
vibration by expiratory currents of air from the lungs. 3
The framework of the larynx: is formed of cartilages connected by
fibrous tissue. The ericoid cartilage, resembling a signet ring in shape,
is connected with the upper cartilage of the trachea by a fibrous
membrane ; the broad part of the cartilage is situated posteriorly.
The thyroid cartilage is formed of two laminz which join in the middle
line anteriorly and are separate behind. The lower part of the posterior
border of each lamina articulates with the outer aspect of the cricoid
cartilage, so that the latter rotates to a limited: extent on the ‘thyroid.
The arytenoid cartilages, right and left, are pyramidal in shape, the base
‘of each articulating with a facet on the upper border of the posterior
portion of the cricoid. The corniculate cartilages are small, and
articulate with the apices of the arytenoid cartilages. The cwnezform
cartilages are elongated in shape and lie in the aryepiglottic folds. The
epiglottis is leaf-shaped, the stalk being attached to the recess of the
‘thyroid cartilage, and the flattened portion extending nearly vertically
upwards.
The thyroid and cricoid cartilages and the: greater part of each
arytenoid consist of hyaline cartilage. The apical parts of the arytenoids,
the corniculate cartilages, the cuneiform cartilages, and the epigiostis
consist of yellow elastic fibro-cartilage.
The lining membrane of the larynx is thrown into two antero-
posterior horizontal felds from the recess of the thyroid cartilage in
front to the anterior or vocal process of the base of each arytenoid
cartilage behind. These are known as the vocal cords (vocal folds), and
the slit which they bound is called the glottis (fig. 47). Ata slightly
higher level there are two parallel folds, the false vocal cords (ventricular
folds). The recess between the true and false vocal cords is called the
ventricle.
The mucous membrane of the larynx is lined by columnar ciliated
epithelium, but the anterior surface and upper half of the posterior
surface of the epiglottis, the vocal cords, and scattered patches of the
it OO i lage
THE ORGANS OF SENSE. 149
membrane above the level of the- glottis are covered by stratified
squamous epithelium. | . .
The glottis is opened by the posterior crico-arytenoid muscles, which
by their contraction rotate the arytenoid cartilages on their vertical
axis in such a way that the vocal processes are turned outwards.
Rotation of these cartilages in the reverse direction is effected by
the lateral crico-arytenoid
muscles, contraction of
which approximates the
vocal cords. The closure
of the glottis is assisted
. Hyoid bone,
by the contraction of the
arytenoid muscles, which,
by approximating the aryte-
noid cartilages, bring to- Outline of
se appendix of
gether the posterior ends ventricle.
of the vocal cords. The Vasco a
crico-thyroid muscles by Venéricle:
3 . Vocal cord.
their contraction cause the Thyro-aryte-
cricoid to rotate on the D%deus muscle.
thyroid cartilage, so that
the broad part of the former
is drawn downwards and
backwards and the vocal
cords are made tense. The
general action of the thyro-
arytenoid muscles is to draw
the arytenoid cartilages to-
wards the anterior part of
the thyroid and so relax Fic. 47. —Coronal section of larynx and upper part
the vocal cords. of trachea. (Gray’s Anatomy.)
The movements which
occur in the larynx in connection with swallowing and breathing will
be referred to in the descriptions of deglutition and respiration. The
changes which occur in the larynx during voice production are observed
with the aid of the laryngoscope. This instrument consists of a mirror
which can be held in position in the pharynx so as to give a reflected
image of the interior of the larynx to the eye of the observer. In such
an image the vocal cords appear white, the false vocal cords more pink
in colour. |
When a note is produced the vocal cords are brought close together,
and an expiratory current of air causes them to vibrate. The current
I50 ESSENTIALS OF PHYSIOLOGY.
of air must have a certain pressure, and this is produced by contraction
of the muscles of the thorax and abdomen which are concerned in
expiration. |
The note produced may vary in loudness, pitch, and quality or
timbre. The degree of loudness varies with the force of the expiratory
current. The pitch is determined: partly by the length and partly by
the tension of the vocal cords, In children the pitch is relatively high,
because the cords are short. At the time of puberty the larynx in-
creases considerably in size, more so in the male than in the female, and
as a result a boy’s voice “breaks,” that is, becomes much lower in pitch.
The possible variation in pitch in any individual is on an average about
two octaves. This is mainly due to variations in the tension of the
vocal cords brought about by the reciprocal action of the crico-thyroid
and thyro-arytenoid muscles. The pitch is also affected by the length
of the vocal cords free to vibrate, this being determined by the move-
ments of the arytenoid cartilages. Further, the force of the expiratory
current influences the pitch, the stronger the blast of air the higher
being the note produced. ’ |
The quality of the note is due to the resonance produced in the
various resonating chambers, the air in the chest vibrating with the
lower notes and that in the mouth and pharynx and in the accessory
sinuses of the nose with the higher pitched notes. Hence the terms
chest notes and head notes used in connection with singing.
The sounds which constitute speech are due to modifications of the
laryngeal simple note, and are brought about by alterations in the shape
of the mouth and in the adjustment of the lips and teeth. The vowel
sounds are continuous vibrations, whereas the formation of consonants
depends on the interruption of vibrations.
Forthe production of the broad ‘‘a” vowel sound, the mouth cavity
is widely open ; for the “‘i” (ee) sound, the space between tongue and
palate is much reduced ; for “‘u” (00), the posterior part of the tongue
is raised against the palate. |
Consonants are classified as dental, guttural, or labial, according to
the position at which the interruption of the laryngeal note takes place.
Thus “t” and “‘d” are dentals, “jp” and “b” are labials, and “g¢” and
“k” are gutturals. |
In whispering there is no phonation; the glottis is open, and the
words produced are the result of the modification of the air current by
the speech mechanism.
THE ORGANS OF SENSE. I5!
SECTION V.
PROPRIOCEPTIVE SENSES.
Our knowledge of the position of the body is derived partly from
tactile and visual impulses, and partly from impulses reaching the
central nervous system from the posterior part of the labyrinth and
from: the skeletal muscles; the impulses arising in the muscles and
labyrinth are called proprioceptive impulses,
The Labyrinth.—The part of the bony labyrinth behind the cochlea
consists of a cavity called the vestibule, into which the scala vestibuli
opens in front and three semicircular canals open behind. Within the
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Fic. 48,—The membranous labyrinth. (Enlarged.) Gray’s Anatoniy.
bony semicircular canals lie the three membranous semicircular canals
(ducts). The latter open into the membranous utricle, which, along with
the saccule, occupies the vestibule, the utricle and saccule being connected
by the saccus (ductus) endolymphaticus (fig. 48). The semicircular
canals are arranged in three planes at right angles to one another.
The external canal lies in the horizontal plane; the superior vertical
canal and the posterior vertical canal lie in vertical planes at right
angles to one another, as shown in fig. 49.
The canals open into the utricle by five orifices, one of which is .
common to the medial end of the superior and the upper end of the
posterior canal ; each canal has a dilatation or ampulla at one end.
The utricle, saccule, and semicircular canals are lined by flattened
epithelium resting on connective tissue ; in each ampulla the connective
o
152 ESSENTIALS OF PHYSIOLOGY.
«
tissue is thickened at one point to form a projection, which is covered
with columnar epithelium supporting a number of cells provided with
hairs, and which is called the crista acustica (septum transversum).
Similar thickenings occur in the utricle and saccule, these being called
maculee acustice ; they have the same structure as the cristz, with the
addition of small concretions of lime, called otoliths, scattered among
the processes of the hair
cells. The fibres of the
vestibular nerve are dis-
tributed to the criste
and macule acustice,
and end in fibrils round
the hair cells. ;
The bony labyrinth
contains perilymph, and
the membranous cavities
contain endolymph ; the
membranous structures
are attached to the bony
labyrinth by fibrous
strands,
The functions of the
semicircular canals have
been ascertained chiefly
by experiments on
pigeons, in which they
are easily accessible. If
the horizontal canals
2 sR i are destroyed, the head
Fic, 49.-Figure from Ewald showing the situation of Ha ‘ id
the three semicircular canals in the skull of the O° ates trom side to
pigeon. (Starling’s Principles of Physiology.) side in a horizontal
A, plane of anterior or superior semicircular canal ; P, plane of :
: posterior; and E, that of horizontal canal. plane , after section of
Note that the anterior canal of one side and the posterior the posterior or superior
canal of the other side are in the same plane. i
vertical canals, the head
and body are thrown into constant movement in a vertical plane,
so that the animal tends to turn somersaults.
After destruction of all the canals, the animal is in constant violent
movement, but can neither stand, walk, nor fly. After a time partial
recovery takes place, but the symptoms return when the eyes are
bandaged, showing that the partial recovery is due to compensatory
utilisation of the visual sensations for co-ordination.
It is clear from these observations that the canals are essential
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THE ORGANS OF SENSE, 153
for the co-ordination of muscular movements and for the maintenance
of equilibrium. Each movement of the head sets up a stimulus in
one or other of the canals, which passes along the vestibular nerve
to the cerebellum; these impulses serve to co-ordinate and restrain
the movement. In the absence of these guiding impulses the move-
ments become completely uncontrolled, and the sense of the position
of the head is lost. The existence of three pairs of canals in. three
different planes makes it possible for impulses to be aroused in what-
ever plane the head is moved. |
These impulses originate in the movements of the endolymph,
which take place when the head is moved and cause the pressure on
the hair cells of the criste to be increased or diminished. This was
shown by Ewald, who connected a small tube with one canal in such
a way that he was able suddenly to blow into the canal so as to cause
the endolymph to flow towards the ampulla. Every time this was
done, the animal moved its head and eyes in the plane of the stimu-
lated canal and in the direction of the current.
The afferent impulses arising in the semicircular canals: maintain
equilibrium and muscular co-ordination, even after removal of the
cerebral hemispheres. In normal circumstances, however, they also
affect consciousness, and we are aware of the movements of the head.
Interference with these impulses in man by disease of the canals
brings about sensations of giddiness and disturbance of equilibrium. —
The semicircular canals are also concerned with the maintenance of
muscular tone, and more particularly with the variations in tone of
different muscles according to the position of the head and body. In
the rabbit, for example, when the head is raised the tone of the
muscles of the forelimbs is increased, the effect being abolished by
destruction of the semicircular canals.
These reflex variations of muscular tone, which are known as postural
reflexes, play a part in the muscular adjustment of the animal to
changes of position, and assist im the maintenance of equilibrium,
The macule acustice are subject to constant stimulation by the
otoliths. The part of the macula upon which the otoliths exert
pressure varies according to the position of the head, and the stimu-
lation of the hair cells by the pressure of the otoliths is also variable.
The impulses to which these stimuli give rise serve to give informa-
tion as to the position of the head when at rest, or in progressive, as.
distinct from rotatory, movement.
The Muscle Sense.—Even when the eyes are closed we are conscious
of the position of the body and limbs when at rest, and of movement
of the limbs, whether this be active or passive. This consciousness
J
154 ; ESSENTIALS OF. PHYSIOLOGY.
constitutes muscle sense, and is brought about by afferent impulses
constantly passing from special structures in the muscles, joints, and
ligaments. About one-third of the fibres in a nerve to a muscle are
afferent in function, and do not degenerate on section of the anterior
roots. These afferent fibres terminate in the muscles in neuro-
muscular spindles, lying between the ordinary muscle fibres. The
muscle spindles consist of fine muscular fibres surrounded by a con-
nective-tissue sheath, A nerve fibre loses its myelin sheath, pierces
the covering of the spindle, and divides into bundles of fibrils which
make several spiral turns round the muscle fibres, and then end by ar-
borisation. Other nerve endings are found in tendon, and consist in
the arborisation of a nerve fibre around a bundle of tendon fibres.
Sensation of passive movements is due chiefly to impulses arising in
_ the joints, that of active movements to impulses arising in the muscles.
It is through this sense that we are able to form an estimate of weight.
The impulses giving rise to muscle sense ‘also take part in the
co-ordination of muscular movement, as has already been described ;
and their absence may lead to disturbances of equilibrium. |
The maintenance of equilibrium is dependent, therefore, on afferent
impulses from the muscles and the labyrinth as well as from the eyes
and the skin; when the sensations resulting from the impulses from
these different sources are discordant, we experience a feeling of
giddiness, and at the same time equilibrium is disturbed.
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CHAPTER VII.
THE BLOOD.
Aut the active cells and tissues are in intimate relationship with
capillary vessels, through which blood is continually flowing. Both
tissue elements and capillary are bathed with a fluid called lymph, and
a constant interchange of material takes place through the lymph
between the tissues and the blood. On the one hand, oxygen and other
nutritive substances pass from blood to tissue to repair loss of substance,
and to furnish a source of energy, and on the other hand, carbonic acid
and other waste materials pass from the tissues to the blood. An
exchange of water and salts also takes place through the capillary wall
by diffusion. In some organs certain substances, called hormones, are
supplied to the blood, and are carried in it to the glands or muscle
fibres which they are destined to excite. Further, as the blood
circulates, now through glands and muscles in which heat is produced,
now through other structures in which heat loss occurs, it serves to
equalise the temperature of the different parts of the body.
Freshly shed blood isa red, viscid, opaque fluid with a specific gravity
of 1055. The specific gravity may be ascertained with a single drop
of blood by making a mixture of chloroform and benzole, and finding
the proportions of the two fluids in which a drop of blood remains
suspended without tending either to sink or rise. The specific gravity
of the mixture, ascertained by means of a hydrometer, is that of the
blood itself.
When human blood is examined under the microscope, it is seen to
consist of two kinds of corpuscles floating in a pale yellow fluid, the
blood plasma. The corpuscles which are most numerous are the red
blood corpuscles, or erythrocytes. The other variety of corpuscle, the
white blood corpuscle, or leucocyte, is in the proportion of 1 to 500 red.
The red corpuscles, when seen singly, are yellow in colour, but when
massed together they give blood its red appearance. They are circular,
biconcave, non-nucleated discs, each having a diameter of 7:5 thousandths
of a millimetre (7°5 »). In all mammals except the camel tribe, the
F55
*
156 ESSENTIALS OF PHYSIOLOGY.
shape of the red corpuscle is the same as that of the human erythrocyte ;
in camels the corpuscle is oval and biconcave.
The white blood corpuscle is a colourless, nucleated cell, and several
varieties occur in human blood. The most abundant type, forming
about 70 per cent. of the total number of leucocytes, is the polymorpho-
nuclear, so called because the nucleus consists of lobes, connected by
finer strands. This cell is rather larger than the erythrocyte, being
about 10m in diameter. It possesses the power of amoeboid movement,
and, because of its function of ingesting bacteria and foreign particles, is
said to be phagocytic. Its protoplasm contains numerous fine granules
which stain with neutral dyes, and are described as neutrophile. A
somewhat similar corpuscle, in which, however, the nucleus is usually
horse-shoe-shaped, contains large granules which stain deeply with acid
dyes, such as eosin. These cells are called eosinophile. They form from
1 to 5 per cent. of the total number of leucocytes. A basophile variety,
in which the granules stain with such basic dyes as methylene blue, is
found only occasionally in normal blood. — Small and large lymphocytes
form about 25 per cent. of the total number of leucocytes, the small
variety being the more numerous. Lymphocytes are distinguished by
containing a large spherical nucleus surrounded by a small amount of
hyaline protoplasm, which does not contain granules.
Other bodies, called blood-platelets, are found in recently shed blood,
but these cannot be seen when precautions are taken to prevent
the blood coming in contact with foreign substances ; and it is believed
that they are not a formed constituent of normal blood. Blood-platelets
are colourless bodies, one-third to one-half the size of red corpuscles,
and each contains a central group of granules resembling a nucleus. If
examined on metaphosphate agar, the platelets show amceboid movement.
The blood of a healthy man contains about 5,000,000 red corpuscles
in each cubic millimetre, that of a woman about 4,500,000. The
corpuscles are counted by means of a hemocytometer. This consists
of a glass cell of known depth, the floor of which is ruled in squares of
known size. The blood is diluted 100 times with a slightly hypotonic
solution of sodium sulphate, which prevents coagulation, and the cell is
filled with the mixture. The corpuscles settle on the squares and can
be counted under the microscope. The volume corresponding with each
square and the dilution of the blood being known, the number of
corpuscles per cubic mm, of blood can be calculated. The number is
diminished by hemorrhage and in certain diseases, and is increased by
living at high altitudes.
The white corpuscles number about 10,000 in each cubic millimetre
of blood. They can be counted by means of the hemocytometer, the
bak ——— ee
THE BLOOD. | 187
blood being diluted ten times with a saline solution similar to that used
for red corpuscles, but ee a little methylene blue to stain the
leucocytes. 3
THE RED BLOOD CORPUSCLES.
Each red corpuscle is soft, and alters its shape readily so that it
can pass through even the narrowest capillary vessels. It is also
elastic, and readily regains its shape when the compressing influence is
removed. Two views are held as to the intimate structure of the
erythrocyte. According to one, the corpuscle consists of a sponge-like
framework (stroma) containing hemoglobin, the blood pigment, loosely
combined with the stroma. Schafer’s view, on the other hand, is that ©
the corpuscle consists of an envelope containing the hemoglobin in
solution in its interior. Whichever view be adopted, it is clear, from
the behaviour of the corpuscle in presence of reagents, that its super-
ficial layer behaves to some extent as a semi-permeable membrane
readily allowing the passage of water but not of salts. Thus, if red
blood cells are placed in.a fluid the salt content of which is markedly
below that of blood plasma, water passes into the corpuscle and
distends it, so that ultimately the membrane ruptures and the hzmo-
globin is discharged. On the other hand, if the surrounding fluid is
hypertonic, for example 2 per cent. NaCl, water passes out of the
corpuscle, which becomes shrunken and crenated in consequence. In
‘9 per cent. NaCl, which is isotonic with mammalian blood Pe the
cell is unaltered.
The envelope of the corpuscle is dissolved by weak alkalies or by
ether, and this makes it probable that it is of a fatty nature. Bile
salts (which are solvents of. fats), amyl alcohol, soaps, higher fatty
acids, and saponin or sapotoxin also dissolve the red corpuscles, setting
free the hemoglobin. The same result can be attained physically by
alternate freezing and thawing. The setting free of the hemoglobin
by any of these means is called hemolysis.
Certain physiological substances also bring about hemolysis and
have been termed hemolysins. Snake venom, and frequently the serum
from an animal of another species, act in this way. Moreover, the
serum of an animal A, which is not naturally hemolytic for the blood
of another animal B, may be made hemolytic for the blood of that
animal, if A has been inoculated with blood from the species B some
days before the experiment is made. Thus rabbit’s red corpuscles are
not broken up by the serum of a guinea-pig. If, however, rabbit’s
blood has been previously injected into a guinea-pig, the serum from
the latter becomes hemolytic for rabbit’s red corpuscles,
158 ESSENTIALS OF PHYSIOLOGY.
Serum which is either naturally or artificially hemolytic loses its
power to dissolve red corpuscles if it is heated to 55° C. But such
serum can have its hemolytic power restored by the addition of serum
from a normal animal. The hemolytic power of any serum therefore
depends upon the presence of two substances, one which is present in
normal serum and is destroyed at a temperature of 55° C., and is usually
called complement ; and a second which is stable at 55° C., and may be
produced in an animal by injection of the corpuscles of another animal.
Heemoglobin may thus be set free from the erythrocytes and pass
into solution in the surrounding fluid in three ways :—
(1) By a physical process, as by dilution with water or by alternate
freezing and thawing.
(2) By chemical means, for example, the solution of the lipoid
stroma of the corpuscles by bile salts, amyl alcohol, soaps, or
other reagents.
(3) By physiological agents, called heemolysins, the exact mode of
action of which-is not known. |
As the result of hemolysis by any of these methods the blood is
said to be ‘laked.” The hemoglobin is in solution, and the blood,
previously opaque on account of the reflection of light from the
erythrocytes, becomes transparent.
THE CHEMICAL COMPOSITION OF RED BLOOD CORPUSCLES.
The red corpuscles may be ob-
tained in sufficient quantity for
analysis by centrifugalising blood
and washing the deposit with ‘9
per cent. NaCl. They are found to
consist of 63°3 per cent. of water
and 36:7 per cent. of solids. Heemo-
globin forms 95 per cent. of the
dry solids, the remainder being
made up of nucleoprotein, lecithin,
cholesterol, fatty acid, and inorganic
salts, the most abundant of the
latter being potassium phosphate.
The stroma is, therefore, as has
already been pointed out, largely
of a lipoid nature.
Fic. 50.—Hemoglobin crystals, mag- Hemoglobin is a compound of
pit, A From, Qian a eee globin, which is a protein belong-
sate Tianhe, 2, from the guinea: ing to the group of histones, with an
ee.
THE BLOOD. 159
iron-containing substance, hematin. The molecule of hemoglobin
is a very large one, and its formula is given by one authority as_
CrsgHyoo3N 195,53%e0>,5 Although it is a colloid, hemoglobin crystallises
fairly readily. The crystals vary in shape in different animals, but in
all cases they belong to the rhombic system. In man they are rhombic
prisms, in guinea-pigs they form tetrahedra (fig. 50). Hemoglobin is
purple in colour, is soluble in water, and its solutions, examined with
the spectroscope, show a broad absorption band in the green between
Frauenhofer’s lines D and E (fig. 51, sp. 2).
i ORD Eb F G
Sp. 1 , Oxyhemoglobin.
: Hemoglobin.
3 Methzemoglobin.
4 Alkaline
methemoglobin.
Acid hematin
5 in ether.
6 Alkaline hematin
in rect. spt.
7 Reduced hematin
é (heemochromogen),
Acid
8 heematoporphyrin.
9 Alkaline
hematoporphyrin.
Fic, 51.—Spectra of hemoglobin and its derivatives. (From Mac Munn’s
Spectrum Analysis, )
The most important property of hemoglobin is its affinity for
oxygen, each molecule combining with two atoms of oxygen to form
oxyhemoglobin. The combination is a loose one, for the attached
oxygen is given up if the solution containing the compound be exposed
to a vacuum, or if a reducing agent, such as ammonium sulphide, be
added to it, HbO, (oxyhemoglobin) again becoming Hb (hemoglobin).
In the living body the same reduction takes place as the blood circulates
through the capillaries of the active tissues. Oxyheemoglobin is charac-
terised by a scarlet colour, and its spectrum exhibits two absorption
bands in the green, between the D and E lines (fig. 51).
The oxygen-carrying power of hemoglobin depends upon hematin
160 ESSENTIALS OF PHYSIOLOGY.
the prosthetic group of the molecule, and the value of hematin in this
respect is determined by the presence of iron. The actual amount of
iron is small. Heematin forms about 4 per cent. of the hemoglobin
molecule, and iron accounts for about 11 per cent. of the hematin, or
about 0°05 per cent. of blood itself. Iron has on this account been
described as the gold currency of the body, and it is carefully preserved,
Red corpuscles are being continually destroyed in the liver, and: the
broken-down material is excreted to a large extent in the bile. Hematin
appears in the bile in an iron-free form as bilirubin, the primary: bile
pigment, the iron being retained by the liver cells. The iron is eventu-
ally used in the formation of new. erythrocytes, this taking place in the
- red marrow.
Owing to the high atomic weight of iron, the Seaport: of a
substance like hematin would be attended with difficulty if it were
not combined with a large protein molecule. By means of its com-
bination with globin the weight of the iron is distributed, and the
resulting compound can -be floated along in the blood stream without
_ difficulty. :
Heemoglobin has an affinity for invbon monoxide 130 times as great
as its affinity for oxygen, and when it is exposed to air containing even
a minute quantity of the former gas, it forms with it a stable compound,
~ carboxyhwemoglobin (HbCO): HbCO differs slightly in colour from HbO,,
strong solutions having a more florid appearance, and weak solutions
retaining a pink colour, whereas the same dilution of HbO, has a yellow
tinge. Solutions of HbCO, when examined spectroscopically, exhibit
two bands in the green, slightly nearer the violet end than those
presented by HbO,. HbCO is unaffected by the na Buon of ammonium
sulphide.
Heemoglobin forms a still more stable Sonipound with nitric oxide,
HbNO, but this is purely a laboratory product, and is only of theoretical
interest. |
When a solution of oxyheemoglobin is treated with ferricyanide of
potassium, a volume of oxygen is given off corresponding with the dissoci-
able oxygen of the oxyhzmoglobin molecule, and the solution becomes
brown, its spectrum showing a characteristic band in the red in addition
to other bands (fig. 51). The brown substance is called methemoglobin.
It contains the same amount of oxygen as oxyhemoglobin, but more
firmly combined, and attached in all probability to a different part of
the molecule. Haldane suggests that in the case of oo. the
dissociable oxygen is in the molecular form i and in methemo-
ee aes ee
Cid thle inital
THE BLOOD. 161
globin the same oxygen is combined as two separate atoms, HDC The
addition of potassium ferricyanide to a solution of oxyhemoglobin
probably leads to the removal of the dissociable oxygen, and then re-
oxidises the hemoglobin into the more stable form of methemoglobin.
When ammonium sulphide is added to a solution of methzemoglobin,
the colour changes to red and later to purple. The intermediate red
stage gives a spectrum corresponding with that of oxyhemoglobin, the
purple product is hemoglobin, and this yields oxyhemoglobin when
it is shaken with air. By means of hydrazine hydrate it can be shown
that methemoglobin contains less dissociable oxygen than oxyhemo-
globin. The latter compound, when treated with hydrazine, gives off a
volume of nitrogen corresponding with the volume of dissociable oxygen.
HbO, + N,H,. H,0 = N,+3H,0 + Hb.
When methemoglobin is treated in the same way it never yields more
than half the amount of gas given off from a similar weight of oxy-
hemoglobin (Buckmaster). The explanation of this fact is still obscure.
If a solution of hemoglobin (or oxyhemoglobin) is warmed with an
acid or an alkali, the globin is converted into metaprotein, and hematin
is set free. In the pure condition hematin is a dark brown or black
amorphous substance, insoluble in water, soluble in acids or alkalies.
In acid solution its spectrum shows, besides other absorption bands, a
characteristic band in the red, nearer the red end of the spectrum than
that given by methemoglobin (fig. 51). The spectrum of the alkaline
solution exhibits a rather faint band just to the red side of the D line.
On the addition of ammonium sulphide, alkali hematin is converted
into reduced alkali hematin or hemochromogen, the spectrum of which
shows two absorption bands in the green, some distance to the violet
side of the D line, the band nearer D being the more distinct of the two
(fig. 51). As these bands can be seen in extremely dilute solutions,
the formation of hemochromogen constitutes a delicate spectroscopic
test for blood pigment.
As has already been said, hematin contains the iron of the Hb
molecule, and it has had the formula C,,H,,N,FeO, assigned to it. It
forms a compound with hydrochloric acid, aitccatiuride of hematin
or hemin, which is easily obtained by heating blood with glacial acetic
acid in the presence of sodium chloride. Hemin occurs in dark brown
rhombic erystals (fig. 52), and its formation is utilised as a medico-
legal test for blood. In the reduced condition, hematin will recombine
with globin to form hemoglobin or a substance indistinguishable from
II
162 ESSENTIALS Of PHYSIOLOGY.
hemoglobin, provided the globin has not been destroyed by the re-
agents used to dissociate the hemoglobin molecule.
If hematin (or hemoglobin) be treated with a strong mineral acid,
iron-free hematin or hematoporphyrin,
xy C,,H,.N,0,, is formed. Acid solutions
of this substance show a spectrum
f “Kk * with two absorption bands, one on
, \ 4 os A either side of the D line, that to the
4. 7 ¥ N ie red side being the narrower (fig. 51).
"A 2 ™~ .;~ The spectrum of alkaline solutions is
/ wf } somewhat similar to that of methe-
ra Pa a i 1, moglobin (fig. 51). “Heematoporphyrin
.
win 7S Tart 4X \\- Pp : ‘ ,
Pay fk y occurs occasionally in the urine in
+ sulphonal poisoning, its spectrum in
of] sa. such cases being of the alkaline type.
. Two similar substances are found in
Tre ei (Droyer.) Brot, Qaan’s the body, hematoidin in old blood
Anatomy. clots, and bilirubin, the primary bile
pigment ; they are formed from hemo-
globin and are said to be identical, each containing one atom less of
oxygen than does hematoporphyrin.
THE ESTIMATION OF HAMOGLOBIN.
The estimation of the amount of hemoglobin contained in a sample
of blood is usually carried out by a colorimetric method. The apparatus
used is called a hemoglobinometer, and consists of two tubes, one of
which is sealed and contains a standard dilution of ox blood, the hzmo-
globin of which has.been converted into carboxyhemoglobin. A little
distilled water and a measured quantity of the blood to be examined
are placed in the second tube, which is graduated. The blood, laked
by the water, is exposed to coal gas, and in this way the hemoglobin
is converted into carboxyhemoglobin, It is then diluted with distilled
water till the tints of the two tubes are alike, and the level of the fluid
in the graduated tube is read. If the latter is at the figure 100, the
amount of hemoglobin is said to be normal or 100 per cent. If it is
over 100, the blood is abnormally rich in hemoglobin ; if below 100, it
is deficient in hemoglobin. The figure 100 per cent. is purely empirical,
and the amount of hemoglobin actually present in blood is normally
about 14 per cent.
If the number of red corpuscles in the blood is ascertained at the same
time, the value of the heemoglobin content of each corpuscle can be stated.
Thus if the number of corpuscles is the normal five millions per cubic
—
teen ee are ee
THE BLOOD. 163
millimetre and the hemoglobinometer gives a reading of 50 per cent.,
each corpuscle only contains half the normal amount of hemoglobin.
THE ORIGIN AND FATE OF RED BLOOD CORPUSCLES.
(1) In early embryonic life red blood corpuscles are formed in
areas, known as ‘blood-islands,” lying in the area vasculosa of the
blastoderm. The blood-islands lie between the mesoderm and the
entoderm, and are said to be derived from the latter. They consist
of branched cells which unite to form a syncytium, their nuclei mean-
mv l em! e meo
meg m m @€ am’ e”
Fie. 53.—Red marrow of young rabbit. Magnified 450 diameters. (From
Schifer’s Lssentials of Histology. )
é, erythrocytes; e’, erythroblasts; e”,a coloured cell undergoing mitotic division; 7, a poly-
morphonuclear "leucocyte ; mM, ordinary myelocytes; m’, myelocytes undergoing mitotic
division ; 0, an eosinophile myelocyte; meg, a giant- cell or megakaryocyte.
while dividing and each new nucleus becoming surrounded by proto-
plasm containing hemoglobin. The coloured cells thus formed are
known as erythroblasts, and are the red corpuscles of the embryo.
They multiply by division. Jn later embryonic life similar nucleated
coloured cells, or erythroblasts, are found undergoing division in the
sinus-like blood-vessels of the liver, and also in the pulp of the spleen.
Non-nucleated erythrocytes, like those developed in post-natal life, are
formed inthe embryo in connective tissue. The connective-tissue cells
become coloured by the formation of hemoglobin, and the coloured
protoplasm is subdivided into a number of dises, or erythrocytes, which
7
164 ESSENTIALS OF PHYSIOLOGY.
lie free in the hollowed interior of the cell. Adjacent cells have
meanwhile become united to form a syncytium, and the hollows
become continuous‘ along the connecting RANCH es, so that a system
of blood-vessels is formed. :
(2) In post-natal life nucleated red corpuscles or sdeh ectaaatn are
found in the red marrow of bone (fig. 53). These are constantly
undergoing mitotic division; the cells thus formed lose their nuclei
by atrophy or extrusion and pass into the blood capillaries, which
in the marrow probably have incomplete walls. Nucleated red cor-
puscles may pass into the blood stream after birth in certain diseases
in which a rapid destruction of blood corpuscles is taking place.
The duration of the existence of a single red corpuscle is not known,
but there is evidence that large numbers of these cells are destroyed
daily to form the pigment of the bile. Fragments of broken-down
erythrocytes are also of constant occurrence in the cells of the spleen.
Moreover, the pigment of hair and of the coloured parts of the skin is
believed to be derived from hemoglobin. Blood corpuscles are also
lost by accidental hemorrhage, in disease, and, in the female, by
menstruation. The deficiency brought about in all these ways, normal
or abnormal, is as a rule rapidly and completely made good by the
activity of the bone marrow, which, in post-natal life, is the only source
of the red corpuscles. When an unusually large and rapid formation
‘of red corpuscles is required, for instance after severe hemorrhage, the
red marrow increases in amount, and replaces the yellow marrow to
some extent. ;
THE WHITE BLOOD CORPUSCLES.
The distinguishing feature of the polymorphonuclear leucocytes is
their power of ameboid movement, a power which is shared by the
eosinophile cells, and to a much less extent by the lymphocytes. The
leucocytes are able to make their way through the capillary walls
between the epithelial cells and wander out into the tissue fluids of the
body. They are especially susceptible to certain chemical stimuli
(chemiotaxis), and are found to concentrate in large numbers round
various chemical substances placed in their neighbourhood. For
example, they appear in force where pathogenic bacteria are active, and
serve a useful purpose in surrounding and destroying these germs, and
thus constitute an important protective mechanism for the body. When
the leucocytes fail to overcome and ingest the bacteria, they may them-
selves be destroyed by the bacterial toxins. The same function of the
removal of useless or harmful material is shown in other ways. For
example, the removal of the tail of the tadpole is effected by leucocytes,
fe alta Maen Si dia
THE BLOOD. 165
and in the mammalian body dead cells and organic foreign substances,
such as buried catgut ligatures, are ingested by the same cells. On.
account of this property of ‘eating up” bacteria and dead matter, poly-
morphonuclear leucocytes are included under the term phagocytes.
The leucocytes consist largely of proteins, cell-albumin, cell-globulin,
and nucleo-protein; they also contain a little glycogen, with some
neutral fats, and lecithin and cholesterol. The inorganic salt’ which
is most abundant in their composition is, as in the case of the red
corpuscles, potassium phosphate.
In the embryo leucocytes are derived from cells which resemble the
erythroblasts but are colourless. In post-natal life the polynuclear
and eosinophile cells, as also the basophile cells when they occur, are
derived from the special cells of red bone marrow called myelocytes ;
the lymphocytes are derived from lymph glands and lymphoid tissue
generally, including that of the Malpighian bodies of the spleen. The
nuclear changes which accompany cell division can be seen in definite
areas, known as germ centres, in the lymphoid tissue of the lymphatic
glands and elsewhere.
The condition called Jlewcocytosis, or increase of the number of
leucocytes in the blood, occurs normally during the digestion of a
protein meal. It also takes place in many infective diseases, being
accompanied by overgrowth of the red marrow, in which the poly-
morphonuclear cells are formed. The increase in the number of
polymorphonuclear leucocytes in the blood is part of the process by
which the body resists and overcomes infection by micro-organisms.
THE BLOOD PLASMA.
When blood is shed, it rapidly becomes viscid and in a few minutes
sets to form a clot. It is therefore necessary to use means to retard
or prevent clotting in order to obtain plasma for examination and
analysis. The various methods which are used for this purpose will be
described in connection with coagulation. .
Plasma is a pale yellow fluid, and has a specific gravity of about
1030, considerably lower than that of blood asa whole. The red
corpuscles have a specific gravity of about 1090, and therefore sink
if blood which is prevented from coagulating is allowed to stand. It
is found that the proportion of H and OH ions in plasma or blood is
about equal, and therefore its reaction is neutral. It must be noted, ©
however, that although blood gives a neutral reaction with phenol-
phthalein, it is alkaline to litmus, because litmus acid is strong enough
‘to displace the acid from sodium bicarbonate, which is an important
constituent of plasma, and to combine with the sodium.
166 ESSENTIALS OF PHYSIOLOGY.
On analysis, plasma is found to contain a large number of sub-
stances, some of which, particularly fibrinogen, appear to be essential
constituents of the plasma itself, some are food stuffs being conveyed
to the tissues, some are waste products being carried to excretory
organs, and others are hormones, enzymes, and bodies of like nature.
A list of the principal constituents is given in the following table :—
The Composition of Biood Plasma.
Water, 92 per cent.
Proteins—serum albumin, serum globulin, fibrinogen—6 to 8 per cent.
“ Dextrose, 0:15 per cent. |
Neutral fats.
Urea (0°02 to 0°05 per cent. as lecithin, cholesterol, lactic acid, and
other bodies.
Hormones.
Enzymes—lipase, etc.
Inorganic salts—chlorides, sulphates, phosphates, and carbonates of
sodium, potassium, calcium, magnesium, and iron.
Pigment and aromatic substances.
Gases—oxygen, carbonic acid, and nitrogen.
The Proteins of Plasma.—lIf an equal volume of a saturated solution
of sodium chloride is added to plasma, and the mixture is allowed to
stand, a sticky white precipitate separates out, consisting of fibrinogen.
This substance is a globulin, and the precipitate may be dissolved in
weak salt solution. Fibrinogen exists in much smaller quantity than
the other proteins, forming about 0°3 per cent. of the plasma. If
plasma is allowed to clot, a comparatively insoluble, stringy substance
called fibrin is formed, and, if this is removed, the fluid which remains
is plasma minus fibrinogen, and is called serum.
When serum is treated with an equal volume of a saturated solution
of ammonium sulphate, a precipitate of serwm globulin is obtained.
This precipitate is found to be a mixture of two substances, one of
which, euglobulin, is a true globulin, while the other, pseudo-globulin,
resembles albumin in being soluble in distilled water. If serum from
which serum globulin has been removed is saturated with ammonium
sulphate, a further precipitate of serwm albumin is obtained. The
filtrate from this contains no other protein. Fibrinogen coagulates at
about 56° C., serum globulin at 75°, and serum albumin at a slightly
higher temperature.
Although albumin and globulin can be separated from serum by
chemical methods, there is reason to believe that in the serum itself
these two substances are combined to form one—serwm protein.
— SC CU
pen, 4 "
ah he
THE BLOOD. 167
The Osmotic Pressure of Blood Plasma.—It has already been pointed
out that the osmotic pressure of the plasma is the same as that of
the corpuscles, so that it is also the same as that of the blood
as a whole. It has been found by experiment that the proteins of
plasma have a slight osmotic pressure, but the osmotic pressure. is
chiefly due to the inorganic salts. One method of ascertaining the
osmotic pressure of blood is to determine how much lower its freezing
point is than that of water, and in the case of blood it is found
to be 0°56° C., which is equivalent to that of a 0°9 per cent. solution
of sodium chloride.
If the osmotic pressure of the blood is higher than that of the
tissues, water will pass from the tissues into the blood in the capillaries,
and salts will diffuse from the blood into the tissues. If the osmotic
pressure of the blood is lower than that of the tissues, the reverse
processes will occur, water passing from: blood to tissues, and salts from
tissues to blood. This interchange is an important factor in the main-
tenance of the balance between the intake and output of water and salts.
PROTECTIVE AND OTHER SUBSTANCES IN THE PLASMA.
A large number of substances, when introduced under the skin or
directly into the circulation (but not when given by the mouth), give
rise to the formation by the tissues and the setting free in the blood-
stream of products which tend to destroy or to precipitate the substance
introduced, or to neutralise its action. The products thus formed are
called antibodies, those which excite their formation being called
antigens. Antigens are colloidal, and almost any protein, including
harmless bodies such as egg-white and caseinogen, can act as an
antigen. Crystalloid substances of small molecular weight, such as
sugar, seem unable to give rise to antibodies.
If a little human blood serum is injected into a rabbit on several
occasions at intervals of a week, the blood serum of the rabbit acquires
the power, when tested in vitro, of precipitating the proteins of human
serum, but not those of the serum of other animals. The substance
thus formed in the rabbit’s blood is called a precipitin. If the rabbit
is injected with sheep’s serum, the precipitin formed will precipitate
sheep’s serum in vitro, but not that of any other animal. The precipitin
acts, therefore, only on the serum of an animal of the same species as
that from which blood is taken for injection into the rabbit ; and the
reaction is said to be specific. Since this reaction is not only one of
the most delicate known tests for the presence of blood, but also makes
it possible to ascertain the species of animal from which the blood was
o
‘168 ESSENTIALS OF PHYSIOLOGY.
derived, it has been used in medico-legal cases to ascertain whether
blood, for example on clothes or weapons, is of human origin or not.
Again, the poisons (toxins) formed by bacteria give rise, when
introduced into the body, to antibodies which neutralise the toxin.
If, for example, a minute amount of diphtheria toxin is injected at
intervals into an animal, the latter forms antitoxin in considerable
amount. ‘This antitoxin is able to neutralise diphtheria toxin, and such
an animal will now survive the injection of a huge dose of toxin, many
times larger than that which would previously have killed it ; and it
is said to be tmmune to that toxin. In this example the toxin and
antitoxin combine directly with one another. In other cases, however,
the antibody which is formed does not itself destroy the antigen, but:
forms a link between the antigen and a substance present in normal
serum and known as complement; the complement, thus linked on to
the antigen, is able to destroy it. Antigens of this kind include
bacteria, red blood corpuscles and tissue cells, and the antibodies are
called lysins. The formation of hemolysin (p. 157) is an example of
this process. :
The capacity to form antibodies, which can destroy or neutralise
bacteria and their toxins, is one of the fundamental means by which
human beings are enabled to resist or to recover from diseases of
bacterial origin.
Anaphylaxis.—If a small dose of an antigen, such as egg-white, is
injected into an animal, and after an interval of sixteen days or more,
a second, even smaller dose is given, the animal becomes extremely ill
and may die in a few minutes. In guinea-pigs, which are particularly
sensitive, there is a marked fall of blood pressure, extreme constriction
of the bronchioles, and convulsions. This hypersensitiveness of an
animal to a second dose of an antigen is known as anaphylaais.
The Reaction of the Blood.—The acid characters of a substance
such as hydrochloric acid in solution are due to the presence in it of
free hydrogen ions (H ions); similarly the alkaline characters of an
alkali such as caustic potash depend upon the presence of free hydroxyl
ions (OH ions). Ina perfectly neutral solution, the two kinds of ions
are present in equal amount. In any aqueous solution, whatever its
reaction, the product of the proportion of H and OH ions present is a
constant figure, but in an acid solution the H ions will be in excess,
whereas in an alkaline solution they will be relatively few as compared
with the OH ions. It is customary to express the reaction of a fluid
in terms of its concentration in H ions, this being indicated by the
formula C,. In an acid solution the relative concentration of H ions
is large, whereas in an alkaline solution it is small.
ee ee te sae
ergot
a A ta em em
~ >
han 2 otha a —_
“Aas
- en oe
TE LE a PS VS ees ictialiy eS ale epee oe
TR Tey eee
THE BLOOD. 169
The reaction of blood to litmus is alkaline, but when it is deter-
mined accurately in terms of H ion concentration, it is found to be-
almost the same as that of distilled water. Under various conditions
the reaction may alter slightly, and these changes produce marked
physiological effects in the body, although they are usually too slight to
affect an ordinary indicator, such as litmus. Further, the presence in
blood of proteins and phosphates makes it possible for a considerable
amount of acid or alkali to be added to blood without any appreciable
change being produced in the H ion concentration. The reason is that
the acid or alkali thus added combines with proteins or phosphates to
form compounds which do not undergo ionic dissociation, and therefore
does not alter the concentration of H ions, by which the reaction of the
blood is ultimately determined. For example, Na,HPO, can. be partly
converted into NaH,PO, on the addition of acid, with little or no
alteration in the number of free H ions present in the solution.
THE COAGULATION OF THE BLOOD.
When blood is shed, it becomes more viscid, and within three to
ten minutes it begins to set into a jelly-like clot. The clot gradually
contracts, expressing a yellow fluid, the serum, as it does so; and
within ten to forty-eight hours the process results in a shrunken, firm
clot floating in the expressed serum. If coagulation has taken place
slowly, so that the corpuscles have had time to settle, the upper part of
the clot will be paler than the deeper part, because the lighter
leucocytes do not sink so quickly as the heavier red cells.
If a drop of blood be placed on a slide and covered with a cover-
slip, the process of clotting may be observed microscopically. It. is
found that the red corpuscles become aggregated into rouleaux, and
that between the aggregations delicate threads of fibrin make their
appearance. Clotting thus consists in the formation of a meshwork
of threads of fibrin, entangling the corpuscles; and the subsequent
shrinking of the clot is due to the contraction of the newly formed
fibres. .
Fibrin may be obtained in quantity by whipping a large volume of
freshly shed blood with a bundle of twigs, when the fibrin adheres to
the twigs as it forms. Blood treated in this way will not clot
subsequently, and is spoken of as defibrinated blood. The fibrin, when
washed free of blood pigment, is a white, stringy substance, easily |
stretched and possessing considerable elasticity. It. is insoluble in
water and in dilute salt solutions, but dissolves slowly in 5 per cent.
sodium chloride. It swells up and slowly dissolves in 0°2 or 0°4 per
cent. hydrochloric acid, with the formation of acid metaprotein.
170 ESSENTIALS OF PHYSIOLOGY.
The essential change in the coagulation of blood is the conversion
of fibrinogen into fibrin. The former substance is no longer present in
defibrinated blood or in serum. The process of clotting seed therefore
be represented diagrammatically in this way :
Plasma __Corpuscles
\
Fibrin
v Vv
Serum Clot.
When fibrinogen is freed from other substances by repeated precipi-
tation, a solution of the pure substance is found to have lost the
property of spontaneous coagulation. If, however, some blood serum
is added to such a pure solution, clotting will occur. It is clear,
therefore, that at least two substances are necessary for the formation
of fibrin, and that one of these is contained in blood serum. If twenty
volumes of alcohol are added to one volume of serum, a precipitate of
serum proteins is formed, and becomes insoluble in water if allowed to
remain under alcohol for some days or weeks, If this precipitate be
then dried and extracted’ with water, the solution so obtained will, if
added to a solution of fibrinogen, cause the latter to clot. The watery
extract contains a substance of unknown composition, which has been
called thrombin.
We therefore find that the formation of fibrin is due to the inter-
action of fibrinogen and thrombin; and, further, it has been shown
that a combination of the two bodies takes place, because, if excess of
fibrinogen be present, the amount of fibrin formed is proportional to
the amount of thrombin added to it. This latter fact is opposed to the
theory formerly held that thrombin belongs to the group of ferments.
The untenability of the older theory is further shown by the discovery
that the activity of thrombin is not permanently destroyed by the
action of heat. Moreover, fibrin can be partly broken down, thrombin
being set free, by treating it for some time with 8 per cent. .sodium
chloride solution.
Thrombin itself is not contained in the blood-stream of the living
animal. If blood is drawn directly from a blood-vessel into alcohol,
it contains no thrombin, so that the latter body must be produced after
the blood is shed. It has, in fact, been shown that it is derived from
a precursor, which has been called thrombogen, by the combination of
the latter substance with calcium salts. If freshly shed blood is
eat at ws
a aes. a ee eee ee ‘>
v
THE BLOOD. 171
mixed with potassium oxalate solution in such quantity that the
mixture contains 0-1 per cent. of potassium oxalate, the calcium salts —
of the plasma are precipitated as calcium oxalate, and the blood will
not clot. The subsequent addition of calcium chloride is followed by
coagulation. But if oxalate serum is added to oxalate blood, clotting
will take place, because thrombin is already present in the serum
before the potassium oxalate is added.
The formation of thrombin from thrombogen and calcium salts is
brought about, or at least facilitated, by an activating substance called
thrombokinase, which is derived in mammals mainly from the blood
platelets. If oxalate plasma from a mammal is allowed to stand for two
or three days on ice, a precipitate of platelets collects at the bottom of
the vessel. The plasma still contains thrombogen, but will no longer
coagulate on the addition of lime salts. It will clot, however, if some
of the platelet precipitate or an extract of an animal tissue be added
to it along with the lime salts, the extra factor, obtained from the plate-
- lets or tissues, being thrombokinase. Again, the blood of birds contains
no platelets, and will not clot if it is drawn directly from a blood-vessel
without contact with the tissues. On the other hand, if it is allowed to
flow over the adjacent tissues on its passage from the vessel, or if a
little tissue extract is added to it, it will coagulate readily. Thrombo-
kinase is therefore present in nearly all the tissues of the body as well
as in the platelets, and this wide distribution facilitates the protective
clotting of the blood which takes place on wounded surfaces. The
leucocytes have also been supposed to discharge thrombokinase when
blood is shed.
The facts described above in connection with the subject of the
coagulation of the blood are generally accepted, but the exact interpreta-
tion of them put forward by different authors, as well as the nomenclature
applied to the different substances concerned, varies somewhat.
The factors. concerned in coagulation may be diagrammatically sum-
marised as in the following table :—
Thrombogen Thrombokinase Lime salts Fibrinogen
Rs
Re . fs
ec /
y
Thrombin
Fibrin,
*
172 ESSENTIALS OF PHYSIOLOGY.
Thrombogen, lime salts, and fibrinogen exist normally in blood.
When blood is shed, thrombokinase, derived from blood platelets or
from the tissues, brings about the combination of thrombogen and lime
salts to form thrombin; thrombin then combines quantitatively with
fibrinogen to form fibrin. |
The Nature of Thrombin.—For many years thrombin was believed
to belong to the group of ferments, but it has been shown (1) that it is
not destroyed by boiling, and (2) that the amount of fibrin formed is
in proportion to the amount of thrombin present. (1) .Thrombin in
watery solution is not affected by boiling, but thrombin contained in
serum is inactivated by boiling. The inactivated substance in the
latter case, however, is reactivated by the addition of alkali. The
destruction of a ferment, on the other hand, at a temperature of 100° C.
is one of its most characteristic features. (2) It is also characteristic
of a ferment that, if the products of its activity be removed, it will in
time act upon all the substrate which is present. Thrombin, on the
other hand, is used up in the formation of fibrin, so that if there is
an excess of fibrinogen the surplus remains unchanged, unless fresh
thrombin is added.
If serum is allowed to stand for two or three days, the thrombin
contained in it disappears, being converted into a substance which has
been called metathrombin, and which can be reactivated by the addition
of either acids or alkalies, with subsequent neutralisation, or by the
occurrence of putrefaction. The same agencies, that is acids, alkalies,
or putrefaction, will break up fibrin, yielding thrombin.
The fact that blood does not clot in the vessels must be due to one
of two things: either an essential factor for coagulation is not present,
or clotting is prevented by some agent which inhibits the process.
With regard to the former possibility, fibrinogen exists in normal
blood, calcium salts are undoubtedly present, and thrombogen must be
a constituent of the blood in some form. If it be accepted that
platelets are only formed when blood comes into contact with foreign
matter, it would appear that the immediate precursor of thrombokinase
is not in existence in the circulating blood. This has already been
shown to be the case in birds, the blood of which contains no platelets.
There is, therefore, good ground for believing that the circulating blood
contains little or no thrombokinase. As regards the second possibility,
evidence has been brought forward by some observers to show that a
substance, anticoagulin or antithrombin, is a normal constituent of blood,
and that it is formed in the liver. It is stated, for example, that the
injection of thrombokinase or of thrombin into a blood-vessel results in
the production of such an antibody, so that the blood becomes defivient
——
ear
tn
=<
ae
THE BLOOD. 173
in coagulating power, just as a precipitin is produced as the result of
the injection of foreign protein. Hence it has been inferred that small
quantities of thrombin are continually being produced in the blood,
thrombokinase being set free by the breaking down of white blood
corpuscles and of the tissues generally. The presence of thrombin
leads to the formation of antithrombin in the liver, and in this way
clotting in the blood-vessels is prevented.
Conditions which Accelerate Clotting.—The rate of coagulation is
accelerated (1) by a certain degree of warmth, (2) by agitation of
the blood, and (3) by increasing the extent of the foreign surface
with which the blood is in contact. A practical application of the
latter fact may be made by applying a sponge or cotton wool to a
bleeding surface to aid in the arrest of hemorrhage. It is probable that
the foreign surface facilitates the formation and disintegration of
platelets with consequent. increased production of thrombokinase.
Intravascular Clotting.—The rapid injection into the blood-stream
of an animal of a quantity of a saline extract of such a cellular organ
as the thymus or a lymph gland, leads to the coagulation of the blood
throughout the whole vascular system. This result is generally ascribed
to the presence of nucleo-protein in ‘the extract. Small quantities of
a similar extract, slowly injected, have an opposite effect, rendering the
blood incoagulable. The difference in the results, according to the
_ quantity of extract injected, has not been satisfactorily explained.
Intravascular clotting is also produced by the injection of thrombokinase
or of snake venom, but not by moderate quantities of thrombin.
+ Conditions which Retard or Prevent Clotting.—These may be
classified as—
(1) Prevention of contact with a foreign surface.
(2) Removal of one or more of the substances concerned in the '
formation of fibrin.
(3) Interference with the interaction of the substances concerned
in the formation of fibrin.
(4) The use of an anticoagulin, or the production of antithrombin
by the injection of certain substances into the blood-stream
before the blood is shed.
(1) Coagulation is delayed if blood is shed into a vessel the interior
of which is smeared with grease of any kind. It is delayed for a
longer time if the blood is kept in contact with the lining of a blood-.
vessel. If a large vein containing blood is ligatured in two places
and the ligatured portion is excised, the blood in the vein, which is
then known as a “living test-tube,” may remain fluid for days.
(2) (a) Lime salts are precipitated by the addition of potassium
174 ESSENTIALS OF PHYSIOLOGY.
oxalate or of sodium fluoride to blood. (0) Sodium fluoride preci-
pitates not only calcium but also thrombogen, so that fluoride blood
will not clot on the subsequent addition of lime salts. (c) If one
volume of saturated solution of magnesium sulphate be added to four
volumes of blood, and the mixture be allowed to stand for twenty-four
hours, the thrombokinase is precipitated, and clotting ,will not take
place on dilution. For the first few hours after the addition of the
sulphate the clotting is merely retarded by the excess of salt, and the
blood will clot if it is diluted. (d) Sodium citrate may also be used
to prevent coagulation. It forms with calcium a double salt, calcium
sodium citrate, which is soluble, and in which the calcium is combined
with the acidic radical and does not become a free ion. Calcium will
not combine with thrombogen unless it is in the ionised condition.
(3) (a) Coagulation may be prevented by cooling freshly shed blood
to 0° C. (6) The addition to the blood of an equal volume of saturated
solution of sodium sulphate will prevent or delay clotting, but co-
agulation will take place when the mixture is diluted, showing that
the action of the salt is purely mechanical.
(4) Hirudin, a substance obtained by extracting the glands: in the
head of the leech, is an anticoagulin, and will prevent clotting either
if added to shed blood or if previously injected into a blood-vessel.
The injection of peptone will render blood incoagulable, and such blood,
when added to blood shed in the ordinary way, will prevent coagulation
of the latter; but peptone itself has no retarding effect on coagulation
when added to shed blood. From experiments such as these it is
clear that peptone has no direct action in preventing coagulation, but
that it leads to the production in the liver of an anticoagulin, which
is discharged into the circulating blood.
Coagulation Time.—The time taken for coagulation of a sample
of blood may be conveniently estimated by means of Dale and Laidlaw’s
coagulometer. This consists of a short capillary tube containing a
leaden shot. The finger is pricked and blood is run into the tube,
which is then immersed in water at a selected temperature, the ends
of the tube being closed. The tube is moved so as to keep the shot
rolling until the latter stops dead with the tube vertical. The time
between the appearance of blood on the finger and the stopping of the
shot is the coagulation time. It varies from one and a half minutes
at 40° C. to about eight minutes at 19° C. for normal blood.
Suttle wih —_—
THE BLOOD. 175
THE TOTAL QUANTITY OF BLOOD IN THE BODY.
The amount of blood in the body of an animal is ascertained by
collecting and measuring the blood obtained by bleeding the animal.
To the total thus obtained must be added the blood which does not
escape when the animal is bled. This must be dissolved out of the
tissues until no more colour is obtained ; the tint of the mixed washings
is compared with that of a known dilution of the blood, and in this
way the amount of dissolved blood is calculated and added to that
found by direct measurement. Two experiments of this kind have
been performed on the bodies of guillotined criminals, and from these
it was found that the blood in man forms one-thirteenth of the body
weight.
Haldane has devised a method for ascertaining the quantity of
blood in the living subject. This method is based upon the affinity
of hemoglobin for carbon monoxide. A small quantity of blood is
withdrawn and its oxygen capacity is estimated. The subject is then
allowed to breathe air containing a known amount of carbon monoxide,
which is absorbed by the blood. A certain proportion of the hemo-
globin of the blood combines with the carbon monoxide, forming
carboxyhzemoglobin, and thus reducing the oxygen capacity of the
blood to that extent. A small quantity of blood is again withdrawn
and its oxygen capacity estimated. If it is found in this way that
100 grams of blood contain 34, of the absorbed carbon monoxide, then
100 grams obviously form 3'5 of the total blood contained in the body.
By this method the total blood has been calculated to be one-twentieth
of the body weight, so that there is a discrepancy between the results
obtained by the two methods, though the latter method is probably
the more accurate.
CHAPTER VIII.
THE VASCULAR. MECHANISM.
SECTION LI.
THE life of the muscles, of the nervous system, and of every tissue of
the body depends upon their receiving an adequate supply of food and
oxygen; and one of the most important functions of the blood is to
convey oxygen and nutritive material to the tissues and to carry away
carbon dioxide and waste products which are formed by the tissues.
In order that this function may be carried out, the heart and blood-
vessels furnish the mechanism by which a constant circulation of the
blood throughout the body is maintained ; and by means of the central
nervous system the activities of this mechanism can be varied in
response to the ever-changing needs, either of the body as a whole, or
of its different parts. i :
The heart acts as a pump, and drives the blood along the arteries,
through the capillaries and veins and back to the heart. The actual
interchange of nutritive material and waste products between the
blood and tissues takes place solely through the walls of the capillaries,
and the entire circulatory mechanism is adapted to maintain the con-
ditions most favourable to this interchange.
THE HEART AND BLOOD-VESSELS.
The heart is a hollow muscular organ lying in the thorax between
the lungs, and slightly to the left of the middle line of the body. It is
conical in shape, the apex being directed downwards, and is divided by
a septum into right and left halves, which do not communicate directly
with each other. Each half consists of two chambers, an upper thin-
walled auricle (atrium) and a lower thick-walled ventricle. Into the
right auricle open the superior vena cava, bringing blood from the head
and upper limbs, the inferior vena cava, conveying blood from the rest
of the body, and the coronary sinus. The right auricle opens into the
right ventricle by an orifice guarded by a valve with three triangular
176
.
~~ oe
nM * i,
ee ee eee a eS ee
THE VASCULAR MECHANISM. 177
cusps, tricuspid valve, arising from the fibrous junction between the
auricle and ventricle and hanging down into the ventricle. The cusps
consist of connective and elastic tissue, and are so arranged as to permit
the flow of blood from auricle to ventricle, but not in the reverse
direction. From each cusp a number of tendinous threads, chorde
tendinew, pass to be attached to projections of the ventricular wall,
known as the papillary muscles. |
The right ventricle possesses two openings: (1)the auriculo-ventricular
just described, and (2) the opening into the pulmonary artery, which
conveys blood from the heart to the lungs. The latter opening is
provided with a valve having three semilunar cusps composed of strong
fibrous and elastic tissue. In the centre of the free border of each cusp
is a small fibrous nodule, the corpus Arantii, which serves to strengthen
the valve. When the valve is closed, the free borders of the cusps come
into contact with each other and are pressed together, thereby pre-
venting the return of blood from the pulmonary artery to the ventricle.
The general arrangement of the left side of the heart is very
similar to that of the right. Two pulmonary veins from each lung
open into the left auricle. The auriculo-ventricular valve possesses
only two cusps; it somewhat resembles a bishop’s mitre, and is known
as the mitral valve. The cusps, like those of the right auriculo-
ventricular valve, are connected by tendinous cords with the papillary
muscles and the wall of the left ventricle. Opening out of the left
ventricle is the aorta, which is provided with a valve having three
semilunar cusps similar in structure to those of the pulmonary artery.
The cavities of the heart are lined by a smooth membrane, the endo-
- cardiwm, composed of delicate connective and elastic tissue covered by
flattened endothelial cells.
The substance of the heart consists of muscular tissue bound
together by connective tissue and supplied with blood from the coronary
arteries. It is arranged in sheets composed of fibres, which are built
up of short cylindrical segments or cells ; each cell has an oval nucleus
and shows an indistinct longitudinal and. transverse striation. The
fibres are branched, the branches of adjacent fibres uniting with one
another so that the heart muscle forms a continuous network of cells,
known as a gyncytiwm.
The wall of the auricles is composed of (a) superficial fibres common
to both auricles, and (b) deep fibres, both looped and annular, proper to
each auricle; the annular fibres form muscular rings around the open-
ings of the great veins. The auricles are joined to the ventricles by
strong fibrous rings, which encircle the auriculo-ventricular orifices, and
by a band of modified muscular tissue, known as the auriculo-ventricular
12
178 ESSENTIALS OF PHYSIOLOGY.
bundle, or bundle of Has, the importance of which will be considered
later. veloc
The muscular fibres of the ventricles are arranged in a very complex
manner. A superficial stratum runs in a spiral direction from the
fibrous rings uniting the auricles and ventricles to the apex of the
heart; here the fibres form a whorl and then ascend in the inter-
ventricular septum and on the inner surfaces of the ventricles to end
in the papillary muscles. Between the layers thus formed are deeper
fibres, most of which are arranged in an @-shaped manner, springing
from the papillary muscles of one ventricle and ending in the papillary
muscles of the other ventricle; they are united by muscular strands
with the layers of the superficial stratum. The muscle is so arranged
that, when contraction occurs, the cavities of the ventricles become
smaller. -
The wall of the left ventricle, which drives the blood through the
greater part of the body, is about three times as thick as that of the
right ventricle, which drives the blood only through the lungs; the
thin-walled auricles merely discharge their contents into the relaxed
ventricles. The capacity of the two ventricles is approximately the
same, amounting in each case to a maximum of 140 c.c., and is rather
larger than that of the auricles
The pericardium isa fibrous sac enclosing the heart, attached below
to the diaphragm and lined by flattened cells; where the great vessels
pass through it the epithelial layer is reflected and covers the surface
of the heart. The smooth inner wall of the sac is moistened by a little
lymph (pericardial fluid), and the movements of the heart are carried
out with hardly any friction. The pericardium serves to prevent over-
distension of the heart, when it is being filled by the inflow of blood
from the veins.
The Blood-vessels.—The arteries, which convey blood from the
heart to the capillaries, are thick-walled tubes made up of muscular and
elastic tissue. A medium-sized artery shows three coats—outer, middle,
and inner. The outer coat is composed of fibrous tissue. The middle
coat consists of smooth muscle fibres arranged circularly, and of yellow
elastic fibres. The inner coat consists of flattened endothelial cells
united edge to edge by a cement substance, some loose connective
tissue, and a thick elastic lamina next the middle coat. The middle
coat of the large arteries, such as the aorta, contains a larger propor-
tion of elastic tissue and a correspondingly small amount of muscle,
whereas- that of the small arteries (arterioles) is purely muscular.
The capillaries form a dense network round and among the tissue
elements in almost every part of the body, and consist of a single layer
in wae os
: Reo nies
oF ee ee ee a eae, eee ne ee
il Ba neines:
nebo
‘ “a —
di sacri Sm
THE VASCULAR MECHANISM. 2179 %
of flattened cells united by cement substance. Their calibre varies
slightly, but the average diameter is little wider than that of a red.
corpuscle,
The veins also possess three coats, but their walls are thinner and
contain much less muscular and elastic tissue than the arteries, and
are strengthened by the presence of a considerable amount of fibrous
tissue, especially in the outer coat. Many veins have -valves consisting
of fibrous tissue covered on each surface by endothelial cells, and so
arranged that they allow blood to flow towards the heart, but prevent
any flow in the opposite direction.
The arteries remain patent when divided, and a high internal
pressure is required to distend their thick muscular and elastic walls,
whereas the thin-walled veins collapse when opened, and become
distended under a very low pressure.
THE COURSE OF THE CIRCULATION.
The heart beats rhythmically from seventy to seventy-two times a
minute. The. beat consists of the contraction of the auricles, followed
almost immediately by that of the ventricles, and is succeeded by a pause,
during which -the-whole-heart-is-eompletely relaxed. The contraction of
the auricles and ventricles is spoken of as auricular or ventricular systole,
the period during which the heart is relaxed being called the diastole.
At each beat the ventricles expel blood into the aorta and pulmonary
artery, from which it is distributed by the former to the body as a whole
and by the latter to the lungs.
The blood entering the aorta from the left ventricle is conveyed by
the arteries arising from it to the capillaries of the various organs of
the body, with the exception of the lungs. From these organs it is
returned by veins, which unite with each other, eventually forming the
ven cavee, which open into the right auricle. The blood passes from
the right auricle into the right ventricle, from which it is forced into
the pulmonary artery; it then flows along the subdivisions of this
artery through the pulmonary capillaries into the pulmonary veins
(two for each lung), and thence into the left auricle. From the left
auricle the blood enters the left ventricle and is again sent out into the
aorta (fig. 54). |
In the abdomen, the blood passes through a double set of capillaries,
The veins which receive blood from the digestive tract and spleen unite
to form a single large vein, the portal vein, which on reaching the liver
again breaks up into capillaries; these open into itne hepatic. veins
which join the inferior vena cava.
180 ESSENTIALS OF PHYSIOLOGY.
The complete circulation consists therefore of two parts, the one
from the right side of the heart through the lungs and back to the left
side of the heart, known as the pulmonary or lesser circulation, the
other from the left side of the heart throughout the rest of the body
and back to the right auricle, forming the
systemic or greater circulation. As the
blood traverses the lungs it takes up
oxygen, becoming scarlet in colour (arterial
blood). Arterial blood is found in the
pulmonary veins, in the left side of the
heart, and in the systemic arteries. Dur-
ing its passage through the capillaries in’
the various tissues the blood loses much
of its oxygen, receives carbonic acid, and
becomes darker in colour (venous bleod).
The venous blood’ is carried along the
systemic veins to the right side of the
heart and into the pulmonary artery to
take up a further supply of oxygen from
the lungs.
——> SYSTEMIC VEINS (VEN CAVA)
THE BLOOD PRESSURE.
When an artery is cut across, the
blood spurts out from its central end (the
end nearest the heart) with considerable
force for some distance; and, evidently,
the blood contained in the arteries is exert-
ing a high pressure upon the vessel walls. When a vein is,divided, the
blood escapes from its peripheral end in a slow steady stream.
Fic. 54.—Diagram showing the
course of the circulation.
The arterial blood pressure can be measured by allowing the blood —
to flow into a vertical glass tube tied into the central end of an artery,
The blood will be seen to attain a height of three or four feet or more,
and to show oscillations corresponding with each heart beat. The
method is unsatisfactory, partly because the clotting of the blood in
the tube soon brings the experiment to an end, partly because, in a
. small animal, the loss of blood from the body may interfere with. the
circulatory mechanism. It is customary, therefore, to place in the
artery a small cannula, filled with half-saturated sodium sulphate solu-
tion to delay clotting, and to connect the cannula with one limb of a
U-shaped tube (“manometer”) containing mercury, which is so heavy
that a column of mercury 100-150 mm. high suffices to counter-
OS le eee
a.
THE VASCULAR MECHANISM. — 181
balance the pressure of the blood in: the artery, and to prevent
the blood escaping into the cannula. A writing point .attached to
a float resting on the mercury in the other limb of the manometer
can be used to record the pressure on a moving blackened surface
(kymograph). |
The following method is used (fg. 55). An artery (such as the
carotid or femoral) is exposed in an anesthetised animal, and the flow
of blood is shut off by a clip. The artery is ligatured about 2-3 cm.
beyond the clip, opened between the clip and the ligature, and a
cannula containing a half-saturated solution of sodium sulphate is tied
into it. This cannula is connected by thick rubber tubing with a ~
i \
rete vari
Fic. 55. Apparatus for taking a blood-pressure tracing,
A, cannula inserted in an artery ; B, pressure bottle; C, mercurial manometer.
bottle containing a half-saturated solution of sodium sulphate, and with
one limb of the manometer. By raising the bottle the cannula and
connecting tubing can be filled with sodium sulphate solution under
such pressure that the column of mercury in the limb to which the
float is attached rises from 100 to 150 mm. higher than the column in
the limb connected with the artery. The connection between the pres-
sure bottle and the manometer is then shut off by a screw-clamp, and
the clip is removed from the artery. The column of mercury rises or
falls slightly until the pressure counter-balances that of the blood, and
the writing lever remains at a constant level, except for slight oscilla-
tions with each heart beat and with the respiratory movements (fig. 56).
The difference in height of the columns of mercury in the two limbs
represents the mean arterial blood pressure. ¢
182 ; ESSENTIALS OF PHYSIOLOGY.
The pressure in the large veins may be determined in a similar
manner, except that, as the pressure is low, the whole manometer is
usually filled with sodium sulphate solution. Observations made in
this way show that whereas in a systemic artery the mean blood
pressure may vary from 100 to 140 mm. Hg and alters slightly with
Fic, 56.—Blood-pressure tracing.
each heart beat and respiratory movement, the venous pressure amounts
only to a very few mm. Hg and is not affected by the heart beat. It
is found that the arterial pressure is highest in the aorta, rather less
in the medium-sized arteries, and that there is an abrupt fall of pressure
in the arterioles ; in the capillaries the pressure is low, and finally there
BPe—--
O | O
H a O ae ERE, |» i
LV RA
Fic. 57.—Scheme of blood pressure. (Starling’s Principles of Physiology. )
LV, left ventricle ; A, arterioles ; C, capillaries; V, veins; RA, right auricle ;
OO, line of no pressure; BP, blood pressure.
is a steady fall of pressure in the veins, until, in the large veins near
the heart, it may actually be negative, that is, less than the atmospheric
pressure. These differences of pressure are diagrammatically repre-
sented in fig. 57. The pressure in the pulmonary artery varies from
20 to 25 mm. Hg, and on the average is about one-sixth of that in
the aorta or its main branches.
We may now consider the factors which are concerned in this
distribution of pressure, and in the conversion of the jerky flow
THE VASCULAR MECHANISM. 183
of blood in the arteries into a continuous flow in the capillaries
and veins.
They are (1) the beat of the heart, (2) the elasticity of the arteries,
and (3) the peripheral resistance.
The action of these factors is a purely mechanical one, and can be
reproduced in an artificial scheme such as is shown in fig, 58. A
reservoir R containing a coloured fluid is attached to a horizontal
rubber tube S T open at its other end, and a number of vertical glass
tubes open at the top are connected with this tube. As the fluid flows
from the reservoir, it rises in the vertical tubes to a height correspond-
ing with the pressure at that point, and the line A B joining the top of
the columns of fluid in the tubes shows that there is a uniform fall of
pressure along the rubber tube. If a screw clip is placed on the rubber
tube at C and gradually tightened, thereby introducing a resistance to
the flow of fluid along the tube, the pressure rises on the proximal side
and falls on the distal side of the clip, the pressure godine along the
tube being indicated by the dotted line A, D, B.
When the flow of fluid from the reservoir is made intermittent by
alternately compressing and releasing the connection between it and
the tube at short intervals, the fluid in the vertical tubes K, L, M, and
N, between the reservoir and the resistance at C, shows corresponding
oscillations in height, whereas in the tubes O and P, beyond ©, these
oscillations are absent, and fluid flows from the end of the rubber tube
in a steady stream.
In the body the reservoir is represented by the heart, which at each
beat sends into the aorta a certain quantity of blood (in man about
**
184 ESSENTIALS OF PHYSIOLOGY.
60 ¢.c.). The peripheral resistance is due to friction between the flow-
ing blood and the walls of the vessels, the amount of friction varying
inversely with the bore of the vessels and directly with the velocity of
the blood. The peripheral resistance caused by this friction is very
. large in the arterioles, which are numerous and of small bore, and in
which the blood flows rapidly. In the capillaries the blood flows so
slowly that, although their calibre is very minute, the resistance is
much less than in the arterioles; and in the large arteries it is com-
paratively slight.
With each beat of the heart an additional quantity of blood enters
the arterial system, and, if there were no peripheral resistance, an equal
quantity would instantly escape through the arterioles into the capillaries.
But the resistance offered by the arterioles is so great that when the
blood is forced during systole into the already distended arterial
system, there is not an immediate escape of a corresponding amount
from the arterioles into the capillaries. Most of the force of the heart
is expended in further distending the arteries in order to accommodate
this additional blood sent into them, and the pressure within them
rises. In the interval between two heart beats the distended arteries
shrink by virtue of their elasticity, thereby forcing blood through the
arterioles, and the pressure falls slightly. The result is that the flow
of blood along the capillaries and veins takes place both during and
between the heart beats as a steady stream, whereas blood enters the
aorta only during the heart beat, and: the flow along the arteries
is jerky.
When the amount of blood entering the arterial system during
systole is equal to that leaving it, partly during systole and partly
during diastole, the mean arterial pressure remains steady. Any
increase or decrease in the amount of blood entering the aorta at each
beat will tend to raise or lower the arterial pressure. Similarly, dilata-
tion of the arterioles will diminish the resistance to the escape of biood
into the capillaries and the arterial pressure will fall, whereas constric-
tion of the arterioles will produce a rise of pressure.
Although the driving force of the heart pump is sufficient to propel
the blood round the body and back to the heart, its action is normally
assisted (1) by the respiratory movements, which will be considered
later (page 283), and (2) by skeletal muscular movements, Every
muscular movement tends to squeeze blood along the veins ‘towards
the heart, any reflux being prevented by the valves with which these
veins are provided,
Mean Systemic Pressure,—When the heart ceases to beat the
arterial pressure falls, the venous pressure rises, and finally there is
~~. . . eee 2 ee
THE VASCULAR MECHANISM. 185
a uniform pressure throughout the vascular system, the height of which
will ‘depend upon the amount of blood present in the vessels. This
is called the mean systemic pressure. If more blood is added to the
vascular system, the mean pressure will rise, and vice versa. When
the heart starts to beat, it pumps blood into the arteries, which
become distended, because at first the outflow from the arterioles is
less than the amount of blood forced into the aorta; and the arterial
pressure rises. Asa result of the transference of blood from the veins
to the arteries the venous pressure falls below the mean pressure. It
follows, first, that under normal conditions the arterial and venous
pressures vary inversely with one another, and, secondly, that the
arterial, capillary, and venous pressures will all vary with the total
volume of blood in the vascular system, although the venous and
capillary pressures are those chiefly influenced.
BLOOD PRESSURE IN MAN.
Arterial Pressure.—The highest blood pressure occurring during
the cardiac systole is called the systolic pressure ; the pressure corres-
ponding with the end of diastole is the dzastolze pressure, the difference
between the two being called
the pulse pressure. The sys-
tolic pressure is measured by
means of a Riva-Rocci sphyg-
momanometer. This consists
(fig. 59) of a leather band
about four inches wide, inside . 4
which is a rubber bag com-
municating with a mercurial
manometer and _ connected
with a small pressure bulb.
Attached to the bulb is a
screw by which the bag can be put into communication with or shut
off from the external air. .
The band is fastened round the upper arm. The observer feels the
radial pulse with the fingers of one hand, while with the other he
squeezes the bulb and distends the bag with air, until the pressure is
just sufficient to obliterate the brachial artery and the radial pulse
disappears. When this occurs, the pressure in the mercurial manometer
is noted. The screw attached to the bulb is then gently turned, the
air slowly escapes, and the pressure falls; when the radial pulse is just
perceptible, the pressure in the manometer is again observed. The
mean between the two readings is the systolic arterial pressure. This
"Gy |
on,
Fie. 59.—Riva-Rocci sphygmomanometer.
(From Messrs Hawksley. )
186 ESSENTIALS OF PHYSIOLOGY. |
represents the pressure on the outer wall of the artery which exactly
balances the greatest pressure within the artery during systole, and at
which the lumen of the artery is just obliterated. |
The diastolic pressure can be approximately measured with the
same instrument by observing the height of the manometer when the
oscillations of the column of mercury with each heart beat are maximal.
When this happens, the pressure in the bag is just equal to that in the
artery at the end of diastole (diastolic pressure); the artery collapses
between the beats and then expands almost fully during systole. Thus
the lowest level of the manometer“Setween the beats gives a record
of the diastolic pressure ; and by measuring the diastolic and systolic
_ pressures, the pulse pressure, which is the difference between them, can
be determined. The systolic pressure in the healthy adult varies in
Sicsamel
(OWE iD
Fie, 60.—(Starling’s Principles of Physiology.)
the large arteries, such as the brachial, from 100 to 110 mm. Hg; it
becomes higher with increasing age, and at fifty, even in health, is
about 140 to 150 mm. Hg. It is temporarily raised during muscular or
mental work, and falls again during rest. '
‘Venous Pressure.—A flat rubber bag having a hole through the
centre of each flat surface (fig. 60) is placed over a peripheral vein
and covered by a glass plate, the junctions being made air-tight with
glycerol ; a tube leads from the bag to a manometer and to a pressure
bulb. Air is blown into the bag, the giass plate being firmly held in
position ; when the pressure reaches a certain height the vein collapses,
and the reading of the manometer represents the venous pressure.
The same method, a smaller bag being used, may be employed to
determine capillary pressure. The average capillary pressure is from
15 to 40 mm. Hg. The venous pressure varies from 8 to 10 mm. Hg
in the smaller veins ; in the large veins near the heart it is only 1 or
2 mm. Hg, and may be negative.
mun By ds sai Pais “
ee ee ee ee a Te ee Per = -
= s
eto ee
THE VASCULAR MECHANISM. . 187
VELOCITY OF THE BLOOD FLOW.
The average rate of flow in a river depends upon the pressure
gradient between its source and the sea; and in the same way the
actual velocity of the blood stream is determined by the driving force
of the heart pump, that is, upon the amount of blood expelled from it
at each beat. If the heart is beating feebly the velocity will be small,
whereas if the heart is beating strongly and rapidly, for example during
muscular exercise, the velocity may be considerable. These differences
are of importance, since the rate at which oxygen is carried from the
lungs to the various tissues of the body is largely determined by the
rapidity of the circulation.
The relative velocity of the blood flow in the arteries, capillaries,
and veins is determined solely by the total width of the channels
through which the blood is flowing. Since the same quantity of blood
has to pass in a given time through each cross-section of the bed of
the vascular system, it is obvious that the smaller the cross section
the greater must be the velocity of the blood flow. For the same
reason the water flows rapidly in a river where the channel is narrow,
and slowly where the channel widens out into large pools.
When an artery divides, each branch is smaller than the parent
artery, but the total cross section of the two branches is larger than
that of the parent artery. The total cross section of the vessels thus
increases with each branching, and in the capillaries it has been
estimated to be about 800 times as great as that of the aorta. The
sectional area of the veins gradually decreases as they unite to form
larger vessels, and that of the large veins entering the heart is approxi-
mately twice as great as that of the aorta. It has been found that.
whereas the average velocity of the blood in the large arteries is about
400-500 mm. a second, it varies from 4-1 mm. a_-second in the capil-
laries, and is from 200-250 mm. a second in the large veins.
In a small organ, such as the kidney or submaxillary gland, the
velocity of the flow of blood is also modified by local changes in the
arterioles. Dilatation of the arterioles lessens the resistance to the
flow of blood through the organ without affecting the general arterial
blood pressure, and since the resistance is lessened in that organ as
compared with other organs in the body, a short-cut is provided between
the arteries and the veins; hence the blood flows through the organ
with increased velocity and in increased amount. This result, which
furnishes an apparent exception to the general statement made above,
is only true when the organ is so small that alterations in the calibre
of its arterioles do not appreciably affect the general blood pressure.
188 7 ESSENTIALS OF PHYSIOLOGY.
Methods of measuring the Velocity of ‘the Blood Flow.—(1)
Ludwig’s Stromuhr.—This consists of two glass vessels A and B
connected at the top (fig. 61). On A is a mark c, the capacity of
the vessel below the mark being exactly known. The vessels are fixed
at their lower ends into a metal disc H, placed upon a similar dise N,
and capable of being rotated upon the latter through two right angles,
The openings a’ and 0’ in the upper disc fit exactly over those (a and 4)
in the lower disc; from these openings in the lower disc arise two
tubes F and G, The experiment is carried out as follows. A clip is
placed on an artery, which is then divided and connected at one end
Fic, 61.—1, Ludwig’s stromuhr ; 2, Diagrammatic representation,
with tube F, at the other with tube G.’ The tube and the vessel A, which
communicate with the proximal end of the artery, are filled with olive oil
up to the mark c, and the remainder of the apparatus is filled with
defibrinated blood. The blood is then allowed to flow through F into
A, thus driving the oil over into B and sending the defibrinated blood »
into the peripheral end of the artery. As soon as the blood leaving
the artery reaches the mark c, the disc H is turned rapidly through
two right angles, and the blood flowing from the artery now drives the
oil back into A. When the oil again occupies its original position, the
disc is once more rotated through two right angles. This process is
repeated as often as necessary, the experiment being carried on for any
desired period ; clotting of the blood can be prevented by the previous
,
oa ore, ae
f, = —
er ee, ee >;
Pe ee ee ee
ae hoes
ae Sake
throw an image of the menisci of the columns of*
-THE VASCULAR MECHANISM. 189
injection of hirudin into the animal. The diameter of the artery is
then measured. From these data the TeneHy of the blood flow can be
_ calculated by means of the formula
volume (passing through the stromuhr) per second
Velocity =
oloetes sectional area of blood-vessels.
If the capacity of the bulb up to the mark ¢ is 5 cc. and it was
filled six times in a minute, then the amount of blood passing through
the instrument would be 30 c.c. in one minute, or 4 ¢.c. in one second,
Supposing the diameter of the artery to be 2 mm., the sectional area is
rr’, and rate of flow can be calculated as follows :—
0 c¢.c. —_ 500 c.mm.
31416x12 31416
Many other instruments have been devised, of which the most
useful is the photohematachometer of Cybulski. This consists of two
vertical tubes united at the top, and opening
below into a horizontal tube, as shown diagram-
matically in fig. @2. The proximal end of an
artery is attached to the instrument at A, the
blood escaping at B into the distal end of the fi)
Velocity = = 159 millimetres per second.
artery. The blood will rise higher in the tube C
than in the tube D, the difference in height of the
two columns being directly proportional to the |[¢ D
velocity of the blood flow in the artery. A graphic
record is. obtained by allowing a beam of light to
fluid on to a moving photographic plate. To de-
termine the absolute velocity of the blood, the
instrument must be calibrated. It has the advan-
tage of giving not merely the average velocity of
the blood flow, but also the variations during ven-
tricular systole and diastole. In one experiment,
the velocity varied from 250 mm. per second during 74, B
systole to 127 mm. in diastole.
SS °©™aay
THE CIRCULATION IN THE CAPILLARIES. :
Fic. 62. — Diagram
On observing the flow of blood through the showing the prin-
‘ pais : ciple of Cybulski’s
small arteries and veins in the mesentery or web photohematacho-
of a frog, the red corpuscles are seen to occupy meter.
the central part of the vessels (axial zone), and to
be moving more rapidly than the peripheral layer of blood, in which are
found most of the leucocytes. The formation of the axial zone is due to the
Igo ESSENTIALS OF PHYSIOLOGY.
fact that the specific gravity of the-red cells.is higher than that of the
plasma. The lumen of the capillaries is so small that no axial zone
is present. The velocity of the blood flow in the capillaries can be
directly observed with the aid of an eye-piece micrometer by noting
the time taken by a red corpuscle to travel a given distance, and varies
from 0°5 to 0°38 mm. per second; and as the ayerage length of a
capillary is from 0'4 to 0°38 mm., any one corpuscle traverses the
capillary in one second. During this time, the interchange of oxygen
and of carbonic acid and of nutritive and waste material between the
blood and the tissues takes place through the capillary wall. The
capillary pressure is intermediate between that in the arteries and
veins, and since the capillaries open directly into the veins the
pressure in them is very easily influenced by a rise or fall in venous
pressure. |
If an irritant, e.g. dilute acetic acid, is applied for a moment or two
_ to the surface of the frog’s mesentery, the minute arterioles and the
_ capillaries soon become dilated, and the blood flows more rapidly through
them. Presently the leucocytes begin to adhere to the capillary walls,
and some of them make their way through the interstices between the
epithelial cells into the surrounding tissue. At the.same time, the
capillary wall seems to alter so as to offer more resistance to the flow of
blood ; and the flow slackens and may cease, although the capillaries
and arterioles are still dilated. After a time, the capillaries become
filled with a mass of red and white cells, much of the plasma passing
through the walls of the capillaries into the lymph. Sometimes the
red corpuscles are also forced through the. capillary walls, this being
called diapedesis. This series of events forms part of the process of
inflammation, which is defined as the response of the tissues to an
injury provided the latter does not cause death at once. The injury
may be mechanical or may be brought about by chemical substances,
including bacterial products.
THE TIME OF THE CIRCULATION. ~
One of the best methods for determining the circulation time is to
inject into one jugular vein a strong solution of methylene blue; the
jugular vein on the opposite side is exposed and allowed to rest on a
strip of white paper. The interval between the injection and the
moment at which the blue colour becomes visible in the opposite vein
is observed ; it varies from 15 to 2 0 seconds,
Another method consists in sending a compensated electric current
through a section of an artery, e.g. the left carotid, and through a galvano-
meter. A little concentrated salt solution is injected into the opposite
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se Ta 2
a es.
ao
-s- “OGM =r Slee a eer Se TE has:
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THE VASCULAR MECHANISM. IgI
carotid artery. When the salt is injected, it increases the conductivity
of the blood, and as soon as the solution reaches the left carotid artery
the electric current passes more easily through the artery, the compen-
sation is upset, and the needle of the galvanometer is deflected.
The time thus measured represents, however, merely the time in
which a blood corpuscle can complete the shortest possible circuit in the
circulatory system ; and the average circulation time of the blood as a
whole is probably twice or thrice as long as that found by these methods.
SECTION II.
THE PHENOMENA OF THE NORMAL HEART BEAT.
(1) Changes in Form.—Observation of the heart, exposed in an
anzesthetised animal, shows that the beat begins with contraction of the
great veins near the heart, and is followed immediately by the contraction
of both auricles (atria), including their appendages. After a brief
interval, known as the auriculo-ventricular interval, the ventricles
contract synchronously, assuming the form of a short truncated cone.
During their contraction the ventricles become shorter from above
downwards, and, as the position of the apex remains almost unaltered,
the auricles are pulled down towards the apex, and the aorta and
pulmonary artery are stretched longitudinally. At the same time, the
cross section of the base of the ventricles alters, becoming more nearly
circular and smaller. When the contraction of the ventricles ceases,
the whole heart remains for a short time at rest (diastole).
(2) The Sequence of Events within the Heart.— During diastole the
blood is flowing steadily into the right auricle from the great veins, and
also from the auricle into the right ventricle, into which the cusps of
the tricuspid valve are hanging. When the auricle contracts, it empties
most of its contents into the ventricle, any reflux of blood along the
great veins being prevented by the simultaneous contraction of the
muscular rings round their termination. The ventricle, which is now
full of blood, almost immediately contracts, and the cusps of the tricuspid
valve, which had already been carried towards each other by eddies set
up behind them as the blood flowed from the auricle into the ventricle,
are driven firmly into apposition by the pressure of the blood, their
thin borders being tightly pressed together so that no blood can escape -
into the auricle. The contraction of the papillary muscles keeps the
chord tendinez taut, thereby preventing any inversion of the valves
under the ventricular pressure. . |
The ventricle is now a closed cavity, and remains so until the
LA
.
192 ESSENTIALS OF PHYSIOLOGY.
pressure exerted by the contracting muscle upon the contained blood
rises higher than that in the pulmonary artery. As soon as this
happens, the semilunar valves open, and blood flows into the pulmonary
artery from the ventricle, which becomes nearly but. not quite empty.
When the systole of the ventricle ends and its walls relax, the pressure
in its cavity very rapidly falls below that in the pulmonary artery ; and
the semilunar valves close, effectually preventing any reflux of blood
into the ventricle. A fraction of a second later, thepressure in the
ventricle becomes less than that in the right ‘auricle, the auriculo-
ventricular valve opens, and blood, which during the ventricular systole
has been flowing into the auricle from the veins, again begins to enter
the ventricle. The valves open or close with the slightest difference
of pressure on either side. During ventricular systole the efficiency
1 : 2
Fic. 68.—Diagram to show the position of the mitral valyes
in diastole (1) and in ventricular systole (2).
A, auricle ; V, ventricle.
of the tricuspid valve is assisted by the diminution of the cross
section of the base of the heart. A similar series of changes takes
place simultaneously in the left side of the heart (fig. 63).
The series of events just described constitutes a cardiac cycle, and
occupies on the average a period of- 0-8 second, The ,,cycle may be
regarded as beginning with the auricular systole, which lasts 0:1
second, and is followed by the ventricular systole, which: lasts approxi-
mately 0:3 second ; during the remainder of the cycle, 0°4 second, the
heart is completely relaxed. When the heart is beating infrequently
the duration of the cycle is lengthened, and when the heart is beating
frequently it is shortened. These differences are due almost entirely
to variations in the time occupied by the diastolic pause, the time
taken up by the systole of the auricles and ventricles being remarkably
constant.
(3) Cardiac Impulse.—If the hand be placed on the chest in man,
THE VASCULAR MECHANISM. 193. Pa,
an impulse will be felt corresponding with each heart beat. It is
most distinctly felt, and is often also visible, in the fifth intercostal
Space, about an inch below and slightly internal to the nipple. The
impulse is due to a combination of two causes. In the first place, the
left ventricle, which lies in contact with the chest wall, and is soft and
flabby during diastole, becomes hard and tense with the onset of
systole. The sudden hardening of the ventricle gives a push to
the soft tissues of the chest wall with which it is in contact, thereby
giving rise to the cardiac impulse. In the second place, the curved
aortic arch tends to straighten out when the tension within it is raised
by the entrance of blood from the heart. The same phenomenon may
Fic. 64.—A cardiograph. (From Messrs Baird & Tatlock.)
be readily observed in any curved elastic tube filled with fluid, into
which more fluid is suddenly forced. The posterior end of the aortic
arch rests against the vertebral column and ribs, and cannot alter its
position ; the anterior end, to which the heart is attached, . therefore,
pushed more firmly against the chest wall.
Although this impulse is often spoken of as the apex beat, the
area of the left ventricle which is in contact with the chest wall is
some distance above the actual apex of the heart.
A graphic record of the cardiac impulse (cardiogram) is obtained
by means of an instrument known as a cardiograph. One form of
cardiograph (fig. 64) consists of a tambour, the membrane of which
is provided with an ivory button which can be placed on the chest at
the position of the cardiac impulse: the tambour is connected with a
second tambour provided with a recording lever. “Another method of
13
194 ESSENTIALS OF PHYSIOLOGY.
obtaining a cardiogram is to use a polygraph (p. 208), and to place
over the region of the cardiac impulse the small metal receiver which
is generally used for recording the venous pulse. The general form
of the tracings is shown in fig. 75, but it varies considerably with the
pressure used and with the spot at which the instrument is applied
to the chest.
(4) The Heart Sounds.—If a stethoscope is applied to the front
of the chest, two sounds are heard with each beat of the heart. They
are often compared with the sounds libb dtp, the first being long and
low-pitched, the second short and sharp. The time relation between
nm ma
o ©
no &
3A 32
>o > 2
2 5 23
£ e
Diastole. S S
dN. ~
F X A A
| |
Blood flowing’ ystol
intoauriclesand | of Systole jof ~
. las ,
ventricles Aur- Ventricles. astole
from jveins, icles,
01 O02 O38 O04 058 O06 07 OS O98 108ec,
~-[] OO I teat Sounds
tao dip Liabb he at nar hn dip
_ Fic. 65.—Diagram of events constituting a cardiac cycle. (Starling’s
Principles of Physiology. )
the heart sounds and the other events occurring during the cardiac
cycle (fig. 65) has. been determined in the following manner, A
stethoscope is connected with an apparatus similar to the receiver of
a telephone; the vibrations of air in the stethoscope, set up by
the heart sounds, throw the membrane of the receiver into vibrations
and so alter the contact between the silver and carbon which form
part of the receiver, and through which a current is passing. The
current also passes through an electro-magnet, which pulls on a
disc connected with the membrane of a recording tambour. Each
vibration of the membrane of the receiver alters the strength of the.
current passing through the electro-magnet and the pull which it
exerts on the iron disc and then on the membrane of the tambour. In
this way the vibrations of the receiver caused by the heart sounds can
be recorded. If such a record is obtained simultaneously with a
cardiogram, it is found that the first sound occupies nearly the whole
CE le el ee ee ee ~ *
SO EE OE
ee
THE VASCULAR MECHANISM. 195
of the systole, and that the second sound lasts for a brief space at the
commencement of diastole.
The first sound is due to two causes, namely, (1) the vibrations set
up by the closure of the tricuspid and mitral valves, and (2) the
contraction of the muscular wall of the ventricles. When the tricuspid
or mitral valves become diseased so that they fail to close, the first
sound is largely replaced by a “blowing” noise, known as a murmur
or bruit. That the contraction of the heart muscle contributes to the
first sound is shown by the fact that, during the contraction of the
bloodless, excised heart, a faint sound can be heard with the stethoscope.
The duration of the first sound almost to the end of systole furnishes
additional evidence that its origin is partly muscular. The relative
importance of the valvular and muscular factors is still a matter of
discussion. The part of the first sound due to the muscular con-
traction is not peculiar to the heart, since a similar sound is produced
by any note of low pitch, and may be heard on listening to a contract-
ing voluntary muscle.
_ The second sound is due entirely to vibrations set up in the semilunar
valves by their sudden closure at the end of systole, and is replaced by a
murmur if these valves are diseased, or if, in an animal, they are hooked
‘back and prevented from closing. The first sound is most distinctly
heard near the apex beat ; the closure of the aortic valves is best heard
in the second right intercostal space close to the sternum, and the
closure of the pulmonary semilunar valves in the second left inter-
costal space.
(5) Endocardiac Pressure.—The pressure within the auricles and
ventricles rises during systole and falls in diastole, and the variations
in pressure are closely bound up with the other events taking place
during the cardiac cycle. The changes in pressure occur so rapidly
that a slowly moving fluid, such as mercury, fails to record them ac-
curately, although a maximum and minimum mercury manometer may ~~
be employed to ascertain the highest and lowest pressure occurring
during a cardiac cycle.
In the early observations of Chauveau and Marey a cardiac sound,
consisting of a long rigid tube having at its lower end a bulb of very
thin rubber supported on a metal framework, was passed along the
jugular vein into the right. ventricle, or along the carotid artery into
the left ventricle, of a horse. The upper end of the sound was attached
to a Marey’s tambour (fig. 66), which consists of a shallow metal cup
having a small lateral opening and covered by a thin rubber membrane
on which rests a light lever. The whole apparatus contains air, and
any rise of pressure in the ventricle compresses the rubber bulb, thereby
ee
2
196 ESSENTIALS OF PHYSIOLOGY.
raising the membrane of the manometer and the lever: the movements
of the lever are recorded graphically on a kymograph.
Fie. 66.—Marey’s tambour. The writing point of the lever is not
shown. (From Messrs Baird & Tatlock. )
The method is unsatisfactory, because, in the first place, it is only
applicable to large animals, and, secondly, because, owing to the com-
pressibility of the air contained in the apparatus and to the oscillations
of the membrane of the manometer, waves are produced upon the
tracing which sometimes render it inaccurate.
These drawbacks are considerably diminished in Hiirthle’s method,
the essential features of which are (1) the use of a very small manometer
withga thick rubber membrane (fig. 67), (2) the substitution for the
sound of a tube opening directly
into the ventricle, and (3) the
including the tambour, with fluid
(half-saturated sodium sulphate
solution).
Kia: 67. Hirthle’s manometer. A still better manometer has
! recently been devised by Piper,
in which instrumental errors are almost completely excluded (fig. 68).
It consists of a metal cannula A, 6 cm. in length, containing a trocar
B, by the aid of which the cannula can be thrust through the wall of
the auricle or ventricle into the cavity of the heart. It is filled with
saline solution containing hirudin, and even the smallest air bubble must
be excluded. The cannula expands at C to form a small chamber, one
wall of which is covered by a thick stretched rubber membrane D; to
the outer surface of this membrane a tiny plane mirror E is attached.
When the cannula has been pushed into the heart and tied in
position, the trocar is withdrawn and the tap F is closed so as to
prevent the escape of blood; the cannula is then fixed with a clamp.
As the endocardiac pressure varies, the membrane bulges or shrinks
slightly and the position of the mirror alters; these movements are
recorded and greatly magnified by throwing on to the mirror a beam
of light, which is reflected on to a kymograph covered with photographic
paper and excluded from other sources of light.
filling of the whole apparatus, °
eae
THE VASCULAR MECHANISM. 197
The advantages of this method are, first, that the movements of
the membrane are directly proportional to the variations in pressure,
and by the elimination of a lever are recorded without inertia, and,
secondly, that when the membrane is thrown into vibrations these are so
rapid (250 per second) that they cannot
possibly be mistaken for oscillations pro-
duced by changes in the endocardiac
pressure, and are sorapidly damped that
they practically do not occur when the
pressure is recorded. A similar cannula
may be passed into an artery so as to
record the changes in arterial pressure.
Fig.'69 represents a record, obtained
by this method, of the pressure changes
in the left auricle and ventricle and
7 in the aorta during a cardiac cycle.
The middle curve representing intra-
ventricular pressure shows at 1 a slight
elevation (not always present) due to
the auricular systole. This is followed
almost immediately by the systole of
the ventricle, which begins at 2 and
occupies the period between 2 and 3 ;
it usually lasts from 0°25 to 0°3 second.
At first the curve rises very steeply ;
the auriculo-ventricular valves close
at the point a, and from a to 6 the
ventricle is a closed cavity. At 6 the
intra-ventricular pressure becomes
higher than that in the aorta, the
semilunar valves open, and blood flows
from the ventricle into the aorta during
the whole period from 6 to 3, when the
ventricle ceases to contract. The !' ®:- as at ene noe gil
notch in the ascending part of the
tracing immediately after the point 4 is due to the fact that, when the
semilunar valves open, the escape of blood from the ventricle is momen-
tarily impeded by the inertia of the column of blood in the aorta.
The portion of the tracing from 6 to 3 lasts about 0°18 second, and
is generally known as the systolic plateau; it resembles a plateau,
however, only when the arterial pressure is low, and its usual shape
would be more accurately described as the systolic arch.
EL
198 ESSENTIALS OF PHYSIOLOGY.
When the systole ends at 3, the intra-ventricular pressure rapidly
falls ; a short distance down the descending part of the tracing, namely,
at the point c, the pressure in the ventricle falls below that in the aorta,
and the semilunar valves close; their closure does not alter the form
of the intra-ventricular record, though it causes a series of small
oscillations in the aortic pressure. At the point 4, the pressure in the
ventricle falls below that in the auricle, the mitral valve opens, and
blood flows into the relaxed ventricle.
Auricular Pressure.—A record of the pressure changes in the
auricle presents three main waves (fig. 69). The first corresponds with
Aorta
Lett
vantrscia
Left \
auricle, }
Fic. 69.—Simultaneous record of the changes of pressure in the aorta, left ventricle
and left auricle. (From Piper.)
the auricular systole, the second with the sudden closure of the auriculo-
ventricular valves, and the third, which occurs toward the end of ventri-
cular systole, is due to the filling of the auricle with blood while the
auriculo-ventricular valves are still shut.
The maximum pressure in the left ventricle of the dog is usually
from 140 to 160 mm. Hg, and in the right ventricle from 25 to 30
mm. Hg.
THE OUTPUT AND WORK OF THE HEART.
When the ventricles contract, they force blood into the aorta and
pulmonary artery against the blood pressure in these vessels. In so
doing the heart performs work, the amount of which may be deter-
mined by the formula W=Q x R, where W is the work done, Q is the
THE VASCULAR MECHANISM. 199
amount of blood expelled from the ventricles (output of the heart) at
each beat, and R is the resistance against which the heart is working ;
R is approximately represented by the average arterial pressure.
The output of the heart may be indirectly measured by enclosing
the heart in an apparatus (cardiometer) which fits closely round the
base of the ventricles, and is connected with some form of tambour
and recording lever. When the ventricles contract and expel blood.
into the arteries, their volume diminishes, and, since the apparatus is
air-tight, there is a corresponding fall of the recording lever.
One of the simplest forms of cardiometer is a glass vessel resembling
a large thistle funnel, the mouth of which is covered by a rubber
membrane with a hole in its centre. When the heart is placed in the
cardiometer, the border of the membrane fits closely in the auriculo-
ventricular groove, forming an air-tight junction. The cardiometer is
—— a= | ——
pe I —_ |
Fic. 70.—Piston recorder. (Diagrammatic, )
LSS SS
SSS
attached to a piston recorder, which is more sensitive than an ordinary
tambour, and consists of a vulcanite piston fitting closely in a cylinder,
the latter having an opening at its lower end by which it can be
connected with the cardiometer: a light counterweighted lever is
attached to the piston (fig. 70). The piston moves very easily, and its
excursions are proportional to the changes in volume of the heart. In
order to obtain a quantitative measurement of the output of the heart
the instrument is calibrated.
Another and direct method of determining the output of the heart is
the heart-lung preparation (fig. 71), devised by Knowlton and Starling.
The common carotid artery, the descending aorta, and the inferior vena
cava are ligatured, and cannule containing a solution of hirudin in normal
saline solution are placed in the innominate artery and the superior
vena cava. The blood leaving the left ventricle through the innominate
artery passes through a thin rubber tube A; this can be compressed to
any desired extent by means of a pump C and pressure bottle D, the
resistance thus offered to the flow of blood through the tube replacing the
200 ESSENTIALS OF PHYSIOLOGY.
resistance of the arterioles. The cannula in the artery is also attached
to a mercurial manometer F, which records the arterial pressure. The
small cylinder E contains air, which forms an air cushion and to some
extent replaces the elasticity of the arterial wall. After traversing the
tube-A, the blood enters a cylinder B in which it is warmed and from
which it passes into the superior vena cava, and then through the lungs
_to the left side of the heart. The circulation is thus confined to the
heart and lungs, artificial respiration being maintained by a pump.
#)
nn
php ie
Fic. 71.—Arrangement of apparatus in the heart-lung preparation.
(Knowlton and Starling.) Description in the text.
The output of the left ventricle in a given time, e.g. five seconds, can be
measured by clamping the tube G, opening the clip on H, and allowing
the blood to flow into a graduated vessel instead of into the cylinder ;
and if the rate of the heart is observed, its output at each beat can be
calculated.
In man, the output of the heart has been indirectly determined in
the following manner. The individual takes a deep breath of air
containing a certain amount of nitrous oxide, which is very soluble
in blood. After a few seconds he expires deeply, a sample of the
expired alveolar air being collected in the manner described on p. 251.
“ :
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-THE VASCULAR MECHANISM. 201
He then holds his breath for twenty to thirty seconds and again
expires deeply, a second sample of alveolar air being collected. The
total amount of air in the lungs at the beginning and end of the
(experimental) period between the two expirations is determined by
indirect means. During this period nitrous oxide is taken up in
solution by the blood as it passes through the lungs, its solubility
being such that 1 c¢.c. of blood, if exposed to an atmosphere of pure
nitrous oxide, will take up 0°43 c¢.c. of the gas.
From these data the amount of blood passing through the lungs in
a minute can be calculated. To take an example, the volume of air
in the lungs at the beginning of the experimental period is 3°25 litres,
and contains, as shown by the analysis of the first sample, 12 per cent,
nitrous oxide; the total quantity of nitrous oxide in the air of the
ven, 60 = 12 _ 390 c.c. At the end of the period the total
volume of air in the lungs is 3:0 litres ; the percentage of nitrous oxide
in the second sample of expired air is 10 per cent. ; and the lungs contain
300 ¢.c. nitrous oxide. Thus 90 c¢.c. nitrous oxide have been taken up .
by the blood ; and the mean percentage of nitrous oxide in the air in
12 per cent. + 10 per cent.
lungs is
the lungs is 5 = 11 per cent.
With this percentage of nitrous oxide in the air of the lungs, each
1 c.c. of blood passing through them will take up ge = 0-047 c.c.
nitrous oxide; and in order to take up 90 c.c., 1°9 litres of blood must
have passed through the lungs during the experimental period. If the
experimental period lasted twenty-seven seconds, the flow of blood
through the lungs per minute is 4°2 litres. This figure represents
the output from the right ventricle during that period, and if the
4200 c.c.
Win
60 c.c. per beat. This figure may be taken as representing the average
output of each ventricle in man, since in hentia the output of the two
ventricles is the same.
The mean arterial pressure in man is about 100 to 110 mm. Hg.
From these data the work done by the heart at each beat can be
calculated. Thus Q x R=60 grm. x 0'100 metre x 13°6 (specific gravity
of mercury being 13°6 times that of blood)=81°6 grm. metres. This
figure represents the work done by the left ventricle. If the pressure
in the pulmonary artery be taken as 20 mm. Hg, the work done by
the right ventricle will be 60 grm. x 0:02 metre x 13-6 = approximately
16:4 grm. metres. S
pulse rate is 70 per minute, the output per beat will be
¢
202 ESSENTIALS OF PHYSIOLOGY.
The total work of the heart, therefore, is in this instance about
98 grm. metres per beat.
The heart expels blood not only against the peripheral resistance,
but with a certain velocity. The work (W) done in imparting this
MY?
velocity to the blood is measured by the formula W=——, where
M= mass of blood expelled, V =its velocity, g=the force of gravity ;
it amounts to approximately 1 per cent. of the total work of the heart
and is practically negligible.
We may now consider the conditions which determine the output
and the work of the heart. |
The Output of the Heart.—It has already been pointed out that,
when a skeletal muscle contracts, the contractile stress developed in it is
proportional to the initial length of the muscle fibre, 7.e. its length just
before it begins to contract, and that if the muscle is stretched by means
of a weight it contracts more forcibly. The heart muscle behaves in
exactly the same way, the only difference being that, in the case of the
heart, the initial length of its muscle fibres depends upon the stretching
of the fibres produced by the contained blood. Hence the greater the
amount of blood present in the heart at the beginning of systole, the
greater the initial length of the fibres, and the greater the force with
which they contract. The result is that within wide limits the amount
of blood expelled from the ventricles at each beat is determined solely
by the amount. entering the heart during diastole. This amount is
increased (1) by deeper respiratory movements, whereby more blood
is sent into the heart with each inspiration; (2) by muscular move-
ment, which drives blood along the veins towards the heart; and
(3) by any increase in the total volume of the circulating blood, such
as is produced by the injection of saline solution into a vein, During
exercise the more forcible respiration and the active muscular move-
ment lead to a much larger output of blood than during rest. There
is a certain optimum filling of the heart and initial length of the
muscle fibres at which the mechanical efficiency of the muscle during
contraction is at its best; excessive filling of the heart may dilate
it to such an extent during diastole as to diminish its efficiency, and
its output falls. |
The output of the heart is not affected, except for a moment or
two, by alterations in the arterial blood pressure unless these are
extreme. The first effect of a rise of blood pressure is that for a few
beats the ventricle empties itself less completely, its volume during
diastole being thus increased. This distension of the ventricle during
diastole increases the length of the fibres, causing them to contract
dS ete eset Nt EB oot
ane + nk retin oa patie OL se
THE VASCULAR MECHANISM. 203
more strongly during systole, and the output of the heart again
becomes as large as it was at the lower arterial. pressure (fig. 72).
The capacity of the ventricle to maintain its normal output in
spite-of a greatly raised arterial pressure is spoken of as its ‘‘ power
of compensation,” and is of the utmost importance. In its relation to
the body as a whole the output of the heart is one of the fundamental
facts of the circulation, since it determines the supply of oxygen to
the tissues; and if the output were diminished whenever the blood
pressure rose, for example during exercise, the high blood pressure
=“ Beis htt A aaED te Aaa oop SACI PRR) =p gaa
Fic. 72.—Output of heart. (Knowlton & Starling.)
A, volume of ventricle: B, arterial pressure; C, output of left ventricle; D, time in seconds.
would necessarily involve a smaller and possibly inadequate supply of
oxygen to the tissues.
If the arterial pressure becomes very high, the dilatation of the
left ventricle during diastole is so great as to diminish its efficiency
when it contracts, the output of the heart falls, and the left auricle
empties itself less completely into the already distended ventricle.
In consequence, the left auricle contains more blood, the pressure
within it rises, and blood passes less readily from the lungs into the
auricle and accumulates in the pulmonary veins and capillaries. This
accumulation of blood in the lungs is spoken of as their “ reservoir”
action, and helps to prevent excessive dilatation of the left side of
the heart. ‘
204. ESSENTIALS OF PHYSIOLOGY.
The compensatory power of the heart enables it to adapt itself to
transient variations in arterial blood pressure. When the work of the
heart is permanently increased by a continuously high blood pressure,
the heart wall hypertrophies, just as skeletal muscles enlarge as the
result of exercise. The effort of prolonged increase in the work done
by the heart is seen in the hypertrophy observed in athletes (athlete’s
heart). .
When the heart beats rapidly the diastolic pause is shortened, less
blood enters it between the beats, and its output per beat is diminished,
Hence its output in a given time (e.g. a minute) is not necessarily
greater when the heart is beating rapidly than when it is beating slowly.
The increased rate of the heart, however, which usually accompanies
increased diastolic filling, is of advantage, since by preventing over-
distension during diastole it enables the heart to contract under the
most favourable conditions as regards its mechanical efficiency.
Work of the Heart.—Since the work done by the. heart is measured
by its output multiplied by the arterial pressure (Q xR), it will be
altered either by a rise or fall of arterial pressure, or by variations in
the output of the heart, or by these two factors varying simultaneously.
Whenever the arterial blood pressure rises the heart does more work,
since the output remains unchanged. Again, increased filling of the heart
brought about by any of the causes already mentioned, by increasing
its output, will add to its work; and the larger output from the heart
tends in itself to raise the arterial blood pressure, and thus to increase
still further the work done. In muscular exercise there is both a rise
of arterial blood pressure and a greater diastolic filling of the heart.
In man, during exercise, the output of the heart per beat may be
90 to 100 c.c., and since the heart is beating much more rapidly, its
output per minute may be two or three times as great as during rest,
and its work is enormously increased. These facts indicate how im-
portant it is that persons, whose hearts have become less efficient owing
to disease, should be kept at rest.
SECTION III.
THE PULSE.
If the arterial system consisted of a series of rigid tubes, the blood
forced into it from the heart would, in accordance with the laws of hydro-
statics, cause an instantaneous rise of pressure throughout the whole
system, and an equal quantity of blood would at once escape from the
distal end of the system. Owing to the fact that the arteries are dis-
tensible, only a fraction of the blood entering the arterial system at
ee a a
THE VASCULAR MECHANISM. 205
each beat is forced through the arterioles during the beat, and much of
the force of the heart is expended in further expanding the already
distended arterial system to accommodate the extra blood sent into it..
The expansion of the arteries starts at the root of the aorta, and
proceeds as a wave along the whole arterial system, gradually dying
away before the capillaries are
reached. This wave of expansion
constitutes the pulse. It travels at
a rate of 6 to 8 metres a second,
and is independent of the movement
of the main mass of blood along the
arteries, the velocity of which rarely
exceeds half a metre a second.
The pulse can be felt, and often
seen, in the superficial arteries of
the body, e.g. the radial artery ; in |
order to study it more exactly a D
graphic record may be obtained by A
Fie. 73. —Dudgeon’s sphygmograph,
; slightly diagrammatic. Explanation
a sphygmograph, of which many of figures in text.
forms exist.
Dudgeon’s sphygmograph, which is illustrated diagrammatically in
fig. 73, may be attached by a band round the wrist in such a way that
the small metal plate A rests on the skin over the radial artery. The
movements of the arterial wall are magnified by the series of levers,
a
% — eee oe cm gee ree S
means of an instrument known as
a, 6, c, and are recorded by the
free end of the lever c, which
writes on a moving strip B of
blackened (smoked) paper. The
paper is moved by means of a
small clockwork arrangement ;
and by means of the dial D
Fic. 74.—Pulse tracing from the the pressure of the plate A on
yorinl eatery the artery can be adjusted.
A typical pulse tracing thus
obtained is seen in fig. 74. It shows:a sharp rise from a to 8,
succeeded as a rule by a steady fall, interrupted at ¢ by a small
notch which is immediately followed by a slight wave. The rise a to
6 constitutes the primary or percussion wave; the wave following
e is the dicrotic wave, the notch just preceding it being the dicrotic
notch. Other small waves, some preceding the dicrotic wave (pre-
dicrotic), and others following it (post-dicrotic), sometimes occur; they
*
206 ESSENTIALS OF PHYSIOLOGY.
are due to slight oscillations of the stretched arterial walls, which are
magnified and distorted by vibrations set up in the sphygmograph. The
portion of the wave from 4 to ¢ is normally descending, and the pulse
is called katacrotic; when the primary wave abakigoa to rise almost
until the dicrotic pata is reached, the pulse is said to be anacrotie.
The primary wave is caused by the sudden expansion of the artery
during the cardiac systole, the form of the wave depending upon the
peripheral resistance. After the first abrupt rise of arterial pressure
the blood usually escapes through the peripheral resistance more
rapidly than it enters the arterial system from the heart, and the pulse is
katacrotic. When the peripheral resistance is very high, e.g. in old age
or in Bright’s disease, blood continues to enter the aorta during systole.
more rapidly than it passes through the arterioles ; the arterial pressure
continues to rise almost to the end of systole, and the pulse is anacrotic.
In the aorta the primary wave begins coincidently with the opening
of the semilunar valves and the escape of blood into the aorta, as may
be seen in fig. 75, which gives a simultaneous record of the endo-
cardiac pressure and of a pulse tracing. Since this wave of expansion
travels from the aorta to the peripheral vessels, the percussion wave
begins in the smaller vessels an appreciable time later than in the
aorta. This can be readily observed by taking simultaneously two
pulse tracings, one from the carotid artery and one from the radial
artery at the wrist. By measuring the interval with the aid of a time
marker and noting the difference in distance from the heart of the points
at which the tracings are made, the velocity of the pulse wave can be
measured. Thus, if the interval is one-tenth of a second and the differ-
ence in distance is 0°6 metre, the velocity is 6 metres per second. The
length of the wave is from 5 to 6 metres.
Fig. 75 also shows that the dicrotic wave occurs immediately after
the closure of the semilunar valves, and as no corresponding wave is
present in the endocardiac pressure tracing, the dicrotic wave must
have its origin in the arterial system. It has been thought that as
the primary wave passed along the arteries it was reflected, as a
secondary wave, from the obstructions set up wherever the arteries
branched, and particularly from the arterioles where the branchings are
very numerous. This small reflected wave, starting at the periphery,
was believed to travel back towards the heart as the dicrotic wave.
Such a wave is actually produced in an artificial scheme of the circula-
tion where the pulse wave beats upon a peripheral resistance.
If this view were correct, the distance between the primary and
dicrotic waves would naturally be less in the peripheral arteries near the
seat of origin of the reflected wave than in the aorta. But the interval
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THE VASCULAR MECHANISM. — 207
between the summits of the primary. and secondary waves is of the
same length in pulse tracings taken from the same individual at the
“game time, whether the artery examined be near or far from the heart ;
for example, it is the same in the carotid and the dorsalis pedis axthciong
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Systole Diastole
Fie. 75.—Diagram (after Hiirthle) showing simultaneous cardiographic,
endocardiac, and aortic curves. (Starling’s Principles of Physiology.)
I., cardiogram ; II., endocardiac pressure; III., aortic pressure; IV., aortic
pressure, corresponding with dotted endocardiac curve in IT.
Hence the dicrotic wave must arise at the same point as the primary
wave, and since the primary wave starts at the root of the aorta, the -
dicrotic wave must start there also.
It is brought about in the following manner. When the left
ventricle ceases to contract, the column of blood travelling along the
*
208 ESSENTIALS OF PHYSIOLOGY.
aorta continues to move in virtue of its momentum, thereby producing
a slight fall of pressure at the root of the aorta; and the wall of the
artery shrinks a little. This slight fall of pressure immediately causes
a reflux of blood against the semilunar valves, closing them, and from
the closed valves the blood rebounds, thereby producing a small
secondary expansion of the aorta; this expansion travels along-the
arterial system and forms the-dicrotie-wave. The height of both the
primary and dicrotic waves is largely determined, first, by the elasticity
of the arteries, and, secondly, by the degree of distension of the arteries
between successive heart beats. If they have become rigid (from old
age or disease), their capacity for expansion will obviously be diminished ;
and if they are already greatly distended by a high mean arterial blood
pressure, their capacity for further distension will also be decreased.
The conditions most favourable to the appearance of a marked dicrotic
pulse, therefore, are (1) a strongly beating heart, (2) a moderate blood
pressure, (3) highly elastic arteries. These conditions are often very
fully realised in young adults during fever. —
The rise of pressure caused by the entrance of blood into the arterial
system with each heart beat produces its maximum effect where it
— first enters it, namely, at the root of the aorta, and the wave of ex-
pansion is largest at this point. Some of this rise of pressure is used
up in expanding the first section of the aorta, and the force tending
to distend the next segment will be slightly less ; this process continues
from segment to segment along the arterial system, the wave gradually
becoming smaller and smaller, until in the capillaries it has entirely
disappeared and no trace of any pulse is visible.
VENOUS PULSE.
A venous pulse is normally present in the great veins near the
heart, and direct observation of the jugular vein shows two visible pulse
waves for each heart beat. In order to obtain a record of the venous
pulse and to interpret it, a simultaneous tracing of the venous pulse
and of the radial pulse is obtained by means of the polygrapli. This
consists of a clockwork arrangement, whereby a continuous record of
the venous and radial pulses can be obtained on a moving sheet of
paper. It is provided also with a time-marker, which records on the
paper. A small metal cup with an opening at its base is pressed on to
the skin over the jugular vein just above the clavicle, and is connected
with a tambour attached to a lever which writes. on the recording
surface. A sphygmograph, attached to the wrist, is also connected by
rubber tubing with a similar tambour and lever, and the two levers are
pe ne ee ee
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= 2S
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ras Oe Bee Ns Lk) ee gk i a a i, 2 i a kL i i i
THE VASCULAR MECHANISM. 209
arranged to write one above the other. Fig. 76 represents a tracing
of the venous pulse taken by this method,
The venous pulse shows three waves ; the first rise, a, corresponds.
with the auricular systole, the second, c, is simultaneous with the
beginning of the ventricular systole, and the third more rounded wave,
v, is due to the gradual filling of the auricle towards the end of
ventricular systole. The waves correspond with the changes of
pressure in the auricle, being transmitted along the column of blood
in the vein.
The venous pulse is confined to the large veins, and as the arterial
pulse is extinguished in the arterioles, there is normally no pulsation in
the medium-sized and smaller veins. The arterial pulse wave may,
however, extend into the smaller veins under certain conditions, When
the chorda tympani nerve is stimulated, for example, the arterioles of
the submaxillary gland are dilated, and not only is the amount of blood
a,c.
NR Ne ss
Ve
nous Pulse
Fie. 76.—Time-marker records + secs.
flowing through the gland increased, but the pulse wave extends into
the veins coming from the gland. The transmission of the pulse into
the veins is due, in this and similar cases, to the diminution of the
resistance in the arterioles, which is a consequence of their dilatation.
SECTION IV,
- THE CAUSATION OF THE HEART BEAT,
We have now to consider how the beat of the heart originates, and
. the conditions upon which its normal rhythm depends and by which
that rhythm can be modified. For this purpose the slowly beating and
relatively simple heart of the frog has proved to be of great value.
Anatomy of the Frog’s Heart.—The frog’s heart consists of a sinus
venosus, two auricles (atria), a ventricle, and a bulbus arteriosus. The
blood from the venze cavee enters the sinus, passes into the right auricle,
and thence into the ventricle. The blood coming from the lungs enters
the left auricle and passes into the ventricle. The ventricle opens into
the bulbus arteriosus, from which the aortic arches arise and distribute
the blood to the entire body, including the lungs. .The cardiac muscle
14
210 ESSENTIALS OF PHYSIOLOGY.
consists of small spindle-shaped fibres showing cross striation, which is
very indistinct, particularly in the sinus venosus.
Two nerves, the right and left vago-sympathetic nerve trunks, enter
the heart at the sinus, and become connected with a small mass of
nerve cells which lies close to the sino-auricular junction and is known
as Remak’s ganglion. Nerve fibres from this ganglion pass along the
septum between the auricles to enter two similar ganglia (Bidder’s
ganglia), lying close to the auriculo-ventricular junction. Scattered
nerve cells are also found in the interauricular septum and in the upper
part of the ventricle, but are absent from its apical half: The fibres
issuing from all these groups of nerve cells terminate in relation
with the muscular fibres of the heart.
THE BEAT OF THE FROG’S HEART.
If the brain of a frog is destroyed and the heart exposed, it can be
seen that each beat consists of a regular sequence of events, namely, (1)
contraction of the sinus, followed by that of (2) the auricles, (3) the
ventricle, and finally (4) the bulbus arteriosus. When the whole heart
is carefully excised from the body and placed in a watch glass containing
salt solution (0°65 per cent. NaCl), it continues to beat in a normal
fashion for some time. On separating the sinus from the rest of the
heart, by cutting through the sino-auricular junction, the sinus
continues to beat as vigorously and at the same rate as before, whereas
the auricles and ventricle cease to beat.
After a short time the auricles and ventricle again begin to beat,
but at a slower rate than the sinus. If the ventricle is cut away from
the auricles, the latter continue to beat, while the ventricle after one
or two beats usually.comes to a standstill. After an interval of half
an hour or more the ventricle may again begin to beat, and it will do
so more readily if it is stimulated by an occasional pin-prick. The rate
of the ventricular beat is slower than that of the auricles. The apical
half of the ventricle, if isolated, will never again start to beat of its own
accord. This experiment makes it clear, first, that the rhythmic beat
of the heart can be carried on quite independently of the central nervous
system, and secondly, that this power of rhythmic contraction is most
fully developed in the sinus.
It was formerly supposed that the beat originated in the nerve cells
of the heart, from which a constant stimulus was sent out to the heart
muscle, and that the muscular fibres responded to this stimulus by a
series of rhythmic contractions. This is the newrogenic theory of the
cause of the heart beat. The view now generally held, however, is that
THE VASCULAR MECHANISM. 211
the cardiac muscle possesses an inherent power of rhythmic contraction,
which is most marked in the sinus and least so in the ventricle, and
that this rhythmic power can continue absolutely independently of
either the central nervous system or the nerve cells in the heart,
although, as will be seen later, it can be influenced by impulses passing
along the nerves to the heart. This view is known as the myogenic
theory of the heart beat. The myogenic theory has been accepted for
the following reasons.
(1) In the first place, it was shown by Gaskell that a strip of the
ventricle of the tortoise, if kept stretched and moist, can be made to
beat rhythmically, and will then continue to beat without any external
stimulus, although subsequent histological examination of the strip
shows that it contains no nerve cells. In the same way the apical half
of the frog’s ventricle, which is free from nerve cells, although it will
not beat spontaneously, can be made to contract rhythmically if it is
fed with fluid through a cannula at a pressure sufficient to put tension
on the muscle fibres.
(2) Secondly, it is possible in the frog’s heart to remove almost
completely the ganglia of Bidder and Remak without disturbing the
cardiac rhythm in any way.
(3) Thirdly, if, in a normally beating heart, successive single stimuli
are applied to the ventricle more frequently than the rate at which the
heart is beating, the rhythm of the heart can be reversed, so that the
beat starts in the ventricle and passes to the auricle, and then to the
sinus. Such a reversal of rhythm is quite incompatible with the
neurogenic theory of the heart beat, since it contradicts the general
law (law of forward direction) that nervous impulses can pass through
a synapse only in one direction.
It may be concluded, therefore, (1) that the rhythmic contraction of
the heart is myogenic in origin, and (2) that, although all parts of the
heart possess some rhythmic power, the beat normally always starts in -
the sinus, in which this power is most fully developed. The sinus sets
the pace of the heart, and the ventricle responds to the stimuli reaching
it, doing its work under the control of the sinus.
The Propagation of the Beat.—In the frog the muscular tissue of
the whole heart is continuous, but the power of the tissue uniting the
sinus and the auricles and the auricles and ventricle to conduct impulses
is less than that of the rest of the heart. The impulses starting from
the sinus are slightly delayed, therefore, in their passage to the
auricles, and again in their passage from auricles to ventricle, so that
there is a distinct pause between the contractions of the sinus and
auricles, and auricles and ventricle respectively.
212 ESSENTIALS OF PHYSIOLOGY.
THE PROPERTIES OF CARDIAC MUSCLE.
If a fine thread is tied round the apex of the frog’s heart and
attached to a light lever supported by a spring, a graphic record of the
heart movements can be obtained ; this shows a small fall correspond-
ing with each auricular contraction, and a larger fall corresponding
with each ventricular systole (fig. 77). A thread, tightly tied at the
junction between the sinus and the auricles, will now bring the auricles
and ventricles to a standstill for a variable time, since the ligature
prevents the passage of the normal rhythmic stimuli from the sinus to
the rest of the heart; this is known as the Stannius ligature. The
quiescent ventricle may then be used to study the properties of cardiac
muscle as compared with those of skeletal muscle.
The contraction of cardiac muscle shows the following characters :—
(1) When the ventricle is stimulated with single shocks of gradually
increasing strength, it is found that with a
vi certain strength of current the heart gives
Nh | \\ \ N\ a) a beat. If a stronger current is used, the
yV VV \ Vy | resulting beats are not increased in extent
—> or force. The observation that if the heart
beats at all in response to a stimulus its con-
Pid 77 tiecing of the Rox: traction is maximal, whatever the strength
mal heart beat in the frog, Of the stimulus, is known as the “all or none
law.” It is due to the fact that the heart
muscle is a syncytium, and that a stimulus applied to any point will, if
it is effective, spread over the whole muscular tissue of the heart. The
heart gives the best beat of which it is capable at any moment, but the
force with which it beats will be influenced by various considerations,
including the nutrition of the fibres. The ‘‘all or none law” simply
means that the heart beat is maximal for the conditions under which
it is placed at any moment, and not that it remains constant through
life. In this respect cardiac muscle behaves. like the individual fibres
of a skeletal muscle which also obey the “all or none law” (p. 22).
(2) If the resting heart is stimulated by successive induction shocks
at an interval of 5 to 10 seconds, the height of the second contraction is
‘rather greater than that of the first. At the third or fourth contraction
a maximum is reached, and succeeding contractions are all of the same
height. This phenomenon is sometimes called the ‘staircase effect,”
and is due to the beneficial influence of the first two or three stimuli
on the contractile power of the heart muscle. The same effect may be
observed in skeletal muscle in similar circumstances.
(3) When the heart is made to beat by a single induction shock,
THE VASCULAR MECHANISM. 213
and a second shock is sent into the heart during the systole evoked by
the first stimulus, the second stimulus produces no visible effect on the
heart, which is said to be “refractory.” This “refractory period”
extends from the instant when the first stimulus is applied until the
end of the systolic phase. Owing to the length of the refractory period
of cardiac muscle as compared with that of skeletal muscle, it is *
impossible to tetanise the heart, since only those stimuli which fall
during the diastolic period are effective. Fissede
The refractory period also accounts for the effect observed in the Ss
normally beating heart when a single shock is sent into the ventricle
at the beginning of diastole. In this case the ventricle responds with
an extra beat, and the stimulus coming down from the sinus to produce
the next ventricular systole falls within the refractory period of this
V
Fic. 78.—Normally beating frog’s heart tracing. A single induction shock,
applied to the ventricle at A, caused an extra systole, followed by a com-
pensatory pause. Downstroke=systole,
extra beat and is ineffective. There is sete eames a long pause, which
is known as the “compensatory pause,” between the extra beat and the
next beat originating from the sinus (fig. 78).
(4) The force of the contraction is greatly influenced, as in the case
of skeletal muscle, by the tension to which the fibres are exposed, that
is, by the length of its fibres. If the heart is isolated and perfused ©
with saline solution (0°65 per cent. NaCl), the force of the ventricular
contraction will vary with the pressure under which the fluid is allowed
to enter the heart. The higher the pressure, the greater will be the
amount of saline solution entering the ventricle during diastole ; and
the pressure of the fluid will put tension on, and thus increase the
length of, the muscle fibres. The heart muscle has the power of.
responding to the increased tension by contracting more strongly,
with the result that it empties itself as completely as it did when
the pressure of the perfusing fluid was low. Increase of internal
tension is thus a stimulus to contraction, and it is for this reason that
the apical half of the frog’s heart contracts rhythmically when it is
214 ESSENTIALS OF PHYSIOLOGY.
filled with fluid under pressure. The heart of the snail is so susceptible
to this stimulus that it will not beat at all unless its fibres are under
tension. |
THE MAMMALIAN HEART.
In the mammalian heart the sinus, although present in early
embryonic life, does not exist as a separate structure after birth, but
is represented by a mass of specialised tissue, lying close to the entrance
of the superior vena cava into the heart, and extending a little way
along the sulcus terminalis of the right auricle. This tissue is known
as the sino-auricular node.
It has already been mentioned that connecting the agribles and ven-
tricles is a band of tissue known as the bundle of’His. This bundle
starts near the opening of the coronary sinus into the right auricle, its
point of origin being called the auriculo-ventricular node, It passes along
the top of the interventricular septum for a short distance, and then
divides into two branches, one of which runs down the right and the
other down the left wall of the septum immediately under the endo-
cardium. In some animals, e.g. the calf, it can be readily dissected out
in this part of its course as a thin band, paler than the rest of the
ventricular muscle. It soon breaks up into a number of very fine
branches which pass partly to the papillary muscles, and are partly
distributed over the rest of the wall of the heart. The extent of this
branching is well seen in fig. 79. Microscopically, the fibres making
up the terminal branches of the bundle are larger and paler than
ordinary cardiac muscle fibres, and only the peripheral part of the fibre
shows cross striation, the centre being protoplasmic in character; the
fibres are called Purkinje’s fibres. :
The Rhythm of the Mammalian Heart.— Although the heart is pro-
vided with many nerve cells and receives in addition a nerve supply
from the central nervous system, its rhythm in the mammal, as in the
frog, is in all probability of myogenic origin and depends solely upon
the inherent rhythmic power of the muscle itself. It has been shown, for
example, that provided they are adequately supplied with oxygenated
blood, strips of mammalian ventricle will continue to beat for some
hours although they contain no nerve cells. Evidence to the same
effect is furnished by the heart of the embryo chick, which begins to
beat some days before any nerves are present in it.
The impulse normally starts in the sino-auricular node, and, travel-
ling over the walls of the auricles, reaches the auriculo-ventrigulay node.
From this node, the impulse passes along the bundle of His to the
ventricles. The importance of the bundle of His is manifested by the
Ment
THE VASCULAR MECHANISM. 215
effects which follow either disease of the bundle in man, or division of
the bundle in animals. In man the continuity of the bundle may be
partially or completely destroyed by disease, the result being known as
partial or complete “heart block.” In partial heart block one out of
every two or three auricular beats is conducted to the ventricle, the latter
beating at half or a third the rate of the auricles (2:1 or 3: 1 rhythm).
In complete heart block the rhythm of the auricles is unaffected,
Fic. 79.—Diagram to show the distribution of the bundle of His (in red) in the
wall of the left ventricle. (After Tawara. )
whereas the ventricles beat at a rate varying from 20 to 40 per minute;
the patient usually exhibits characteristic symptoms (Stokes-Adams
disease), and a simultaneous record of the venous and radial pulse,
taken with the polygraph, shows that the rhythm of the ventricles is
quite independent of that of the auricles.
When the bundle is divided in an animal, the rhythm of the
auricles remains unaltered, whereas the ventricles immediately begin to
beat at a slow rate having no relation to that of the auricles. Partial
heart block is sometimes seen in asphyxial conditions, even when the
bundle is intact. Partial or complete heart block can also be induced
in the frog’s heart by compressing the tissue uniting the auricles and
°
216 ESSENTIALS OF PHYSIOLOGY.
ventricle so as to lessen or abolish its conductivity. The mammalian
ventricle differs, however, from that of the frog in possessing a more
pronounced rhythmic power, and when functionally isolated from the
auricles, it immediately begins to beat with its own rhythm. ~
It is evident that the bundle of His is essential for the propagation
of the wave of contraction from the auricles to the ventricles.
The Electrical Changes in the Heart.—The time relations of, and
the course taken by, the wave of contraction, as it travels from the
sino-auricular node over the heart, can be easily demonstrated, not
only in the lower animals, but even in man, by studying the electri-
cal changes which take place at the same time. These changes
may be recorded by connecting two parts of the heart, for instance.
the auricles and ventricles, with some form of galvanometer. For
this purpose, the string galvanometer (p. 24) is now most gener-
ally used.
The resting heart is isoelectric, that is to say, the auricles and the
ventricles are at the same ‘potential, and the thread of the galvanometer
is at rest. When the auricles contract, a difference of potential is set
up between them and the ventricles, and a current passes in the heart
from auricle to. ventricle, and through the galvanometer from ventricle
to auricle. During systole of the ventricles and diastole of the auricles,
the current passes in the opposite direction. Cardiac muscle thus
resembles skeletal muscle in that its contraction is preceded by an
electrical change, the contracting part being galvanometrically negative
(but electro-positive) to the resting part. The difference in character
of the galvanometric tracings of the electrical changes in the heart
and in skeletal muscle is due, partly to the prolonged contraction of
cardiac muscle fibres, partly to the complex arrangement of these |
fibres in the heart wall. |
In order to obtain a record of the electrical changes in the human
heart, one end of the thread of the galvanometer is connected with a
vessel containing salt and water into which the right arm of the subject
is placed. The left leg is placed in a’similar vessel connected with the
other end of the thread. The right arm is regarded as conducting the
electrical changes at the base of the heart, while the changes occurring
at the apex are conveyed down the left leg to the apparatus. The
apparatus is so arranged that in these circumstances an upward move-
ment of the shadow of the thread on the photographic record means
that the base of the heart is negative to the apex, and is therefore
contracting, while the apex is at rest.
Fig. 80 represents an electro-cardiogram obtained with this instru-
ment, and shows three upstrokes for each cardiac cycle. The first
ee ee
El i ee, ee ee
THE VASCULAR MECHANISM. 217
wave, P, corresponds with the systole of the auricles, which, when they
contract, become galvanometrically negative to the resting apex. The
second wave, R, occurs at the beginning of the ventricular systole, and
is due to the systole commencing at the base of the ventricle, which
becomes negative to the apex. As the wave of contraction travels to
the apex, the thread returns
to its resting position and
remains steady for a short
time, during which the whole =
ventricle is in contraction, 3=———— = ———
The final rise at T is dueto == aero a
the systole lasting longer at py¢, 30.—Electro-cardiogram from human heart.
the base than at the apex, (Hume.) Explanation of letters in text.
particularly round the root of
the aorta and pulmonary artery. The records obtained in the lower
animals show the same general form; in the frog and tortoise the
prolonged contraction of the ventricle is accompanied by a long period
between P and T during which the heart is isoelectric.
The Nutrition of the Heart Muscle.—The nutrition of the heart
muscle is dependent on the amount and composition of the nutrient
70
fluid supplied to it, that is, in normal conditions, of the blood. The
influence of alterations in
the composition of the nu-
trient fluid are very easily
studied in the isolated frog’s
heart, the muscular wall of
which is nourished entirely
by interchanges between
the muscular fibres and the
blood flowing through the
heart. For this purpose a
cannula, such as that shown
in fig. 81, is tied into an
auricle, and the heart is
excised. The cannula is
, .', attached to a bottle con-
Fic. 81.—Diagram of apparatus for perfusion of | . ,
frog’s heart. ‘taining a suitable perfusion
fluid, which enters the
heart at a pressure of 1 to 2 cm. of water. The fluid expelled through
the bulbus arteriosus from the ventricle flows over the surface of the
heart, keeping it moist. The apex of the ventricle is connected by
a thread with a recording lever. - .
2
218 ESSENTIALS OF PHYSIOLOGY.
The perfusing fluid most commonly used is known as Ringer’s fluid,
and has the following composition :— |
NaCl Se 0-65 per cent.
KCl ae Sant | kts ee
CaCl, ' sy ORE
A trace of sodium carbonate.
If 0-1 per cent. dextrose is added to this solution, it is called Locke’s
fluid. :
By means of this or similar methods, the influence of changes in the
composition of the perfusing fluid upon the rate and force of the beat.
can be readily ascertained. The muscle of the frog’s heart possesses a
large store of energy, and will continue to beat for a long time without
any fresh supply of nutrient material so long as the perfusing fluid
contains oxygen. The force of the beat is dependent, however, not
only on the oxygen supply, but also on the presence or absence of
certain salts and on the reaction of the perfusing fluid. —
(1) If the heart is perfused with a solution free from calcium, the
contractions of the ventricle gradually become feebler, and the heart
soon stops in diastole. When calcium, but not potassium, is present,
the ventricle after a short time fails to relax completely during diastole,
and eventually may come to a standstill in a fully contracted state.
The presence of both calcium and potassium, as, for example, in Ringer’s
fluid, seems to be essential for the maintenance of the normal beat.
(2) The heart is extremely sensitive to slight changes in the
reaction, that is, the H ion concentration, of the perfusing fluid. It
beats most forcibly when the perfusing fluid is neutral; if the fluid is
made slightly acid or slightly alkaline, the beats become smaller.
In the mammal the heart receives its blood supply through the
coronary arteries, along which blood is flowing during both systole and
diastole. The conditions which modify the supply of blood to the
heart by the coronary vessels can be readily studied in the heart-lung
preparation (p. 199). In the first place, the amount of blood flowing
through the coronary vessels varies directly with the blood pressure in
the aorta, Secondly, it is increased ‘by the addition of adrenalin to the
circulating blood, this substance dilating the coronary vessels. Thirdly,
carbonic acid and other metabolic products of muscular activity dilate
the coronary vessels. By one or other of these means the blood supply
to the heart is increased whenever it does more work. ‘
Ligature of a coronary artery, or of one of its main branches, is
followed almost immediately by stoppage of the ventricles, and the
heart cannot be made to beat again. 7
o
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On a Sale te eh le tL SN Cate een Bake s -
tne Pree cise OE Ae AE Ta a ited el A rent
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coe:
THE VASCULAR MECHANISM. 219
The force of the mammalian heart beat is also influenced by changes
in the reaction of the blood. If the tension of carbonic acid in the
blood is greatly increased, the heart relaxes more completely during
diastole and contracts less forcibly during systole, with the result that
its output is diminished, A decrease in the tension of carbonic acid, on
the contrary, leads to the accumulation of blood in the great veins, less
blood enters the heart wee diastole, and the output of the heart is
diminished.
Both in the frog and in the mammal, the rate of the heart is
increased by a rise and decreased by a fall in the temperature of the
fluid circulating through it. With this exception, changes in the
character of the blood have little or no effect on the rate of the
heart beat.
SECTION V.
THE REGULATION OF THE VASCULAR MECHAN ISM.
In order that the various tissues of the body may receive an
adequate supply of nutritive material and oxygen, it is essential that
the blood supply to the different organs should be varied in accordance
with their needs. This end is attained by means of the central nervous
system, which can modify the rate of the heart and the calibre of the
arterioles in response either (1) to external stimuli, or (2) to impulses
arising in the different parts of the body itself, or (3) to changes in the
character of the circulating blood. This latter factor also directly
influences the force of the heart beat and the calibre of the vessels.
THE INNERVATION OF THE HEART.
The nerves supplying the heart are (1) the vagus, and (2) branches
from the sympathetic system. The sympathetic fibres arise in the
frog from the white ramus of the third spinal nerve, and have their
cell station in the corresponding sympathetic ganglion. From the
ganglion they pass upwards to join the vagus close to its exit from the
skull ; the combined vagus and sympathetic fibres form a single nerve
on each side, the vago-sympathetic, which runs to the heart (fig. 82).
The fibres of the vagus nerve: have their cell station in the ganglia of
the heart itself.
In the mammal the vagus gives off branches in the thorax,
which .run direct to the heart, in which their cell stations lie. The
sympathetic fibres leave the spinal cord by the second and third
thoracic white rami; their cell stations are in the stellate ganglion,
from which post-ganglionic fibres run directly to the heart.
220 ESSENTIALS OF PHYSIOLOGY.
The Vagus.—When a weak stimulus is applied to the peripheral
portion (7.e. the end towards the heart) of the divided vago-sympathetic
nerve in the frog, the heart immediately beats more slowly: a stronger
stimulus brings the heart to a standstill. When the stimulus ceases, the
heart begins to beat again, at first feebly, but soon more strongly than
before the stimulus was applied. This effect of the vagus is known as
inhibition,” since the nerve, when stimulated, checks or inhibits the
normal rhythm of the heart. |
In the mammal, stimulation of the peripheral end of the vagus
produces the same effect, and, if the blood pressure is being recorded,
Fie. 82.—Origin of the nerves to the frog’s heart.
1, 2, 3 and 4 are spinal nerves.
the tracing shows a marked fall of pressure (fig. 83). When the
stimulus is removed, the heart begins to beat again, and the blood
pressure returns to, or even rises above, its original level. Sometimes,
especially if the stimulus is prolonged, the ventricle may again begin
to beat slowly even during the stimulation. This phenomenon is known
as ‘vagus escape,” and is due to the fact that the ventricle is beginning
to beat independently with its own normal slow rhythm.
In the frog the vagus fibres supply not only the sinus and auricles,
but also the ventricle. In many, and probably in all mammals, the
vagus fibres are distributed to the auricles, but not to the ventricles ;
stimulation of the vagus in this case affects only the auricles directly,
and the ventricles stop beating because they no longer receive the
normal stimulus from the auricles. As a rule the vagus affects chiefly
the sino-auricular node, inhibiting the impulses normally originating at
this point, and the whole heart is brought to a standstill. Occasionally
a
a ee eS ee
et hl
ei th he
m1 ae ea eee Ne State, Ser eee ae we Vere,
nein deh Senet
ee ee
THE VASCULAR MECHANISM. 221
it seems to act mainly by lessening the conductivity of the bundle of
OTEK, and the auricles continue to beat at their usual rate, whereas the
ventricles beat infrequently or not at all.
The Sympathetic Fibres.—Stimulation of the sympathetic nerves,
either in the frog or in the mammal, quickens the heart, and for this
reason they are called accelerator nerves; usually the force of the heart
Hurthles
Manometer
NWA
aves
_ Seconds
Stat et ett tte ot state Betis ete beat
Fic. 83.—Stimulation of peripheral end of one vagus at X.
Note the inhibition of the heart and fall of blood pressure. B.P., arterial pressure recorded
by mercurial manometer.
is also increased. The effect is only produced after a latent period of
some seconds, and lasts for a little time after the cessation of the
stimulation (fig. 84). In the mammal the blood pressure may rise
slightly or may be unaffected.
Cardiac Reflexes.—The efferent fibres of the vagus arise from a
collection of nerve cells lying in the medulla oblongata, and known as
the vagus centre. Impulses are constantly passing from the centre
down the vagus; these exert a restraining force on the rate of the
heart beat and tend to inhibit it. This action of the vagus centre is
222 ESSENTIALS OF PHYSIOLOGY.
described as its tonic inhibitory action, and is particularly well marked
in the dog and horse. On section of the vagus nerves the tonic action
is abolished and the heart beats more rapidly. The tone of the vagus
L,.Accelerator
12:35.
SEE EE
Fic. 84.—Upper tracing shows acceleration of the heart due to
stimulation of an accelerator nerve.
centre can be reflexly increased or diminished. by afferent impulses
reaching it from various parts of the body. The most important
afferent paths are (1) the depressor nerve, (2) afferent fibres running in
Vagus CE.8.
Secs.
SeGeretis to eats hk ae ea tr ke, th eet ee te re
Fic, 85.-—Reflex slowing of the heart due to stimulation of central end
of one vagus. The other vagus is intact,
the vagus from the lungs and from the heart itself, and (3) many
sensory nerves.
(1) The depressor nerve is purely afferent, and, starting in the
walls of the aortic arch, it runs up the neck on each side, in some
animals (rabbits) as a separate nerve, in others bound up with the
THE VASCULAR MECHANISM. 223
vagus trunk, to end in the medulla oblongata. If the nerve is
divided, stimulation of its central portion (7.e. the end towards the
brain) causes a fall of blood pressure and slowing of the heart. The
slowing of the heart is due to a reflexly produced increase of vagus
tone, and does not occur when the depressor nerve is stimulated after
section of the vagi.
(2) Stimulation of the central end of one vagus usually causes slowing
of the heart, provided the other vagus is intact (fig. 85).
(3) The stimulation of the central end of almost any sensory nerve,
which in a conscious animal would give rise to pain, causes reflex
hy
¥V\A
' A,
VY\A »
A
anh
Vy }
ty
Fic. 86,—Reflex acceleration of the heart and rise of blood pressure
caused by stimulation of central end of the (divided) sciatic nerve.
quickening of the heart (fig. 86), owing mainly to diminution of the tone
of the vagus centre and partly to stimulation of the accelerator nerves.
Stimulation of the central end of the splanchnic nerves or of the fifth
nerve, however, causes slowing of the heart, an~effect which is some-
times seen to follow a severe blow on the abdomen, and which is also
readily produced by irritation of the nasal mucous membrane.
The tonic action of the vagus centre is also increased when the
general blood pressure is raised, and the heart is slowed, possibly as the
direct effect of the increased blood pressure on the vagus centre.
This relationship between the blood pressure and the pulse rate is
known as Marey’s law, which states that ‘the pulse rate varies
inversely with the blood pressure.” Exceptions to this law are observed
(1) during muscular exercise and (2) as a result of painful stimuli.
224 ESSENTIALS OF PHYSIOLOGY.
The accelerator nerves also exert a tonic influence on the heart, tending
to quicken it, and when they are divided the heart beats more slowly.
The rate of the heart is also modified by impulses passing to the vagus
centre from the higher parts of the brain, the acceleration occurring
during excitement, for example, being brought about by this means.
The Action of Drugs on the Heart.—The action of drugs on the
heart is most easily studied in the frog. The rate of the heart beat is
slowed by pilocarpine or muscarine, which stimulates the vagus endings
in the heart, this effect being abolished by atropine, which paralyses
these endings, so that after its administration stimulation of the vagi
has no effect on the rate of the heart. Atropine has no action on the
accelerator nerve endings. Nicotine first stimulates and then paralyses
the cell stations of the vagus in the heart; and if it is painted on the
heart, stimulation of the vagus is ineffective, since the impulses passing
along it are blocked at the cell stations. Adrenalin stimulates the
nerve endings of the accelerator nerve, thereby inerenene both the
force and the frequency of the heart beat.
THE INNERVATION OF THE BLOOD-VESSELS.
The Vaso-constrictor Nerves.—If the ears of a rabbit, preferably
a light coloured one, are examined, it will be observed that when one
cervical sympathetic nerve is divided, the ear on that side almost
immediately becomes flushed. The central artery and its branches
can be seen to become wider, many small vessels previously invisible
come into view, and the whole ear becomes warmer than the opposite
one. Stimulation of the peripheral end of the cervical sympathetic
nerve causes an immediate constriction of the blood-vessels, many of
which disappear from view, and the ear becomes paler and cooler than
that of the opposite. side.
This experiment, which was first carried out by Claude Bernard,
shows that the cervical sympathetic nerve contains fibres which run to
the blood-vessels of the ear, and which, when stimulated, cause con-
striction of the arterioles by the contraction of their muscular walls.
It proves, further, that normally the muscular coats of the arterioles
are neither fully relaxed nor fully contracted, but are in a state of
partial contraction, which is spoken of as tone. The tone of the
arterioles exists only so long as they are in connection with the central
nervous system, and is dependent upon impulses passing from the
nervous system. ‘The nerve fibres which carry these impulses to the
arterioles, and which when stimulated increase their tone, causing
them to constrict still further, are called vaso-constrictor nerves.
In other organs the presence of vaso-constrictor nerves, and the
—
ee
THE VASCULAR MECHANISM. 225
effect of section or stimulation of these nerves on the calibre of the
arterioles has been ascertained, not by direct ocular observation, but by
determining the amount of blood flowing through the organ in a given
time. The volume of blood (V) flowing through an organ in a given
time varies directly with the mean arterial pressure (P) and inversely
with the resistance (R) in its arterioles, and is represented by the
formula V« =) Hence the rate of blood flow through a small organ
such as the kidney may be altered in one of two ways. On the one
hand, in the absence of any active change in its arterioles, a rise of
the general arterial blood pressure will force more blood through the.
arterioles of the kidney. On the other hand, if the general pressure
remains constant, dilatation of the renal arterioles will lead to an
increased rate of blood flow through the kidneys by lessening the
resistance to the flow of blood. In experiments on the rate of blood
flow through an organ it is necessary, therefore, to record both the
rate of flow and the general arterial pressure, in order to ascertain
whether the alterations in flow are due to local changes in the arterioles,
or to changes in the general arterial pressure, or possibly to a com-
bination of these factors.
The amount of blood flowing from an organ in a given time may
be directly measured by allowing the blood escaping by the veins to
pass along a graduated tube. Thus if 2 c.c. of blood flow into the
tube in 4 seconds, the rate of flow is 30 c.c. per minute. This method is
very useful inthe case of small organs such as the kidney or sub-
maxillary gland.
Another method is to record the variations in volume of the organ.
These variations depend almost entirely upon the amount of blood
present in the organ at any moment, and this will alter with the degree
of dilatation or constriction of its blood-vessels, The organ is placed in
an air-tight box or instrument known as a plethysmograph, provided
with a small opening which is connected with a tambour and a record-
ing lever. When the organ expands, the air in the box is driven along
the tube into the tambour, thereby raising the lever; shrinkage of the
organ has the opposite effect. The form of. plethysmograph varies
with the shape of the organ which is being studied. The one generally
used for the kidney, and known as an oncometer, is made of vulcanite
with a glass lid; in one side is a groove through which the renal
vessels and nerves can pass (fig. 87). The box is made air-tight by
filling the interstices with vaseline. A glass tube passing through its
wall is connected with a piston recorder (p. 199). Fig. 88 represents
a record of the kidney volume thus obtained, simultaneously with a
15
226 ESSENTIALS OF PHYSIOLOGY.
general blood-pressure tracing, and shows the effect of stimulating the
peripheral end of a divided renal nerve.
By one or other of these methods it is found that division of the
nerves passing to the kidney produces an increase of its volume and
an increased flow of blood
through it, whereas stimula-
tion of the nerves has the
opposite effect. Since in these
experiments the general
arterial pressure remains prac-
tically unaltered, the changes
in the rate of blood flow produced by section or stimulation of the
renal nerves must be brought about by alterations in the calibre of
the arterioles of the kidney.
Experiments of this kind show that the arterioles of almost every
organ in the body are supplied with vaso-constrictor nerves. The tone
Fic. 87.—Oncometer.
Fic. 88.—Tracing of arterial blood pressure (1), and kidney volume (2).
Between X and X the 10th thoracic nerve-root was stimulated,
causing a decrease in kidney volume. (From Practical Physiology,
by Pembrey and others.)
of these vessels is controlled by a centre, the vaso-motor centre, lying
in the medulla oblongata; nerve fibres pass from the cells of this
centre down the spinal cord, to end in all probability round cells in the
lateral horn in the dorsal region and round corresponding cells in the
lumbar region, These cells give off small medullated fibres, which
leave the spinal cord by the anterior roots and enter the white rami
THE VASCULAR MECHANISM. 227
communicantes to form part of the sympathetic system ; their subse-
quent course has already been described (p. 101). _
The mean arterial pressure is determined largely by the resistance —
offered by the arterioles to the flow of blood through them; it rises
when they constrict, and falls when they dilate. Hence stimulation of
the vaso-motor centre by causing constriction of arterioles all over the
body produces an enormous rise of blood pressure ; destruction of the
centre is followed by dilatation of the arterioles, and the blood pressure
falls to 40 mm. Hg or less. The centre lies in the floor of the fourth
ventricle, its lower limit in the rabbit being about 4 mm. above the
apex of the calamus scriptorius, and its upper limit about 4 mm. higher.
Its position has been ascertained experimentally by observing the effect
on the blood pressure of transection of the brain-stem at various levels.
Section through the pons or upper part of the medulla oblongata does
not affect the blood pressure; when the section passes through the
upper end of the centre it produces a slight fall of pressure, and if a
section is made a few millimetres lower the fall of pressure is maximal.
On division of the spinal cord in the cervical region, all the arterioles
are cut off from the vaso-motor centre, and the fall of blood pressure is
as great as after destruction of the centre itself. When the transection
is made in the dorsal region, only those arterioles which receive vaso-
constrictor nerves from the spinal. cord below the lesion will lose their
tone ; and the fall of arterial pressure becomes less marked the lower
the level at which the spinal cord is divided, transection in the lower
lumbar region having no effect upon the mean arterial pressure.
If an animal is kept alive for some hours or days after transection
of the spinal cord, its arterioles gradually recover their tone and the
blood pressure returns to a normal level. This is brought about by
means of subsidiary vaso-motor centres in the spinal cord, which are
called into play when the medullary centre is put out of action. On
subsequent destruction of the spinal cord, the blood pressure falls almost
to zero.
The only arterioles in the body which are not known to be influenced
by vaso-constrictor nerves are the cerebral and coronary vessels. Re-
cent observation has shown that, contrary to the views formerly held,
vaso-motor fibres are distributed to the pulmonary vessels ; their exist-
ence has been demonstrated by means of adrenalin (p. 234), which stimu-
lates the endings of the vaso-constrictor fibres and produces constriction
of the arterioles. The addition of adrenalin to the blood flowing through
the lungs is followed by constriction of the pulmonary arterioles and a
rise of pressure in the pulmonary artery, and if the lungs are perfused
under a constant pressure the outflow of blood is diminished.
a
228 _ ESSENTIALS OF PHYSIOLOGY.
The Vaso-Dilator Nerves.—In many parts of the body the arterioles
are supplied not only with vaso-constrictor but also with vaso-dilator
nerves, stimulation of which produces dilatation of these vessels owing
to relaxation of their muscular walls. The chorda tympani nerve, for
example, sends vaso-dilator fibres to the vessels in the sub-maxillary
gland, and when it is stimulated the blood flow through the gland is
increased, and may become four or five times as large as that taking
place before stimulation of the nerve. Since the general blood pressure
remains unaltered, this increase in the blood flow through the gland
must be due to dilatation of its arterioles. Vaso-dilator fibres are also
found in the nerves supplying the other salivary glands, the tongue, and
other structures in the head. Similar fibres leave the spinal cord by
the anterior roots of the second and third sacral nerves, and stimulation —
of these nerves, which are called the nervi erigentes, causes dilatation
of the blood-vessels of the generative organs and the rectum.
The vaso-dilator nerves show two important points of difference
from the vaso-constrictor nerves. In the first place, mere section of
the nerves produces no obvious effect upon the calibre of the blood-
vessels, so that, unlike the vaso-constrictors, the vaso-dilator fibres do
not appear to exercise a continuous influence upon the tone of the
arterioles. Secondly, the cell stations for these nerves lie, not in the
sympathetic ganglia, but close to or even within the organ whose
arterioles they supply. |
In the instances just given the nerves contain only vaso-dilator
fibres, but in the nerves supplying the limbs both vaso-dilator and
vaso-constrictor fibres are present. Stimulation of the peripheral end
of a nerve, such as the sciatic, usually causes vaso-constriction, though
the existence of vaso-dilator fibres can be demonstrated in one of the
following ways :— ~ .
(1) If the sciatic nerve is divided and its peripheral end stimulated
immediately, the arterioles constrict, but when the nerve is stimulated
two or three days after section, the arterioles dilate. This result is due
to the fact that the constrictor fibres degenerate and cease to carry
impulses earlier than do the dilator fibres.
(2) If the sciatic nerve is stimulated with single induction shocks
repeated at intervals of one to two seconds, these shocks stimulate
only the dilator fibres, and the arterioles dilate.
(3) The dilator nerves are excited more readily than the con-
strictor nerves by mechanical stimuli, such as pinching the nerve.
The constriction or dilatation of the arterioles, thus produced, de-
creases or increases the amount of blood flowing through the vessels of
the limb, and alters the volume of the limb.- These changes in volume
THE VASCULAR MECHANISM. 229
can be readily recorded by enclosing the distal part of the limb in a
plethysmograph of suitable shape, which is connected with a tambour
and a recording lever.
Bayliss has shown that the vaso-dilator fibres to the limbs leave the
spinal cord by the posterior roots, and that stimulation of the peripheral
m™ Volume of
S limb.
Blood
ia pressure.
Fic. 89.—Stimulation of peripheral end of 7th lumbar posterior root.
(Bayliss. )
end of the posterior roots causes marked dilatation of the arterioles
(fig. 89). The posterior root fibres starting in the skin and deep
tissues normally carry impulses from these structures to the spinal
cord and brain. When stimulation of the peripheral end of the
Fie. 90.—-Scheme to show path of antidromic impulses (axon reflex).
posterior root causes dilatation of the arterioles, the impulses must
pass towards the periphery, that is, in. the opposite direction to that
taken by the impulses from the skin. For this reason the impulses
running towards the periphery and causing vaso-dilatation have been
called antidromic. In normal circumstances a stimulus applied to the
skin at A (fig. 90) will give rise to an impulse .passing along the
230 ESSENTIALS OF PHYSIOLOGY.
sensory nerve B into the spinal cord; in its course each nerve fibre
gives off a branch C, which ends in the walls of the arterioles (D) of
the limb. The impulse passing along the fibre B also passes along C
and relaxes the muscle of the arterioles. Since nerve fibres can con-
duct impulses in both directions, stimulation of the posterior root
fibres at E gives rise to an impulse which, travelling down the nerve,
passes by the branch C to the arterioles at D, and causes them to relax.
We see, therefore, that whether the stimulus is applied at the periphery
A or at E, the impulse reaches the arterioles along the branch C. The
effect of stimulation at A, which is not a true reflex, is called an axon
reflec. If the posterior root fibres become degenerated peripherally to
the ganglion, the axon reflex disappears, and a stimulus applied to the
skin at A causes no dilatation of the subcutaneous vessels.
This reflex is of great importance to the body. As is well known,
an irritant (e.g. a mustard blister) applied to the skin causes dilatation
of the cutaneous vessels and reddening of the skin. The dilatation of
the vessels is part of the means by which the tissues protect themselves
against injuries or irritants, and if the vascular changes do not occur,
owing to degeneration of the peripheral sensory fibres, the damage done
by the irritant to the tissues may be much more severe.
Vaso-dilator fibres are also present in the sympathetic system itself,
although their presence is not readily demonstrated owing to the greater
abundance of vaso-constrictor fibres ; but when the endings of the latter
are paralysed by the drug ergotoxin, stimulation of the splanchnic
nerves causes vaso-dilatation and a fall of blood pressure.
The vaso-dilator fibres seem to be concerned mainly, though not
entirely, with bringing about an increased flow of blood in individual
organs, whereas the vaso-constrictor fibres, controlled by the vaso-motor
centre, regulate the.tone of the arterioles of the body as a whole.
Influences Affeeting the Vaso-motor Centre.—The vaso-motor centre
is extremely susceptible both to impulses reaching it from other parts
of the nervous system, whether these reach it from the higher parts of
the brain or from the peripheral nerves, and to changes in the character
and amount of the blood passing to the brain. Its activities are con-
stantly varying in response to these stimuli, in such a way that the
mean arterial pressure is raised to meet special needs of the body, and
is prevented from falling below the level necessary for the adequate
supply of blood to the tissues, and more especially to the brain.
(1) Nervous Stimuli.—The depressor nerve is a purely afferent
nerve, originating in the root of the aorta, and passing to the brain.
Electrical stimulation of its central end causes a lessening of the activity
of the vaso-motor centre, decrease of the tone of the arterioles, and a
ot
ae ee ee ee ae
4
THE VASCULAR MECHANISM. = 231
fall of blood pressure; this occurs whether the vagus nerves haye
been previously divided or are intact. In all probability, when the
arterial blood pressure is very high, impulses are set up in the endings
of the nerve in the stretched aortic wall which reflexly lower the blood
pressure, and thus lessen the strain placed upon the heart. The
passage of impulses along the depressor nerve in these circumstances
can be observed by means of the string galvanometer.
Increased activity of the centre and a rise of blood pressure are
brought about by stimulation of most sensory nerves (see fig. 86), and also
by impulses passing to it from the cerebral cortex during muscular
exercise and in. violent emotional excitement, such as fear or anger,
(2) The Composition of the Blood.—The vaso-motor centre is extremely
sensitive to changes in the composition of the blood supplying it, being
stimulated by lack of oxygen or by the presence of an excess of carbonic
acid in the blood. The effect of lack of oxygen and of excess of carbonic
acid is seen in its most extreme form in asphyxia (p. 270), but even
a slight excess of carbonic acid stimulates the centre, and leads to
constriction of the arterioles and a rise of blood pressure. The same
effect is produced when the reaction of the blood, measured by its
H ion concentration, becomes less alkaline; and the injection into the
blood stream of small amounts of an acid, such as lactic acid, may
produce a considerable rise of blood pressure. Further, the vaso-motor
centre is stimulated whenever the amount of blood passing through the
brain in a given time diminishes.
During asphyxia the blood-pressure tracing often shows, in addition
to the oscillations caused by the heart beat, two other groups of waves.
In the first place, the blood pressure shows oscillations corresponding
with the respiratory movements ; they are still present in a curarised
animal, and are due to impulses passing by irradiation from the excited
respiratory centre to the vaso-motor centre. They are called Traube-
Hering, curves. Secondly, much larger waves, known as Mayer
curves, are seen, and are due to rhythmical variations in the activity
of the vaso-motor centre; they are often present after severe
hemorrhage. | 3 .
The subsidiary vaso-motor centres, unlike the chief centre, are
extremely insensitive to either nervous or chemical stimuli, and probably
they take little or no part in the vascular changes normally occurring
in the body, though their activity can be excited by asphyxia.
In whatever way the activity of the vaso-motor centre is increased,
the constriction of the blood-vessels which is produced is most pro-
nounced in the abdominal organs. The splanchnic nerves send con-
strictor fibres to the blood-vessels of almost the whole of the abdominal
232 '\ ESSENTIALS OF PHYSIOLOGY.
viscera, and the total capacity of these vessels is so large that the
amount of. blood contained in them forms a great proportion of the
total blood in the body. Further, the general arterial pressure is more
markedly altered by section or stimulation of the pplatiglins nerves
than of any other nerve in the body.
Hence the maintenance of the mean arterial pressure at a constant.
level, in spite of the varying influences which are brought to bear upon
the vaso-motor centre in daily life, is largely effected by changes in
the degree of constriction of the arterioles supplied by the splanchnic
nerves, and known as the splanchnic area. For example, when the
depressor nerve is stimulated, the fall of pressure which occurs is due
mainly to the dilatation of the arterioles in the splanchnic area. When
the blood supply to the centre is deficient, the resulting rise of blood
pressure is caused primarily and chiefly by constriction of the arterioles
of the abdominal organs. During muscular exercise an increased flow
of blood through the skeletal muscles is required and takes place, and
the vessels in the splanchnic area are constricted, more blood being
diverted into the muscular system. On the contrary, during digestion
the digestive organs require an abundant blood supply, and the vessels
of the skin are contracted, while the arterioles of the digestive tract are
relaxed. It is for this reason that severe exercise taken immediately
after a meal tends to disturb digestion.
- When the arterioles of the abdominal organs constrict in response to
stimulation of the splanchnic nerve, they naturally contain less blood
than before, and the blood thus squeezed out of them by their con-
striction has to be accommodated in other parts of the vascular system.
Much of it passes into the arteries, thereby distending them more fully,
and so raising the arterial pressure. This factor is an additional
cause of the rise of blood pressure which is brought about by stimula-
tion of the splanchnic nerves.
Influence of Gravity on the Circulation.—If a thin-walled,
cylindrical rubber bag is filled with fluid and held with its long axis
vertical, the fluid, under the influence of gravity, tends to accumulate
at, and to distend, the lower end of the bag. In the body also, the
blood tends to accumulate in the most dependent parts, and if a rabbit.
is held up by its ears the blood accumulates in the abdominal area,
particularly in the large veins, and the arterial blood pressure falls
(fig. 91). Asa result, the amount of blood passing through the brain
in a given time is inadequate, and its functions are seriously interfered
with, so much so that it is said to be possible to kill a hutch rabbit by
holding it up in this position for a short time, death being due to
anemia of the brain.
= a a ie eA ibd Se ee
THE VASCULAR MECHANISM. 233
The influence of gravity is antagonised completely in man, and to a
lesser extent in most animals, by means of a compensating action on the
part of the vaso-motor centre. When a man rises from the horizontal
to the standing position, the blood tends to accumulate in his
abdomen, and the supply of blood to the brain is diminished. This
diminution at once stimulates the extremely sensitive vaso-motor
centre, which sends out impulses constricting the arterioles of the
splanchnic area, thereby forcing the blood out of this area into the rest
of the body, including the brain. Conversely, the splanchnic arterioles
A hae
4 tty 4,
‘ yl
rie Wy
i
tay
ny
Fig. 91.—Aortic blood pressure. Effect of postung. (L. Hill.) From Practical
Physiology, by Pembrey and others.
A-B, vertical, head up; B, horizontal; C, vertical, head down; D, horizontal.
relax to some extent whenever an individual changes from the vertical
to the horizontal position.
This reaction on the part of the vaso-motor centre to any change
in the position of the body as regards gravity, is so rapid and complete
that we are not normally aware of its existence. The temporary giddi-
ness, which-is often noticed by individuals who are anemic or run
down on changing suddenly from:a horizontal to a standing position,
is due to the fact that the response of the vaso-motor centre to the
change of position is slower than usual, and that for a few moments the
brain is inadequately supplied with blood. In the same way “ fainting”
is in many cases caused by temporary diminution of the activity of
the vaso-motor centre, so that the blood pressure falls and the blood
supply to the brain is deficient, causing loss of consciousness. The
234 ESSENTIALS OF PHYSIOLOGY.
compensating action of the vaso-motor centre for the effects of gravity
is also inefficient in aneesthetised persons.
Owing to the influence of gravity, the arterial pressure in the
femoral artery of an individual in the erect position is much higher
than that in the brachial artery. The constriction of the arterioles of
the legs, however, is so great that the pressure in the capillaries and
veins of the leg and foot is no higher than that in the hands. The
flow of blood from the foot and leg back to the heart against the force
of gravity is greatly assisted, and indeed made possible, by muscular
movement: each muscular movement squeezes the blood along the
veins towards the heart, and the valves prevent any reflux. In persons
‘who are compelled to stand still for any length of time, or in whom
the valves are defective, the veins tend to become dilated and varicose.
The Effect of Hemorrhage.—Any considerable loss of blood from
the body lessens the amount present in the arterial system, and the
output of the heart at each beat decreases; the arterial pressure falls,
and the supply of blood to the brain becomes inadequate. The vaso-
motor centre is at once stimulated, causing increased constriction of
the arterioles ; at the same time fluid passes from the tissues into the
blood, and the arterial pressure rapidly regains its normal level.
After a very severe hemorrhage these compensatory mechanisms are
inadequate, and the blood: pressure remains low.
The Influence of Adrenalin.—The structure and functions of the
suprarenal glands are dealt with on p. 400, but it is necessary to
mention at this point their influence on the circulation. These glands
producea substance, adrenalin, which can be extracted from them and
obtained in a pure form. A minute amount of adrenalin (e.g. 0°01
or 0°02 mgr.), injected into a vein, stimulates the nerve endings of all
the fibres of the sympathetic system, including those which supply the
arterioles, and causes extreme vaso-constriction of all the arterioles
except the coronary vessels, which are dilated, and the cerebral vessels,
which are unaffected ; and if the vagus nerves have been divided a
huge rise of blood pressure is produced. ‘The suprarenal glands receive
fibres from the splanchnic nerves, and when a splanchnic nerve is
stimulated, some of the adrenalin present in the suprarenal gland passes
into the suprarenal vein and so into the blood stream, and gives rise
to the effects just described. It is clear, therefore, that whenever a:
splanchnic nerve is stimulated the ensuing rise of blood pressure is
partly due to the increased peripheral resistance brought about by the
direct action of the splanchnic nerve on the abdominal blood-vessels,
and is partly caused by the constriction of arterioles all over the body
by the adrenalin set free into the blood stream. The influence of
THE VASCULAR MECHANISM, 235
these two factors is seen in the form of the blood-pressure tracing,
which often shows, as it rises, a small notch or step (fig. 92) ; the first
part of the rise is due to the direct action of the splanchnic nerve; the
rise above the notch is due to adrenalin. Owing to the setting free of
adrenalin, stimulation of a splanchnic nerve causes diminution in the
volume of the limbs. These effects are produced not only when the
splanchnic nerve is divided and its peripheral end is directly stimu-
lated, but also when the impulse passing along the splanchnic nerve
originates in the vaso-motor centre itself, as in asphyxia. After
extirpation of the suprarenal glands, adrenalin can no longer be set
free into the blood stream, and stimulation of the splanchnic nerve
causes a much smaller rise of blood pressure; the vaso-constriction is
limited to the abdominal vessels, and the blood-vessels of the limbs are
passively dilated by the higher arterial pressure (fig. 93).
yt Nyon
Rey ee See
etl bel Poel Noe st vt tat aye ny evel yt hy ae
sel a ne a eg MeN ehh
Fi nts i
Nh im '
Fic, 92.—Blood-pressure tracing; showing effect of stimulation of left
splanchnic nerve, (von Anrep.)
Shock.—After severe injuries or profuse hemorrhage an individual
may pass into the condition known as shock. The characteristic
symptoms of shock are a low arterial blood pressure, and disappearance
of many of the normal reflexes; the pulse is rapid and feeble, the
respiration is shallow, and the temperature is low. A similar condition
sometimes occurs after surgical operations, and can be experimentally
produced in animals. Its causation is not understood, though various
explanations have been offered to account for the low blood pressure,
which forms such an important part of shock. The low blood pressure
is not due to paralysis of the vaso-motor centre, since the centre
responds to afferent stimuli almost as readily as in the normal animal.
It has been suggested that in shock the veins dilate and that the
blood accumulates in them, leaving the arterial system comparatively
empty: in these circumstances the arterial pressure will be low, and
the nutrition of the brain will be impaired. Other explanations have
also been offered, none of which is entirely satisfactory.
236 ESSENTIALS OF PHYSIOLOGY.
Local Changes in the Arterioles.—The variations in the activity of
the vaso-motor centre, brought about by the means already described,
are chiefly directed to regulating the mean arterial pressure and to.
providing an efficient supply of blood to the brain. Alterations in the
calibre of the arterioles in any one organ of the body as distinct from
the body in general are sometimes due to the presence of vaso-dilator
nerves, as, for example, the chorda tympani. There is, however, another
factor of great importance. Generally speaking, increased functional
activity of any organ of the body is accompanied by dilatation of its.
arterioles and an increased flow of blood through it; this: is brought
Fie. 98.—Stimulation of a splanchnic nerve after removal of the suprarenal glands,
(von Anrep. )
L.V., volume of leg enclosed in a plethysmograph ; B.P., arterial blood pressure.
about partly, or even wholly, by the direct action upon the walls of
the arterioles of the waste products (metabolites) formed by the organ:
during its activity. They include carbonic acid, lactic acid, and
probably other substances, and experiment has shown that when these
products are added to the blood passing through an organ, e.g. the
heart or skeletal muscle, its arterioles dilate. This local mechanism
provides a means by which the increased demands of a tissue for
nutritive material and oxygen, when it is active, are met by an increase
in the amount of blood passing through it.
THE CEREBRAL CIRCULATION.
The circulation of the blood through the brain is peculiar in two:
respects. In the first place, the brain is enclosed in a rigid case, the
skull, which it almost completely fills, and, secondly, there is at present
no conclusive evidence that the arterioles of the brain are under the
—_—
—*
THE VASCULAR MECHANISM. 237
control of the vaso-motor centre, although the existence of nerve
plexuses round them has been observed histologically. The presence
of cerebro-spinal fluid in the skull allows for a slight increase in the
volume of the blood in the cerebral blood-vessels, since a rise of
pressure within these vessels distends them to a certain extent, and
forces some of the cerebro-spinal fluid out of the skull into the sheaths
of the nerve trunks. Apart from this increase, which amounts only
to 2 or 3 «c., the amount of blood in the cerebral vessels cannot
be increased, since the capacity of the skull is constant, and the
brain, which, together with the blood-vessels, practically fills it, is
incompressible,
Any increase or decrease in the amount of blood supplying the
brain is brought about entirely by variations in the veloevty with which
the blood flows through the cerebral vessels. It has already been
pointed out that if the width of the bed through which the blood is
flowing remains constant, the velocity of the flow will vary directly with
the pressure which tends to drive the blood along the vessels. In the
body this pressure is the general arterial pressure, and since the volume
of the cerebral vessels remains constant, the velocity of the flow of
blood through them will depend entirely on’ the arterial pressure, pro-
vided there is no obstruction to the escape of blood from the cerebral
veins. If the arterial pressure rises, the arteries become rather more
distended and occupy more space, and as the total volume of the
cerebral vessels remains unaltered, the expansion of the arterioles
must be accompanied by narrowing of the capillaries and veins. The
narrowing is not sufficient to cause any obstruction to the escape of
blood through these vessels, and the amount of blood flowing through
the brain is greatly increased. If the general arterial pressure falls,
the velocity of the blood flow through the brain diminishes, and the
amount of oxygen reaching it in a given time is correspondingly
decreased. When the supply of oxygen falls, the vaso-motor centre is
stimulated, causing general vaso-motor constriction, and thereby raising
the blood pressure to such a level that the flow of blood through the
brain is again sufficient to provide an adequate supply of oxygen.
Thus the blood supply to the brain is determined almost entirely by
conditions outside the brain itself, being increased whenever the
abdominal vessels are constricted and diminished when these dilate.
The brain is protected from lack of oxygen by the vaso-motor centre in
the manner just described.
If the blood supply to the brain becomes inadequate, the respiratory
centre is also stimulated, and the increased respiratory movements
bring more blood to the heart, and so enable it to expel more blood at
238 _ ESSENTIALS OF PHYSIOLOGY.
each beat, and thus to assist the vaso-motor centre in raising the blood
pressure. c
The pressure of the contents upon the wall of: the skull is equal to
that within the capillaries of the brain. It cannot be greater than the
capillary pressure, since in these circumstances the capillaries would
collapse and the flow of blood through the brain would cease. The
intra-cranial pressure can be raised by any obstruction to the escape of
blood from the cerebral veins, or by the presence within the skull of a
foreign body, such as a blood clot. In the latter case the pressure
within the skull may rise sufficiently to compress the capillaries or even
actually to obliterate them. Such compression, which inevitably
diminishes the blood supply to the brain, may cause loss of conscious-—
ness and other serious symptoms. .
SUMMARY OF THE ESSENTIAL FEATURES OF
THE CIRCULATION.
The most important functions of the circulatory mechanism are (1)
to maintain a continuous slow stream of blood through the capillaries
so as to provide the most favourable conditions for the passage of oxygen
and nutritive material from the blood to the tissues and of waste pro-
ducts into the blood ; and (2) to keep the arterial pressure at such a
height as to ensure an adequate supply of blood to all parts of the body.
The carrying out of these functions in response to the varying needs of
the tissues and to stimuli reaching the body from the. outer world is
chiefly effected through the nervous system, which controls the rate of
the heart and the calibre of the arterioles. In the attainment of this
end, the heart and arterioles are made to co-operate with each other.
For instance, if the peripheral resistance becomes very great, the rise in
blood pressure stimulates the vagus centre, the heart beats more slowly,
and the pressure falls to its normal level (Marey’s law). Again, if the
heart is beating against a high peripheral resistance, its work is
lightened by impulses passing along the depressor nerve, which reflexly
diminish this resistance. If the blood pressure falls, it is restored to
the normal level by increased activity of the vaso-motor centre, which is
assisted in many cases by an increased output of blood from the heart.
These regulative mechanisms depend for their efficiency upon the
nutrition of the heart itself, and upon the maintenance of its normal
rhythm. Broadly speaking, the rate of the heart is determined by the
influence of the nervous system, and the force of the beat by the state
of nutrition of its muscular fibres. The latter depends almost entirely
upon the amount and character of the blood supplied to the heart
by the coronary vessels; if its nutrition is impaired the heart beats
OO a a
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THE VASCULAR MECHANISM. 239
more and more feebly, fails to expel its contents into the aorta, and
finally the circulation may come to a standstill. Thus the nutrition of
the heart muscle is ultimately the most important feature of the -
circulatory mechanism.
SECTION VI.
THE FORMATION OF LYMPH.
Except in the spleen, and possibly the liver, the blood does not come
into direct contact with the cells of the tissues. It is separated from
them not only by the walls of the capillaries, but by a fluid called lymph
or tissue fluid, which lies between the capillaries and the tissue cells
themselves. From these spaces the lymph passes into narrow channels
(lymphatic vessels) lined by endothelial cells. These channels unite
and finally end in a single vessel, the thoracic duct, which opens into
the junction of the left jugular and subclavian veins, and conveys the
lymph from the greater part of the body into the blood stream. The
lymph from the right side of the head and neck, and the right fore-limb
passes into a vessel which opens into the junction of the right jugular
and subclavian veins. The lymph has been described by Foster as a
‘middle man,” since, on the one hand, it receives from the blood oxygen
and dissolved nutrient materials and passes them on to the tissue cells,
and, on the other hand, it receives from the tissue cells carbonic acid
and other waste products and returns them to the blood stream. The
interchange of material between the blood and the tissues takes place
by diffusion (p. 14), and in this way the tissues are nourished without
any increase necessarily taking place in the amount of tissue fluid.
THE COMPOSITION OF LYMPH.
Lymph can be collected by placing a cannula in the thoracic duct
of an animal, such as a dog or horse. If the animal has not been
recently fed, the lymph is a clear, colourless fluid having a specific
gravity of about 1015, and usually clots when allowed to stand. It
contains some lymphocytes, 4 to 5 per cent. of protein, the proteins
being the same as those in blood serum, and also various salts and
extractives. After a meal the lymph is milky in appearance, owing to
the presence of large numbers of minute fat globules. The fat is
derived from that taken in the food, which, after absorption from the
digestive tract, passes into the intestinal lymphatic vessels (lacteals).
In their course the vessels pass through the lymphatic glands, in which
lymphocytes are formed ; these enter the lymph stream and are carried
into the blood.
240 ESSENTIALS OF PHYSIOLOGY.
THE FORMATION OF LYMPH.
Although the exchange of material between the blood and the tissues
does not necessarily increase the amount of lymph in the tissue spaces,
it is found that, in point of fact, lymph is constantly being formed in
the body, and, after passing along the lymphatic vessels, is returned
to the blood stream along the thoracic duct. The formation of lymph
has been attributed by some writers to secretion by the walls of the
capillaries, and by others to the action of purely physical processes such
as filtration and osmosis. If the latter view is correct, a rise in capillary
pressure should lead to an increase in the formation of lymph ; and this
is found to be the case.
The pressure in the capillaries is much more easily altered by a rise
in the venous pressure than by a rise in the mean arterial pressure ;
a rise in venous pressure, by obstructing the escape of blood from the
capillaries, at once raises the capillary pressure. Hence a large rise in
capillary pressure can be produced by ligaturing the vena cava or portal
vein ; and this is followed by a great increase in the flow of lymph from
the ditivaia duct.
Again, when a large quantity of saline solution is injected into the
circulation, the blood is not only increased in amount, but becomes
more watery, the condition being called hydremic plethora. The arterial
pressure remains almost unaltered, but the veins are distended to con-
tain the greater part of the fluid thus added to the circulation, and the
venous pressure rises; as a result the pressure in the capillaries also
rises, and the flow of lymph from the thoracic duct becomes very rapid.
Hydremic plethora may also be produced by injecting into the
blood a strong solution of dextrose or other crystalloid body ; this
raises the osmotic pressure of the blood, and water passes by osmosis
from the tissues into the blood, thereby increasing its volume and
raising the capillary pressure. In these circumstances a great increase
takes place in the flow of lymph from the thoracic duct, as is seen in the
following table :—
HyDRa&MIC PLETHORA.
Periods of Ten Arterial Blood Pressure in Inferior
Minutes. Pressure, Vena Cava, | Flow of Lymph.
100 mm. Hg 12 mm. water | 3°0 cc.
40 grm. dextrose dissolved in 50 c.c. water injected into a vein.
2 105 mm. Hg 180 mm. water 33°0 c.c.
3 120 9 ” 50 29 oe) | 31°0 ”
4 118 4, 5; 25 4s | 20°0 ,,
THE VASCULAR MECHANISM. 241
From these experiments it may be concluded that the walls of the
capillaries form a membrane through which lymph can be filtered off,
and that the amount of fluid which passes through the membrane in a.
given time depends directly upon the capillary pressure.
Another factor in the formation of lymph is the variable readiness
with which filtration takes place through the capillaries in different
parts of the body under the same pressure. The least permeable
capillaries are those of the limbs, the most permeable being those of
the liver; and almost all the lymph flowing from the thoracic duct of a
resting animal is formed in the liver and digestive tract. The perme-
ability of the capillaries can be increased by the injection of various
substances called lymphagogues, including peptone and leech extract.
The injection of one or other of these substances into the blood stream
leads to an increased formation of lymph, although the capillary
pressure, after a short time, is almost unaltered. The lymph-is derived
almost entirely from the liver, the capillaries of which become more
permeable, as is shown by the fact that, if the lymphatic vessels of the
liver are ligatured, the subsequent injection of peptone does not increase
the formation of lymph.
The permeability of the capillaries is also increased when their nutri-
tion is impaired, e.g. by lack of oxygen, and this may give rise to dropsy.
The formation of lymph is also dependent upon the metabolism
of the tissues themselves. The injection into the blood of bile salts,
for example, leads to the secretion of bile by the liver, and the flow of
lymph from the thoracic duct is increased. This is not due to raised
capillary pressure or to changes in the permeability of the capillaries,
but is brought about in the following manner. In normal circum-
stances the osmotic pressure of the tissue cells, the lymph, and the
blood is almost the same. When the metabolism of the liver is
increased, metabolic products are formed in the liver cells and diffuse
into the lymph, raising the osmotic pressure of the lymph and liver cells
as compared with that of the blood. Consequently water passes from
the blood into the lymph, and this fluid is increased in amount and
gives rise to a larger flow from the thoracic duct. Similar results
have been observed in other organs, and probably increased functional
activity of any tissue in the body leads to increased formation of
lymph. We may conclude, therefore, that lymph formation is not
a secretory process, but is brought about by purely physical processes,
namely, filtration and osmosis; and the factors concerned in its pro-
duction are (1) the capillary pressure, (2) the degree of permeability of
the capillary walls, and (3) the metabolic activity of the tissues. There
is no reason to suppose that in health the permeability of the
16 —
242 ESSENTIALS OF PHYSIOLOGY.
capillaries alters, and therefore the formation of lymph is increased
chiefly by variations in the first and third of these factors.
ABSORPTION FROM THE TISSUES.
If a saline solution containing some readily recognisable substance,
such as potassium iodide, is injected under the skin or into the pleural
or peritoneal cavity, it rapidly disappears, and the presence of
potassium iodide can be demonstrated in the blood or urine some
time before it appears in the lymph flowing from the thoracic duct.
This experiment makes it clear, first, that water and substances in
solution can be readily absorbed, and, secondly, that the absorption does
not take place into the lymphatic vessels, but through the capillary
walls directly into the blood. Similarly, tissue fluid may be absorbed
through the capillary walls; after hemorrhage, for instance, the
volume of the blood is rapidly brought back to the normal by the
passage of fluid from the tissue spaces into the blood. This process
depends upon the fact that proteins exert an osmotic pressure, which,
though very small in comparison with that of a solution of crystalloid
bodies, is yet appreciable. The osmotic pressure of the crystalloids in
blood and lymph is much the same, but, owing to the percentage of
protein in blood being higher than that in lymph, the blood has a slightly
higher osmotic pressure, and fluid tends to pass from the lymph into the
blood. At the same time, fluid is being filtered through the capillary
wall from the blood into the lymph at a rate varying with the capillary
pressure. These two processes, namely, filtration and absorption, tend
to balance one another and to keep the amount of tissue fluid constant.
The balance may be disturbed either by a rise or by a fall of capillary
pressure. In the former case, the amount of tissue fluid is increased,
whereas, if the capillary pressure falls, for instance after severe
hemorrhage, the amount of fluid absorbed exceeds that which is filtered
through the capillary walls ; and the volume of the blood increases at
the expense of the lymph and tissues.
The absorption of saline solution, placed under the skin, is brought
about partly by simple diffusion of the dissolved substances into the
blood, and partly by the osmotic pressure exerted by the proteins in
the blood.
THE FLOW OF LYMPH.
The tissue fluid is formed under a pressure which is probably rather
less than that in the capillaries, and this pressure tends to drive the
lymph towards the thoracic duct; the pressure in the duct where it
THE VASCULAR MECHANISM, 243
opens into the great veins is at most 2 to 3 mm. Hg, and may be negative.
Other and more important factors are muscular and respiratory move-
ments. Every muscular movement, by compressing the lymphatic ©
vessels, forces the lymph on towards the thoracic duct. With each
inspiration, the abdominal pressure rises and the intestinal lymphatic
vessels are compressed ; at the same time, the pressure on the thoracic
duct becomes negative, and lymph is sucked into the chest.. The effect
of these movements is assisted by the presence of valves in the larger
lymphatic vessels, which prevent any backward flow of lymph.
CHAPTER IX.
THE RESPIRATORY SYSTEM.
SECTION I.
RESPIRATION consists in the transference of oxygen from the atmo-
spheric air to the tissues of the body, and of carbonic acid from the
tissues to the outer air. In man and most vertebrates, the oxygen and
carbonic acid are carried to and from the tissues respectively by the
blood, which, on the one hand, receives oxygen in the lungs and gives
it up to the tissues, and, on the other hand, receives carbonic acid from
the tissues and gives it up in the lungs. In fishes and many in-
vertebrates, the lungs are replaced by gills. The transference of oxygen
from the atmosphere into the blood and of carbonic acid from the
blood into the atmosphere is called external respiration, the interchange
of gases between the blood and the tissues being termed internal or
tissue respiration.
THE STRUCTURE OF THE AIR PASSAGES AND LUNGS.
The respiratory system consists of the lungs and the air passages
leading to them, namely the mouth and lower half of the nasal cavity,
the upper part of the pharynx, the larynx, trachea, and bronchi. The
trachea is a wide tube about 44 inches in length in man, and is lined
by stratified epithelium, the inner layer being ciliated. The epithelium
rests upon a thick basement membrane, beneath which is a layer of
elastic fibres running longitudinally ; external to this membrane is
areolar tissue in which lie many small glands, which secrete mucin.
The trachea is strengthened by C-shaped hoops of cartilage, and on its
posterior wall is a layer of unstriped- muscle, the fibres of which run
transversely. The cartilaginous rings keep the lumen of the trachea
patent, and prevent its occlusion by external pressure. The fluid
formed by the small mucous glands moistens the inner surface of the
trachea, and serves also to catch bacteria or particles of dust, which
244 |
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THE RESPIRATORY SYSTEM. 245
are carried in with the inspired air; the cilia, by their movement,
carry the fluid up the trachea into the pharynx.
The main bronchi are similar in structure to the trachea.
In the lungs the bronchi branch in a tree-like manner, the final
ramifications opening into the pulmonary air cells. The larger intra-
pulmonary bronchi are lined by columnar ciliated epithelium resting
on a basement membrane. Lying under this basement membrane are
longitudinally disposed elastic fibres with loose connective tissue. More
externally is a layer of smooth muscle fibres arranged circularly, the
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(From Gray’s Anatomy.)
bronchial muscle. External to the bronchial muscle -is a fibrous coat
containing scattered, irregular plates of hyaline cartilage.
The smaller bronchi (bronchioles) have no cartilaginous plates, but
their muscular coat is well marked.
Each bronchiole leads into a small number (three or four) of wider
thin-walled spaces, lined by flattened epithelium, and called atria. Out
of each atrium open two or three blind diverticula, each of which is
called an infundibulum. The walls of the infundibula are studded with
hemispherical sacs known as alveoli, which are lined by flattened, non-
nucleated, epithelial cells. Between adjacent alveoli there is a dense
network of capillaries, supported by a small amounf of fine connective
246 -ESSENTIALS OF PHYSIOLOGY.
and elastic tissue; the network of capillaries is thus common to the
two adjacent air cells, and the blood in the capillaries is separated from
the air in the alveoli merely by two thin layers of epithelium. In
birds, even the alveolar epithelium appears to be absent, the blood and
air being separated solely by the capillary wall.
The branches of the pulmonary artery accompany the bronchi, and
open into the alveolar capillary network, from which blood is carried
back to the left auricle by the pulmonary veins. Oxygenated blood is
supplied to the bronchi by the bronchial arteries.
The lungs nearly fill the thoracic cavity, the space between them
being occupied by the heart, great vessels, and other structures. Each.
lung is covered by a thin membrane, consisting of a superficial layer.
of flattened epithelium resting on connective tissue; the membrane
is known as the pleura, and is reflected at the root of the lung on
to the chest wall. Each pleura thus forms a closed sac, the walls
of which are normally in apposition ; their inner surfaces are moistened
by a small amount of fluid, resembling lymph, and glide over one
another with every movement of the chest wall and lung.
THE RESPIRATORY MOVEMENTS. .
The thorax is a completely closed box which alters in shape and
size with each respiratory movement ; with inspiration it becomes larger
in all its diameters, vertical, antero-posterior, and transverse, returning
to its former size during expiration. This increase in size is brought
about partly by the upward movement of the ribs, partly by the descent
of the diaphragm. ®
The diaphragm consists of a muscular sheet with a tendinous
central portion. In the position of rest it forms a dome projecting
towards the thoracic cavity, and when it contracts the summit of the
dome, namely the tendinous portion, is drawn downwards from 1 to 2 em.,
thus increasing the vertical diameter of the chest. The extent to which
the central tendon can be drawn downwards is limited by the resistance
of the abdominal viscera, and when the limit is reached, the direction
of the pull of the costo-sternal muscle fibres of the diaphragm is
reversed so that the lower end of the sternum and the movable ribs
are raised, The spinal fibres of the diaphragm, the lower attachment
of which is a fixed point, still exert a downward pull upon the central
tendon,
The Ribs.—At the beginning of inspiration the first pair of ribs
and the manubrium sterni are fixed by the resistance of the cervical
structures, and the second to the fifth pairs of ribs are drawn upwards
by the contraction of the external intercostal muscles and the serratus
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THE RESPIRATORY SYSTEM. 247
posticus (posterior) superior. Since the ribs slant downwards and
forwards from their vertebral articulation, this upward movement
carries forward the sternum and increases the antero-posterior diameter
of the chest. At the same time the ribs rotate slightly round the axis
represented by a line drawn from their vertebral to their costal attach-
ments, and their lower borders, which in the expiratory position are
inverted, become everted. The costo-chondral angle is also opened out.
By these means the transverse diameter of the chest is enlarged.
The lower ribs are raised partly by the external intercostal muscles,
partly also by the contraction of the diaphragm, and partly by the
interchondral portion of the internal intercostal muscles.
Thus the muscles concerned in quiet inspiration are the diaphragm,
the external intercostal muscles, the interchondral portion of the internal
intercostal muscles, and the serratus posticus (posterior) superior.
The entrance of air into the lungs is also assisted by widening of the
glottis, and, if the breathing is at all laboured, by dilatation of the ale
nasi. In forced inspiration, other muscles such as the trapezius, pectoral
muscles, sterno-mastoid, and rhomboids are called into play.
During quiet expiration, the chest returns to its former shape and
size, mainly on account of the elasticity of the chest wall and lungs,
and of the abdominal wall and abdominal contents; the downward
movement of the ribs during expiration is also assisted by the con-
traction of the costal part of the internal intercostal muscles, which
pass downwards and backwards from each rib to the one immediately
below it. In forced expiration the accessory muscles employed are
mainly those of the abdominal wall.
Quiet respiration in men is carried out principally by the movements
of the diaphragm. In women the larger part is played by the move-
ments of the upper ribs, chiefly on account of wearing of tight clothing,
which interferes with the movement of the diaphragm and lower ribs.
THE EFFECT OF THE RESPIRATORY MOVEMENTS
ON THE LUNGS.
The passage of air into and out of the lungs during respiration is
brought about by purely mechanical causes, which can be roughly
illustrated by the aid of an artificial model. A thin-walled rubber bag
is placed in a glass vessel closed by a cork; the bag is attached to a
glass tube, which passes through the cork and is open at the top.
The bottle is connected by another tube with a mercury manometer
and by a third tube with a suction pump, by means of which air can
be sucked out of it.
248 ESSENTIALS OF PHYSIOLOGY.
At the outset of the experiment the air in the bottle is at the same
pressure as that of the atmosphere, and the bag is collapsed. If a little
air is sucked out of the bottle, the pressure falls, and, since the pres-
sure within the rubber bag remains unaltered, a difference of pressure
is set up on its inner and outer surfaces. The bag expands, air being
sucked into it along the glass tube to fill the extra space thus provided,
until the pressure within it and outside it becomes nearly equal; but
the pressure outside the bag is finally a little less than atmospheric
pressure, because a part of the pressure in the bag is used up in
overcoming the tendency of its’ stretched elastic wall to collapse.
When more air is sucked out of the bottle the rubber bag expands:
still further, but the pressure in the bottle remains negative, that -
is, less than atmospheric pressure. When the bottle is opened,
the pressure on each side of the bag becomes the same, and it
collapses.
In the body the lungs take the place of the bag, the bottle is
represented by the chest, and the changes in the pressure on the outer
surface of the lungs are brought about by alteration in the size of the
chest cavity. If a small tube, connected with a manometer, is passed
- through the chest wall of an animal into the pleural cavity, the
pressure within the chest is seen to be lower than that of the atmo-
sphere ; at the end of expiration the difference is usually about 6 mm.
Hg. When the chest enlarges during inspiration, the pressure on
the outer surface of the lungs diminishes, and, as the pressure within
them remains unchanged, they expand still further. Owing to the greater
force required to bring about this additional expansion of the lungs, the
pressure in the pleural cavity is further diminished, and amounts on an
average to 730 mm. Hg. The negative pressure in the pleural cavity
thus varies from -6mm. Hg during expiration to - 30 mm. Hg or
more during inspiration, and represents the pressure required to over-
come the tendency of the expanded lungs to collapse by virtue of their
elasticity. When the lungs expand further during inspiration, air rushes
in to fill the additional space; during expiration air is expelled. The
expansion of the lungs during inspiration is due almost entirely to
the enlargement of the infundibula and atria, and the portions of the
lungs which expand most are those in contact with the diaphragm and
ribs ; the dorsal and mediastinal surfaces and the apex are much less
expansile.
If the chest is opened, either during life or after death, the pressure
on both the outer and the inner surfaces of the lungs is that of the
atmosphere, and owing to their elasticity the lungs collapse. In the
condition known as emphysema, the elastic tissue of the lungs atrophies
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THE RESPIRATORY SYSTEM. 249
and they do not collapse when the chest is opened. In the new-born
infant, the lungs, even in their collapsed condition, fill the chest. As_
the child grows, the capacity of the chest increases more rapidly than
does the size of the lungs; and the lungs of the adult are considerably
expanded even at the end of expiration.
Since with each expansion of the chest the lungs increase in size
so as to fill completely the extra space thus provided, the amount of
air entering the lungs at each breath varies with the extent of the
respiratory movement. In quiet respiration it amounts to 350-500 c.c.,
and is spoken of as t¢da/ air. The additional volume of air, which can
be taken into the lungs by forced inspiration, amounts on an average to
about 1500 c.c. and is called complemental air. The largest amount of
air which can be expelled from the lungs by the most violent expiration,
made at the end of an ordinary breath, is termed swpplemental air ; it
varies in different individuals from 1000 to 1500 ¢.c. Even after the most
forcible expiration, a considerable amount of air—usually about 1000 c.c.
—still remains in the lungs, and is spoken of as resedual air. The
total volume of air which can be taken into and expelled from the
lungs by the most forcible inspiration and expiration, namely, the sum
of the tidal, complemental, and supplemental air, is termed the wtal
capacity of the chest, and is from 3000 to 3500 c.c. These figures are
obtained by allowing the individual to breathe into a spirometer, which
is a small gasometer provided with a graduated scale. When the
subject breathes into or out of the spirometer, the air chamber rises
or falls, the increase or decrease of its contents, thus produced, being
read off on the scale.
Tidal air 500 cc. . \
Complemental air 1500 ¢c.c. . } Vital capacity 3500 c.c.
Supplemental air 1500 c.c. {
Residual air 1000 c.c.
The normal_rate of respiration in adults is 15 to 18 a minute.
Expiration follows inspiration immediately, and is succeeded by a
slight pause before the next inspiration begins. Children breathe more
rapidly, the rate in the infant being about 40 a minute.
Ordinary quiet breathing is usually called ewpnea, and an increase
in the depth of the respiratory movements is called hyperpnea ; if these
movements are not only deeper, but also laboured, the term _dyspnea_is
applied to them. A temporary cessation of breathing is known as
apnoea, . .
If the ear is placed in contact with the chest wall, a faint sound—
the vesicular murmur—is heard during inspiration ; it is believed to be
_
250 ESSENTIALS OF PHYSIOLOGY.
produced by the passage of air through the larger respiratory passages,
the sound being modified in its conduction through the substance of the
lung. On listening over the trachea and large bronchi, the sound is
louder and is audible both during inspiration and expiration. The
character of the sound is altered in disease of the lungs or pleura.
SECTION II.
THE CHEMISTRY OF RESPIRATION. ©
The composition of expired air differs considerably from that of the
atmosphere, the average difference being as follows :—
Nitrogen. | Oxygen. | Carbonic Acid.
Inspired air. 79 20°96
0°04
Expired air .- . 794 | 16°50 4°]
4°46 4:06 —
_ The increased percentage of carbonic acid in expired, as compared
with inspired, air is 4°06, the difference between the percentage of
oxygen in inspired and expired air being 4:46, so that the total volume
of the air expired is less than that inspired. It is for this reason that
the relative amount of nitrogen in expired air is slightly increased.
The ratio of the amount of carbonic acid leaving the body to the amount
of oxygen taken into the body and not reappearing in expired air, is
known as the respiratory quotient, and is usually expressed as at
ete NA PET CA ‘
In the table it is a = 0°90. ‘ Its significance will be discussed
subsequently (p. 343).
In addition to containing less oxygen and more carbonic acid, the
expired air is fully saturated with water, and is at the body temperature.
It has been stated that small quantities of poisonous substances are also
present in expired air, and that the accumulation of these substances in
crowded and ill-ventilated rooms is the cause of the headache and other
uncomfortable symptoms experienced in these circumstances. Careful
experiments have shown that this is not the case, and that the
symptoms are due partly to the accumulation of carbonic acid in the
air, and make their appearance when this reaches 0:1 per cent. or more,
and partly, as Leonard Hill suggests, to a high temperature and lack of
i i eee SP ee ep Oe ot io
THE RESPIRATORY SYSTEM. 251
currents of air in the room. They may be avoided by constant renewal
of the air in rooms by adequate ventilation. |
The expired air comes partly from the lungs, partly also from the
respiratory passages, namely the bronchi, trachea, pharynx, and nose.
Since the air in these passages undergoes very little change in composi-
tion during respiration, they are known as the “dead space,” the
capacity of which varies from 130 to 150 c.c. and is very constant for the
same individual. The air contained in the dead space is expelled during
the first part of expiration, the air expelled during the latter part of
expiration, particularly if this is forcible, coming directly from the
alveoli of the lungs. Since the interchange of oxygen and carbonic.
acid between the blood and the air in the lungs takes place solely in
the alveoli, it is of great importance to ascertain the composition of the
alveolar air.
Haldane has devised a simple apparatus for collecting samples of
alveolar air in man. It consists of a piece of rubber tubing about
1 inch in diameter, 3 or 4
feet long, and provided ——_ : MOUTH-PLECE.
with a mouthpiece. About
2 inches from the mouth-
piece the tube is connected
with a sampling tube
(fig. 95), which has pre- Fie. 95.
viously been made vacuous.
The subject breathes normally for a few moments, and then, at the end
of a normal inspiration, he expires deeply through the mouthpiece and
instantly closes it with his tongue. The upper tap of the receiver is
at once opened, and air rushes into it from the tube; the tap is then
closed, and the sample can be analysed, A second similar experiment
is made, in which the subject expires deeply at the end of a normal
expiration, and another sample of air is obtained. The mean of the
analyses of the two samples gives the average a of the
alveolar air.
The reason for taking two samples is that at the end of inspiration
the lungs contain a maximum percentage of oxygen, whereas at the
end of expiration they contain a maximum percentage of carbonic acid.
The amount of carbonic acid in alveolar air obtained by this method
varies from 5 to 6 per cent. in different individuals, but is remarkably
constant in the same individual; the amount of oxygen is usually
13 to 14 per cent. _ Alveolar air thus contains considerably less oxygen
and more carbonic acid than ordinary expired air, the reason being that,
in the expired air, the alveolar air is mixed with the contents of the
SAMPLING TUBE.
.
252 ESSENTIALS OF PHYSIOLOGY.
dead space, the composition of which differs but little from that of the
atmosphere. 3
The air is analysed by shaking up a known volume with caustic potash,
which absorbs the carbonic acid; the diminution in volume represents the
amount of carbonic acid present in the air. The air is then shaken up with
pyrogallic acid, which absorbs oxygen, and the diminution in volume is again
measured. The residual gas is regarded as being nitrogen.
THE GASES IN THE BLOOD.
Before discussing the means by which the interchange of oxygen and
carbonic acid between the blood and air in the lungs is effected, it is
necessary to determine, first, the amount of these gases in the blood, and,
secondly, the conditions in which they are held in it.
If blood is exposed to a vacuum, there is a considerable evolution
of gas, which may be collected and analysed. For this purpose, some .
form of gas pump is usually employed, the composition of the gas
being subsequently ascertained by exposing it first to potash, which
absorbs carbonic acid, and secondly to pyrogallic acid, which absorbs
oxygen. By this means it is found that 100 volumes of blood give off
about 60 volumes of gas, the composition of which varies according
to whether the blood is arterial or venous. The total quantity of
oxygen which 100 volumes of blood can take up or give off is called its
oxygen capacity.
GASES IN THE Bioop (Doe).
Oxygen. Carbonic Acid. | Nitrogen.
Arterial blood
100 volumes yield . .| 20 vols. 40 vols. | 1-2 vols.
of Venous blood
) yield . .| 8-12. ,, 46-48 __,, 1-2 ,,
This method necessitates the use of comparatively large quantities
of blood (10 to 20 c.c.) in order to give accurate results ; and Barcroft
has devised an apparatus, by means of which the estimation of the
blood gases can be carried out with very small quantities (1 ¢.c. or even
0-1 c.c.) of blood. The method has the advantage of being readily
applicable to man, and depends upon the fact that, when potassium
ferricyanide and a trace of alkali (usually ammonia) are added to blood,
all the oxygen previously in combination with hemoglobin is evolved,
and the amount of oxygen given off from a known volume of blood can
eae ise ale
?
;
,
|
5 am
hiatal iia att te Te te
THE RESPIRATORY SYSTEM. 253
be measured. The hemoglobin then takes up an equivalent amount
of oxygen from the reagents, being converted into methzmoglobin.
The probable course of the reaction is represented by the equation
HbO, +4K,Fe(CN),+4KOH=HbO, +4K,Fe(CN),+2H,0+0,..
Oxyhzemoglobin Methzmoglobin
Barcroft’s differential apparatus consists of a manometer, of which the bore
is 1 mm., and which is provided with a scale graduated in millimetres.
Attached to each limb is a small bottle, which is detachable, and by means of
taps each bottle can be connected with or shut off from the outer air. The
manometer is partially filled with clove oil.
(1) To determine the oxygen capacity of a sample of blood, 2 c.c. of dilute
ammonia and 1 c.c. of the blood to
be examined are placed in each
bottle, and the bottles are shaken so
that the blood is thoroughly laked
and saturated with oxygen. The
stoppers are carefully greased, and
0-2 c.c. of a saturated solution of .
potassium ferricyanide is placed in
the reservoir in the stopper of one
bottle A. The bottles are then
attached to the manometer and
placed in a water bath, the taps
being open. When the reading of
the manometer becomes constant,
the taps are closed, thereby exclud-
ing the bottles and the manometer
from the outer air, and the apparatus
is tilted so that the ferricyanide runs
into the blood in A, which gives off
its oxygen. The level of the clove
oil falls in the limb attached to the
bottle A and rises in the opposite
limb, and the bottles are replaced in
the water bath until the readings Fyg, 96.—Barcroft’s blood-gas apparatus.
become constant. If the difference (From Barcroft, Respiratory Function
of level on the two sides is 60 mm., of the Blood.)
this difference, multiplied by the
constant of the apparatus (which may be taken- as 3:0), represents the amount
of oxygen given off by the blood; thus 60 mm.x3:°0=180 c.mm. oxygen.
Since 1 c.c. of blood gives off 0°18 c.c. of oxygen, 100 c.c. of blood will give off
18-0 c.c. oxygen, and this is its oxygen capacity. We have taken 3:0 as the
constant of the apparatus, but it must be remembered that each apparatus has
its own constant, which may be slightly greater or less than 3°0.
(2) To determine the amount of oxygen present in a given sample of blood,
2 c.c. of dilute ammonia are placed in each bottle; 1 c.c. of the blood to be
examined is carefully placed under the ammonia at the bottom of one bottle A,
and thus kept from contact with the air. 1 c.c. of fully oxygenated (saturated)
blood is placed in the other bottle B.. The stoppers are greased, the bottles
are attached to the manometer and placed in a water bath, the taps being open.
When the reading of the manometer becomes constant, the taps are closed and —
the apparatus is shaken in order to lake the blood in each bottle. The blood
in bottle B, being already fully saturated, takes up no more oxygen when
brought into contact with the air in the bottle. Ifthe. blood in the bottle A
is already fully saturated with oxygen, it takes up no more oxygen, and the
254. ESSENTIALS OF PHYSIOLOGY.
reading of the manometer remains unchanged ; but if it is not fully saturated,
it takes up some oxygen, and the level of the clove oil rises in that limb of the
manometer. The manometer is replaced in the water bath till the reading is
constant. If the difference in the level of oil in the two limbs is 20 mm., the
blood must have taken up 20 mm. x 3°0=60 c.mm. of oxygen ; and if its total
oxygen capacity is ascertained, the degree to which it was previously
saturated can be calculated.
Thus, if the oxygen capacity of 1 c.c. blood was 0°18 ¢.c., and it took up in
the foregoing experiment 0°06 c.c. oxygen, it must have previously contained
0°12 c.c., and its percentage saturation was 100 x eae per cent.
THE CONDITION IN WHICH THE GASES ARE HELD
IN THE BLOOD.
Theoretically, the gases in the blood might be either simply
dissolved in it or chemically combined with some constituent of the
blood. In order to decide which of these possibilities is the correct one,
it is necessary to consider first the conditions which modify the amount.
of any gas present in a fluid such as water. On exposing water to the
air, a certain amount of oxygen and nitrogen is dissolved in it. Con-
fining our attention to oxygen, the amount dissolved in a known
volume of water depends upon (1) the pressure exerted by the oxygen
upon the surface of the water, (2) the temperature of the water, and
(3) the capacity of water to dissolve oxygen. The capacity of water to
dissolve any gas is constant for the same gas, provided the pressure of
the gas and the temperature of the water remain unchanged. Some
gases, such as carbonic acid, are very soluble in water, others such
as oxygen and nitrogen are only slightly soluble. The coefficcent of
solubility of a gas is defined as the amount of that gas which is dissolved
at a given temperature in 1 c.c. of the liquid, when the pressure of the
gas on the liquid is 760 mm. Hg.
Since the capacity of water to dissolve any gas is constant, the
amount of the gas dissolved at a given temperature (which will be
assumed to be constant) varies directly with the pressure of the gas
on the surface of the liquid. When water is exposed to pure oxygen at,
atmospheric pressure, the pressure of oxygen on the water is 760 mm.
Hg; if it is exposed to atmospheric air, both the oxygen and nitrogen
exert a pressure, which is proportional to their percentage in the air.
In such a mixture of gases, the proportion of the total pressure exerted
by the oxygen on the walls of the vessel containing it, or on: the surface
of a fluid, is called the partial presswre or tension of oxygen, and it is
measured by determining (1) the percentage of oxygen in the gaseous
mixture, and (2) the total pressure of the mixture. The partial pressure
na) Ld AE De ae a Aa tes elit AS fend Ot See
— me
ES a ee ee ee ae
eS aS SS se ee) ee
THE RESPIRATORY SYSTEM. 255
of oxygen in atmospheric air is, therefore, 20 per cent. of 760 mm. Hg,
namely 152 mm. Hg.
The Tension of Gas in a Fluid.— When water is exposed to a
gaseous mixture containing oxygen, the molecules of oxygen tend to
pass into the liquid and be dissolved ; at the same time, the molecules
of oxygen already in solution tend to pass from the water into the
gaseous mixture. This tendency of the molecules of oxygen to leave
the fluid is called the tension of oxygen in the fluid. When these two
opposing processes are equal, the gaseous mixture and the fluid are in
equilibrium, and the amount of oxygen dissolved in the fluid remains
constant. In these circumstances, the tension of oxygen in the fluid is -
equal to the partial pressure of the oxygen in the gaseous mixture to
which the fluid is exposed.
The tension of oxygen in a fluid cannot be measured directly, but
is determined by placing samples of the fluid in a series of closed vessels,
containing oxygen at various known partial pressures, and finding in
which vessel the fluid neither gives off nor takes up oxygen. The tension
_of oxygen in the fluid in this vessel is equal to the partial pressure of
the oxygen in the gaseous mixture to which the fluid is exposed ; and
if the amount of oxygen in the gaseous mixture is 5 per cent. and the
total pressure of the gaseous mixture is 760 mm. Hg, the partial
pressure of oxygen is 38 mm. Hg; this is equal to the tension of
oxygen in the fluid. The forms of apparatus which have been devised
for measuring the tension of a gas in a fluid are called aérotonometers.
The Tension of Oxygen in Blood.—In the case of water, oxygen is
held in simple solution, and, if the temperature is constant, the amount of
oxygen in the water varies directly with the partial pressure of oxygen
in the air to which it is exposed. If the pressure of oxygen is doubled,
twice as much is dissolved in the water. When similar experiments
are carried out with blood, the amount of oxygen present in the blood,
as determined by exposing it to a vacuum in a gas pump or by
Barcroft’s apparatus, is not proportional to the partial pressure of
oxygen in the gaseous mixture to which the blood is exposed. For
example, when the blood is in equilibrium with air containing oxygen
at a partial pressure of 100 mm. Hg, it will contain about 18 volumes
of oxygen per 100 volumes of blood. If the partial pressure of oxygen
is reduced to 50 mm. Hg, the blood will contain 14°5 volumes of
oxygen per 100 volumes of blood.
Since the amount of oxygen in blood is not dzrectly proportional to
the partial pressure of oxygen in the air to which the blood is exposed,
it is evident that oxygen is not held in blood_simply in_solution.
Further, a given volume of blood can take up many times as much
256 ESSENTIALS OF PHYSIOLOGY.
oxygen as an equal volume of water, Hence it is clear that oxygen
must form an unstable compound with some: constituent of the blood ;
this constituent is hemoglobin, and a solution of pure hemoglobin can
take up from the air as much oxygen as blood containing the same
amount of hemoglobin. 1 gram of hemoglobin can combine with
approximately 1°34 ¢.c. oxygen, though this figure varies slightly in
different animals, probably on account of slight differences in the
character of the protein part of the hemoglobin molecule.
If 100 c.c. of blood are exposed to an atmosphere of pure oxygen in
a closed vessel at atmospheric pressure, they take up about 20 c.c. of
oxygen. When the oxygen is slowly withdrawn from the vessel, so
that the pressure on the surface of the blood gradually falls, the blood
remains almost unaltered until the pressure falls to about 100 mm. Hg.
With a further fall of pressure the blood rapidly gives off its oxygen,
and, when the pressure falls to zero, all the oxygen has been evolved.
Evidently the combination of hemoglobin with oxygen is a reversible
one ; hemoglobin gives off its oxygen when the pressure of oxygen is
low, and takes it up when the pressure is high. This reversible action
is usually indicated thus, HbO,— Hb + O,. It is to this power that
heemoglobin owes its value for respiration. In the alveoli of the lungs
the partial pressure of oxygen is relatively high, and the blood becomes
almost fully saturated with oxygen. The partial pressure of oxygen in
the tissues is low, and, when the blood is carried round in the circulation
to the tissues, the hemoglobin dissociates, the oxygen set free being
taken up by the tissues. The effect of a varying partial pressure of
oxygen upon the combination between hemoglobin and oxygen is shown
in fig. 97. The curve shows the percentage of hemoglobin present
as oxyhemoglobin with varying pressures of oxygen; when all the
hemoglobin is in the form of oxyhemoglobin, it is said to be fully
saturated. For pressures above 100 mm. Hg, the curve becomes
almost a straight line.
This curve, which is known as the dissociation curve of hemoglobin,
is obtained in the following manner. A small quantity of blood is
placed in an aérotonometer containing a mixture of oxygen and nitrogen
at atmospheric pressure. A suitable form of tonometer is that shown
in fig. 98; it consists of a glass cylinder, which can be rotated in
water kept at a constant temperature. When the cylinder is rotated,
the small amount of blood, previously placed in it; spreads out into
a thin film over the inner wall of the cylinder, and after a short time
the blood and gas come into equilibrium, and the blood neither takes
up nor gives off oxygen. The cylinder is removed from the water
bath, and the percentage of oxyhemoglobin in the blood is estimated
—
" - oo —e
eer ? .
. ee a oe ee
ni _—
—_——
iain t ee ei POTS. pte
THE RESPIRATORY SYSTEM.
257
by Barcroft’s blood-gas apparatus. If, for example, the partial pressure
of oxygen in the mixture of gases placed in the aérotonometer is
40 mm. Hg (corresponding with about 5 per cent. of oxygen), and
Percentage saturation
with oxygen
100
rw
w
oO
70 90 6100
Oxygen pressure
mm,
10 20 30 40 50 60
(From Barcroft, Respiratory Function of the Blood, )
oglobin is red, and reduced hemoglobin is purple. —
Fic. 97,—Dissociation curve of hemoglobin dissolved in water at 37° C.
more readily in blood than when simply dissolved in water.
17
iy
'— the percentage of oxyhemoglobin in the blood, when in equilibrium
is mixture, is 70 per cent., that point can be marked on the
comparison of the two curves shows that oxyhemoglobin dissociates
When
the partial pressure of oxygen is 20 mm. Hg, blood contains only
30 per cent. of its hemoglobin in the form of oxyh#moglobin, whereas ©
in a watery solution of pure hemoglobin 72 per ‘vent. exists as
'*
258 ESSENTIALS OF PHYSIOLOGY.
oxyhemoglobin. This difference is chiefly due to the presence of salts,
particularly potassium salts, in the blood, which render the combination
between hemoglobin and oxygen more unstable; when hemoglobin
is dissolved in water containing the same salts as those normally
present in blood, its dissociation curve is identical with that of blood.
ak The other factors, which alter the form of the dissociation curve of
blood, are (1) the;temperature of the blood, and (2) the presence of
carbonic or other acids. The higher the temperature of the blood
and the greaterfthe percentage of carbonic or other acids, the more
100
90
i
h oN
80 AL Bal ea He
ee a
ae fy B ea : rm): \
60 | / ~ Hf 4 : \
50 : Wr
40}- / :
30 N
pakte
al
4
0 10 #0 30 40 50 60 70 80 90 100
Oxygen pressure in mm. of mercury.
Percentage saturation with oxygen.
re
0
Fic. 99.—Dissociation curves (A) of hemoglobin dissolved in water at
37° C., and (B) of blood at 37° C
readily does the oxyheemoglobin dissociate. The effect of carbonic
acid and of lactic acid is shown in fig. 100. These factors not only -
modify the extent to which oxyhemoglobin dissociates at the same
partial pressure of oxygen, but also the rate at which it loses oxygen
and becomes reduced; when an indifferent gas such as nitrogen
is bubbled through a solution of blood, the blood becomes reduced
much more rapidly if it contains an excess of carbonic acid or a small
amount of lactic acid. The influence of carbonic or other acids on the
readiness with which oxyhzmoglobin dissociates, when the pressure of
oxygen to which it is exposed is low, is of great physiological importance.
As the blood passes through the capillaries, it not only gives off
oxygen, but also receives from the tissues carbonic acid and frequently
/
THE RESPIRATORY SYSTEM. 259
lactic acid. The greater ease with which the oxyhemoglobin dis-
sociates in these circumstances increases the supply of oxygen available
for the tissues ; and this effect will be especially marked and beneficial —
when the tissues are functionally active and are giving out large
amounts of carbonic acid and require a larger supply of oxygen.
Although these conditions profoundly influence the rate of trans-
ference of oxygen from the blood to the tissues, they do not appreciably
affect the amount of oxygen taken up by the blood as it passes through
the lungs. The partial pressure of oxygen in the lungs is normally
; aan
oy | oe ae omens
& PBS mast
on
JY es ——
Dp ~
3 6S
-
Sg
oO
me
~
a
Percentage saturation with oxygen.
w
S ©
ee ee
ee
a
ee
0 10 20 30 40 §0 60 70 80 90 100
Oxygen pressure in mm. mercury.
. 100.—A, B, and C show the effect of varying tensions of CO, in
blood ou the dissociation curve of oxyhemoglobin. The dotted
line D is the dissociation curve of oxyhemoglobin in blood to
which 0°025 per cent. lactic acid was added.
FI
Q
just over 13 per cent. (about 105 mm. Hg), and a consideration of
fig. 100 shows that at this pressure of oxygen the saturation of the -
blood will be practically the same, whether the blood is normal or
whether it contains an excess of carbonic acid or a trace of lactic acid.
In addition to the oxygen thus combined with hemoglobin, the
blood contains oxygen in simple solution in the plasma, amounting at
the body temperature to about 0°36 c.c. oxygen per 100 c.c. of blood.
Carbonic Acid in Blood.—The tension of carbonic acid in blood,
when determined by means of an aérotonometer, is equal to a partial
pressure of about 5 per cent. of carbonic acid. If the carbonic acid
were simply dissolved in the blood, the latter would. contain at this
260 ESSENTIALS OF PHYSIOLOGY.
tension only about 24 volumes of carbonic acid per 100 volumes of
blood; but since 100 volumes of blood, when exposed to a vacuum,
give off from 40 to 48 volumes of carbonic acid, the bulk of this must be
in a state of chemical combination. It is combined partly as sodium
bicarbonate, partly with the proteins of the blood plasma, and partly
with hzemoglobin, the proportions in inorganic and organic combination
being approximately equal. The proteins in the plasma can act either
as weak acids or as weak alkalies, and as the blood passes through the
lungs it loses carbonic acid and the proteins combine with the sodium
thus set free ; when the blood takes up carbonic acid from the tissues,
the combination between sodium and protein is broken, and the carbonic
acid unites partly with the sodium and partly with the protein. In
this way, the reaction of the blood remains practically constant, in
spite of the varying amount of carbonic acid which it contains.
Owing to this action of protein, when blood is exposed to a vacuum the
whole of the carbonic acid which it contains is evolved, whereas when a
pure solution of sodium bicarbonate is exposed to a vacuum only half
the carbonic acid is evolved. In the former case the sodium enters
into combination with the protein; in the latter it forms sodium
carbonate, which is stable in vacwo. _
The dissociation curve of carbonic acid in blood, when exposed to
varying partial pressures of carbonic acid in the surrounding air, has
the same general form as that of oxyhzmoglobin, the dissociation being
most rapid when the partial pressure of carbonic acid varies from
0 to 30 mm. Hg. Oxygenated blood gives off its carbonic acid more
readily than deoxygenated blood.
The blood also contains about 24 volumes of carbonic acid per
100 volumes of blood in solution,
THE EXCHANGE OF GASES IN THE LUNGS.
‘The air in the lungs is separated from the blood by the walls of the
pulmonary capillaries and by the alveolar epithelium; and each of
these membranes is extremely thin. ‘Two theories have been held as
to the means by which oxygen passes from the alveoli into the blood
and carbonic acid from the blood into the alveoli, On the one hand, it
has been supposed that the tension of oxygen in arterial blood is higher
than that in the alveolar air, and that the pulmonary epithelium must
therefore secrete oxygen into the blood. On the other hand, it is
widely held that the tension of oxygen in arterial blood is always lower
than that in the alveolar air, and that oxygen passes from the lungs
into the blood by the purely physical process of diffusion. These
divergent views arose from the fact that the observers employed
THE RESPIRATORY SYSTEM. ” 261
different methods to determine the tension of oxygen in arterial blood
and obtained discordant results.
An improved form of aérotonometer, which can be used to measure |
the tension of oxygen in circulating ‘blood, has been devised by Krogh,
and the lower part of it is shown in fig. 101.- It consists of a cannula A,
which is attached to the central end of an artery ; the blood flows from
the cannula into a bulb B, from the top of which passes off a narrow
eraduated tube, A small air bubble C is placed. in the bulb, and the
blood flows through the bulb and is returned to the distal end of the
artery by the tube D, clotting being prevented by the injection of
hirudin. An interchange of gases takes place between the blood and
the bubble, and, owing to the small size and rela-
tively large surface of the latter, the gases in
the blood and the bubble soon come into equili-
brium, this being reached when the size of the
bubble remains constant; it is then withdrawn
into the graduated tube, and its composition is
analysed.
Since the oxygen of the blood is in equili- =:
brium with that in the bubble, the tension of HES
oxygen in the blood can be ascertained by deter- pete
mining the percentage of oxygen in the bubble ioe
and the total pressure to which it is exposed. If — (= forse
the arterial pressure is 100 mm. Hg, and the ==[ 0
bubble contains 10 per cent. of oxygen, the partial ,.. , eons nee,
pressure of oxygen is 10 per cent. of 860 mm. | meter.
Hg (atmospheric +arterial pressure), namely 86
mm. Hg. The tension of oxygen in the blood is thus equal to 86 mm.
Hg, or about 11:3 per cent. of oxygen at atmospheric pressure.
By means of this apparatus, it has been found that in animals the
tension of oxygen in arterial blood is distinctly less than that in alveolar
air, the difference being usually | to 3 per cent. of an atmosphere (fig. 102).
It has been estimated that in man the oxygen tension of venous blood
is about 40 mm. Hg, whereas the partial pressure of oxygen in alveolar
air is usually about 105 mm. Hg. ‘Taking into account (1) the rate at
which oxygen can diffuse through a membrane similar to that separating
the blood from the alveolar air, and (2) the enormous surface area of
the alveoli, it has been calculated that this difference in the tension of
oxygen in the lungs and in the venous blood respectively is more than
sufficient to provide for the passage of oxygen from the alveolar air into
the blood by simple diffusion.
Although this view is generally accepted, some authorities are still
’
©
262 ESSENTIALS OF PHYSIOLOGY.
of opinion that it is true only for the resting individual, and that during
exercise, when the need for oxygen is greater, or, at high altitudes, when
the supply of oxygen is deficient, the tension of oxygen in the blood
is higher than that of alveolar air, and that this difference is due to
the secretion of oxygen into the blood by the pulmonary epithelium.
Similarly, it has been found that the tension of carbonic acid in
venous blood corresponds with a partial pressure of about 46 mm. Hg,
and is very slightly higher than that in alveolar air (fig. 102). If the
20
a + 7~—— ——-. -o-7r" ~—
19 gk, LETS sae tk Segue hn PSE
-——— mae ~-— re
~~
ee
(a eee | ES DE EE Y
220 30 40 50 3 0 20 30 HW 50 4% W 20 30 40
Fic. 102.—The tension of gases in the alveolar air and in blood. (Krogh,)
From Barcroft, Respiratory Function of the Blood.
The dotted lines represent the tension in the alveolar air, and the continuous lines
show the tension in the blood. During three periods the amount of CO, in
inspired air was increased.
tension of carbonic acid in alveolar air is raised, the tension in the
blood shows a corresponding rise and still remains slightly higher than
that ig the alveolar air.
In man the difference between the tension of carbonic acid in venous
blood and in alveolar air is probably about 6 mm. Hg. This difference
is much smaller than that between the tension of oxygen in alveolar
air and venous blood respectively, but carbonic acid diffuses through a —
membrane such as that separating the air in the lungs from the blood,
twenty-five times as quickly as oxygen; and the difference between the
29 005 29 O05 | — «£4 % C02 in inspired air
oe 28 mat Pet Eien hee mee
ie li NL ON en a 26
THE RESPIRATORY SYSTEM. 263
tension of carbonic acid in venous blood and in alveolar air, slight
though it is, is sufficient to allow carbonic acid to pass from the blood
into the air in the lungs by simple diffusion.
Owing to the difference in the tension of oxygen and of carbonie
acid in the blood and in alveolar air, the venous blood as it flows
through the lungs takes up oxygen and leaves the lungs almost fully
saturated ; at the same time it loses 6 to 8 volumes of carbonic 8 per
100 volumes of blood.
7
SECTION III.
THE REGULATION OF THE RESPIRATORY MOVEMENTS,
Oxygen is continually passing from the alveolar air into the blood,
and is replaced by the entrance of air from the atmosphere during
respiration. Similarly, carbonic acid is constantly diffusing from the
blood into the alveolar air, and with each expiration a certain amount
is expelled from the lungs. Assuming that the amount of air entering
and leaving the lungs with each breath is 400 c.c., and that the expired
air contains 4 per cent. of carbonic acid, then with each respiration
16 c.c. of carbonic acid are removed from the body. Since the composi-
tion of alveolar air remains constant, it is evident that 16 c.c. of carbonic
acid must, in the same time, have passed from the blood into the air
in the lungs. When an additional amount of carbonic acid is formed
in the body and taken up by the blood, it passes into the alveolar air
and tends to raise its percentage. Hence, if the percentage of carbonic
acid in alveolar air is to remain constant, as it actually does under
normal conditions, the amount of air passing into and out of the lungs
at each breath must be correspondingly increased. If 32 ¢.c. of carbonic
acid pass from the blood into the alveolar air during a breath, they can
be removed if the amount of expired air is 800 c.c. containing 4 per
cent. carbonic acid. In the same way, the needs of the body for more
oxygen are met by a larger amount of air entering the lungs at each
inspiration. This process, whereby any accumulation of carbonic acid
or deficiency of oxygen in the alveolar air, and consequently in the
blood, is prevented, is under the control of the central nervous system,
and is regulated in such a way that an excess of carbonic acid, or a lack
of oxygen, leads to deeper and more rapid respiratory movementm
The Respiratory Centre.—The rhythmic alternation of inspiration
and expiration is brought about by the contraction of the respiratory
muscles, which are controlled and co-ordinated by a centre — the
respiratory centre—lying in the grey matter of the floor of the fourth
ventricle near the apex of the calamus scriptoriys. From this centre,
264, ESSENTIALS OF PHYSIOLOGY.
impulses pass along the nerves supplying the respiratory muscles,
namely, the vagus to the muscles of the larynx, the cervical nerves to
the muscles of the neck, the intercostal nerves to the intercostal
muscles, and the phrenic nerves to the diaphragm.
The centre is bilateral, each half controlling the muscles of the
corresponding: side, the two halves being connected by commissural
fibres. Its position has been determined by observing the effect upon
the respiratory movements of transection of the brain stem or spinal
cord at various levels. When a section is made across the pons or
medulla oblongata at any point above the level of the striz acustice,
respiration is unaffected. If the spinal cord is divided at the upper
end of the cervical region, the respiratory muscles supplied by nerves
above the section, e.g. those which dilate the ale nasi, continue to
contract. Destruction of the medulla oblongata, in the region of the
apex of the calamus scriptorius, is at once followed by cessation of all
respiratory movements. The region occupied by the centre is not
sharply defined, but it is undoubtedly closely connected with the
sensory nuclei of the vagus nerves. |
There is no evidence of .the existence of subsidiary centres in the
spinal cord.
The eentre continues to send out rhythmic impulses to the
respiratory muscles when it is cut off from afferent impulses reaching
it either from the higher parts of the brain or from the spinal cord. If,
however, the brain stem is divided below the pons, and the vagus nerves
are also divided, the respiratory movements are replaced by a series of
inspiratory spasms, and the animal dies after a short time. It is
doubtful, therefore, whether the centre can be regarded as acting
automatically in the absence of all afferent impulses, though this view
has been taken by some writers.
The question is one of théoretical rather than practical interest,
since, in ordinary circumstances, a constant stream of afferent impulses
is reaching the centre and modifying its activity; and hardly any
nervous centre in the body is more easily influenced in this way than
the respiratory centre. For instance, the mere directing of one’s
attention to the respiratory movements is sufficient to alter their rate
or depth. The respiratory centre, like the vaso-motor centre, is also
extremely sensitive to any changes in the composition of the blood
supplying it. These two factors which modify its activity, namely, (1)
the composition of the blood, and (2) nervous impulses from the higher
centres or from the peripheral nerves, . will be considered separately.
The former affects primarily the depth, and the latter the rate of
respiration.
OO are eee ee ee
eg tl
5
;
5
Se rs ee ee in
THE RESPIRATORY SYSTEM. 265
Methods of Recording Respiratory Movements.—In order to study
these changes, it is desirable to obtain a graphic record of the rate
and depth of the respiratory movements; and numerous methods
have been devised for this purpose.
(1) In man the respiratory movements can be recorded by a stetho- °
graph, one form of which consists of a small metal cylinder, provided
with a lateral opening and closed at each end by a rubber membrane ;
the lateral opening is connected by rubber tubing with a tambour.
Strings are attached to the centre of each rubber membrane, and are
passed round the chest and tied. Each expansion of the chest causes
the strings to pull upon the rubber membrane, so that the capacity of
the cylinder increases and the lever of the tambour falls; during
expiration the membranes return to their former position. The same,
apparatus can be used to record the respiratory movements in the
lower animals.
(2) Another method, used in animals, is to connect the side piece
of a cannula, inserted into the trachea, with a tambour; with each
inspiration air is sucked out of the tambour, and the lever falls.
(3) In rabbits, a small slip of the diaphragm on each side is inserted _
into the xiphisternum ; and by separating the xiphisternum from the
sternum, this strip of muscle can be isolated without interfering with
its vascular or nervous connections. It contracts synchronously with
the rest of the diaphragm, and by connecting the xiphisternum with
the membrane of a tambour by means of a thread, each contraction of
the slip can be recorded and serves as an index of the movements of
the diaphragm as a whole (Head’s method).
THE CHEMICAL REGULATION OF RESPIRATION.
The two most important changes in the composition of the blood
which alter the respiratory movements are: (1) variations in the
tension of carbonic acid, and (2) a fall in the tension of oxygen.
(1) The Tension of Carbonic Acid.—If an animal is allowed to
breathe air containing 2 to 3 per cent. carbonic acid, the respiratory
movements become much deeper, and after a short time usually more
frequent ; such an experiment is illustrated in fig. 103. If the experi-
ment is made on man, it is further found that the percentage of
carbonic acid in the alveolar air remains constant. The immediate
effect of breathing air containing an excess of carbonic acid is to
increase the percentage in the alveolar air, thereby diminishing the
passage of carbonic acid by diffusion from the blood into the air in
the lungs. As a result, the tension of this gas in the blood rises; this
increase in tension stimulates the respiratory centre to increased
266 ESSENTIALS OF PHYSIOLOGY.
activity, and the amount of air passing into and out of the lungs with
each breath may be doubled or trebied. In this way a larger amount
of carbonic acid is expelled from the lungs with each expiration, and
the mean tension in the lungs is kept at the same level.
The centre is extremely sensitive to the slightest rise in the tension
of carbonic acid in the blood, and a rise of 0°2 per cent. of an atmo-
sphere in the pressure of this gas in alveolar air doubles the ventilation
of the lungs, that is to say, the amount of air passing into and out of
rh ANA
NYODANY NYY \
A |
Fie. 103.—The effect of CO, on the respiratory movements in the normal animal.
The inspired air contained 3 per cent. CO, during the period marked on the tracing.
them at each breath. The capacity of the respiratory centre to react
to any increase in the tension of carbonic acid in the blood passing to
it is very great, as is shown in experiments in which the amount of
carbonic acid in the air breathed is gradually increased. As seen in the
following table from Haldane, the percentage in alveolar air remains
constant until the amount in the inspired air exceeds 3 per cent.; beyond
this point the percentage of carbonic acid in the alveolar air rises, and the
ventilation of the lungs becomes enormously increased.
| Average Fre-
| quency of
Respiration
per Minute.
| Ventilation of |
Alveoli., Normal]
tuken as = 100. |
Percentage of
Carbonic Acid
in Inspired Air.
Percentage | Average Depth
of CQ, in of Respira-
Alveolar Air. tions in c.c.
|
|
|
|
|
|
Fe) |
7
0°03 per cent. 5°6 percent. | 673 14 100 |
1°52 5°55 ,, 793 15 137
2°02 —,, 5°6 = | 864 | 15 | 153 |
hy Sena 5°5 ¥ 1216 | 15 | 226 |
| |
yd a eere 59 5a | 1330 | 14 | 278
yk eee 6:2 ¥ | 1771 | 19 | 498
6°02 Sat, | 2104 | 27 857
> |
THE RESPIRATORY SYSTEM. 267
The effect of carbonic acid in stimulating the respiratory centre is
also seen during muscular exercise. During exercise the muscles form
a large amount of carbonic acid, which passes into the blood and raises
the tension of this gas in the blood. This rise of tension stimulates
the respiratory centre, and the respiratory movements become deeper
and more frequent, with the result that a larger amount of carbonic
acid is carried off in the expired air. Most observers have found a
considerable rise in the tension of carbonic acid in alveolar air during
exercise. According to the recent observations of Krogh, however, the
tension remains practically unaltered, and the response of the respiratory
centre to the stimulus of carbonic acid is ‘as delicately adjusted during
exercise as during rest. As the result of several observations on the
same person, Krogh obtained the following figures for the alveolar
tension of carbonic acid :—
Tension of Alveolar 66.)
Rest ‘ yoke! S 9S - 506% ‘
Exercise . ; : . 510 \
We may, therefore, regard the regulation of the respiratory move-
ments and of the amount of air passing into and out of the lungs as
dependent, under normal conditions, upon the tension of carbonic. acid
in the blood which supplies the respiratory centre; and since the
tension of carbonic acid in the respiratory centre itself must vary with
that in the blood, the ultimate stimulus to the respiratory movements
is evidently the tension of carbonic acid in the respiratory centre.
Since the slightest rise in the tension of carbonic acid in the blood
increases the respiratory movements, a diminution might be expected
to lessen the respiratory movements by diminishing or abolishing the
stimulus to the centre; and such is found to be the case.
When an individual takes a number of deep breaths (forced
respiration), more carbonic acid is removed from the lungs than is
entering them from the blood. Hence the tension in alveolar air, and
in blood, falls to such a level that it no longer stimulates the respiratory
centre ; and respiration ceases for a short time (apnoea). During the
period of apnoea, carbonic acid continues to reach the blood from the
tissues, and the tension in the blood and in the alveolar air gradually
rises, until it again reaches a level sufficient to stimulate the respiratory
centre; when this occurs respiration recommences (fig. 111).
Apnoea can also be produced in animals by repeated inflation of the
lungs, whereby carbonic acid is, so to speak, washed out of the lungs
and its tension in the alveolar air falls.
A marked fall in the tension of carbonic acid in the alveoli gives rise
268 ESSENTIALS OF PHYSIOLOGY.
to the condition known as acapnia, in which the blood pressure is very
low and the tone of the muscular walls of the digestive tract and of
other unstriped muscles is. lost; the symptoms closely resemble those
of surgical shock (p, 235).
The activity of the respiratory centre is excited, not only by a
rise in the tension of carbonic acid, but also by the addition to the blood
of other acids, such as lactic acid. When 0-02 to 0-04 per cent. lactic acid
is added to the blood, a considerable increase is produced in the depth
of the respiratory movements even though the tension of carbonic acid
in the blood is unaltered. It has been shown, indeed, that carbonic —
acid stimulates the respiratory centre, not in a specific manner, but.
because it is an acid and increases the concentration of H ions in the |
‘ blood. Any other acid, by acting in the same manner, produces the same
effect, and when lactic acid is added to the blood the centre is stimulated
by the combined effect of this acid and of the carbonic acid already
present in the blood. Since the respiratory movements are increased
without any corresponding rise in the production of carbonic acid, the
tension of carbonic acid in the blood and alveolar air falls.
(2) The Tension of Oxygen in the Blood.—The normal tension of
oxygen in alveolar air is 105 to 110 mm. Hg. At this tension the blood
leaving the lungs is almost fully saturated with oxygen, and the dis-
sociation curve of blood (p. 258) shows that, even when the oxygen tension
is reduced to 70 mm. Hg, the blood still contains 90 per cent. of its heemo-
globin as oxyhemoglobin. Experiment shows, in fact, that atmospheric
air containing only 12 to 13 per cent. oxygen, which corresponds with
about 8 per cent. oxygen in the alveolar air, can be breathed without dis-
comfort and without any alteration in the respiratory movements. When
the percentage of oxygen in alveolar air falls below this figure, the
breathing becomes deeper and considerable hyperpneea may be produced.
At the same time, the individual becomes cyanosed and may feel giddy,
or may even lose consciousness.
These symptoms are the result of the imperfect oxygenation of the
blood. The mere lack of oxygen in itself does not act as a stimulus
to the respiratory centre, and the hyperpnoea is caused indirectly by
the passage of lacticacid into the blood. Lactic acid is normally formed
in the body, and in the presence of an adequate supply of oxygen it is
subsequently oxidised to carbonic acid and water. When the supply
of oxygen is insufficient, the lactic acid passes into the blood, and, by
increasing its H ion concentration, stimulates the respiratory centre ;
the respiratory movements become deeper, and more air enters the
lungs at each breath. The deeper breathing raises the tension of
oxygen in the alveolar air, and thus increases the amount of oxygen
, a es
ar
i ee
a ee ee ere eae
—— 2s
THE RESPIRATORY SYSTEM. 269
taken up by the blood. Thus the lactic acid, by producing hyperpneea,
serves to protect the tissues from the ill effects of a lack of oxygen.
‘The presence of an excess of oxygen in alveolar air has no effect on
the respiratory movements, and wide variations may occur in the
partial pressure of oxygen in the alveolar air without any appreciable
change taking place in the respiration; even when pure oxygen is
breathed, respiration is unchanged in a healthy individual. It is
evident, therefore, that, provided the supply of oxygen is adequate, the
depth of the respiratory movements is normally regulated by the tension
(partial pressure) of carbonic acid in the alveolar air and in the blood.
EFFECT OF CHANGES IN PARTIAL PRESSURE OF OXYGEN AND CARBONIC ACID.
Alveolar Pres- | Alveolar Pres-
Barometric ok oF tere of | sure of CO, in | sure of O, in
Pressure. Al weglae Ae aicantes she Percentage of | Percentage of
; * lan Atmosphere.| an Atmosphere,
1| 646°5 mm. Hg 6°61 13°19 5°23 10°41
2| 755 5 5°95 13°97 5°53 13°06
3 | 1260 3°52 16°79 5°64 28°84
Further, this: tension is extremely constant, though, as shown in
the foregoing table, the actual percentage of carbonic acid in the lungs
varies with the barometric pressure.
For example, when the barometric pressure is 1260 mm. Hg, the
alveolar air contains 3°52 per cent. carbonic acid; this represents a
3°52 x 1260
760
partial pressure of = 5°6 per cent. at 760 mm. Hg pressure.
ASPHYXIA.
The effects of a rise in the tension of carbonic acid and of lack of
oxygen in the blood are seen in their most extreme form in asphyxia,
and affect not only the respiratory, but also the circulatory system.
Asphyxia may be brought about by occlusion of the trachea, by the
absence of oxygen in the air breathed, and in other ways, and three
stages are usually described.
(1) Stage of Hyperpnea.—During this period, which lasts from
+ to 1 minute, the respiratory movements gradually increase in depth,
and soon involve not only the muscles usually employed in respiration,
but also the accessory muscles. The respiratory movements during
this stage are co-ordinate, and show an alternate inspiratory and
expiratory rhythm. Consciousness is lost at the end of this stage; the
270 ESSENTIALS OF PHYSIOLOGY.
stage passes into
(2) The Stage of Expiratory Convulsions.—During this period
every muscle which can assist expiration is called into action, and at the
same time convulsive movements of the limbs take place. This period :
lasts about a minute, and is succeeded by :%
(3) The Stage of Exhaustion, during which the animal lies passive,
the muscles are flaccid except for an occasional deep inspiration, the
pupils are widely dilated, and all reflexes are absent. The respiratory
movements become less frequent, and at the end of 14 to 2 minutes death .
takes place. The blood after death is almost free from oxygen. |
If the blood pressure is observed in an animal during asphyxia, the |
vagi being cut, it may be seen that towards the end of the first stage
the blood pressure rises rapidly, and soon reaches a very high level, which
is maintained for a short time; towards the end of the second stage it
begins to fall, and continues to fall steadily until the animal dies.
This sequence of events is brought about in the following manner.
At the beginning of asphyxia, the accumulation of carbonic acid in the
blood excites the respiratory centre, producing hyperpnea. Towards
the end of the first stage the increasing deficiency of oxygen begins to
make itself felt, leading to the further stimulation of the respiratory
centre and to loss of consciousness. In the second stage, the lack of
oxygen stimulates the whole of the central nervous system, giving rise
to the general convulsions which are observed; this effect is soon
succeeded by paralysis, resulting from the prolonged deficiency of
oxygen, and ending in death.
- The vascular changes are also due partly to excess of carbonic acid,
partly to lack of oxygen, as is shown in figs. 104 and 105, These figures
make it clear that a rise of blood pressure similar to that seen in
asphyxia (fig. 104, A) is produced either when the animal is allowed to
breathe air containing no oxygen and no excess of carbonic acid
(fig. 104, B), or when it breathes air containing an adequate supply of
oxygen and a large excess of carbonic acid (fig. 105, A), or, finally, when
lactic acid is injected into the circulation (fig. 105, B).
The rise of blood pressure is due to stimulation of the vaso-motor
centre, which produces general constriction of the arterioles ; adrenalin
also is set free into the blood stream, and contributes to the rise of
pressure. By enclosing a loop of intestine in a plethysmograph it may
-be shown that, with the onset of asphyxia, the volume of the loop
diminishes owing to constriction of its blood-vessels, and that this con-
striction persists until the death of the animal. The final fall of blood —
pressure must be due, therefore, to failure of the heart ; owing partly 7
|
;
[
expiratory movements become more and more ferepern and the first i
:
r
—
THE RESPIRATORY SYSTEM. 271
Fic. 104.—Blood-pressure tracing. (Mathison.)
A shows the effect of asphyxia ; B shows the effect of breathing pure nitrogen.
B
pdt oh hte bed
— 220
Js
"th fin ft if!
ie
. 900M
Us
teshsdeade Pele ih |. fe
Fie, 105.—Blood-pressuie tracing. (Mathison. )
A shows the effect of excess of COQ. in inspired air; B shows the effect of injecting
lactic acid into the circulation.
272 ESSENTIALS OF PHYSIOLOGY.
to the resistance offered by the high blood pressure, partly to the direct
effect of lack of oxygen upon the nutrition of the heart itself, its output
gradually diminishes, it becomes more and more distended, and finally
ceases to beat. When the vagus nerves are intact, the vaso-constriction
leads to reflex slowing of the heart, and the rise of blood pressure is
smaller; the slowing of the heart lessens the strain thrown upon it,
and life is prolonged for a minute or so longer. During asphyxia the
conductivity of the bundle of His is often diminished, producing a
condition of heart block, so that the auricles may beat twice or thrice
as frequently as the ventricles.
The increased activity of both the respiratory and vaso-motor centres
in asphyxia is, undoubtedly, the result of increased H ion concentration .
of the blood ; it must be noted, however, that the respiratory centre is
the more sensitive, and may be excited by an excess of carbonic acid
too slight to affect the vaso-motor centre.
THE NERVOUS REGULATION OF RESPIRATION.
The respiratory centre can be influenced by nervous impulses
_ reaching it (1) along the vagus, (2) from the higher parts of the brain,
and (3) from other afferent nerves; the impulses affect primarily the
frequency of the respiratory movements.
(1) The Vagi.—The influence of the vagus nerves is most easily
studied in the rabbit, by recording the movements of an isolated slip
of the diaphragm (p. 265). When a record of the respiratory movements
is obtained by this method, it may
be seen that division of the vagus
nerve is followed by a decrease in
the frequency of respiration, al-
though each breath is deeper than
before. Electrical stimulation of
; the central end of one vagus may
Fic, 106.—Stimulation of central end
of one vagus between X and X. then produce one of two effects
The slip of diaphragm remains in upon the respiratory movements,
the relaxed, expiratory condition,
Sometimes, especially with a weak
stimulus, or with a constant ascending current, the inspiratory move-
ments are inhibited (fig, 106), and the slip of diaphragm remains in
the expiratory position, that is, it is relaxed. More often the inspiratory
movements are increased (fig. 107), as is shown by the fact that the
diaphragm contracts and may remain in the inspiratory position.
These experiments make it clear, first, that the vagus contains
afferent’ fibres carrying impulses to the respiratory centre, which
® THE RESPIRATORY SYSTEM. 273
| modify the rate of respiration, and, secondly, that these impulses are of
two kinds, one tending to cause inspiration and the other expiration.
These fibres have their origin in the lungs, and their endings can be
stimulated byinflating, or sucking air out of, the lungs. Fig.108 illustrates
the effect of suddenly distending the pulmonary alveoli with air; the
diaphragm remains re-
laxed, inspiratory move-
ments cease, and the whole
chest is in the expiratory
position. This effect,
which is produced only
when the vagus nerves are
intact, and lasts only as
. : . Fie. 107.—Stimulation of the central end of one
long ”" the distension per vagus between X and X. The diaphragm
sists, is known as vagus enters into continued contraction (inspiratory
apnoea. Conversely, when position).
air is sucked out of the lungs (negative ventilation) the diaphragm
is thrown into contraction, and the inspiratory movements are
increased.
It is evident that distension of the pulmonary alveoli stimulates the
endings in the lungs of afferent fibres which pass up the vagus to the
respiratory centre, and inhibits in-
spiration and brings about expira-
[ tory movements. On the contrary,
collapse of the pulmonary alveoli
YOUU ek stimulates the endings of afferent
—_> fibres running in the vagi and con-
veying impulses to ‘the respiratory
Fic. 108.—Apncea produced by sudden centre whereby inspiratory move-
Slstpsiog of fhe Inge betvern X monte are evoked.
The existence of such impulses
can be further demonstrated by connecting the vagus with a string
galvanometer ; with each inspiration a deflection of the thread takes
place, which indicates the passage of an impulse along the nerve ; and,
under certain conditions, a similar deflection may also be observed
during expiration. In normal respiration these two sets of fibres are
alternately stimulated, each inspiration sending impulses along one set
of fibres and reflexly causing expiration, whereas each expiration gives
rise to impulses which bring about another inspiration. The electrical
variations in the vagus seem to show that the impulses leading to
expiration are the more important and more pronounced.
Although these impulses help to maintain the normal rhythm of the
18
274 ESSENTIALS OF PHYSIOLOGY)
respiratory movements, they are not essential, and respiration continues
after they are prevented from reaching the respiratory centre by
division of the vagi. Their principal function seems to be that of
rendering the centre more sensitive to the normal stimulus of carbonic
acid, and of controlling the respiratory discharge from the centre to
the respiratory muscles. In their absence, the respiratory centre is not
stimulated until the tension of carbonic acid in the blood rises higher
than in the normal animal, although the resulting respiration, when it
does occur, is exceedingly forcible ; moreover, the normal adjustment of
the respiratory movements in response to any considerable increase in the
Respir.
tion.
BALatY! EVEL RELA
SUVA V\
Awl
i Winn
.A a AA a AAR AANAAND | Na Se
AT AVAVAPPPATATATAAYAVAYAFAVAVAVATITATATA MATA ACA May ain blood
Whi} if | ry VV ¥Y
PVVVYV EVV V ENE yee : pressu
Ww
_ Fic. 109.—Respiratory movements in rabbit with vagi divided. Between the arrows,
the inspired air contained 10 per cent. CO,. Note that the rate of respiration
is not increased. (Scott.)
tension of carbonic acid in alveolar air and blood is no longer efficiently
carried out.
During muscular. exercise, for example, a normal animal breathes
not only more deeply, but also more frequently, and is thus able to
expel from the lungs all the additional carbonic acid reaching them
from the blood ; after section of the vagi the rate of respiration remains
unaltered, the ventilation of the lungs is inadequate, and the percentage
of carbonic acid in the alveolar air rises. The same result is seen
when an animal, with its vagi divided, breathes air containing an excess
of carbonic acid (fig. 109), the ventilation of the lungs being much less
than in the normal animal, as is seen in the following table :—
VENTILATION OF THE LUNGS.
Total Ventilation per Minute.
Composition of Inspired Air. Rabbit with Vagi
Normal Rabbit. Divided.
Q) Leonor RIP vik 1368 c.c. 1305 c.c.
(2) Air to which 8°6 per cent.
CO, was added . 2613 5, 1596 ,,
THE RESPIRATORY SYSTEM. 275
(2) Impulses from the Higher Parts of the Brain.—In an anesthe-
tised animal, the brain stem may be divided in the pons or upper part
of the medulla without any obvious change being produced in the
respiratory movements. Nevertheless, impulses from the higher parts
of the brain can greatly modify respiration, and the effect on respiration
of emotional states, such as anger or excitement, is often very marked.
Again, the respiratory movements become deeper and more frequent
at the very beginning of severe muscular exercise ; even the first breath
taken after the onset of muscular work is much deeper than the respira-
tion during rest (fig. 110). These changes occur too quickly to be
xX 3)
AAT
WEG LLL) $I LUL L L a
Fie. 110,—Effect of muscular work on the respiratory movements. (Krogh.)
The work begins at X. Tracing to be readffrom right to left. 4
brought about by an increase in the tension of carbonic acid in the
blood, and recent observation shows that they are due to impulses
passing from the cerebral cortex to the respiratory centre, which render
it more sensitive than before to the presence of carbonic acid in the
blood. Hence the normal tension of carbonic acid, acting on the
unusually sensitive centre, calls forth deeper and more rapid respirations.
By this means the amount of oxygen entering the lungs and passing to
the blood and tissues is increased at the very beginning of exercise ; and
the muscles are able to take up from the blood, without delay, the
additional oxygen which they need for their increased activity.
(3) The respiratory movements may also be modified by impulses
reaching the centre from almost every region of the body. Thus pain-
ful stimuli usually produce hyperpnea, whereas impulses passing along
the fifth nerve and the nerves from the. upper respiratory passages
tend to inhibit respiration ; these nerves may be stimulated by irritant
vapours, such as that of ammonia. The superior laryngeal nerve
supplies sensory fibres to the glottis; and stimulation of its endings,
for instance, by the entrance of a crumb into the glottis, inhibits in-
Spiration and causes violent expiratory efforts. Electrical stimulation
of the central end of the nerve brings about the same effect.
276 ESSENTIALS OF PHYSIOLOGY.
Summarising the various factors which influence respiration, we see,
first, that the normal stimulus to the respiratory centre is the tension
of carbonic acid in the blood passing to the centre: an increase of this
tension stimulates the centre, and when the tension falls respiration
ceases. Secondly, when the blood is deficient in oxygen, lactic acid
passes into the blood and also stimulates the centre. Thirdly, impulses
passing along the vagus nerves from the lungs help to maintain the
normal rhythm of respiration, and make the centre more sensitive to
the chemical stimulus of carbonic acid. Finally, impulses from the
higher parts of the brain, or impulses reaching the centre by afferent
nerves may, and do, modify the respiratory movements.
APNGA.
Reference has already been made to the inhibition of the respiratory
movements, which is known as apnoea, The most important condition
which gives rise to apnoea is a fall in the tension of carbonic acid in the
openebreckr
ie SF
Pitre rrr gen rn rrereterre RP ArIRer TTC TnIT Tne:
fe
pene benenpcereememnernapnerbeerdrreeypereneerspeer
CeO te a ea Ny ee ene
st a ak tI wa ik A a
Fic. 111.—Forced breathing in man, followed by apnoea and subsequently by
Cheyne-Stokes breathing. (Haldane and Douglas. )
blood ; this occurs whenever the ventilation of the lungs is increased
without any corresponding rise in the production of carbonic or lactic
acid in the body, and may therefore be brought about either, in man,
by voluntary forced breathing (fig. 111), or, in animals, by vigorous
artificial respiration. In these circumstances the apnoea may last for
1 to 2 minutes, respiration beginning again as soon as the tension of
carbonic acid in the alveolar air returns to its normal level. That it
is due solely to a fall in the tension of carbonic acid is clearly shown
by the fact that, when the inspired air contains an excess of carbonic
acid, forced breathing is not followed by apnoea.
A totally different form of apnoea is that produced by sudden dis-
tension of the lungs, and known as vagus apnoea (p. 273). This is
— ee
ry Pee ah
yr
THE RESPIRATORY SYSTEM. 277
due to the stimulation of the endings in the lungs of the afferent fibres
of the vagus which inhibit inspiration.
Apnea also occurs during deglutition, and lasts for a period of
about 6 seconds ; the afferent impulses pass along the glossopharyngeal
nerve. The effect of this inhibition is to prevent particles of food
being drawn into the lungs by an inspiratory act during the process
of swallowing. |
Another interesting example of the adaptation of the respiratory
mechanism to the needs of the animal is seen in ducks. When a duck
plunges its head into water in search of food, the respiratory move-
ments are inhibited; and this form of apnoea can be reproduced
experimentally by placing the duck in the vertical position with its
head downwards. The afferent impulses which travel to the respiratory
centre and inhibit respiration arise in the muscles of the neck and in
the labyrinth of the ear ; after section of the afferent nerves from these
muscles or destruction of the labyrinth, this form of apnoea can no
longer be evoked.
CHEYNE-STOKES BREATHING.
This form of breathing (fig. 112), which is not infrequently observed
in human beings living at high altitudes or suffering from various
diseases, more especially those affecting the circulatory system, has the
following characters. After an apneic pause respiration begins, the
breaths being shallow at first and gradually increasing in depth till
they reach a maximum. They then become smaller, and in a short
time cease altogether, being succeeded by a period of apnea. Cheyne-
Stokes breathing can also be produced experimentally in healthy
persons as a result of prolonged forced breathing, as is seen in fig. 111 ;
the immediate effect of the forced breathing is a period of apnma, and
when breathing recommences, it often exhibits the periodic character
just described.
The phenomenon is caused by lack of oxygen in the blood. During
the period of forced breathing, the tension of carbonic acid in the
alveolar air falls considerably, leading to prolonged apnea, during
which the oxygen tension in the blood sinks and the supply to the
respiratory centre is inadequate ; as a result lactic acid is formed in
the respiratory centre, and, together with carbonic acid, stimulates the
respiratory centre, although the tension of carbonic acid alone is
insufficient. Respiration begins.again, and the oxygen taken into the
lungs and into the blood oxidises the lactic acid. At the same time,
however, the deeper breathing removes some carbonic acid from the
lungs, the chemical stimulus to respiration again disappears, and the
278 ESSENTIALS OF PHYSIOLOGY.
breaths become smaller and finally stop. During the next apneic
pause, the blood again becomes deficient in oxygen, and a fresh forma-
tion of lactic acid takes place in the respiratory centre.
When this form of breathing occurs in disease, it can be removed
either by allowing the patient to breathe nearly pure oxygen, which
improves the nutrition of the respiratory centre and prevents the
formation of lactic acid, or by the administration of air containing a
slight excess of carbonic acid, which, by raising the tension of this gas
A TUTTEREERIDITI ROS ESSIE TIERS EU REPO RSEECR ISI LL ISPS EERSSPRSCERERORICRSIOPRliteitisraliieinisietinieeiiiiiestieeeiieiiieiti ii iiity
Fic. 112.—Cheyne-Stokes respiration. (Pembrey and Allen.) From Practical
Physiology, by Pembrey and others.
in the alveolar air and blood, increases the strength of the stimulus
to the respiratory centre.
SECTION IV.
TISSUE RESPIRATION.
The processes of external respiration, which have been thus far
considered, are so adapted that the blood, when it reaches the capillaries,
is almost fully saturated with oxygen. More important, however, is
tissue respiration, which consists in the transference of oxygen from the
blood to the tissues and the return of carbonic acid from the tissues to
the blood.
Oxygen passes from the blood to the tissues by diffusion, and the
amount which is available for the tissues depends largely upon the
extent to which oxygen is set free in the plasma by the dissociation of
oxyhemoglobin. If the tension in the tissues were zero, as was formerly
supposed, the conditions during the passage of blood through the
capillaries would be the same as if it were exposed to a vacuum, and
the dissociation of oxyhzemoglobin would be almost complete. Recent
observations have shown, however, that the actual tension of oxygen
in the tissues varies from 10 to 40 mm. Hg, being highest in the glandular
ee ee ea
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en
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f
:
:
:
:
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THE RESPIRATORY SYSTEM. 279
tissues and least in the muscles; this tension is so low that, as the
blood traverses the capillaries, its oxyhemoglobin undergoes dissocia-
tion to a considerable extent. The oxygen thus set free passes into
solution in the blood plasma and diffuses into the tissues. Further, the
rate at which oxyhemoglobin dissociates at this tension is increased by
the addition to the blood of carbonic acid, and often of lactic acid, as it
passes through the capillaries. hy
In addition to these two factors, the amount of oxygen available
for the use of the tissues depends upon the rate at which the blood is
flowing through the capillaries.
Carbonic acid also passes by diffusion from the tissues, in which its
tension is high, into the plasma, in which its tension is much lower.
The tension of carbonic acid in the tissues is ascertained indirectly by
measuring its tension in bile or urine, and is about 8 to 9 per cent. of an
atmosphere, whereas its tension in blood is only 5 to 6 per cent. of an
atmosphere. |
The Consumption of Oxygen by the Tissues.—The tissues are
constantly taking up oxygen, and the amount used by any tissue, for
example the kidney, can be determined by ascertaining, first, the
difference in the quantity of oxygen present in 1 c.c. of the blood
entering and leaving the organ respectively ; secondly, the amount of
blood flowing through the organ in a given time ; and, thirdly, the weight
of the organ. The degree to which the blood is saturated with oxygen
is measured by Barcroft’s blood-gas manometer. The rate of blood
flow through the organ is ascertained by observing directly the quantity
escaping from it in a given time. To take an example, if | c.c. of
arterial blood contains 0°18 c.c. oxygen, and 1 c.c. of blood from the
renal vein contains 0°13 c.c. of oxygen, each c.c. of blood passing
through the kidney loses 0°05 c¢.c. oxygen. Supposing the rate of
blood flow through the kidney to be 50 c.c. per minute, the amount of
oxygen taken up by the kidney cells is 2°5 ¢.c.; and if the weight of
the kidney is 30 grams, 1 gram of kidney uses 0°08 c.c. oxygen
per minute. Experiments made in this way show that the amount of
oxygen consumed by the different tissues of the body varies greatly ;
the heart and kidney use more oxygen than any other organ.
O, ConsumeD BY 1 Gram OF THE TISSUE PER MINUTE.
Skeletal muscle (resting) . ’ . 0°003 c.c.
a (active) ‘ 4 . 0°03 c.c.
Feat muscle ; ; ; 2 - 0°05 c.c.
Kidney. . : . . 0:03-0:06 c.c.
Submaxillary gland (resting) : . +0°023 c.e.
280 ESSENTIALS OF PHYSIOLOGY.
Provided that the flow of blood to an organ is sufficient to supply
the amount of oxygen which it needs, a further increase in the flow of
blood through it does not increase the consumption of oxygen. For
instance, the rate of blood flow through the submaxillary gland may
be increased tenfold without any rise taking place in the oxygen
consumption of the gland. The oxygen consumption rises, however,
whenever the functional activity of a tissue becomes greater, and in the
case of skeletal muscle it may be increased tenfold during and just
after muscular contraction. <A similar rise occurs when the heart does
more work, or when the secretory activity of the submaxillary and other
glands is evoked.
OXYGEN CONSUMED BY THE SUBMAXILLARY GLAND.
Oxygen Used by Rate of Blood Flow
1 grm. of Gland per through the Gland
Minute. in c.c. per Minute.
(1) Resting. 0-023 c.c. 0°35 ¢.c.
(2) Resting, with dilated
blood-vessels_. 0:024 ,, re Cine
(3) During the secretion
of saliva . ; tad Bee
The increased consumption of oxygen occurs not only during the
period of secretion or of muscular work, but for some time after this is
08
06
"04
202
¢)
0 50 100 §=150 200
Fic. 113.—Ordinate= volume of oxygen used in c.c. per minute; abscissa=time in
seconds ; upper signal=duration of flow of saliva; lower signal=duration of
chorda stimulation. Blackened area represents the oxygen used by the salivary
gland. (From Barcroft, Respiratory Function of the Blood.)
over (fig. 113); and the oxygen is apparently used mainly-in the
carrying out of chemical changes, whereby the gland or muscle stores
up potential energy and is restored to its previous condition. Every
increase in the functional activity of an organ is also accompanied by
dilatation of its blood-vessels, which, as already mentioned (p. 236), is
me eh tala | Te) ee he a a, Ee a Pas A" a ie CO eT ae? Cer yy Ore) > a) ed 2g _
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THE RESPIRATORY SYSTEM. 281
brought about by the action of the metabolic products set free in the
tissue upon the vessels of the organ ; this increases the supply of blood
and, therefore, of oxygen to the active tissues. .
The oxidative changes in the body normally take place in the tissue
cells and not in the blood itself. When methylene blue is injected
into an animal and the animal is killed a few: minutes later, the blood
is coloured blue, whereas the tissues show no change of colour.
Although methylene blue is a comparatively stable substance, the
avidity of the tissues for Oxygen is so great that they are able to reduce
it with the formation of a colourless reduction product. On exposing
the tissues to the air, methylene blue is re-formed, and the tissues
become deeply stained.
Another method of demonstrating the fact that oxidation does not
take place in the blood is to allow it to stand for a short time, and to
observe whether any of its oxyhzmoglobin is reduced. In normal
animals the reduction is almost negligible, and evidently no oxygen is
used up in carrying out metabolic changes in the blood itself.
SECTION V.
THE EFFECT OF CHANGES IN BAROMETRIC PRESSURE.
(1) The Effect of Lowered Pressure.— When a person ascends from
near sea level to a height of 5,000 to 10,000 feet or more, he is apt
to suffer from symptoms which are generally described as mountain
sickness: these symptoms are headache, mental confusion, blueness
of the lips, and nausea or vomiting. They may occur when the
individual ascends to this height in a train, and are therefore not
due to muscular exercise, although they may become more severe when
exercise is taken. They are caused entirely by lack of oxygen. With
increasing altitude the barometric pressure, and therefore the partial
pressure of oxygen in the atmosphere and in the alveolar air, gradually
fall; and when the alveolar pressure of oxygen falls to about 60 mm.
Hg, the symptoms just described make their ‘appearance. Similar
symptoms are produced in animals and in man, when they are placed in
closed chambers and the barometric pressure is gradually reduced. If
the individual remains at a high altitude, the symptoms pass off in
the course of a day or two, and after a time exercise may be taken
without their recurrence.
The adaptation of the body to the altered conditions is. brought
about by changes in both the circulatory and respiratory systems. In
the first place, the lack of oxygen leads to the passage into the blood of
o
282 ESSENTIALS OF PHYSIOLOGY. .
lactic acid, which stimulates the respiratory centre ; and the breathing
becomes deeper, with the result that the partial pressure of carbonic
acid in the alveoli decreases and that of oxygen rises. At a high
altitude, when the alveolar oxygen pressure is low, even a small rise in
the partial pressure of oxygen appreciably increases the extent to which
hzemoglobin can be saturated with oxygen as the blood passes through
the lungs. For example, if the alveolar pressure of the oxygen rises
from 40 mm. Hg to 45 mm. Hg, the saturation of hemoglobin may
be increased from 70 per cent. to 75 per cent. At the same time,
acceleration of the heart takes place, and consequently the blood is
carried more rapidly through the lungs and round the body.
Owing to these changes, the blood not only takes up more oxygen
in the lungs, but transfers it more rapidly from the lungs to the
tissues. The presence of lactic acid also assists the dissociation of
oxyheemoglobin in the capillaries, so that more oxygen is available for
the tissues. .
Secondly, a more gradual compensatory process takes place in
persons who remain at a high altitude for a long period. The number
of red corpuscles and the percentage of hemoglobin in the blood are
increased, and the oxygen-carrying capacity becomes much greater. In
one series of observations, made at a level of 14,000 feet, the percentage
of hemoglobin in the blood rose in the course of some weeks from 115
to 154. When the subjects returned to a low level, the percentage of
hemoglobin rapidly fell to normal.
The extent to which adaptation takes place varies in different
individuals; in many cases the supply of oxygen to the tissues is
barely sufficient during rest, and exercise frequently leads to an in-
adequate supply of oxygen to the tissues and brings on mountain
sickness. This occurs less readily, however, in the trained than in the
untrained person, since the former uses his muscles more economically,
and therefore his démands for oxygen are not so great.
(2) The Effect of Raised Pressure.—Men engaged in the building
of bridges and tunnels are often compelled to work in caissons, which
are filled with compressed air to prevent the inrush of water. They
suffer no inconvenience, and the respiratory movements are not affected
while they are in the caisson, even though the pressure may be three
or four atmospheres. Under this pressure, however, the blood dissolves
an increased amount of oxygen and nitrogen, and when a man leaves
the caisson, and the pressure to which the blood and tissue fluids are
exposed is reduced to one atmosphere, most of the nitrogen, previously
in solution, is evolved as bubbles, which may obstruct the flow of blood.
along the blood-vessels or through the heart.
THE RESPIRATORY SYSTEM. 283
The symptoms caused by this obstruction, and known as caisson
disease, are very varied, and include paralysis, severe abdominal pain,
and collapse. The disease is prevented by allowing the man to pass”
from the caisson into a special air chamber, in which the pressure is
gradually lowered to that of the atmosphere so as to prevent any
sudden evolution of nitrogen.
CARBON MONOXIDE POISONING.
The affinity of carbon monoxide for hemoglobin is about 130
times as great as that of oxygen; and when air containing even a
small percentage of carbon monoxide is breathed, the oxyhemoglobin
is replaced by carbon monoxide hemoglobin, and asphyxia is produced.
The fatal effects of breathing air containing coal gas, in which carbon
monoxide is present, are brought about in this way; but death may
often be prevented by the administration of pure oxygen, which not
only increases the amount of oxygen dissolved in the blood and carried
to the tissues, but, by its mass action, gradually displaces the carbon
monoxide from its combination with hemoglobin.
SECTION VL
THE INFLUENCE OF THE RESPIRATORY MOVEMENTS
ON THE CIRCULATION.
On examining a tracing of the blood pressure, it is often noticed
that the pressure shows oscillations corresponding with each respiratory
movement, rising a little with each inspiration and falling during
expiration. Further, the pulse is more frequent during inspiration
and less frequent during expiration (fig. 114). The difference in the
pulse rate is due to a slight diminution of the tone of the vagus during
inspiration, which allows the heart to beat more rapidly ; it is abolished —
by section of the vagi, but the respiratory oscillations of the blood
pressure are not affected by this procedure. |
At the end of expiration, the pressure inside the cavity of the
chest is slightly below atmospheric pressure and the pressure on the
walls of the great veins and of the heart is negative, whereas the
pressure in the jugular vein, for example, is slightly higher than that
of the atmosphere. Owing to this difference of pressure in the vessels
within and outside the chest, blood tends to be sucked along the great
veins and into the heart. With each inspiration the negative pressure
is increased, and the flow of blood into the heart becomes more rapid.
The thin-walled auricles are also slightly dilated. by the negative
284 ESSENTIALS OF PHYSIOLOGY.
pressure, and the flow of blood to the heart is thus further assisted,
the thick-walled ventricles and arteries being practically unaffected.
As the result of the additional blood reaching the heart during
inspiration, the amount of blood expelled from the heart at each beat
becomes larger, and the arterial pressure rises. Conversely, during
expiration the negative pressure diminishes again, less blood is sucked
into the heart, its output is smaller, and the arterial pressure falls.
A second factor which conduces to the rise of arterial pressure
is the descent of the diaphragm during inspiration ; this, by diminish-
ing the size of the abdominal cavity, raises the intra-abdominal
pressure and squeezes blood out of the abdomen along the inferior
vena cava into the heart. ;
According to some observers, a third factor also takes a part in
producing the rise of blood pressure during inspiration, namely,
, changes in the size of the
fiults \ vessels forming the capil-
Blood press lary network in the lungs.
an The lung is exposed on
its inner surface to atmo-
spheric pressure, and on its
Rie ~ _. outer surface to a pressure
Patctpimciag lve ——_/Y_~—so which is less than atmo-
Fic. 114.—Effect of respiratory movements on spheric pressure. An in-
arterial blood pressure. (Starling’s Principles Grease in this difference of
of Physiology.)
pressure, such as occurs
during inspiration, will dilate the capillaries in the wall of the lungs,
blood will flow through them more rapidly, and a larger quantity
will reach the left ventricle in a given time; the additional blood
thus reaching and. expelled from the left ventricle will raise the
arterial pressure. Conversely, during expiration, when the difference
of pressure on the two sides of the lung wall diminishes, the capillaries
will shrink, blood will flow less rapidly through them into the left
ventricle, and the blood pressure will fall. Other writers, however,
consider that changes in the calibre of the capillaries play no part
in the variations of blood pressure which accompany the respiratory
movements; they regard these variations as being due entirely to
alterations in the amount of blood which is drawn into the heart by
the negative pressure on the walls of the great veins, and is driven
out of the abdominal veins by the contraction of the diaphragm.
Eqpiration| Inspiration
The negative pressure during inspiration varies with the depth of |
the inspiration, and, when the breathing is very forcible, the rise and
fall of blood pressure during inspiration and expiration may become very
eee coke
THE RESPIRATORY SYSTEM. 285
marked. The rise of pressure begins just after the beginning of
inspiration, and continues for a short time during expiration, so that
the variations in blood pressure are not quite synchronous with the
respiratory movements. This delay in the rise of blood pressure is
due to the fact that, when more blood enters the right auricle at the
beginning of inspiration, it has to travel through the lungs before it
reaches the left ventricle and is expelled into the aorta. Again, at the
beginning of expiration there is, for the same reason, a slight delay in
the diminution in the amount of blood sent out by the left ventricle.
In man the alterations in blood pressure produced by the respira-
tory movements are of very complex origin, and the effects are not so
constant as those just described for animals; they vary with the type
of respiration, a costal inspiration causing a fall, and a diaphragmatic
inspiration causing a rise of blood pressure.
SECTION: VII.
MUSCULAR EXERCISE.
The supply of oxygen to the tissues and the removal of carbonic
acid are effected by the conjoint action of the respiratory and circulatory
systems’; and, in order that the tissues may receive their due supply of
oxygen, it is necessary, not only that the blood in its passage through
the lungs should become fully saturated, but that an adequate amount
of blood should be carried to the tissues. We find, therefore, that the
demands of the body as a whole for an increased amount of oxygen are
met by alterations in both the respiratory and circulatory systems, and |
that for this purpose the two systems exert a correlated action, which
is very clearly illustrated in muscular exercise. Many of these changes
have already been considered, but they may conveniently be summarised
here.
During muscular exercise the contracting muscles require a large
amount of oxygen and give off much carbonic acid. By means of
increased respiratory movements sufficient oxygen reaches the alveolar
air to replace that which passes into the blood, and sufficient carbonic
acid is removed from the lungs to keep its, percentage in the alveolar
air almost unchanged ; and the blood, during exercise, is almost as fully
oxygenated as during rest, the hemoglobin being at least 90 per cent.
saturated with oxygen.
The adjustment of the respiratory mechanism to the increased
needs of the muscles is brought about (1) by impulses passing from the
cerebral cortex to the respiratory centre at the very Qgutset of exercise,
286 ESSENTIALS OF PHYSIOLOGY.
which increase the sensitiveness of the centre to carbonic acid (p. 275) ;
and (2) by the chemical stimulus of the additional carbonic acid pro-
duced in the muscles. In addition, the respiratory centre is stimulated
by lactic acid, the amount of which in the . blood increases during
muscular exercise, if the latter is at all severe. During rest, the blood
in man contains only minute traces of lactic acid, and this is not in-
creased by moderate exercise, such as walking, though running for a
few minutes raises the amount of lactic acid in the blood, and lactic
acid may be detected in the urine. Notwithstanding the great increase
in the supply of oxygen to the actively contracting skeletal muscles, the
latter are unable to oxidise all the lactic acid which is formed in them in
these circumstances. The excess of lactic acid passes into the circula-
tion, and not only stimulates the respiratory centre, but also (p. 258)
renders the blood more readily dissociable and enables the muscles to
obtain oxygen more easily.
The changes in the circulation during exercise are acceleration of
the heart and a rise in the mean arterial pressure, associated with
dilatation of the blood-vessels to the muscles. The acceleration of the
heart is due primarily to diminution in the tone of the vagus centre
and, therefore, of the restraining influence which it normally exerts
on the heart. It is seen at the very beginning of exercise, being
brought about in all probability by impulses passing from the cerebral
cortex to the vagus centre, and is particularly well marked in animals,
such as the dog, which are accustomed to take severe exercise.
Subsidiary factors which contribute later to the acceleration are (1)
an increase in the tone of the accelerator centre, and (2) the setting
free of adrenalin into the circulation. At the same time, the increased
respiratory movements and the muscular movements increase the
amount of blood reaching the heart, and its output becomes larger.
The rise in the arterial blood pressure is due partly to the increased
output of the heart, and partly to constriction of the splanchnic
vessels ; and, since the blood-vessels to the muscles are dilated, the
flow of blood is largely diverted from the abdominal organs to the
skeletal muscles.
Owing to these factors, the velocity of the blood flow through
the lungs, and indeed through the whole body, becomes greater; and
within a given time a much larger quantity of oxygen can be taken
up by the blood as it passes through the lungs, and can be transported
to the tissues, than during rest.
During exercise a large amount of heat is evolved in the muscles,
and the temperature of the body may rise to 100° or 101° F. ; this rise
in temperature tends further to accelerate the heart, and is one of
—<——
THE RESPIRATORY SYSTEM.
the reasons why the pulse rate remains rapid for some time after the
exercise has come to an end.
Second Wind.—It is a matter of common knowledge that, in a
trained person, the respiratory discomfort which occurs soon after the
beginning of exercise usually passes off in a few minutes, and the
exercise can then be continued for a long period without further incon-
venience, the individual being said to have gained his second wind.
Its causation has been much discussed, and is not fully understood,
though it is probably due in part to the fact that a trained person
uses his muscles more economically than an untrained person, and
therefore produces less carbonic acid. The observation that the
tension of carbonic acid in the alveolar air may fall with the onset of
second wind suggests that such is the case. If the muscles are used
more economically after a brief period of exercise, the decrease in the
production of carbonic acid will lessen the stimulus to the respiratory
centre, and will account for the absence of panting and respiratory
distress in second wind. It is probable that adjustment also takes
place in the circulation, since the process of training consists essenti-
ally in the better adaptation of the circulatory system to withstand
extra strain. |
CHAPTER X.
THE DIGESTIVE SYSTEM.
SECTION I.
THE NATURE OF DIGESTION.
THE process of digestion consists essentially in the splitting up of the
molecules of the food-stuffs into a large number of much smaller
molecules which, partly because of their smaller mass, are easily
absorbed through the mucous membrane of the digestive tract. Thus
the digestion of fat results in the splitting up of the molecule of neutral
fat into one molecule of glycerol and three molecules of fatty acid, a
single protein molecule is broken up into a very large number, probably
a hundred or more, of molecules of amino-acids, while one molecule of
starch is subdivided into about two hundred molecules of dextrose. In
all cases the process is one of hydrolysis, and in the case of protein and
_ starch it takes place in a series of stages. Similar changes can be
brought about by chemical means, such as boiling with mineral acids,
but the digestive juices achieve their results more rapidly and
effectively, at the temperature of the body, by means of certain active
agents known as enzymes or ferments.
ENZYMES.
No enzyme has yet been isolated and analysed, and therefore
nothing definite is known as to the composition and constitution of
these bodies except that they are not proteins. They exist, however,
in great variety in animal and vegetable cells, and they possess very
definite properties. (1) They are colloidal substances, and do not
diffuse through animal membranes. (2) They only act in solutions,
having no effect in the dry state. (3) As a rule each enzyme is
specific ; it will produce its own definite effect and no other. Thus the
ptyalin of saliva converts starch into maltose, but has no action on
protein. (4) Enzyme action is markedly affected by temperature. It
is most’ effective, in the case of the enzymes of the animal body, at
288
THE DIGESTIVE SYSTEM. 289
37° to 40°C.; and it is destroyed by the temperature of boiling water.
(5) Enzymes will act indefinitely if the products of their activity are
‘removed, that is, they are not used up and do not form permanent
compounds with the substances on which they exert their activity. In
other words, no definite proportion of enzyme to substrate, that is the
material acted upon, is necessary; but the greater the proportion of
enzyme to substrate the more quickly is the final result attained.
The foregoing characteristics are the principal tests by which an
enzyme may be distinguished. But some further points in connection
with the origin and action of ferments have to be noted, and help to
throw light on the probable mode of action of these bodies. Enzymes
are produced in living cells, and they either exert their action on
substances present in the cell body, or are turned out of the cell, as in
the case of the digestive juices, to exercise their function elsewhere.
That function is to be regarded as the acceleration. of a process which
tends to go on, it may be with infinite slowness, in the absence of an
enzyme. The enzyme may be compared with the grease on the slip-
ways when a ship is launched, in that by its presence it facilitates a
process brought about by quite independent forces. The particular
process expedited by the presence of an enzyme is generally that of
hydrolysis, in which a large molecule takes up water and then the
resulting compound splits into smaller molecules. But in the case of
many, if not of all, enzymes, this process may be reversed, the smaller
molecules being condensed again with the loss of water. For example,
the enzyme maltase, if added to a solution of maltose, will convert most
of this sugar into dextrose. But if the same ferment is added to a
solution of dextrose it will convert a certain proportion of the latter
into maltose. The action of maltase is, in fact, to bring about a
certain definite proportion between the maltose and dextrose in
solution, the proportion, or, as it is called, the equilibriwm point,
varying with the concentration of the solution. Thus maltase, acting
upon maltose, might convert it entirely into dextrose if the latter
substance were removed at the same rate as it was produced; or, on
the other hand, the same enzyme, acting upon dextrose, might con-
vert the latter entirely into maltose, provided the maltose were being
simultaneously removed. This reversible enzyme action is of great
importance in the body, where, for example, dextrose is at one time
converted into glycogen by a synthetic process, while later the glycogen
is once more turned into dextrose by a process of hydrolysis.
Enzymes, then, are bodies of unknown constitution, which facilitate
certain chemical reactions without providing any energy for these
reactions and without being used up in the process. They are, in fact,
19
290 ESSENTIALS OF PHYSIOLOGY.
organic catalysts, and some suggestions as to their mode of action
may be obtained from the study of. the methods by which the simpler
inorganic catalysts produce their effects. Two examples of the latter
may be taken: (1) the effect of spongy platinum in bringing about the
combination of hydrogen and oxygen to form water, and (2) the
_ oxidation of indefinite quantities of SO, to form SO, by the inter-
mediation of * nitrogen peroxide. The platinum acts as a catalyst
because of its physical properties. Its low surface tension leads to a
concentration of the gases at its surface, whereby the molecules are
brought into close contact and their chemical interaction is favoured.
With this may be compared the first stage in the action of an enzyme,
in which a physical union takes place between enzyme and substrate
by the process known as adsorption.
The nitrogen peroxide, on the other hand, forms an intermediate
chemical compound, After yielding part of its oxygen to oxidise SO,,
and thus becoming reduced to nitric oxide, the latter substance takes
‘up oxygen from the air to form nitrogen peroxide once PRISTY, and so
the process is repeated indefinitely.
SO, + NO, =SO,+ NO
NO+0=N0O,.
The enzymes known as oxidases possibly act in a somewhat similar
way to the nitrogen peroxide in this reaction. Many oxidases are
believed to consist of an organic peroxide combined with a peroxidase,
The latter splits off oxygen from the peroxide, which again takes up
oxygen, and the process is repeated. In the case of enzymes it is found
that, with a limited amount of ferment, the same amount of substrate
is acted upon in a given time whether the observation is made early or
late in the process. It would, therefore, appear that the mode of
action of the organic catalyst is comparable with that of the inorganic
catalyst which acts chemically, the enzyme probably being adsorbed by
the substrate, which is then capable of taking up water and splitting
up, with the result that the enzyme is once again set free.
In the case of most enzyme reactions the velocity of the process
tends to diminish after it has gone on for a time. An enzyme has not
only a specific affinity for its particular substrate, but it may also have
an affinity for the products of the reaction, and by forming com-
binations with these it may be put out of action. By-products are also
formed in some enzyme reactions, and these, for example acids or
alkalies, may either increase or decrease the power of the enzyme itself
and so modify the rate of the process. In some cases these by-
products destroy the enzyme and so bring the reaction to an end.
THE DIGESTIVE SYSTEM. 29!
THE STAGES OF DIGESTION.
The activities of the digestive tract aretwoin number. First, there
is a motor mechanism by means of which the contents of the tract are
moved progressively from mouth to cesophagus, stomach, small intestine,
and large intestine, and are finally expelled. Secondly, there is a series
of secretory glands, which produce the digestive juices met with in the
different regions of the tract. These juices have no effect on water and
inorganic salts, but their enzymes bring about hydrolytic changes in
the protein, carbohydrate, and fatty constituents of the various food-
stuffs. Food is mixed with saliva in the mouth and is then quickly
passed on to the stomach, in which it remains for some time. During
the early part of its stay in the stomach, the starch of the food under-
goes the preliminary stages of digestion through the agency of the
saliva. Gastric juice is also secreted, and originates digestive changes
in the proteins and, to a certain extent, in the fats, while it gradually
destroys the salivary enzyme.
The next stage consists in the forwarding of the stomach contents _
into the small intestine, where the pancreatic juice and bile carry the
digestive changes in the carbohydrates and proteins a stage further, and
complete the digestion of the fats.
Finally, the intestinal juice effects the concluding stages of
carbohydrate and protein digestion, and the hydrolytic products of
carbohydrates, proteins, and fats, together with inorganic salts and water,
are absorbed through the wall of the small intestine. Undigested
substances and certain waste matters reach the large intestine, where
much of the remaining water is absorbed, the residue constituting the
feeces.. Under normal conditions, such of the constituents of a meal as
are not absorbed begin to reach the large intestine four to five hours after
the ingestion of the meal, and the residues are finally expelled from
twelve to twenty hours later, so that all the processes described above
are going on simultaneously, there being as a rule an interval of not
more than four or five hours between meals during the day.
SECTION II.
CHANGES IN THE FOOD IN THE MOUTH.
The changes which take place in the food in the mouth are chiefly
mechanical, its stay in this region being so brief that the saliva has not
time to produce an appreciable chemical effect. Salivary digestion
takes place mainly in the stomach in the interval before the enzyme is
destroyed ‘by the gastric juice. In the mouth the food is masticated,
that is, it is broken up and formed into a pulpy mass by the vertical,
*
292 ESSENTIALS OF PHYSIOLOGY.
lateral, and antero-posterior movements of the jaws, the various frag-
ments being directed in turn by the muscular movements of the cheeks,
lips, and tongue, so that they come between the opposing teeth. At
the same time, saliva is being poured out in considerable quantity, and
is being intimately mixed with the food by the same grinding process.
After mastication the insalivated food is collected into a bolus on the
tongue by further movements of the cheeks, lips, and tongue itself, and
in this condition it is ready for the process of swallowing.
The Composition of Saliva.—Saliva, which is the mixed secretion of
the parotid, submaxillary, and sublingual glands, together with that of
the buccal glands scattered about the mucous membrane of the mouth,
is a viscid, colourless, cloudy fluid, with a slightly alkaline reaction, and
an average specific gravity of about 1005. It contains asa rule over
99 per cent. of water and less than 1 per cent. of solid constituents.
The latter consist of coagulable proteins, mucin, a diastatic enzyme
called ptyalin, and inorganic salts. Calcium salts are present in con-
siderable proportion, and are responsible for the formation of tartar on
the teeth. Traces of potassium thiocyanate are often present, and may
be recognised by the red colour which they give with ferric chloride.
The viscidity of saliva is due to the mucin it contains; the cloudiness
is the result of the presence of numerous squamous epithelial cells from
the mucous membrane of the mouth, and of the so-called salivary cor-
puscles. The latter are granular spherical cells resembling leucocytes,
some of which are probably derived from the lymphoid tissue of the
tonsils, while others may come from the salivary glands themselves.
The Functions of Saliva.—The saliva serves other purposes besides
that of digestion, and in the dog it serves other purposes only, there
being no ptyalin in the saliva of that animal. By its admixture with
the food it facilitates mastication and deglutition. By keeping the lips
and tongue moist, it is of service in those movements which are essential
to the function of speech ; and, by dissolving substances which affect
the sense of taste, it renders these capable of stimulating the gustatory
end-organs.
Tn these cases the use of saliva is a mechanical one, whereas in its
digestive function it gives rise to chemical change. Ptyalin acts upon
starch, converting it into dextrin and maltose. Raw starch is not
acted upon by saliva, which cannot dissolve the cellulose capsules of the
starch granules, and to enable ptyalin to produce its effect upon starchy
foods, these must be boiled or otherwise cooked. When starch granules
are heated in the presence of water, they swell, and the cellulose
capsules are ruptured so that the starch passes into a pseudo-solution.
If a little saliva is mixed with some boiled starch paste-in a test-
— es ee
THE DIGESTIVE SYSTEM. 293
tube and the mixture kept at 40° C., the changes which occur may be
conveniently observed, The original starch solution is slightly opalescent. _
Within a few seconds it becomes clear, but if a drop of the clear
solution be added to a drop of dilute iodine a blue colour results, as in
the case of starch which has undergone no digestive change. This, the
first stage in the process, is that of soluble starch. A little later, a
drop of the fluid gives a purple colour with iodine, later still a reddish
brown, and finally an “‘achromic point” is reached, that is, a stage
when the digest does not give a colour reaction with iodine. The
digestive power of any particular saliva can be estimated by the time
taken to reach the achromic point.
The purple and reddish brown reactions with iodine indicate the
presence of erythrodextrin, at first mixed with a certain amount of
unaltered starch so that the dextrin reaction is complicated by the blue
starch reaction, but later without any such admixture. If, at any
time after the indications of the presence of dextrin appear, a little of
the solution be boiled with an alkaline solution of cupric sulphate,
reduction of the latter will take place, a precipitate of yellow cuprous
oxide being formed. This reaction indicates the presence of a reducing
sugar, maltose. In the achromic stage the solution is found to contain
maltose and a form of dextrin, which gives no colour reaction with
iodine and is therefore called achroo-dextrin. The ultimate result of
the digestion usually consists of about 80 per cent. of maltose and
20 per cent. of achroo-dextrin, and although the proportion of the
latter may be reduced in favourable circumstances to 5 per cent., the
conversion of starch into maltose by ptyalin is never complete.
The salivary digestion of starch consists in the taking up of water
by the starch molecule, and for each molecule of water taken up a
molecule of maltose is split off. In this way the original molecule
becomes progressively smaller, and passes through a series of dextrins
which are grouped as erythro- and achroo-dextrins according to their
reaction with iodine, finally reaching the stage of maltose and achroo-
dextrin, on which ptyalin has no further action. The process may be
diagrammatically shown thus :—
Starch.
| .
Soluble starch,.
|
Erythro-dextrins Maltose.
Achroo-dextrins Maltose. :
*
204 ESSENTIALS OF PHYSIOLOGY.
Or it may be epitomised in the formula
(CygH 9901 0)100 + 80H,O = 80C,,H5,0,; + (CygH 9919) 20:
Starch Maltose _ Achroo-dextrin.
Evans’ method of determining the amylolytic power of saliva is based
upon a calculation of the amount of maltose formed from a given quantity of
starch in a definite time. 5 c.c. of mixed saliva are diluted to 50 cc. with
distilled water, and the mixtute is filtered. 3 c.c. of the diluted saliva are
added to 50 c.c. of a 3 per cent. solution of neutral soluble starch, which is at
a temperature of 46° C. Digestion is allowed to proceed at 46° ©. for ten
minutes, and is then stopped by the addition of a little sodium hydrate. The
copper-reducing power is then determined, and the amount of maltose formed
is calculated.
The digestive action of ptyalin on starch is most energetic in a
neutral medium. Hence it is favoured by the addition of a trace of
acid to normal alkaline saliva, Like other ferment processes, it is
arrested by a high temperature, and ptyalin-itself is destroyed by the
slightest excess of hydrochloric acid, even less than is contained in
gastric juice. The optimum. temperature for the action of ptyalin is
46° C. When a meal is taken, it forms a fairly compact mass in the
stomach, but is gradually penetrated by the gastric juice; as this
penetration takes place, the ptyalin is gradually destroyed, but about
half an hour elapses before salivary digestion in the centre of the
mass is finally terminated.
THE SECRETION OF SALIVA.
There is a constant production of saliva in sufficient quantity to
keep the buccal mucous membrane moist, but in certain circumstances
the flow is largely increased. The conditions which are followed by
more profuse production of saliva are (1) the presence of food in the
mouth, and (2) the sight, smell, or thought of food. Food in the mouth
is followed so promptly by the increased salivary flow that it must
necessarily produce its effect through the nervous system. The second
group of conditions likewise obviously produce their effect by means of
a nervous mechanism. It is clear, therefore, that the secretion of
saliva is brought about by a reflex mechanism, the afferent nerves
usually being those connected with the buccal mucous membrane, and
the efferent nerves those passing to the various salivary glands. .
Each salivary gland has a double nerve supply. The chorda
tympani nerve is distributed to the submaxillary and sublingual glands
(fig. 115), and the parotid receives a branch from the auriculo-temporal
nerve. In addition, each gland receives fibres from the sympathetic
system. Ifa cannula is placed in, the duct of the submaxillary gland
and the chorda tympani is divided, no flow of saliva is observed. But
THE DIGESTIVE SYSTEM. 295
if the distal portion of the nerve be stimulated, a profuse flow of saliva
follows within two or three seconds, while at the same time the blood-—
vessels of the gland are dilated, and the output of carbonic acid and the
intake of oxygen are both increased. Section of the sympathetic fibres
is likewise followed by a negative result, while stimulation of the distal
portion in the dog leads to the production of a very small flow of viscid
saliva, accompanied by constriction of the blood-vessels. In view of
these experiments it might appear that the primary effect of the
stimulation of the chorda tympani was the dilatation of the vessels, and
that the flow of saliva was due to filtration.
MAL dit
Hf Aue
Mi
fie
Hill
Ln
; nap!
wily
Mtn
Fic. 115.—Scheme of nerve supply of submaxillary gland.
VII N, facial nerve; C, T., chorda tympani; L, lingual nerve; G, Langley’s ganglion ;
8, sympathetic fibres to the gland.
Filtration is a purely physical process in which fluids pass through
a permeable membrane under the influence of pressure. In the process
of secretion, on the other hand, work is done by cells; these take up
material from the lymph which bathes them and is derived from the
blood, effect chemical changes in that material, and discharge the re-
sulting products in the form of a “secretion.” In the case of the
salivary glands, several facts indicate that the process is a secretory
one. (1) The cells of the salivary gland, as will be described more
fully later, accumulate granules during the period when saliva is not |
being poured out, and discharge them during the period of activity.
(2) The consumption of oxygen by a salivary gland is increased during
the production of saliva, indicating that work is being done. (3) Two
of the constituents of saliva, mucin and ptyalin, do not exist in the
296 ESSENTIALS OF PHYSIOLOGY.
blood, and must be formed in the gland. (4) The pressure in the duct
of the gland during activity may be greater than the pressure in the
carotid artery, and therefore much greater than that in the capillaries
of the gland. (5) The molecular concentration of saliva in inorganic
salts is only about half that of blood, whereas if saliva were a filtrate
the concentration of salts would be the same as that of blood. (6)
Secretion may be obtained in the absence of blood. If the head of a
Fic. 116.—Effect of stimulation of the chorda tympani on the volume of the
submaxillary gland. (Bunch.)
a, volume of gland; 0, blood pressure.
rabbit be cut off and the chorda stimulated immediately, a flow of
saliva is obtained. (7) The blood-vessels may be dilated without any
secretion taking place. In an animal to which atropine has been
administered stimulation of the chorda is followed by no secretion of
saliva, although the vessels become fully dilated.
It has been suggested that, whereas the nervous factor undoubtedly
originates the salivary flow, other factors come into play to assist in its
continuance. Stimulation of the chorda is followed by a temporary
diminution in the volume of the gland, although the blood-vessels are
THE DIGESTIVE SYSTEM. 297
dilated (fig. 116), This shrinkage must be due to the first discharge of
material from the secretory cells. As a result of the loss of fluid of
lower molecular concentration, the osmotic pressure in the cells them-
selves is raised and water is attracted to them from the lymph. In
this way the molecular concentration of the lymph is also increased,
and therefore water is attracted from the blood. Moreover, as the
result of the secretory activity of the cells, large molecules are being
split up into a large number of smaller molecules, and the discharge of
these metabolites into the lymph tends still further to raise the osmotic
pressure and ultimately the amount of that fluid.
If the chorda tympani has been divided, a flow of watery caliva
begins from one to three days later, and continues for five or six weeks
(paralytic secretion), when the gland atrophies and the secretion
ceases. If the chorda of one side only has been divided, a similar
secretion is said to take place in the opposite gland, an antiparalytic
secretion, the explanation of which is not clear.
Division and stimulation of the nerves to the parotid and sub-
lingual glands give parallel results to those described above for the
submaxillary gland. v
The physiological centre for the reflex mechanism of salivary secre-
tion is situated in the medulla oblongata. The cranial efferent fibres
take origin in the nucleus of the nervus intermedius. Those for the
submaxillary and sublingual glands join the facial nerve and leave it
in the chorda tympani, which subsequently joins the lingual branch of
the fifth nerve. The fibres for the parotid join the trunk of the glosso-
pharyngeal nerve, and, leaving it by its tympanic branch, reach their
destination after passing through the tympanic plexus, the Vidian nerve,
the otic ganglion, the second division of the fifth, and the auriculo-
temporal nerve. The cranial fibres for the salivary glands belong to
the autonomic nervous system, and in the case of the submaxillary and
sublingual glands the post-ganglionic fibres take origin in Langley’s
ganglion and the submaxillary ganglion respectively. This can be
proved by painting these ganglia with nicotine, after which stimulation
of the chorda tympani is without effect on the glands.
The pre-ganglionic fibres of the sympathetic nerves to the salivary
glands pass down the spinal cord, leaving it by the anterior roots of
the upper three thoracic nerves to join the sympathetic chain. They
run in the cervical sympathetic nerve up to the superior cervical
ganglion, where the post-ganglionic fibres arise. These pass on the
walls of the external carotid artery to the various glands.
~*~
298 ESSENTIALS OF PHYSIOLOGY.
THE CHANGES WHICH ACCOMPANY SECRETION
IN THE SALIVARY GLANDS.
(1) It has already been pointed out that a temporary diminution in
the volume of the gland follows stimulation of the secretory nerve.
This is followed by an expansion due to the dilatation of the blood-
vessels. Further, secretion is accompanied by histological changes and
by an alteration in electrical potential.
(2) Histologically the salivary glands are of two types—serous glands,
which produce a fluid containing proteins and ptyalin, and mucous
glands, the secretion of which contains mucin. The salivary glands of
the dog, however, secrete no ptyalin. The parotid in man and most
animals belongs to the serous type ; the submaxillary and sublingual are
: generally mucous, but the former is
serous in the rabbit and mixed in
man. The general structure of a
salivary gland is of the compound
acinous type, each acinus being
lined by columnar or cubical cells,
and the whole being held together
by connective tissue.
If a portion of a mucous gland,
which is in the resting condition, be
teased in 2 per cent. salt solution,
Fic. 117.—Mucous cells from fresh sub- the individual cells are seen to be
ey Seen eee oe. hae ee somewhat columnar in shape and to
Histology. - be filled with large granules which
a, from a resting or loaded gland; 0, from a gwe]] up and disintegrate on the
gland which has been secreting for some
time ; a’, b’, similar cells which have been addition of acetic acid. If the
treated with dilute acid. ; :
gland be hardened in alcohol and
stained sections be examined, no granules are visible, but the body of.
the cell is clear, with a delicate network, and the nucleus is flat and
lies at the base of the cell. In the case of a gland which has been
made to secrete profusely, either by means of electrical stimulation
or by the administration of pilocarpine, the granules are fewer in
number and are found in the part of the cell which abuts on the
lumen of the acinus; and the cell is smaller than that in the resting
condition (fig. 117). The hardened specimen shows a larger propor-
tion of protoplasm, and the nucleus does not lie so close to the base
of the cell. In the process of secretion, therefore, there has been
a discharge of the granules contained in the cell. But from their
behaviour to reagents it is clear that the granules do not represent the
ee
Fak OT i a | ee he ke oe ee
THE DIGESTIVE SYSTEM. 299
final stages of the secretory process, for mucin itself is precipitated by
acetic acid. The material contained in the granules must be a pest si
of mucin, and it has therefore been called mucinogen. .
The cells of the serous type, treated in the same way, are more
cubical in shape, with a central nucleus, and in the resting state are
diffusely filled with much finer granules, which, as they are supposed to
consist of a precursor of ptyalin, have been called zymogenic. As in
the case of the mucous gland, the serous cell becomes smaller during
activity and the granules are diminished in number, especially towards
the base of the cell (fig. 118). In sections of the hardened gland which
have been stained, the cells even in the resting stage always appear to
contain a relatively greater amount of protoplasm than the mucous cells.
In both mucous and serous cells the formation of granules is pre-
Fie, 118. —Alveoli of aserous gland. (Langley.) From Schifer’s
Essentials of Histology.
A, atrest. B, after a short period of activity. C, after a prolonged period of activity.
In A and B the nuclei are obscured by the granules of zymogen.
ceded by the appearance of filaments, basophile in character, which have
received the general name of ergastoplasm. |
(3) In the resting condition of a salivary gland an electrical current
can be detected by the capillary galvanometer, the direction of which in
the gland is from the acini towards the duct. This current, in the case
of the submaxillary gland, undergoes a diphasic variation on stimulation
of the chorda tympani, becoming first increased in intensity and then
reversed in direction. Stimulation of the sympathetic nerve is followed
by a negative variation only.
DEGLUTITION.
The bolus of food formed in the mouth is conveyed through the
pharynx and esophagus into the stomach by the act of deglutition,
which originates as a voluntary process and is continued by nervous
reflexes. In the first or voluntary stage the jaws are closed and the
tongue is raised so as to press against the palate, the latter movement
being due to contraction of the mylohyoid aided by the intrinsic muscles
of the tongue itself. At the same time the base of the tongue is drawn
300 — ESSENTIALS OF PHYSIOLOGY.
slightly backwards by the contraction of the stylo-glossi and palato-
glossi. In this way the bolus is propelled through the opening bounded
by the anterior pillars of the fauces, beyond which the act becomes
involuntary. The later stages are subdivided anatomically into
pharyngeal and cesophageal, but physiologically ney are to be looked
upon as a continuous series of reflexes.
While the bolus is in the pharynx, the soft palate is raised so as to
form an inclined plane and prevent the passage of food into the nares.
At the same time the opening into the respiratory tract is guarded by
the elevation of the larynx under the posterior end of the retracted
tongue, by the constriction of the aperture of the larynx itself, and by
closure of the glottis. The arytenoid cartilages are rotated inwards by
contraction of the lateral crico-arytenoid muscles, and approximated by
contraction of the arytenoideus. They are at the same time drawn
forward by contraction of the thyro-arytenoids, so that the glottis takes
the form of a T-shaped slit. The opening of the larynx is diminished in
size by contraction of the ary-epiglottidean muscle fibres. At the same
time, by the elevation of the larynx and the drawing back of the tongue
the opening is further guarded by the lower part of the epiglottis.
The passage of the bolus through the’cesophagus is effected by
a wave of contraction preceded by relaxation in the muscular wall
of the tube. The cesophagus consists of four coats: (1) a mucous
coat, bounded externally by a layer of smooth muscle fibres arranged
longitudinally, the muscularis mucose, and lined by stratified squamous
epithelium ; (2) a submucous coat of loose connective tissue containing
mucous glands, which supply a lubricating fluid; (3) a muscular
coat, composed of striated muscle in the upper part of the tube
and of smooth muscle in the lower part, and consisting of an outer
longitudinal and inner circular layer ; and (4) a fibrous coat. Owing to
the more rapid contraction of the striated muscular fibres, the bolus travels
more quickly in the upper part of the cesophagus than in the lower part,
The time taken by the act of deglutition is normally five or six
seconds. This may be ascertained by listening over the pharynx or
the cesophagus by means of a stethoscope, the entrance of the food
into the pharynx at the beginning of the act, and again into the stomach
at its close, being characterised by distinct sounds. A more precise
method is direct observation with the aid of Réntgen rays, the food
swallowed in this case being mixed with bismuth so that it gives a well-
marked shadow.
During swallowing there is temporary inhibition of respiratory
movements, and in this way a further safeguard is introduced against
the entrance of food particles into the larynx.
i a
THE DIGESTIVE SYSTEM 301
The nerves chiefly concerned in the first stage of deglutition are
the fifth cranial nerve to the muscles which close the jaws and the
mylohyoid, and the twelfth nerve to the muscles of the tongue. The ©
afferent nerves connected with the reflex act are the second division
of the fifth, the glosso-pharyngeal, and the branches of the superior
laryngeal nerve to the pharynx. The efferent nerves are those which
form the pharyngeal and cesophageal plexuses, the ninth, tenth, and
eleventh cranial nerves. The reflex centre is in the medulla oblongata,
and it appears to consist of a series of centres ; for if the cesophagus be
cut across, stimulation of the afferent nerves will be followed by an
orderly wave of contraction, just as in the intact oesophagus.
The inhibition of respiration which accompanies swallowing depends
on stimulation of the glosso-pharyngeal nerve ; if the latter is divided
and the central portion is excited electrically, the respiratory move-
ments are arrested for a period corresponding with that of a normal
act of deglutition, that is, for five or six seconds.
SECTION III. |
THE STOMACH AND ITS FUNCTIONS.
The stomach forms a dilated portion of the digestive tube capable
of storing considerable quantities of nutritive material, and it thus
obviates the necessity of taking food at inconveniently frequent intervals.
The food remains in the stomach for some hours, and during this period
it is acted upon by the gastric juice, so that, when it afterwards comes
under the influence of the more potent digestive juices found in the
small intestine, the hydrolysis of the protein constituents is already
well advanced, some of the fats have been acted upon, and, as has
already been described, the saliva has effected a conversion of starch
into dextrin and maltose.
The Composition of Gastric Juice.—Gastric juice may be obtained
for analysis by producing a permanent gastric fistula in an animal.
Pawlow’s method is to make an incision in the stomach, separating it
into a larger and a smaller portion. The larger portion is stitched
up and remains in continuity with the digestive tract. The smaller
portion is kept separate from the larger by a layer of mucous membrane,
and is made to open on the surface of the body (fig. 119).! It is found
by experiment that the juice secreted by the small stomach has the
same composition as that produced by the large stomach, and also that —
it is secreted in the same proportional amount when the available extent
1 We are indebted to the kindness of C. Griffin & Co,; Ltd., for permission to
use these diagrams.
\
_ L,, first stage : A—B, incision. II., lesser stomach completed : S, lesser stomach; A, abdominal wall.
302 ESSENTIALS OF PHYSIOLOGY.
of mucous membrane is taken into consideration ; moreover, it has the
advantage of being free from admixture with food. | |
The juice obtained in this way is a clear fluid having a specific gravity
of 1003-5 and an acid reaction. It consists of about 99 per cent.
of water and 1 per cent. of solids, the latter including mucin, proteins,
enzymes, and inorganic salts. The juice also contains free hydrochloric
acid in the proportion of about 0:2 per cent. in man; the percentage is
, I. II.
Peritoneal coat.
Muscular coat. .
Mucous membrane.
Right vagus. ~
Fic. 119.—Pawlow’s method of forming a subsidiary stomach. (From{Pawlow’s
Work of the Digestive Glands.)
rather higher in the dog and other carnivorous animals. The salts are
chiefly chlorides and phosphates of potassium, sodium, calcium, and
magnesium, the base in largest proportion being potassium.
The existence of free hydrochloric acid may be proved by two tests: (1)
A solution of Congo red sided to gastric juice gives a blue colour, showing
the presence of free mineral acid. (2) If a ays of Gunzberg’s reagent
(phloroglucin-vanillin) be evaporated to dryness and a drop of gastric juice
be added to the residue and gently heated, as drying takes place a bright red
colour is developed, proving that the acid is hydrochloric.
THE FUNCTIONS OF THE. GASTRIC JUICE.
It has already been pointed out that the acid of the gastric juice
destroys the ptyalin of the saliva, but the hydrolysis of the carbo-
hydrates of the food may be continued, to some extent at least, by the
hydrochloric acid in the stomach. This chemical hydrolysis, however,
if it occurs, is of less importance than the action of the gastric enzymes
upon proteins, milk, and fats.
The digestive action of gastric juice can be studied, -like thatgof
saliva, by means of experiments in test tubes. Fresh gastric juice
obtained from a fistula may be used, but it is generally more con-
venient to make an artificial extract. For this purpose the mucous
membrane of a pig’s stomach is cut in small pieces and extracted
a
THE DIGESTIVE SYSTEM. 303
with glycerol. By adding some of the glycerol extract to 0'2 per
cent. hydrochloric acid an artificial gastric juice is obtained.
(1) Ifa few flakes of fibrin be placed in a test tube containing such |
an artificial juice and the tube be kept at a temperature of 37° C., it
will be observed that the fibrin gradually swells up and then dissolves.
If the solution is neutralised, a precipitate of acid metaprotein is formed.
If this is removed by filtration and the filtrate. is boiled, a coagulum of
soluble globulin may be formed. When this is removed by filtration,
the solution gives a pink colour on the addition of dilute copper sulphate
and caustic soda, owing to the presence of proteoses and peptones.
Further analysis of this filtrate shows that several varieties of
proteose and at least two kinds of peptone are present. The proteoses
are classified as primary and secondary, the primary group being pre-
cipitated by the addition of an equal volume. of saturated solution of
ammonium sulphate; when this precipitate has been removed by filtra-
tion, the secondary varieties are precipitated by full saturation with the
same salt. When the latter precipitate is removed, the solution contains
peptones only, although, if the digestion be prolonged for several days,
some amino-acids may be present. |
The changes produced in the fibrin are due to the activity of an
enzyme, pepsin, which in the presence of dilute hydrochloric acid brings
about hydrolysis of the protein molecule and breaks it up into smaller
and more soluble molecules. The first’ change produced is the solution
of the fibrin with the production of a substance known as soluble
globulin. This is then changed into acid metaprotein, and, by successive
hydrolytic stages, the various proteoses, and finally peptones, are formed.
Digestion in the stomach never passes beyond the peptone stage, and in
fact the conversion into peptone is not complete when the contents of
_ the stomach are passed on into the small intestine.
The stages of peptic digestion of protein may be represented in
tabular form thus—
Protein
Soluble globulin
Acid metaprotein
Primary proteoses
Secondary proteoses
Peptones
Some proteins which occur in food do not undergo these changes,
Thus elastin is unaffected by peptic digestion in the time available in
\
304 ESSENTIALS OF PHYSIOLOGY.
the stomach. The collagen of connective tissue is probably converted
first into gelatine and then into gelatoses and gelatine peptones. The
protein constituent of the conjugated proteins is usually converted into
proteose and peptone, the prosthetic group being set free. Thus in
the digestion of nucleo-protein by gastric juice an insoluble residue of
nuclein is formed ; in the digestion of mucin (gluco-protein) glucosamine
is found in the products. ’
(2) The effect of gastric juice upon caseinogen, the shieplie -protein
of milk, is peculiar in that there is a conversion of the caseinogen into
a comparatively insoluble substance, casein. This action of gastric
juice has been for many years ascribed to a separate ferment called
rennin, but latterly evidence has been brought forward which suggests
that the formation of casein from caseinogen is due to pepsin itself.
The matter has not been conclusively settled, and it will be convenient
to retain the term rennin in the meantime when describing the effect of
gastric juice on caseinogen.
The action of rennin can be demonstrated by adding a little of an
extract containing this enzyme to a quantity of milk and allowing the
mixture to stand for a time at a temperature of 37°C. In a few minutes
the milk becomes clotted, and after a time the clot shrinks, squeezing out
a clear fluid, whey, which contains all the constituents of milk except
caseinogen and fat. It can be shown that the fat is entangled in the
clot in an unaltered form, so that the coagulation is brought about by
the action of the rennin on the caseinogen. If a little potassium oxalate
is added to milk, the subsequent addition of rennin does not result in
the formation of a clot, but if calcium chloride be added clotting occurs.
Three factors are necessary for the formation of the clot, namely,
caseinogen, rennin, and lime salts. If rennin is added to a solution
of pure caseinogen, and the mixture is kept for a short time at a
temperature of 37° C. and then boiled to kill the enzyme, the addition
of calcium chloride will bring about the formation of casein. Obviously
the enzyme has produced some change in the caseinogen, and the only
factor required to complete the conversion into casein is the addition of
lime salts. There is in the first place, therefore, a conversion of casein-
ogen into “soluble casein” by the action of the enzyme, and secondly,
soluble casein combines with lime salts with the production of insoluble
casein. The process may be represented thus :—
Caseinogen<-Rennin (or pepsin)
Soluble casein Calcium salts
V
Casein
THE DIGESTIVE SYSTEM. | 305
(3) Gastric juice also acts upon neutral fats, but only on those
which are in the form of a fine emulsion, such as yolk of egg or milk.
The fats are split into glycerol and fatty acids by the agency of an
enzyme, lipase. .The fat-splitting function of gastric juice, however, is
limited in extent, and is of relatively small importance as compared
with that which takes place in the small intestine. Pepsin indirectly
assists the digestion of fat by dissolving the cell envelopes of the fat
cells of adipose tissue contained in food. In this way fat is set free and
prepared for the subsequent digestive action of pancreatic lipase.
THE SECRETION OF GASTRIC JUICE.
The mechanism of the secretion of the gastric juice is studied by
the subdivision of the stomach in an animal in the manner already
described (p. 301), the larger part remaining in continuity with the
digestive tract, while the smaller subdivision opens freely on the surface
of the body. In such an animal, it has been shown that secretion of
juice in the large stomach is accompanied by a proportional secretion
of juice in the small stomach, and, further, that the two juices are equal
in digestive power.
The secretion of gastric juice begins aaeart five minutes after an
ordinary meal is taken, and continues steadily during the period of
digestion of the stomach contents. The initial secretion has been
shown to be produced by a nervous mechanism, this being supplemented,
after a variable time, usually twenty to forty-five minutes, by a further
flow excited by a chemical stimulus.
The Nervous Mechanism of Gastric Secretion.—The normal stimulus
which excites the flow of gastric juice is the presence of food in the
mouth. If the cesophagus of a dog is divided and the two ends are
brought to the surface and fixed there, food may be masticated and
swallowed by the animal, but none will reach the stomach. In such a
case the food which is eaten all escapes by the esophageal fistula, and
is spoken of as a sham meal. In an animal provided with both
cesophageal and gastric fistulee, sham feeding is followed by the secretion
of gastric juice, at the same time and in the same way as if the food
reached the stomach. This fact points to the probability that the
first secretion is due to a nervous reflex, a presumption which is further
supported by the observation that, as in the case of the saliva, even the
sight or suggestion of food is followed by the appearance of gastric juice.
The nervous nature of the stimulus is proved by the effect of division of
both vagus nerves to the stomach, after which no-secretion takes place on
sham feeding or on showing food to the animal. Further proof is
3 20
e+
306 ESSENTIALS OF PHYSIOLOGY.
obtained by the production of secretion as a result of stimulation of
the vagus. The nerve is divided in the neck, and, four days later,
when the cardio-inhibitory fibres have degenerated, the distal end of
the nerve is stimulated by a tetanising electric current. Five minutes
after the commencement of stimulation, the gastric juice begins to
flow.
The efferent nerves concerned in the secretion are therefore the two
vagi; the afferent nerves are normally the branches of the fifth and
glosso-pharyngeal to the mucous membrane of the mouth, but stimula-
tion of other sensory nerves, such as those of sight and smell, may also
excite the secretion. The production of gastric juice through the
latter mechanism is spoken of as “psychic” secretion; and the juice
formed is described as ‘‘appetite juice,” since the sight of food in a
hungry animal gives rise to its secretion.
The Chemical Factor in the Secretion of Gastric Juice.—In a dog
provided with a gastric fistula of the kind described above, a fistulous
opening is made into the main stomach, and the two vagi of the animal
are divided so as to preclude the possibility of reflex secretion through
these nerves. It is then found that the introduction of meat into the
main stomach is followed in from twenty to forty-five minutes by a‘ flow
of gastric juice. The same effect is produced by Liebig’s extract of
meat, certain preparations of peptone, or semi-digested bread, but not
by pure proteose or peptone, or by bread, starch, or white of egg.
Mechanical stimulation, or the introduction of a mechanical irritant,
such as sand, is also without effect. The substances which stimulate
secretion are spoken of as secretogogues, and Edkins has shown that
they produce their effect by exciting the cells of the gastric mucous
membrane to produce a hormone. If the pyloric mucous membrane
of the stomach is boiled with water or dilute acid, a decoction is obtained,
_ which, when injected into the blood stream, excites the secretion of
gastric juice. Extracts of the mucous membrane of the body of the
stomach have no such effect. As the result of his experiments, Edkins
concludes that the partially digested food products excite the formation
of a hormone in the cells of the mucous membrane of the pyloric
portion of the stomach ; it is called gastric secretin or gastrin, and
is absorbed into the blood stream, and is carried in the course of the
circulation to the glands of the stomach, stimulating them to produce
their secretion.
The group of hormones, to which gastrin belongs, possess certain
definite properties. They are substances of relatively low molecular
weight, and are easily diffusible. Hach exercises a specific function
in exciting the activity of a particular organ or tissue, and, when its
i ee
THE DIGESTIVE SYSTEM. 307
function is performed, it is rapidly destroyed in the body. It does not
act as an antigen, that is, it does not excite the production of an anti- —
body which would interfere with the performance of its function. The
hormones of the digestive tract are not destroyed by boiling, but are
soon oxidised in the body or in the presence of alkalies.
The Gastric Juice Produced by a Normal Meal.—The nervous
secretion of gastric juice begins about five minutes after food is taken.
By means of this juice digestion in the stomach is initiated and carried
on up to a certain point. The semi-digested products of its activity
excite the formation of gastrin, by the agency of which the production
of the juice is continued as long as the stomach contents require it.
The following table from Pawlow illustrates the relative quantity and
digestive power of the juice secreted (1) after a normal meal, (2) as a
result of the chemical stimulus alone, and (3) after a sham meal, ¢.e. by
the nervous secretion alone. The digestive power is measured by filling
short lengths of capillary tube with egg-white, coagulating the latter
by heat, and placing the tubes, the ends of which are open, in the juice
to be tested for a given time. The digestive power is estimated by
the length of the column of coagulated protein which has undergone
solution.
Normal meal, S
AEA oon . um of |
200 gm. meat ee ENO Sham meal. two last
into stomach. P expts.
Hours.
Quantity | Strength | Quantity | Strength | Quantity | Strength | Quantity
C.C. mm. dig. ¢.c. mm. dig, C.¢. mm. dig. c.c,
1 12°4 5°43 5°0 2°5 7°7 6°4 12°7
2 13°5 3°63 7°8 2°75 4°5 5°3 12°3
3 7°5 3°5 6°4 3°75 06 5°75 70
4 4:2 3°12 5°0 3°75 0°0 0°0 pO.
|
It will be observed that the amount of juice secreted as the result
of a normal meal, shown in the first column, corresponds with the
totals given in the last column for the nervous a.and chemical secretions
obtained separately.
It has already been pointed out that meat excites the production
of gastric secretin and therefore of gastric juice, and that bread, when
it has undergone partial digestion, is also an efficient secretogogue.
Fat, on the other hand, inhibits gastric secretion, and therefore the
flow of juice which results when milk is taken into the stomach is
smaller than that which results from a meal of meat or bread. The
308 : ESSENTIALS OF PHYSIOLOGY.
flow of juice is also inhibited. vel alkalies, ai is excited by acids in
the stomach.
The Origin of the Chief Constituents of the Gastric Juice.—Two
chief types of gland are found in the mucous membrane of the stomach.
The tubular glands of the body of the stomach are relatively straight,
open into short ducts, and possess two kinds of secreting cells. The
gland is lined throughout by cubical, granular cells which are called
chief or peptic cells. Between the chief cells and the basement
membrane there occur at intervals somewhat larger ovoid cells, which
are described as oxyntic because they are believed to secrete the acid
of the gastric juice. The second type of gland oceurs in the pyloric
portion of the stomach. The pyloric glands are also simple tubes,
which are twisted on themselves and open into relatively long and wide
ducts. Moreover, each is lined by one type of cell only, resembling in
structure the chief cells of the glands of the body of the stomach.
The cells lining the general surface of the stomach and the ducts
secrete the mucin of the gastric juice. The enzymes of the juice are
contained in the secretion of the body of the stomach and also in that
of the pyloric portion, and are derived from the chief cells of the glands
of the body and from the cells lining the pyloric glands. Pepsin, how-
_ ever, does not exist in the secretory cells as such, because extracts of
the mucous membrane do not possess marked peptic activity until they
have been treated with acid. It is therefore a precursor of pepsin,
known as pepsinogen, which is found in the secretory cells, and this is
converted into pepsin, after its discharge from the cells, by the bydro-
chloric acid of the gastric juice.
The facts from which it is concluded that the acid itself is derived
from the ovoid cells are (1) that it is most abundant in the middle of
the stomach, where these cells are most numerous, and (2) that it is
absent from the secretion of the pyloric portion of the stomach, where
ovoid cells are also wanting.
Various explanations have been offered as to the method of pro-.
duction of the free acid in the gastric juice. The most probable of these
suggestions is that the acid is derived from the interaction of chlorides
with di-sodium hydrogen phosphate, according to the formula
2Na,HPO, + 3CaCl, = Ca,(PO,), + 4NaCl + 2HCL.
During the early stages of secretion the cells of the gastric glands
become enlarged, the chief cells, and those lining the pyloric glands, are
crowded with secretory granules, and the ovoid cells are distended and ~
clear. As secretion proceeds, all the cells become diminished in size, as
in the case of the salivary glands. It has already been pointed out
a
et
i a eee ore ng eT
Si a en et i
THE DIGESTIVE SYSTEM. 309
that pepsin exists in the gland cells in the form of a precursor, and it
is probable that lipase, as well as rennin, if the latter exists as an in-
dependent ferment, are also represented in the chief and pyloric gland ©
cells as granules of a zymogenic nature.
THE MOVEMENTS OF THE STOMACH.
The movements of the stomach are most conveniently studied by:
direct observation with the aid of Rontgen rays after the administra-
tion of a meal mixed with a quantity of oxychloride of bismuth, In
Fic. 120.—Shape of human stomach, in vertical position, shortly after a bismuth
meal. (Hertz.)
U, umbilicus ; 0, cesophagus ; F, fundus; P.C., pyloric canal; I.A., incisura angularis.
these circumstances the organ is seen to consist of two parts, the axis
of the larger portion ‘being nearly-vertical, and forming an angle with
the smaller pyloric portion, which is again subdivided by a constriction
into two parts, the pyloric vestibule and the pyloric canal (fig. 120).
The pyloric canal is about 3 centimetres in length, while the pyloric
vestibule or antrum is less constant in size.
After the ingestion of a meal, the muscular walls of the body and
310 ESSENTIALS OF PHYSIOLOGY.
fundus (namely, the dome-shaped part of the organ above the entrance
of the csophagus) become tonically contracted. Waves of contrac-
tion occur in the pyloric portion, beginning about the middle of the
stomach and travelling at the rate of about three a minute towards the
pylorus. The stomach contents are propelled by these waves towards
the pylorus, and if the opening is kept closed by the contraction of the
Siw, Vag
MN AHRIA UMM TT TM IMS TET CRIT
Fig, 121,—Tracing showing initial relaxation followed by contraction of the muscular
wall of the stomach on stimulation of vagus nerves, (Elliott.)
sphincter, they return by an axial stream towards the body of the
stomach. In this way complete mixture with the gastric juice is
effected. As digestion proceeds and the food is brought into a more
fluid state, the sphincter undergoes periodic relaxation, opening every
few minutes to allow the passage of the semi-liquid material into the
duodenum. By the tonic contraction of the fundus and body, the food
is little by little subjected to the action of the pyloric mill, and subse-
quently passed on to the duodenum; and as the contents of the
stomach are thus gradually diminished in quantity, the organ becomes
THE DIGESTIVE SYSTEM. 311
more tubular in shape, until finally, four or five hours after the com-
mencement of digestion, the process is complete.
The precise mechanism by which the movements of the stomach
are originated and carried out has not been definitely ascertained.
Branches of the vagus nerves and of the sympathetic system are supplied
to the viscus, and form connections with the plexus which lies between
the layers of the muscular coat. Fine filaments from the latter are
distributed to the muscle fibres. Division of the two vagi is followed
Volume of Stomach
|
66 mm /
Caro tia b&b. r
Stim
Splanchnics
i m mvt i! imi! PALES MMe THT MUI TIDE UTI I TMG Pity stint sf i ipUytfineeNANNOATIMADAED) LH OATONT RIN EE ye tpn ne
Fic, 122.—Tracing showing relaxation of the muscular wall of the stomach ot a cat on
5 oO . . -
stimulation of splanchnic nerves. (Elliott. )
by defective movements, so that the stomach is incompletely emptied
after each meal, Stimulation of the vagi leads to temporary diminution
of muscular tone, followed by increased contraction (fig. 121), whereas
the sympathetic system has on the whole an inhibitory effect on the
muscular wall (fig. 122).
The influence of the central nervous system is, however, not
essential for the stomach movements, for normal contractions may be
observed in the isolated organ placed in warm saline solution. It is still
an open question whether these contractions are myogenic in origin, or
are due to a local reflex through the plexus in the muscular coat. In any
case, the movements may be considered to be under a certain degree of
312 ESSENTIALS OF PHYSIOLOGY.
control by the vagi and the sympathetic, the former having on the
whole a motor and the latter an inhibitory effect.
The periodic opening of the pylorus has been experimentally shown
to be under the control of a local reflex mechanism, depending on the
reaction of the contents in the pyloric portion of the stomach and the
duodenum respectively. When the reaction in the duodenum is acid,
the pyloric sphincter remains tightly contracted ; if, on the other
hand, the reaction on the stomach side of the pylorus is acid and on
the duodenal side it is neutral or alkaline, the sphincter relaxes. Thus
when acid material has passed from stomach to intestine, the pylorus
remains closed until the duodenal contents have been neutralised by
the alkalies present in the bile and pancreatic juice. When neutralisa-
tion has taken place, a further quantity of the gastric contents is allowed
to pass through the pyloric opening. The precise nature of the reflex
by which this is effected is still uncertain. If a quantity of water is
drunk, it does not excite the secretion of gastric juice, the pyloric
_ sphincter is not stimulated to contract, and, if the stomach is empty,
} the fluid begins to enter the duodenum within one or two minutes
of its being taken into the mouth.
SECTION IV.
DIGESTION IN THE SMALL INTESTINE.
. If an experimental meal is given to an animal in which a fistula has
been made just beyond the pylorus, it is found that food begins to pass
from the stomach into the intestine eight to twelve minutes after the
meal is taken. The rate of escape of the food from the stomach is
indicated in the following table:—
lst hour ; } . 932°6 per cent.
2nd _,, % Pe ane ape ee Wig Bec gg o
3rd_s,, een ie ADCO, yy Mas
AGH’ veg, eee ; 87: sagem
5th ,, on ptateee penea cee Rare ato paed
6th ,, ; ris ae
_ If the material collected in this way is analysed, it is found that
67 per cent. of the nitrogen is in the form of proteose and peptone, and
that of the starch of the meal 21 per cent. has been converted into
dextrin and 4 per cent. into sugar. The whole of the protein and carbo-
hydrate of the meal is accounted for, no absorption of these substances
or of fat having taken place in the stomach. The mixture of semi-
THE DIGESTIVE SYSTEM. 313
digested substances which enters the intestine has a yellowish colour and
a semi-fluid consistence, and is immediately subjected to the action of
the pancreatic juice and bile. The secretion of Brunner’s glands and
the intestinal juice are also mixed with the duodenal contents, but the
digestive action of the former is not known to have any importance, and
that of the latter has its chief value in the later stages of the digestive
process. The action of the pancreatic juice and bile must therefore be
considered in the first place.
THE COMPOSITION OF PANCREATIC JUICE.
Pure pancreatic juice may be obtained from an animal either by
means of a temporary fistula, made by introducing a cannula into the
pancreatic duct, or by a permanent fistula. In the dog there are
two ducts, the larger of which opens into the duodenum about an
inch below the entry of the bile duct. Pawlow’s method of making
a permanent fistula is to cut out a patch of the duodenal wall
with the opening of the duct in its centre, stitch up the gap in the
duodenum, and suture the patch with the opening of the duct into
the abdominal wall. -
The pancreatic juice obtained in this way is a clear, limpid fluid,
having a specific gravity of about 1007 and a strongly alkaline |
reaction. The degree of alkalinity is such that equal volumes of
gastric juice and pancreatic juice neutralise each other. The con-
centration of pancreatic juice varies considerably, but it contains
on an average about 4 per cent. of solids. These consist of nucleo-
protein, enzymes or their precursors, and inorganic salts. The
chief salt is sodium carbonate.
THE FUNCTIONS OF PANCREATIC JUICE.
The action of the pancreatic juice on the constituents of the food
may be studied in test tubes, using either the secretion obtained from
a fistula, or an artificial juice made by adding a glycerol extract of the
fresh gland to a:solution of sodium carbonate of such a strength that
the mixture contains 0°5 per cent. of the carbonate.
The Action of Pancreatic Juice on Proteins.—Pure pancreatic juice,
obtained directly from .the pancreatic duct, without contact with the
intestinal mucous membrane, has no action on proteins. If, however,
the juice has flowed over the duodenal mucous membrane or has been
mixed with intestinal juice, it is strongly proteolytic. The pure juice
contains a substance, trypsinogen, which is the precursor of a proteolytic
314 ESSENTIALS OF PHYSIOLOGY.
enzyme, trypsin, and is converted into the latter by another ferment,
enterokinase, produced in the mucous membrane of the small intestine.
Trypsinogen may also be converted into trypsin by other means, for
example by the action of lime salts; or the activation will slowly take
place spontaneously if the juice is allowed to stand. Activation is,
however, effected most rapidly by enterokinase, requiring a few minutes
in the case of this ferment as against over twelve hours by means of
calcium.
If a few flakes of fibrin are placed in a solution containing trypsin
and 0°5 per cent. sodium carbonate in a test tube, and kept at a
temperature of 37° C., the fibrin will begin to be eroded in a few
minutes, and gradually it will become dissolved. The products in
solution will vary according to the time during which digestion has
been allowed to proceed, but generally speaking the course of hydrolysis
is the same as in peptic digestion, except that, as the process takes
place in an alkaline medium, the metaprotein formed is the alkaline and
not the acid variety. A second point of difference is that the inter-
mediate stages are passed through more rapidly in pancreatic than in
gastric digestion, and thirdly, some amino-acid is produced even in the
time available for digestion in the intestine.
In the normal course of digestion in the intestine, the final con-
version of peptone into amino-acids is largely effected by the ferment
erepsin, Which is contained in the intestinal juice ; but almost complete
hydrolysis into amino-acids can be obtained 2n vitro by means of trypsin,
if the digestion is allowed to proceed for three or four days. The
splitting is not quite complete, for even if the digestion has been
allowed to go on for some weeks some amino-acids remain united in
groups of two or more, known as polypeptides. These latter substances
have much smaller molecules than peptones and do not give the biuret
reaction, that is, they do not give a pink colour with copper sulphate
and caustic soda.
If a tryptic digestion of fibrin or casein has been allowed to proceed
for some days, the solution contains the amino-acids derived from these
substances (p. 13). Leucine and tyrosine are most easily demonstrated
in the fluid, and crystallise out readily if the fluid is concentrated.
Tyrosine appears as sheaves of colourless needles, and leucine, which is
the more soluble of the two, occurs in the form of yellowish balls, which
sometimes show concentric and radial striation. Solutions of tyrosine,
when boiled with Millon’s reagent, give a red colour.
The stages of tryptic digestion may be represented in tabular
form thus :—
—————————
A i 9 el
THE DIGESTIVE SYSTEM. 315
Protein
Soluble ‘elobulin
Alkali tars
Primary proteoses
Secondary proteoses
Peptones
Polypeptides and Amino-acids.
The earlier stages of tryptic digestion of protein are most efficiently
carried out in a slightly alkaline medium, but the ferment is active in
either an alkaline or neutral solution. Under natural conditions, the
alkalinity of the pancreatic juice is largely neutralised by the acid con-
tents from the stomach, and the contents of the small intestine through-
out its length are almost neutral. The activity of trypsin diminishes
during the course of normal digestion, since the ferment enters into com-
bination with the products of its own activity, that is, with amino-
acids and peptones, and in this way it becomes inactive. The intestinal
contents taken from the lower end of the ileum show little trace of
tryptic activity. ,
The Action of Pancreatic Juice on Starch.—The action of pan-
¢creatic juice upon starch depends on the presence of an enzyme, amylase.
By means of this ferment starch is converted into maltose, as in the
case of salivary digestion (p. 293), but the action of pancreatic amylase
is more rapid and powerful than that of the ptyalin of saliva. If some
pancreatic extract is added to dilute starch paste kept at a temperature
of 37° C., the soluble starch stage is reached in a few seconds and
erythro-dextrin may be detected in half a minute. Moreover, the
pancreatic juice is capable of digesting unboiled starch, on which saliva
has no action,
As maltose is formed it is further hydrolysed, partly by a ferment
maltase, contained in the pancreatic juicé, and partly by a similar
ferment found im the intestinal juice. Under the influence of maltase
each molecule of maltose takes up a molecule of water and is split into
two molecules of dextrose.
CoH 0), + H,O ag 20H, .0¢.
Pancreatic juice differs, however, from intestinal juice in having no
similar action on the other disaccharides, lactose and cane sugar.
The Action of Pancreatic Juice on Fats.—If perfectly neutral fat,
316 ESSENTIALS OF PHYSIOLOGY.
such as pure olive oil, be shaken up with pancreatic juice, and the
mixture be kept at a temperature of 37° C., the fatty ester will be
hydrolysed, yielding fatty acid and glycerol, and the reaction of the
fluid will become acid. The agent which brings about this change is
an enzyme, /ipase, which is a constituent of the pancreatic juice. Lipase
may be extracted from the fresh pancreas by glycerol, but not by water.
The fat-splitting action of lipase is greatly facilitated by the presence
of bile, taking place four or five times as rapidly when assisted by bile
as in its absence. This acceleration is due to the bile salts, which not
only reduce surface tension and so promote the admixture of the enzyme
and the fats, but also have the property of bringing fatty acids and
soaps into solution.
The digestion of the fats is further assisted mechanically by the
formation of soaps. Some of the fatty acid, which is set free combines
with the alkalies of the intestinal contents to form soap. Segmental
contractions of the intestine lead to the mechanical subdivision of the
fats with the formation of an emulsion. Each fat droplet becomes
coated with a fine film,of soap, which prevents it from coalescing with
others, and in this way the formation of a still finer emulsion is favoured,
and the fat is made more accessible to the enzyme.
The Action of Pancreatic Juice on Milk.—A milk-curdling ferment
has been described as occurring in the pancreatic juice, and it is a fact
that clotting of milk takes place when the. juice is mixed with milk at
the body temperature. It is doubtful, however, whether a separate
rennet ferment is present in the pancreatic secretion, and the milk-
curdling function has been ascribed by some authorities to trypsin. In
any case the clot stage is very brief, for the curd is rapidly dissolved by
the proteolytic action of trypsin; moreover, the presence of rennin
‘in the pancreatic juice would. seem to be unnecessary in view of the
active milk-curdling property of gastric juice.
THE SECRETION OF PANCREATIC JUICE.
In an animal with a permanent pancreatic fistula, a flow of juice is
seen to begin within five to twenty minutes after the ingestion of a
meal. The secretion is largely increased two or three hours later, when
the stomach contents are passing into the duodenum in largest amount,
and it comes to an end in about five hours. The following record of
two of Pawlow’s experiments shows the rate of flow :—
{Bo 6 Re.
i i th ll
ae ee ee
Te
THE DIGESTIVE SYSTEM, 317
PANCREATIC SECRETION AFTER A MEAL OF 600 c.c. oF MILK.
Hour after Feeding. 7 Quantity of Juice in c.c.
Ist 8°75 8°25
2nd . ; : 75 6°0
3rd ; ; Sera Rs See
4th ; ; : 9-0 6°25
5th 2:0 1°5
Total : 4 49°75 “45:3
Bayliss and Starling have shown that the secretion of pancreatic
juice is almost entirely due to a hormone, which they have called
secretin, and which is formed in the mucous membrane of the small
intestine. The production of the hormone is brought about by the
presence of acid in the intestine, and the normal stimulus for its
formation is the passage of the acid gastric contents into the duodenum.
Much work has been done with a view to determining whether a
nervous factor is concerned in the production of pancreatic juice,
comparable with the reflex which brings about the first secretion of
gastric juice ; and Pawlow has shown that stimulation of the vagus will
excite a small flow of pancreatic juice, even when the pylorus of the
stomach is ligatured so as to prevent the passage of the acid contents of the
stomach into the duodenum. The amount secreted as the result of vagus
stimulation is, however, so small that the nervous factor is obviously
of subsidiary importance in the case of the pancreas. It is possible
that the first few c.c. of juice secreted may be nervous in origin, because
the juice first formed appears a few minutes after a meal is taken, and
further, it differs in character from the later formed juice, being more
viscid, richer in ferments and in protein constituents and poorer in
alkali than the latter.
The later flow is, however, the essential secretion, and that it is due
to the formation of a hormone is proved by experiment. If 0°4 per cent.
hydrochloric acid is introduced into the duodenum, a flow of pancreatic
juice is evoked within two or three minutes. The secretion induced in
this way occurs even if the nerves to the intestine have been divided, so
that it must be due to the production of some chemical substance which
is conveyed to the pancreas. This substance is secretin, and it may be
extracted from the duodenal mucous membrane by grinding it with sand
and boiling with 0°4 per cent. hydrochloric acid. If alkali be added to
318 ESSENTIALS OF PHYSIOLOGY.
the boiled fluid till it is almost neutral, the proteins are precipitated,
and a protein-free filtrate may be obtained. Injection of the filtrate
into a blood-vessel of another animal excites the production of pancreatic
juice. From this experiment it is obvious that the hormone is not
destroyed by acid or by boiling; it is, however, readily destroyed
by alkalies. Secretin is-absorbed by the blood. directly, and does not
normally reach the lumen of the intestine; and its introduction into
the duodenum does not lead to a flow of pancreatic juice.
The epithelial cells of the intestinal wall form in the first instance
a precursor of secretin, called prosecretin. Secretin itself is freely
soluble in water and alcohol, but an extract of intestinal mucous
membrane made with either of these fluids contains neither secretin nor
prosecretin. The latter substance, therefore, is insoluble in water and
alcohol, and is converted into secretin on boiling with dilute hydrochloric
acid.
Prosecretin is most abundant in the duodenum, it occurs also to a
considerable extent in the jejunum, and to a less degree in the ileum ;
but near the junction of the small and large intestines it is a Ss i in
very small amount.
The amount of secretin formed in the body, as shown by the volume
of pancreatic juice secreted, varies with the nature of the food. After
a meal of bread or meat the flow of juice is more abundant, and
reaches its maximum more rapidly, than after a meal of milk. The
reason of this difference is that meat or semi-digested bread stimulates
the production of gastric secretin and thus causes a large flow of gastric
juice, whereas milk is a less efficient stimulus to gastric secretion. As
a result, more acid reaches the duodenum after a meal of bread or meat
than after the ingestion of milk, and therefore more secretin is produced
in the former case than in the latter. On the other hand, the fatty
acids formed by the action of gastric lipase on milk are converted into
soaps in the duodenum. Soaps stimulate the production of secretin,
and the delayed maximal production of pancreatic juice after a meal of
milk may be explained by a second formation of secretin in this way,
The Changes in the Pancreas which accompany Secretion.—The
pancreas is a compound tubular gland, and it contains, in addition to
the ordinary secretory tubules, clumps of cells which do not stain
deeply with the ordinary dyes, and which are known as cell-islets
(fig. 130). These islets are supposed to be concerned with the formation
of an internal secretion, and their function will be discussed in con-
nection with metabolism. The secretory tubules of the pancreas are
lined by a single layer of columnar cells, each of which shows two
zones—an outer, which stains with basic dyes such as hematoxylin or
a,
THE DIGESTIVE SYSTEM. 319
toluidin blue, and an inner, filled, in the resting stage of the gland, with
secretory granules. The granules stain well with osmic acid, eosin, or
neutral gentian. ;
After a prolonged period of secretory activity the granules are
greatly diminished in number, and the inner zone of the cell is relatively
and absolutely smaller as compared with the outer zone. Under normal
conditions this diminution of the inner zone does not occur, because the
formation of new granules keeps pace with the extrusion of those
previously formed, so that the appearance of the cells is little altered
by the secretion required for an ordinary meal. The secretory granules
are in all probability zymogenic, and represent the precursors of the
enzymes found in the juice. In the case of trypsinogen, the precursor
has received the name of protrypsinogen.
THE BILE.
The bile is not a digestive juice in the same sense in which saliva and
the gastric and pancreatic juices belong to that category. It is to be
looked upon as an excretion, which incidentally assists the digestive
action of the pancreatic juice. The production of bile is constant,
although the rate of formation varies with the period of the day and
other circumstances, and, as it is formed, the secretion is stored in the
gall-bladder. This reservoir discharges the accumulated bile into the
intestine simultaneously with the great flow of pancreatic juice, which
takes place during the third hour of digestion of a meal. The
mechanism by which this emptying of the gall-bladder is effected has
- not yet been definitely ascertained.
THE COMPOSITION OF THE BILE.
Bile may be obtained for analysis from the gall-bladder of a recently
killed animal, or it may be collected from a gall-bladder fistula during
life. Its composition, however, is not the same in the two cases, fistula
bile being more dilute than bile which has been stored for a time in
the gall-bladder. The difference is shown in the following two saree
of human bile by different observers :—
Fistula Bile, Gall-bladder Bile.
Mucin and pigments . O-148 | Mucin .. : » eee
Sodium taurocholate . 0°055 | Sodium taurocholate . . O87
Sodium 7 Kauai . 0°165 | Sodium glycocholate . . 3°03
Cholesterol . ; ies Cholesterol ; ; ~ 0°35
Lecithin ; , . 0:038 | Lecithin . ; . 0°53
Fats , , .. | Fats . : «O73
Inorganic salts - 0°840 | Soaps hy See ; Cae
Water . : ; J" $G2F
320 ESSENTIALS OF PHYSIOLOGY.
Bile thus becomes more concentrated by the absorption of water
during the time it remains in the gall-bladder. It is a viscid fluid,
golden brown in carnivora, green in herbivora, and possesses a bitter
taste. The viscidity is due to the presence of mucin in human bile,
and of a nucleo-protein in that of the ox and some otheranimals. The
colour depends upon the presence of the bile pigments, bilirubin and
biliverdin. The former is more abundant in the bile of carnivora, the
latter in the bile of herbivora. The proportion appears to vary in
human bile according to the nature of the diet, the brown tint of
bilirubin predominating when a flesh diet is taken, and the green of
biliverdin when the diet is largely vegetarian. The bitter taste of bile
is due to the bile salts, glycocholate and taurocholate of sodium. In
dog’s bile the latter only is present.
The Properties, Source, and Fate of the Chief Constituents of the
Bile.—The mucin of human bile gives a stringy precipitate on the addi-
tion of acetic acid, the precipitate being insoluble in excess of the acid.
The nucleo-protein of ox bile gives a similar precipitate with acetic acid,
but as in the case of nucleo-proteins generally, the precipitate is dissolved
by excess of the acid. The mucin or nucleo-protein of the bile is derived
from the mucus-secreting cells which line the bile ducts and gall-
bladder.
The dle salts are compounds of sodium with glycocholic and
taurocholic acid respectively. These acids may be split up by hydrolysis,
glycocholic into glycine (amino-acetic acid) and cholic acid, and
taurocholic into taurine (amino-ethylsulphonic acid) and cholic acid.
C,,H,3NO, + H,O =CH, . NH, . COOH + C,,H,,0,
Glycocholic acid. Glycine. . Cholic acid.
Taurocholic acid. Taurine. Cholic acid.
If a little syrup of cane sugar be added to a solution of bile salts
in a test tube, and strong sulphuric acid be poured down the side of
the tube so as to lie below the solution, a cherry-red colour appears at —
the junction of the two fluids. The colour is due to a reaction between
cholic acid and furfuraldehyde, the latter being formed by the action
of the sulphuric acid on the cane sugar. The bile salts have the
property of reducing the surface tension of the fluid in which they are
dissolved. This can be shown by comparing the effect of scattering
flowers of sulphur on water and ona solution of bile salts. The sulphur
floats on the water, but sinks immediately in the bile salt solution
(Hay’s test). Further, watery solutions of bile salts readily dissolve
fatty acids. Bile acids are derived from protein sources and are formed
BS lee ee a,
'
' THE DIGESTIVE SYSTEM. 321
in the liver. The larger proportion of the bile salts which pass into
the intestine is reabsorbed and returns to the liver to enter again into
the composition of bile. This is spoken of as the circulation of the ~
bile salts, and not only do these substances enter once more into the
formation of bile, but they also stimulate the liver to further secretion,
that is, they act as cholagogues. A small proportion of the bile salts
is excreted in the faeces in the form of dyslysin, a substance formed
from them by bacterial decomposition, Part of the sulphur excretion
from the body takes place in this way, since this element forms part of
the taurine molecule.
Cholesterol (p. 8) and lecithin (p. 8) are probably largely derived
from the stroma of the red blood corpuscles which are broken down
‘in the liver. Gall-stones, which are of fairly common occurrence in
the gall-bladder, are usually composed chiefly of cholesterol. Normally
the latter substance is excreted in the feeces.
The bzle pugments are characterised by their colour and by the fact_
that they are easily oxidised by nitrous acid, yielding a series of
coloured products (Gmelin’s test). If nitric acid containing nitrous
acid is added to bile in which the pigment present is bilirubin, the
colour changes to green (biliverdin), then to blue, violet, red, and
finally to yellow (choletelin), Bilirubin and biliverdin show. no
absorption bands with the spectroscope. Bilirubin (C,,H,.N,O,)
is identical with heematoidin, a substance formed by the decomposition
of hemoglobin in old blood clots in the body. Its molecule only
differs from that-of iron-free hematin, formed by the action of strong
mineral acids on hemoglobin, in possessing one atom less of oxygen.
Bile pigments are derived from the pigment of the blood -by the
breaking down of the latter in the liver. The facts upon which this
statement is based are (1) the identity of bilirubin with hematoidin,
(2) histological observations on the liver, (3) observations on the
proportion which exists between the rate of destruction of the red
blood corpuscles and the amount of bile pigment formed, and (4) the
effect of injection of hemoglobin into the blood stream. Histological
observations show that the walls of the sinusoids in the liver are
incomplete, so that red blood corpuscles can pass through them ; and
red corpuscles, broken up to a greater or less degree, have been seen
within the liver cells. Moreover, the presence of iron in the liver
cells can be demonstrated by the blue colour produced by treatment
of sections with ferrocyanide of .potassium and hydrochloric acid.
Again, when the destruction of red blood corpuscles is increased, as
in the disease known as pernicious anzmia, or in poisoning by the
injection of pyrogallic acid or toluylene diamine, the amount of bile
2I
322 ESSENTIALS OF PHYSIOLOGY.
pigment produced is excessive and the iron in the liver cells is increased.
Finally, when a solution of hemoglobin is injected into the blood
stream, there is an increased production of bile pigment.
The bile pigments undergo bacterial decomposition in the large
intestine with the formation of stercobilin, the pigment of the feeces.
Some of the latter is reabsorbed, and appears in the urine as a chromo-
genic substance, urobilinogen, from which urobilin is formed by
oxidation. Urobilin itself is identical with stercobilin, and it occurs
in the urine in pernicious anemia and other diseases in which destruc-
tion of red blood corpuscles is excessive.
THE FUNCTIONS OF THE BILE.
It has already been pointed out that bile is not a digestive juice
in the proper sense of the term. It is said to contain a weak amylo-
lytic enzyme, but the action of this ferment is quite insignificant,
Nevertheless the bile exercises important functions in connection with
the digestive process. (1) The acid metaprotein and proteoses result-.
ing from the gastric digestion of proteins are precipitated by the bile
salts in the duodenum. This conversion of a fluid or semi-fluid
material into the solid condition will retard its progress along the
intestine and allow more time for the action of the pancreatic juice.
(2) The bile salts act as a “co-enzyme” to each of the principal
ferments of the pancreatic juice, that is, they increase the rate of the
digestive process without themselves taking any active part init. In
the presence of bile salts the power of the pancreatic amylase to
hydrolyse starch is doubled, and the proteolytic power of trypsin is
similarly increased, while the action of lipase upon fats is quadrupled.
The adjuvant action of bile in digestion is due to the property which
the bile salts possess of reducing surface tension, as well as to their
property of dissolving fatty acids and soaps. By the reduction of
surface tension the contact of enzyme with substrate is promoted,
and this is of especial value in facilitating the access of lipase to oily
fluids. (3) Bile promotes the absorption of the products of digestion,
this property also being due to the bile salts, Free fatty acids are
brought into solution, and in this form are more adapted for passing
through the epithelial cells of the intestinal villi. Moreover, these
cells have their surface tension lowered, and are thus made more
permeable by all the products of digestion. Lecithin and cholesterol,
which are held in solution in the bile, also play a part in promoting
absorption, but the precise way in which they act is not understood.
The importance of the presence of bile for the digestion and absorption
of fat is shown by the fact that when bile is prevented from entering
- THE DIGESTIVE SYSTEM. 323
the intestine, 60 per cent. of the fat of a meal passes into the feces,
as compared with about 5 per cent. under normal conditions. (4) Bile
is said to stimulate the peristaltic movements of the intestine, and (5), —
as already pointed out, the reabsorbed bile salts stimulate the liver to
further secretion.
THE SECRETION OF BILE.
The secretion of bile is a continuous process, and in periods when
digestion is not taking place, bile accumulates in the gall-bladder.
About the third hour after a meal is taken the gall-bladder is
emptied into the lumen of the duodenum, but the mechanism by
which the contents are expelled has not yet been ascertained. Bile
continues to flow into the intestine during the digestive process, and,
later, again accumulates in the gall-bladder. The rate of production of
bile has been studied in animals with experimental fistulee, and also in
man when fistule have formed in the course of disease. In such cases,
however, the normal stimulus due to the reabsorption of bile salts is
wanting. It is found in man that something less than a litre of bile
can be collected froma fistula in twenty-four hours, an amount equal to
that of the juice secreted by the pancreas in the same time.
So far as is known, the secretion of bile is independent of nervous
action, and is excited (1) by the reabsorbed bile salts, and (2) by
secretin. The flow from a fistula is fairly continuous, though it varies
in rate with the period of the day and with other conditions. The rate
of flow in such a case is, of course, unaffected by reabsorption of bile
salts. It is doubled by the introduction of dilute hydrochloric acid into
the duodenum, or by the injection of secretin into the blood stream.
The rate of flow, like that of the pancreatic juice, varies with the nature
of the food, and for the same reason (p. 318), being greatest when a
meat diet is taken, and least when the food consists mainly of carbo-
hydrates. Fatty food, which inhibits the secretion of gastric juice,
excites the secretion of the pancreas and liver, though not to the same
extent as does meat. The influence of fat in this direction is due to
the formation of soap in the duodenum, the latter substance acting as
a stimulus to the production of secretin.
THE FINAL STAGES OF THE DIGESTIVE PROCESS.
Saliva, gastric and pancreatic juice in turn carry the digestion of
the food-stuffs up to a certain point. The pancreatic juice completes
the digestion of the fats; it converts proteins into peptones and amino-
acids ; it is capable of converting starch completely into dextrose, but it
has no action upon cane sugar and lactose. The completion of the
digestive process is the function of the intestinal juices
324 ESSENTIALS OF PHYSIOLOGY.
THE COMPOSITION OF THE INTESTINAL JUICE.
Intestinal juice is obtained, like the other digestive secretions, by
means of a fistula. A segment of intestine of sufficient length is
separated by incisions; one end of the separated portion is closed by
sutures, and the open extremity is sutured into the abdominal wall, the
continuity of the remainder
of the bowel being restored
by stitching the free ends
together. The detached
‘segment retains its normal
blood and nerve supply.
In another method, both
ends of the segment are
left open, and each is
sutured separately into the
abdominal wall (fig. 123).
The juice obtained from
such a fistula has a specific
gravity of about 1010, and
is alkaline. It contains 1
to 2 per cent. of solids,
half of which are organic
and half inorganic, The
| organic solids consist
*~=.mainly of serum albumin, serum globulin, and enzymes. The inorganic
substances are chiefly sodium chloride and sodium carbonate.
.
1 kets ee Ne .
Meg ee
cote ee e ; #
ce ae eee a ee %
: s 3 *
2 AA ake BS ror ae
.2 i BA
Fig. 123.—Scheme of intestinal fistula.
A, first stage of operation ; B, fistula completed.
THE FUNCTIONS OF THE INTESTINAL JUICE.
It has already been pointed out that the intestinal juice converts
trypsinogen into trypsin by virtue of the enzyme, enterokinase, which it
contains, and also that another of its enzymes, maltase, shares with a
similar ferment in the pancreatic juice the function of completing the
digestion of starch by converting maltose into dextrose. The
intestinal juice also contains two enzymes which convert the di-
saccharides, cane sugar and lactose, into monosaccharides. One of
these ferments, invertase, converts a molecule of cane sugar into one
molecule of dextrose and one molecule of fructose. The other, /actase,
hydrolyses lactose in a similar way into dextrose and galactose. Lactase
is most abundant in young animals, at the period of life when lactose
is an important constituent of the dietary. The terminal stages of
the hydrolysis of protein are effected by a ferment, erepsin, existing in
2
THE DIGESTIVE SYSTEM. 325
the intestinal juice. Erepsin acts upon proteoses and peptones, splitting
them up into amino-acids. There is some ground for believing that
the hydrolysis is not quite complete, and that, in the final result, in —
addition to amino-acids, there are some more complex substances called
polypeptides, groupings of amino-acids which are less complex than
peptones, and which do not undergo complete hydrolysis. Erepsin is
also contained in the epithelial cells covering the villi, and it is possible
that the final stages of hydrolysis may occur in these cells.
The chief amino-acids resulting from the digestion of proteins are
leucine, tyrosine, aspartic acid, glutaminic acid, tryptophane, and the
hexone bases, lysine, arginine, and histidine. These, and other sub-
stances belonging to the same group, are linked together to form the
protein molecule, and the differences found to exist between the various
proteins are associated with differences in the proportions of their
constituent amino-acids.
The intestinal juice is produced by the crypts of Lieberkiihn,
tubular glands lined by columnar epithelium, occurring in the mucous
membrane of the small intestine, and opening between the bases of the
villi. There is no evidence that the secretion is influenced by a nervous
factor. If three adjacent loops of intestine are separated from each
other by ligatures, and the nerves to the middle loop are divided, the
latter is found full of fluid after four to sixteen hours, while the
adjacent loops are empty. T'wo days later, however, all three loops are
empty. It appears probable, therefore, that the production of fluid
following the section of the nerves is due to dilatation of blood-vessels
resulting from the division of vaso-constrictor nerves, and that absorp-
tion occurs and the production of fluid ceases as the vessels regain their
tone. No conclusions can therefore be drawn as to an inhibitory or
other influence of the nervous system on the secretion from such an
experiment. The normal stimulus for the secretion of the intestinal
juice is undoubtedly secretin, and possibly also other hormones. This
possibility is supported by the fact that intestinal juice is produced
in the dog about ten minutes after the ingestion of a meal of meat,
and that the flow is increased in the third hour after the food has been
taken. The secretion of intestinal juice can be brought about. by
mechanical stimulation, probably by means of a local nervous mechanism.
THE PROGRESS OF DIGESTION IN THE SMALL
INTESTINE.
Experiments have been made on dogs in which the intestinal con-
tents were withdrawn, by means of appropriate fistule, at different
stages of their passage along the bowel. It was found that after a
326 ESSENTIALS OF PHYSIOLOGY.
test meal 77 per cent. of the protein was converted into proteose and
peptone, and one-half to three-fifths of the starch into dextrin and
sugar, as a result of gastric and duodenal digestion, and that, when the
intestinal contents reached the lower end of the ileum, the digestion of
all the food-stuffs was complete. |
THE MOVEMENTS OF THE SMALL INTESTINE.
The intestinal contents are slowly propelled along the gut towards
the colon, and at the same time they are subjected to a continuous
mixing process. The onward movement is effected by waves of con-
traction which sweep along the muscular coat of the bowel, and con-
stitute what is known as peristalsis, The mixing of the material in
the intestine is brought about by ring-like or segmental contractions,
which are not progressive in character. These movements may be
observed in the living animal or person by the aid of Réntgen rays
after the administration of a bismuth meal. They can also be observed
directly, if the abdomen is opened in an animal immediately after it
has been killed. The movements may be recorded in the living
animal by means of a balloon inserted into the lumen of the bowel
and connected with a writing tambour; contraction of the intestinal
wall compresses the balloon, and air is forced into. the tambour,
thereby raising the lever. Other experimental methods may also
be used.
The Peristaltic Movements.—Any mechanical stimulus, such as
pinching the intestine, will set up a peristaltic wave. The normal
stimulus is a bolus in the lumen of the gut, and it is for this reason
that the indigestible material of vegetable food is of value in promot-
ing peristalsis. There are two features characteristic of the peristaltic
wave. First, it is preceded by a wave of relaxation which begins below
the point of stimulation, the contraction wave itself beginning above
that: point. Second, it always travels in the aboral direction. If
a segment of the intestine be excised and again stitched in position in
a reversed direction, the peristaltic waves in the reversed segment will be
opposed to those of the rest of the intestine, and partial obstruction to
the passage of the intestinal contents will result. Peristalsis still
occurs when all connections with the central nervous system have
been divided, but it is abolished by painting the gut with nicotine
or cocaine. We may conclude, therefore, that it is effected by a local
reflex mechanism connected with Auerbach’s plexus (the myenteric
plexus) in the muscular coat, the stimulus arising from the presence of
a bolus in the intestine, and depending upon either the stretching of
the intestinal wall or the irritation of nerve endings in the mucous
* a>
SE ——
THE DIGESTIVE SYSTEM. 327
membrane, This is the only known local reflex, with the possible
exception of the opening and closing of the pyloric sphincter.
The Segmental Contractions.—The segmental contractions differ
from the peristaltic waves in occurring regularly and rhythmically.
When they are recorded by means of a balloon in the intestinal lumen,
they are found to be most marked at the middle of the balloon where
the tension is greatest. The result of such contractions is to sub-
divide any bolus over which the contraction takes place, and thus
to bring about a thorough admixture of the intestinal contents. Both
layers of the muscular coat take part in these contractions, as well as
Fie, 124.—Tracing showing relaxation of intestinal wall on stimulation of splanchnic
nerve (balloon method). (Bayliss and Starling. )
in the peristaltic waves, and as a result the bowel exhibits pendular or
swaying movements. The segmental contractions have no influence in
promoting the onward movement of the intestinal contents.
It has not been definitely determined whether the segmental move-
ments are myogenic or neurogenic in origin, though the latter seems
the more probable. They are not abolished by painting the intestine
with nicotine or cocaine, and this would point to their being of
myogenic origin; on the other hand, they occur in isolated strips of
muscular coat to which the myenteric plexus is attached, and do not
occur if the plexus is absent, so that the presence of the plexus
would seem to be necessary for them to take place
The Nerves of the Small Intestine.—The small intestine is supplied
by the vagus and the splanchnic nerves, and although, as has been
328 ESSENTIALS OF PHYSIOLOGY.
pointed out, the intestinal movements are independent of the central
nervous system, it can be shown that these nerves exert a controlling
influence. Stimulation of the vagus is followed by contraction of the
muscle of the intestinal wall after a preliminary relaxation, but has no
effect on the ileo-colic sphincter. Stimulation of the splanchnic nerve
results in a general relaxation (fig. 124), but in contraction of the ileo-
colic sphincter. The vagus and sympathetic fibres to the small
intestine ‘are distributed in the first instance to the myenteric nerve
plexus lying between the layers of the muscular coat. The influence
of the central nervous system on the intestinal movements is shown in
their inhibition as a result of pain, and also in their exaggeration in
consequence of emotional conditions. The movements of the intestinal
wall are increased by pilocarpine, which stimulates the nerve endings
of the vagus. The nerve endings are paralysed by atropine, the effect
of which is antagonistic to that of pilocarpine.
The Passage of the Intestinal Contents from the Ileum into the
Cecum.— Observations with the aid of Rontgen rays show that the
material in the small bowel tends to accumulate behind the ileo-colic
sphincter, and that it passes into the cecum in considerable quantity :
when the sphincter relaxes. The immediate cause of the relaxation of
the sphincter appears to be a nervous reflex (gastro-ileac), which follows
the entrance of a fresh meal into the stomach. The delay in the ileum
will obviously favour the absorption of the last traces of nutritive
substances. _
SECTION V.
ABSORPTION IN THE SMALL INTESTINE.
The absorption of the food-stuffs is almost entirely limited to the
small intestine. It has been proved that no water is absorbed in the
stomach, and although some experiments have seemed to indicate that
there may possibly be some absorption of peptone, sugar, and more
especially alcohol in that organ, the quantities concerned are, at the
most, so small as to be negligible. It will be shown later that a large
amount of water is absorbed in the large intestine, and that there is a
possibility of the absorption of small amounts of dextrose there also,
but, under normal circumstances, the material which reaches the large
bowel is free from sugar, as well as all other nutritive digestive
products. :
The progress of absorption can be investigated by the method
already referred to of collecting the intestinal contents by means of
fistulee after the ingestion of a weighed meal. For example, after a
s aes
> .
ntact sina iaiie
THE DIGESTIVE SYSTEM. 329
meal of 200 grams of bread given to a dog, the following results were
obtained :—
Fistula. Obtained. mts
Pyloric . . | 691 grams. 0
Duodenal. a ss 17°45
Jejunal ts GOO 2%, Hf. RTT
Tleum . : Sry AED tor535 67°65
Ceecum , ‘ BO 22.2 55 94°34
The material obtained from the various fistule was of course mixed
with the digestive juices. It is evident that the food material is
practically completely absorbed by the time the lower end of the ileum
is reached. The absorption takes place through the villi of the small
intestine, partly directly into the blood stream, and partly by way of
the lymphatic system, the food material in the latter case reaching the
blood-vessels along the thoracic duct.
A villus is a finger-like projection of the mucous membrane, covered
by columnar epithelium, each cell having a refractive, striated border
on its free end, and resting by its deep extremity on a basement
membrane. In the centre of the villus is a lymphatic vessel, the
central lacteal, commencing by a blind extremity and communicating
with the plexus of lymphatic vessels in the submucosa. Between the
lacteal and the basement membrane are retiform tissue with scattered
leucocytes, and strands of smooth muscle which extend from the
muscularis mucosze and are attached to both basement membrane and
lacteal. A small artery is supplied to each villus and breaks up into
a plexus of capillaries, lying immediately under the basement membrane
and reuniting to form a small vein.
THE PROCESS OF ABSORPTION.
The absorption of the food products is effected by what, in the
absence of a more precise definition, is called the vital activity of the
epithelial cells of the intestinal mucous membrane. Experiments show
that the process cannot be accounted for by filtration, diffusion, and
osmosis. Filtration cannot take place, because the pressure in the blood
capillaries of the mucous membrane is higher than the pressure in the
lumen of the intestine. Again, a solution which is isotonic with the
blood serum undergoes absorption, and an animal will~even absorb its
330 ESSENTIALS OF PHYSIOLOGY.
own blood serum, so that diffusion and osmosis do not provide a com-
plete explanation. 3
Although these physical processes do not account for absorption,
they may nevertheless occur in the intestine. Thus, if the bowel con-
tain a hypertonic solution of salt, water passes from the mucous.
membrane into the solution until the latter becomes isotonic with the
blood, after which it is steadily absorbed. Osmosis may therefore
retard, or on the other hand it may assist, absorption, but the passage
of material from the lumen of the intestine into the villi must be
regarded as due to the activity of the epithelial cells. If, however, the
epithelial cells are injured by means of sodium fluoride, absorption is
entirely regulated by the processes of diffusion and osmosis, and is
therefore incomplete. Histological evidence as regards the absorption
of fats shows that these substances pass through the epithelial cells, and
experiments with dyes soluble in lipoids show that these also enter the
cells themselves. The absorption of the products of protein digestion
is also said to be accompanied by structural changes in the cells, and
probably both amino-acids and dextrose pass through the cell substance.
On the other hand, dyes insoluble in lipoids have been shown to pass
between the cells, and the possibility of the intercellular cement forming
a route for absorption cannot be considered to be absolutely exciuded.
- Generally speaking, therefore, absorption is an active or vital
process, even in the case of water and salts, but it may be assisted or
retarded by the physical processes of diffusion and osmosis.
The Absorption of the Products of the Digestion of Proteins.— The
final stages of the digestive hydrolysis of the food proteins may take
place in the mucous membrane of the intestine after absorption has
begun. It has been shown that during the absorption of a protein
meal the presence of peptone can be demonstrated in the wall of the
bowel. If, however, the mucous membrane be kept at the body
temperature for half an hour before it is analysed, no peptone will be
found in it. Proteose and peptone may be taken up by the epithelial
cells, and may be further hydrolysed by erepsin in the cells themselves
with the formation of amino-acids. Even coagulable proteins may be
taken up by the epithelial cells. It has already been pointed out that
an animal can absorb its own serum, and this will take place when the
intestine has been washed free of enzymes. Similarly egg-albumin
may be absorbed, the amount introduced into the bowel being reduced
by one-fifth in three hours. It is probable that, in these circum-
stances, complete digestive hydrolysis takes place in the mucous
membrane, and that all food proteins are reduced to the amino-acid
condition before being utilised in the body.
Ee ee —
THE DIGESTIVE SYSTEM. 331
The protein derivatives are absorbed into the blood-vessels. After
a protein meal the lymph which can be collected from the thoracic
duct is not increased in amount, nor does it contain an increased |
quantity of protein. On the other hand, the absorption of a protein.
digest is not interfered with by ligature of the thoracic duct. It
is, however, a matter of considerable difficulty to demonstrate the
presence of amino-acids in the blood of the portal vein. Experiments
which have been carried out show that a process of deamination takes
place in the intestinal wall, whereby part of the amino-acids is oxidised,
yielding ammonia and an oxy- or ketofatty acid, which pass into the
portal circulation together with unchanged amino-acids.
The Absorption of the Monosaccharides.—The products of carbo-
hydrate digestion all belong to the group of monosaccharides and are
easily diffusible substances. The chief of these is dextrose, but some
fructose is formed by the hydrolysis of cane sugar, and some galactose
by that of the sugar of milk. All three varieties are absorbed directly
into the blood stream. During absorption of a carbohydrate meal
more sugar is found in the portal vein than in the hepetic vein. More-
over, the absorption of sugar is not accompanied by an increase of
that substance in the lymph of the thoracic duct, and it is not inter-
fered with by the ligature of that structure. Disaccharides are not
absorbed from the intestine.
The Absorption of Fat.—The products of digestion of fat are
fatty acid, held in solution by the bile salts, glycerol, and a little soap.
The absorption of fat differs from that of amino-acids and dextrose
in that it takes place for the most part into the lymphatic system.
During the absorption of a fatty meal the lymphatics of the mesentery
become filled with a milky fluid called chyle, so that they are easily
visible to the naked eye. Chyle collected from the thoracic duct may
contain over 6 per cent. of fat. Ligature of the thoracic duct
diminishes, but does not entirely abolish fat absorption. The absorbed
fat reaches the blood stream by the thoracic duct, and if an animal be
bled during fat absorption, the plasma will be found to be milky from
the quantity of minute fat globules present init. A few hours later
the plasma is again clear, because the small quantity of fat present
in it is held adsorbed by the serum proteins, the remainder having
either been oxidised or transferred to the fat depdts of the body.
About 98 per cent. of the fat taken as food is absorbed, but only 60
per cent. can be recovered in the chyle. The fate of the remaining
40 per cent. is unknown. It can neither be recovered from the blood
nor from the thoracic duct, nor does it appear in the feces.
The bile salts have an important influence on fat absorption, partly
332 ESSENTIALS OF PHYSIOLOGY.
because of their property of holding fatty acids in solution, and partly
because they reduce surface tension and so facilitate the passage of the
fatty material into the epithelial cells. In the absence of bile 60 per
cent. of the fat of a meal remains unabsorbed. The absence of pan-
creatic juice also prevents the absorption of much of the fat, because
fat is not absorbed unless it is acted on by lipase, and the lipase of
gastric juice takes a comparatively small share in fat digestion.
The soaps formed in the intestine are split up by the intestinal
epithelial cells into fatty acids and alkali. The fatty acids are absorbed
and recombined with glycerol in the cells to form neutral fats.
Fat absorption may be studied histologically. During its occurrence
the epithelial cells covering the villi become filled with droplets of fat,
which may be stained black with
osmic acid (fig. 125), red with Schar-
lach R or Sudan ITI, or blue by means
of Nile blue. The droplets consist
of neutral fat, so that a re-synthesis
takes place in the epithelial cell,
the bile salts which held the fatty
acid in solution probably passing
Sess directly into the blood stream.
- Frc. 125.—Mucous membrane of frog’s The fat may be traced through the
Ara Wataean i sly aa? cell into the core of the villus, where
Histology. ) the droplets are finely emulsified
ep, epithelium; sfr, striated border; c, by the lymph in the tissue spaces,
leucocytes; 2, lacteal. Z 3
. and are carried into the central
lacteal. The wandering leucocytes in the villus take up fat droplets,
and this may account to some extent for the 40 per cent. of absorbed
fat which cannot be recovered from the thoracic duct.
Alternate contraction and relaxation of the muscular fibres in the
-yillus tends to propel the contents of the central lacteal towards the
larger lymph vessels in the intestinal submucosa.
It has been demonstrated that hydrocarbons such as parafiin land
petroleum are not absorbed in the intestine.
SECTION VI.
THE LARGE INTESTINE.
Four or five hours after each meal the contents of the small
intestine begin to pass through the ileo-colic junction into the large
intestine. An important factor in promoting the transference of
THE DIGESTIVE SYSTEM. 333
material from ileum to colon is the gastro-ileac reflex. The entrance
of the succeeding meal into the stomach excites the production of |
peristaltic waves in the lower part of the ileum, each wave, as it
reaches the ileo-colic junction, being followed by relaxation of the
sphincter. In this way the contents of the ileum are propelled into
the colon in successive portions, and if they contain bismuth the level
which they attain in the ascending portion of the large intestine can
be observed by means of X-rays to rise towards the hepatic flexure in
an intermittent manner.
The material which thus passes into the large bowel is in the form
of a jelly, coloured by the presence of bile pigment. It normally
contains hardly any nutritive substances, the derivatives of the diges-
tion of protein, fat, and carbohydrates having been almost completely
absorbed in the small intestine. Indigestible substances contained in
the food are present, especially cellulose, together with cast-off epithelial
cells and the unabsorbed portions of the various digestive juices. The
chief secretory waste products are the pigment of the bile, unabsorbed
glycocholate and taurocholate of sodium, and cholesterel.
THE FUNCTIONS OF THE LARGE INTESTINE.
In carnivorous animals the large intestine is short, and its function
is limited to the absorption of water and the consequent reduction in
bulk of the feeces. In herbivora, on the other hand, the large intestine
is of considerable length, and not only absorbs water but serves an
additional purpose. A large proportion of vegetable food-stufts consists
of cellulose, which is not affected by the digestive enzymes. Cellulose
is decomposed in the large intestine of the herbivora by bacterial
action, being converted into fatty acids, which are absorbed and utilised
in the body. Further, in all the higher animals, the cells lining the
simple tubular glands of the large intestine are for the most part of
the mucus-secreting type, and are of service in producing mucin, which
acts as a lubricant and facilitates the passage of the feces along the
bowel. ;
In man the functions of the large intestine include secretion,
excretion, and absorption, and in addition some bacterial decomposition
takes place in its contents. (1) The secreted material, as in the higher
animals generally, is mucin, derived from the tubular glands of the
mucous membrane. (2) The substances excreted by the large
intestine are calcium, magnesium, and iron, chiefly in the form of
phosphates. The amount of calcium excreted by the bowel varies
with the amount contained in the urine. Acid urine, such as occurs
334 ESSENTIALS OF PHYSIOLOGY.
normally in carnivora, and in man when the diet contains a due
‘proportion of protein, holds calcium phosphate in solution, and in
such a case the proportion of calcium excreted by the large intestine
is relatively small. When the urine is alkaline, on the other hand, as
in herbivora, and in man when the diet is largely vegetable, the amount
of calcium excreted by the bowel is greater. Other chemical substances
taken as drugs, for example srauetl: may also be excreted by the
large intestine.
(3) The only substance absorbed in any quantity in the large bowel
is water. The contents of the ascending colon contain no nutritive
substances, but their bulk is fairly large owing to the amount of
fluid which they contain. During their stay in the large intestine
the bulk is greatly reduced, chiefly by the absorption of water. It
is said that 400 ¢.c. are absorbed from the contents of the colon in
twenty-four hours. The possibility of the absorption of nutritive
substances in the large intestine is of importance, because attempts
are frequently made to introduce food-stuffs into the body by means
of rectal injections. Experiments prove, however, that nutritive
material is not absorbed by the large intestine, with the exception
of small amounts of dextrose, which are too minute to be of real
practical value.
(4) The dacteria in the human large intestine act upon cellulose
with the production of lower fatty acids, marsh gas (CH,), carbonic
acid, and hydrogen. Undigested protein residues also undergo bacterial
decomposition with the production of the aromatic bodies, indol (C,H;N),
skatol (methyl-indol), and phenol. It is possible that the fatty acids
derived from cellulose may be absorbed, as they are in the herbivora,
and there is evidence that absorption of indol, skatol, and phenol takes
place, inasmuch as compounds of these substances with sulphuric acid,
the ethereal sulphates, are found in the urine.
THE FACES.
The residues which finally reach the rectum constitute the feces,
and form a solid or semi-solid mass, coloured by the pigment sterco-
bilin, which is derived from bilirubin. The composition of the feeces
has already been indicated. They contain about 65 per cent. of water,
with organic material and inorganic salts. The organic substances
are partly nitrogenous, and partly of a fatty nature and soluble in
ether. The nitrogenous constituents include cholic acid, dyslysin,
indol and skatol, purin bodies, epithelial cells, and dead bacteria. The
lipoids are fatty acids, lecithin, and coprosterin, a body allied to
THE DIGESTIVE SYSTEM. 335
cholesterol. There may be a small quantity of neutral fat. When
vegetable food has been taken, the feecal matter will include un-
decomposed cellulose, but, for the most part, the feces consist of —
substances derived from the digestive tract itself.
THE MOVEMENTS OF THE LARGE INTESTINE.
The movements of the large intestine have been most satisfactorily
studied with the aid of X-rays (fig. 126).! The caecum and ascending
colon are filled, in the manner which has already been described, by
the peristaltic contractions of the ileum, and are entirely passive
during the process. Later, segmental contractions occur in this part
of the bowel, similar in nature to those which take place in the small
intestine. These movements tend to mix the contents and promote
the absorption of water. The transference of the feces from the
cecum and ascending colon to the transverse and descending colon
takes place at long intervals, usually three or four times in twenty-
four hours, by means of peristaltic contractions. These movements
generally follow the entry of food into the stomach, and are ascribed
to.a gastro-colic reflex. Scattered masses may remain in the trans-
verse colon for a time, and these are suddenly transferred to the
descending colon by a peristaltic wave.
The feces remain:for a time in the sigmoid flexure, until, usually
after a meal, a certain amount passes into the rectum and gives rise
to the desire for defecation. By relaxation of the sphincter ani,
accompanied by contraction of the walls of the sigmoid flexure and
rectum assisted by contraction of the voluntary muscles of the abdominal
wall and pelvic floor, the lower end of the bowel is evacuated. The
act is a reflex one, but in the adult it is under the control of the
higher centres. 7
The Nerve Supply of the Large Intestine.—The large intestine
receives its nerve supply from the sympathetic system and from the
pelvic visceral nerves or nervi erigentes. The sympathetic fibres form
the inferior mesenteric nerves, running from the inferior mesenteric
ganglion to the ascending, middle, and transverse colon, and the hypo-
gastric nerves pass from the same ganglion to the rectum. The pre-
ganglionic fibres emerge from the spinal cord by the second, third, and
fourth anterior lumbar nerve roots. .
The nervi erigentes emerge from the cord by the second and third
sacral nerve roots, and are distributed to the whole length of the large
' We are indebted for this diagram to the kindness of the Oxford University
Press.
a
-336 ESSENTIALS OF PHYSIOLOGY.
intestine. Stimulation of the sympathetic nerves causes inhibition of
the tone of the intestinal wall and cessation of the rhythmic movements.
Stimulation of the nervi erigentes causes contraction of the whole length
f
az)
{
40
3
Fic, 126.—Passage of food along the large intestine after a
bismuth meal, as seen by means of X-rays. The numbers
refer to the hours after the meal was taken. (Hertz. )
of the colon. The sympathetic is therefore inhibitory and the pelvic
visceral nerves are motor in function.
As in the small intestine, there are ganglionated plexuses in the
intestinal wall, of which the myenteric, lying between the layers of the
muscular coat, is associated with the local reflex mechanism controlling
the movements of the bowel.
x
ee ee
THE DIGESTIVE SYSTEM. = & Pd
THE VERMIFORM APPENDIX (VERMIFORM PROCESS).
‘In man the cecum is very short and has attached to it a wormlike ©
process, the vermiform appendix. This has a thin muscular coat
and a thicker mucous coat, the latter composed almost entirely of
lymphoid tissue containing scattered tubular glands. The human
appendix is regarded as a vestigial remnant of no functional import-
ance. It is homologous with the long and capacious cecum of herbi-
vora, which serves the useful purpose of retaining food material while
the cellulose undergoes bacterial decomposition. ,
22
CHAPTER XI.
METABOLISM.
SECTION I.
THE food-stuffs, after being digested and absorbed into the blood or
lymph, are carried to the tissues, in which they pass through a series
of complex chemical transformations, the end products of which leave
the tissues and are removed from the body by the lungs and kidneys.
This series of chemical changes constitutes metabolism, and the
metabolic activities of the tissues are of two kinds. On the one hand,
the living tissues are constantly undergoing changes whereby a portion
of their substance is broken down and removed from the body; on
the other hand, this loss is replaced by the building up of fresh tissue
from the. nutritive materials supplied in the blood. The former of
these processes is called katabolism, and the latter anabolism.
These changes involve the consumption of a large amount of
oxygen, and the evolution of energy in the form of heat and muscular
work. The food-stuffs are protein, fat, and carbohydrate, 90 to 94 per
cent. of those consumed on an ordinary diet being absorbed into the
blood stream, and the remainder being lost in the feces. The fats
and. carbohydrates are completely converted into carbonic acid and
water, and the proteins partly into carbonic acid and water, the
nitrogen being excreted as urea and other incompletely oxidised sub-
stances in the urine. The carbonic acid is removed from the body
almost entirely through the lungs, and the water by the lungs,
kidneys, and skin. The changes undergone by the food-stuffs in, the
body may, therefore, be studied in three ways, namely (1) by measur-
ing the total amount of heat evolved in their oxidation, (2) by deter-
mining the quantity of oxygen required for the carrying out of these
oxidations, and (3) by measuring the amount of the end products which
are formed. We may also attempt to follow out the series of changes
taking place in the individual food-stuffs in the tissues themselves,
338
METABOLISM. 339
THE PRODUCTION OF HEAT.
The heat evolved by the complete oxidation of a food-stuff can be
ascertained by means of the bomb calorimeter (fig. 127), which consists
of a metal bomb, A, through the top of which pass two wires, 2 and h’,
connected by a strip of soft iron wire; one gram of the substance,
e.g. dextrose, which is to undergo combustion, is placed in contact with
the iron wire. The bomb is filled with oxygen from an oxygen cylinder,
under a pressure of 7 to 8 atmospheres, and is enclosed in a bath, C,
containing a known volume of
water, which is surrounded by
an air and a water jacket, D.
When a current is passed
through the wires the soft iron
fuses and ignites the sugar,
which is rapidly oxidised to
carbonic acid and water. The
heatevolved in this processraises
the temperature of the water in
which the bomb is placed ; and
if the temperature of the water
before and after the combustion
is observed, the amount of heat
evolved can be calculated, and
is expressed in calories. A large
calorie is the amount of heat re-
quired to raise 1000 grams of
water 1" C. (A small calorie is Fic. 127.—Bomb calorimeter.
the amount of heat necessary to T=thermometer; E=air jacket; B=strip of
raise the temperature of 1 gram AB gag ta
of water 1° C.; this measurement is now seldom used.) If the volume
of water in such an experiment is 1 litre and the rise of temperature
is 4°°1 C., 1 gram of sugar when fully oxidised gives out 4:1 calories ;
and this amount, which is spoken of as the calorie value of dextrose,
is constant. The average calorie value of fat, carbohydrate, and protein
is shown in the following table :— :
fe
1
ih
ie a MATTE]
|
at
Rat
SS
il
Tm
pi tiae
HN
Has
WI
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lh
i
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it
lili
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ff
mn
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HARRAH
| Wey hiqyy
| mihi
||
iii
We
it
NII
il
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Yj
OZ
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TTT
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Tea
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i
Fat, 1 gram = 9°3 calories.
Carbohydrate, 1 gram = 4:1 calories.
Protein, 1 gram = 4°1 calories.
The calorie value of protein when completely oxidised in the calori-
meter is 56 ; but protein is not fully oxidised in the bedy, its nitrogen
340 ESSENTIALS OF PHYSIOLOGY.
being excreted in the urine largely as urea, and to a less extent as
other substances, each of which has a calorie value of its own. In
determining the physiological calorie value of protein, the heat value of
urea and other nitrogenous products must be deducted from the figure
5°6. When this is done, the heat value of protein is reduced to 4:1,
which represents its calorie value in the body. The energy set free in
the body by the oxidation of the food-stuffs appears partly as heat and
partly as muscular work. The energy set free as muscular work may
_ be calculated as heat according to the following equation :—
425 gram-metres of work = 1 calorie.
PS
CALORIMETER
CHAMBER.
lanes Sl a | bade
b
(r Jsoda-Lime| | H,30,, |
Air minus CO, and Water; deficient in Oxygen.
Oxygen ad
Fic. 128.—Diagram to show the principle of the Atwater-Benedict calorimeter.
(After Halliburton.) Starling’s Principles of Physiology.
The heat formed in the body is determined by placing the animal in
a suitable calorimeter. Large calorimeters such as that of Atwater and
Benedict have been constructed, in which a man can live for two
or three days, or longer, and can carry out muscular work. The
Atwater-Benedict calorimeter consists of a room, with double, non-
conducting walls, containing a series of pipes through which water is
flowing at such a rate that the temperature of the room remains
constant (fig. 128). The whole of the heat given off by the man
warms the water passing through the pipes, and is thus carried off.
By measuring the amount of water flowing along the pipes and the rise
in its temperature as it passes through the calorimeter, the heat
evolved by the individual can be estimated. By means of these
methods it is possible to determine, on the one hand, the energy supplied
to the body in the food, and, on the other hand, the energy lost from
the body as heat and muscular work. It is found that these two
6) cE te ee
METABOLISM. 341
balance one another within the limits of experimental error, and that
the energy lost from the body has its origin entirely in the potential _
energy taken into the body in the food. Thus the principle of the
conservation of energy is as true for living beings as it is in the rest of
the organic and the inorganic world.
Calorie Value of :
‘i Day of Material Oxidised |: Calories Lost from
xperiment. in the Body. the Body.
1 2349 2414
2 2345 2386
3 2391 2413
THE RESPIRATORY QUOTIENT AND EXCHANGE.
The respiratory exchange is the total quantity of oxygen taken into
the body and of carbonic acid discharged from the body in a given time.
The respiratory quotient, as already mentioned (p. 250), is the
ratio of the amount of carbonic acid discharged from the body to the
amount of oxygen taken in in a given time. |
In man the amount of oxygen used and of carbonic acid evolved in
the metabolic changes taking place in the body under varying conditions,
such as rest or muscular exercise, can be determined by means of the
calorimeter just described. The air leaving the calorimeter passes
through vessels containing soda lime, which absorbs carbonic acid, the
increase in weight of these vessels giving the weight of carbonic acid
exhaled in a given time. The oxygen used by the individual is replaced
from a cylinder, the amount supplied being measured. A continuous
circulation of air through the chamber is provided by a small pump.
The respiratory quotient and ‘exchange may also be determined
approximately by finding with the aid of a spirometer the average
volume of air breathed in a minute, and by analysing a sample of
expired air. Thus, if a man.breathes 500 c.c. of air at each breath and
his respirations are 16 per minute, he breathes 8 litres per minute.
Assuming that the expired air contains 16 per cent. oxygen and 4 per
cent. carbonic acid, he must have absorbed 5 c.c. oxygen from each
100 c.c. of air breathed, namely 400. c.c.; similarly, he must have
breathed out 320 ¢.c. carbonic acid, and the respiratory quotient is
For small animals the apparatus devised by Haldane and Pembrey,
342 ESSENTIALS OF PHYSIOLOGY.
and shown in fig. 129, may be employed. The animal, ¢.g. a mouse or
guinea-pig, is placed in a vessel through which is drawn a current of
air, freed from carbonic acid and water by passing it through soda lime
and sulphuric acid. The water in the air leaving the chamber is ab-
sorbed by pumice saturated with sulphuric acid, in the tubes BB; these
are weighed before and after the experiment, the increase in weight re-
presenting the water given off by the animal. The carbonic acid
evolved is absorbed in the tube, which contains soda lime, and the water
taken up from the soda:lime is absorbed by sulphuric acid ; these two
tubes are weighed before and after the experiment. The amount of
oxygen taken up by the animal is determined indirectly by subtracting
A B C 3 Seeger ae» M
Fie, 129.—Haldane-Pembrey respiration apparatus for mouse,
(From Practical Physiology, by Pembrey and others. )
' A, soda lime; B, H.SO,; C, chamber for animal; M, gas meter.
the loss in weight of the animal during the experiment from the total
weight of carbonic acid and water given off.
The respiratory exchange serves as an index of the total oxidative
processes taking place in the body, just as the amount of oxygen used,
and carbonic acid evolved, by a single organ indicate the functional
activities of that organ. It is not influenced by the nature of the
food which is consumed, but is very greatly modified by the functional
activity of the animal. During exercise the chemical changes taking
place in muscle are increased, heat is evolved, and more oxygen is used
in the body; since the muscles form about 40 per cent. of the total
weight of the body, the respiratory exchange during exercise may be
eight to ten times greater than during rest.
RESPIRATORY EXCHANGE IN MAN. (BENEDICT AND CATHCART.)
Carbonic acid
Oxygen Absorbed in Discharged in c.c.
c.c. per Minute.
per Minute.
Resting. 242 218
: Moderate work . 1490 1224
Severe work ; 1850 1789
———_
METABOLISM. ah?
Further, the respiratory exchange varies with the size of the animal.
The smaller the animal the greater is its surface relatively to its
weight, and since the body loses heat chiefly from its surface, the relative |
loss of heat must be greater in a small than in a large animal. In order
to make up for this loss the smaller animal must produce a relatively
larger amount of heat, and must use up in this process relatively more
oxygen, than a bigger animal. . It is found, in fact, that metabolism
is more active and the respiratory exchange is greater, weight for
weight, in small animals than in large ones.
Apart from the question of size, the consumption of oxygen is
also relatively greater in growing than in adult animals, since oxygen
is being used not only in the chemical changes necessary to maintain
the weight of the animal, but also in the metabolic processes associated
with growth.
The Respiratory Quotient.—The size of the respiratory quotient
is determined almost entirely by the character of the food consumed by
an animal, If an animal were living solely upon carbohydrate food, all
the oxygen taken into the body would reappear as carbonic acid, and
the quotient would be 1, in accordance with the following equation :—
6CO, _
C.H,,0, + 60, oe 6CO, + 6H,0. 60 cat
k.
If the diet consisted entirely of fat, the oxygen taken into the body
would not all reappear as carbonic acid; some of it would be used in
converting hydrogen into water, and the quotient would be less than
1, the oxidation being represented thus :—
57CO,
CyH,(CigHs40.)s + 800, = 5700, + 52H,0.
Olein.
eat) Ft.,
On a purely protein diet part of the oxygen would be combined
with nitrogen and sulphur, as well as with hydrogen, and the respiratory
quotient would be approximately 0°8. In man living on a mixed diet
the quotient is usually about 0°85, and can be lowered by the exclusion
of carbohydrate, or raised when the diet consists mainly or exclusively
of carbohydrate food. The respiratory quotient is thus of great value,
in that it gives an indication of the nature of the food which is being
oxidised in the body under various conditions. During muscular
exercise, for example, the quotient rises very slightly, and it is evident
that the active muscle uses a rather larger proportion of carbohydrate
than resting muscle. In a large number of experiments made on the
same individual, the average respiratory quotient was 0°85 during rest,
and 0°88 during muscular exercise. ‘
344 - ESSENTIALS OF PHYSIOLOGY.
END PRODUCTS OF METABOLISM.
The whole of the carbon in fat and carbohydrate, and a large pro-
portion of that in protein, is removed from the body in a completely
oxidised form as carbonic acid, while the nitrogen of protein leaves the
body in the form of urea, uric acid, and other substances, which are
excretedin the urine. Since protein contains about 16 per cent. nitrogen,
it is possible, by determining the total amount of nitrogen in the urine,
to calculate the amount of protein from which it has been derived ;
each gram of nitrogen in the urine represents the breaking down
in the body of 6:3 grams of protein. By measuring the output of
nitrogen in the urine and the amount of carbonic acid discharged from
the lungs daily, the total amount of protein, fat, and carbohydrate
which has been oxidised in the body can be calculated, if the respiratory
quotient is known. In health the amount of carbon and nitrogen thus
removed from the body is equivalent to the amount taken in with the
food, provided that the weight of the individual’ remains steady. If
the individual is putting. on weight, some of the carbon taken in the
food is retained in the body as fat or carbohydrate ; when weight is
being lost, more carbon, and possibly more nitrogen, will be discharged
than are taken in with the food.
We may now consider the changes taking place in the various food-
stuffs after their absorption from the digestive tract.
SECTION IL.
THE METABOLISM OF FAT.
The fats taken -by the mouth, after being hydrolysed in the
digestive tract and re-synthesised in the walls of the villi, enter the
thoracic duct as neutral fats and pass into the blood stream. Fora
short. time after a fatty meal the fats circulate in the blood stream, and
may give the blood a slightly milky appearance, but they are rapidly taken
up and stored in the subcutaneous tissues and omentum, which serve
as depdts in which the fat not immediately required by the body can
be kept. The composition of: the subcutaneous fat thus reflects that
of the fat in the food; and if abnormal and easily recognisable fats
are given by the mouth, they can be identified shortly afterwards in —
the fat in the subcutaneous tissue. For this purpose, erucic acid or
fats containing iodine may be used. In man the greater part of the
fat deposited in the body is derived from the fat in food, but it can be
formed, and in herbivora is mainly formed, from carbohydrates, This
METABOLISM. 345
was demonstrated in the classical experiment of Lawes and Gilbert.
These observers took two young pigs from the same litter. One was |
killed, and the amount of fat and protein in its body was determined ;
the other was fed on a diet containing known quantities of protein,
fat, and carbohydrate, and after some weeks it was killed, and the
amount of fat in its body was ascertained.
After deducting the amount of fat taken in the food from that
- present in the animal’s body when it was killed, a large residue remained,
which must have been formed either from the protein or carbohydrate
of the food. It could not have been formed from protein, since the
amount of fat in the animal was larger than the amount of protein
food consumed. The greater part of the fat must, therefore, have
been formed from carbohydrate ; and there is no doubt that in herbivora,
whose diet consists mainly of carbohydrate, the bulk of the fat is
formed in this way. There is no evidence that fat can be formed from
protein, and animals, e.g. dogs, fed on a purely protein diet do not put
on any fat.
The process by which carbohydrate is converted into fat in the body
is not known, but it is probable that the carbohydrate is first broken
down into some simple substance such as acetic aldehyde (CH, . CHO),
or pyruvic acid (CH,.CO.COOH), which are known to be formed from
carbohydrate, and that by the linking up of molecules of these bodies
the fatty acids are synthesised. 3
Their possible formation from aldol is thus shown :—
(1) CH,. CHO+CH,.CHO=CH,.CHOH. CH, CHO (aldol)
(2) CH,.CHOH.CH,.CHO + Ho =CH,.CH,.CH,.COOH + H,O
, Butyric acid.
The fat formed from carbohydrate contains a large proportion of
stearin and palmitin, and has a higher melting point than that
usually deposited in the tissues from the food. For this reason the fat
of cattle is much firmer than that of omnivorous animals, which may
be practically fluid at the body temperature.
The fat in the fat depots cannot be used directly by the muscles or
other tissues, but has first to undergo certain changes in the liver;
these consist in the conversion of saturated into unsaturated fats by
the removal of hydrogen. A saturated fat, e.g. stearin, is one in
which the affinities of all the carbon atoms are satisfied, whereas an
unsaturated fat, such as olein, is one in which. the affinities of two or
more of the carbon atoms are unsatisfied and the carbon atoms are
united by a double bond. ‘
346 ESSENTIALS OF PHYSIOLOGY.
Thus oleic acid has the Jorinnlé: C,H... CH: CH. C,H,,.COOH, and
has one double bond.
The fatty acids present in the liver contain two, three, or even four
double bonds, each of which represents a weak spot in the long chain
of carbon atoms ; and an unsaturated fatty acid tends to split at these
points with the formation of smaller molecules. The unstable fatty
acids formed by the liver are carried to the muscles and other tissues,
in which they enter the complex protoplasm of the living cells, and are
finally oxidised to carbonic acid and water. .
During starvation the fat in the body is used by the tissues as
their chief source of energy: the liver receives more fat than usual from
the fat depéts, and is called upon to desaturate fat in larger amount and
thus render it available for use by the tissues. The fat passes to the
_ liver from the fat depdts more rapidly than it can be desaturated, and
an accumulation of saturated fat takes place in the liver ; this accumula-
tion, which is termed fatty infiltration, readily. occurs in starvation and
equally readily passes off again when food is given. It takes place, not
only in starvation, but whenever the tissues need a larger supply of
fat, for example in diabetes.
The nature of the process whereby the subcutaneous tissues and
omentum take up fat after a meal and give it off to the blood again in
times of need is not known, though it has been supposed to be due to
the reversible action of an enzyme (lipase) in the fat cells.
In a man whose weight remains steady the fat taken in the food is
rapidly used in supplying energy in the body, and little or none of it
is permanently stored in the fat depéts. When weight is being put on,
‘a certain proportion of fat in the food is stored up instead of being
oxidised, and fat may also be formed from carbohydrate. On the
contrary, loss of weight during starvation, or produced by some
other means, involves depletion of the store of fat in the body.
The fat normally stored represents a reserve of potential energy
which can be increased, or which in time of need can be drawn upon
for the use of the muscles and other tissues; and owing to its
high calorie value, the energy which may be thus kept in reserve
is very considerable.
The fats on leaving the liver pass to the tissues, in which they
become combined in such a way that they can no longer be recognised
histologically, although their presence is made manifest by chemical
analysis of the tissue. A healthy kidney, for instance, when stained
with Sudan III may show only a few specks of fat here and there,
although when analysed it is found that 18 to 20 per cent. of its dried
substance is fat. Under the influence of certain poisons (e.g. phos-
METABOLISM. 347
phorus), and in some diseases, the combination of fat with the rest
of the protoplasm is broken, and the fat is set free in such a form that
it can be stained and recognised under the microscope, This change,
which is called fatty degeneration, was formerly believed to be the
result of the formation of fat from protein. Chemical analysis of such
an organ shows, however, that it contains no more fat than would
be found in a healthy organ. Hence fatty degeneration implies, not
the formation of fat from protein, but merely the setting free in a
visible form of previously combined. fat.
The fat is normally completely oxidised in the tissues to carbonic
acid and water; the nature of the changes taking place is not fully
understood, but in all probability the long chain of carbon atoms
constituting a fatty acid is broken down -in stages, two carbon
atoms being split off at each stage. The complete oxidation of fat,
however, is dependent upon the presence of carbohydrate in the
tissue cells.
Acidosis.— When the tissues are deprived of carbohydrate, for ex-
ample during starvation or on a diet free from carbohydrate, the oxida-
tion of fat is incomplete, and intermediate metabolic products are formed
in the tissues and pass into the blood and urine. These products are
B-oxybutyric acid, aceto-acetic (diacetic) acid, and acetone, their
chemical relationship being shown as follows :—
CH, CH, CH,
| | |
CHOH CO CO
| —> | in |
OH, CH, CH,
| |
COOH COOH
B-oxybutyric acid ee Acetone
aci
There is evidence that B-oxybutyric acid and aceto-acetic acid are
normally formed in the body during the metabolism of fat, but are
fully oxidised; in the absence of carbohydrate they pass into the blood
stream, and aceto-acetic acid, instead of undergoing oxidation, is con-
verted into acetone. The origin of these bodies from fat is clearly
shown by the observation that when the diet consists solely of fat their
amount in the urine may become very large. $-oxybutyric acid and
its products are formed to some extent at least in the liver.
The presence of these substances in the blood and urine is known as
acidosis, and is an indication, whenever it occurs, that the supply of
carbohydrate to the tissues is inadequate. In the blood B-oxybutyric
acid and diacetic acid combine with ammonia which would otherwise
e
348 ESSENTIALS OF PHYSIOLOGY. .
be converted into urea, and in acidosis the amount of ammonia excreted
in the urine is increased. By this means the acids are neutralised
and the reaction of the blood remains unchanged ; and the amount of.
ammonia in the urine serves as an index of the extent to which these
acids are being formed in the body.
Test for Acetone in Urine.—A few c.c..of urine are saturated with ammonium
sulphate, and a few drops of ammonia are added ; on the addition of a drop or
two of a freshly prepared solution of sodium nitro- prusside a beautiful purple
is slowly produced, which varies in depth with the amount of acetone present
and fades after a short time. No other substance gives this test, which is
called Rothera’s test.
Test for Diacetic Acid in Urine-—On adding a solution of ferric chloride, in
excess of that required to precipitate the phosphates, the appearance of a se
red colour indicates the presence of diacetie acid.
There is no simple test for B-oxybutyric acid.
_ Since carbohydrate is readily converted into fat in the body, it
might be expected that the converse change would also occur. There
is no definite evidence, however, that fat is converted into carbohydrate,
except perhaps in hibernating animals, in which the respiratory quotient
is extremely low and may be 0°3 or 0-4.
Such a quotient could occur if fat were being transformed into
dextrose, since, in this process, oxygen would be taken into the body
which would not reappear as carbonic acid.
SECTION IIL.
METABOLISM OF CARBOHYDRATE.
The carbohydrates are absorbed from the digestive tract and enter
the blood stream mainly as dextrose, and to a small extent as fructose
and galactose. The arterial blood, when examined, is found to contain
0-1 to 0-2 per cent. of dextrose, and this amount is not increased after —
a carbohydrate meal, even though in such circumstances 100 grams
or more of sugar may be rapidly absorbed from the digestive tract into
the portal blood. The sugar in the portal blood, however, does vary in
amount, being slightly less than that in the general circulation during
starvation, and distinctly greater after a carbohydrate meal; and it
is clear that the sugar absorbed during digestion undergoes some
change as the blood passes through the liver. This change consists in
the conversion of sugar into glycogen, which is stored in the liver cells.
Preparation of Glycogen.—An animal is killed a few hours after
a meal rich in carbohydrate, and the liver is rapidly excised, chopped
into small pieces, and thrown into boiling water. After two or three
minutes the pieces of liver are taken out of the water, ground up with
ee
METABOLISM. 349
sand, and returned to the boiling water, which is made slightly acid
with acetic acid. The mixture is boiled for a minute or two and is
then filtered ; the coagulated proteins remain behind, and the filtrate,
which is free from protein, forms an opalescent solution containing
glycogen. |
Glycogen, like dextrin, gives a mahogany-brown colour with iodine, and
does not reduce Fehling’s solution ; when boiled with dilute mineral acids it is
converted into dextrose. It differs from dextrin, first, in forming an opalescent
solution, whereas a solution of dextrin is clear, and, secondly, in being pre-
cipitated more readily by alcohol. Further, glycogen is precipitated by basic
lead acetate, which does not precipitate dextrin.
If the liver is left for some hours before being treated in the
manner just described, the filtrate contains an abundance of dextrose,
but no glycogen. The amount of glycogen present in the fresh liver
varies with the previous condition of the animal and, if it has been well
fed, may form 10 per cent. of the total weight of the liver. When an
animal has been starved for a few days, and particularly if during this
period it has been made to take exercise, the liver may be almost free
from glycogen. Both in the well-fed and in the starved animal the
percentage of sugar in the arterial blood remains unaltered. These
observations can be confirmed by histological examination of the liver.
When the liver of a well-fed animal is hardened in alcohol and examined
microscopically, the cells are seen to be full of glycogen, which can
be stained with iodine; and the protoplasm may be reduced to a
network in the meshes of which the glycogen lies. An examination of
the liver of a starved animal shows that glycogen is almost or quite
absent.
From these and other observations Claude Bernard, who discovered
glycogen, concluded, first, that the sugar absorbed from the digestive
tract entered the portal blood, and that, as it passed through the liver,
this organ removed the sugar from the blood and stored it as glycogen,
Secondly, he believed that the percentage of sugar in the systemic blood
normally remained constant, and that as the sugar was removed from
the blood by the tissues for their metabolism some glycogen was made
into sugar by the liver: this passed into the general blood stream,
thereby keeping the percentage of sugar in arterial blood at the normal
level. He regarded the rapid conversion of glycogen into sugar after
death as being due to a ferment, the activity of which was no longer
controlled, as it had been during the life of the animal. This view has
been generally accepted, and glycogen may be regarded as a store of
carbohydrate which is increased at each meal, and which is continuously
being drawn upon by the tissues. In all probability the conversion of
glycogen into sugar, and of sugar into glycogen,’ is carried out by a
350 ESSENTIALS OF PHYSIOLOGY.
reversible ferment. We may perhaps regard glycogen as a sort of
current account, which fluctuates from day to day, whereas the store of
fat in the body represents a more permanent reserve, or capital account,
which can be called upon in times of stress.
Although the main source of glycogen is carbohydrate food, it can
also be formed to some extent from protein, since, when an animal is
starved until its liver is presumably free from glycogen, and is then
killed shortly after a large meal of protein, some glycogen is found in
its liver. There is no evidence that glycogen can be formed from fat.
Glycogen is found most abundantly in the liver, but it occurs
in muscles, being especially plentiful in foetal muscles, and it is also
present in the white blood corpuscles. |
FATE OF SUGAR.
The carbobydrate.taken into the body ultimately undergoes one of
two changes. Some of it, more particularly in herbivora, is converted
into fat; the remainder passes from the blood to the tissues, where it
is oxidised and used as & source of energy. There is direct evidence
that sugar is made use of by the tissues. Using the heart-lung pre-
paration (p. 199), Starling found that the normal heart used up sugar
at the rate of about 4 milligrams per gram of heart per hour,
Further indirect evidence to the same effect is furnished by the fact
that the glycogen disappears most rapidly from the liver when the
functional activity of the tissues is greatest. Thus severe muscular
exercise or the convulsions induced by strychnine lead to the rapid
disappearance of glycogen from the liver. Neither the conditions which
determine the taking up of sugar by the tissues from the blood, nor
the intermediate stages in its oxidation are fully known, but it is
probable that lactic. acid is an intermediate product in the conversion
of sugar into carbonic acid and water.
Some light is thrown on the conditions which influence the setting
free of sugar from the liver and its further oxidation in the tissues by
certain abnormal conditions in which sugar appears in the urine, and
which are known as glycosuria.
Glycosuria.—The urine normally contains slightly less than 0:1 per
cent. of dextrose, and does not reduce an alkaline solution of copper
sulphate; the term glycosuria is only used when the urine contains
dextrose in sufficient amount definitely to reduce such a solution.
This may occur in a variety of circumstances, If dextrose is injected
into the circulation or under the skin, the percentage in the blood
rises (hyperglyceemia), and the sugar is at once excreted by the kidneys.
A similar condition, known as (1) Alimentary Glycosuria, is observed
METABOLISM. z 351
when very large amounts of sugar are taken by the mouth. The in-
gestion of starch, even in large quantities, does not lead to glycosuria,
since its digestion and absorption are sufficiently slow to enable the >
liver to convert the sugar into glycogen.
| (2) Adrenalin Glycoswria.—The injection of a small quantity of
adrenalin into the circulation is followed by the appearance of dextrose
- in the urine; at the same time, glycogen disappears from the liver, and
the percentage of sugar in the blood, is increased. Evidently the
adrenalin causes the liver to discharge its glycogen into the blood as
sugar, which is excreted by the kidneys. Glycosuria may also occur
under any conditions in which adrenalin is set free into the blood
stream in larger amount from the suprarenal glands.
(3) Diabetic Punctwre.—Claude Bernard was the first to show that
puncture of the floor of the fourth ventricle in rabbits is followed by
hyperglycemia, glycosuria, and the disappearance.of glycogen from the
liver; if the animal has been previously starved to rid its liver of
glycogen, glycosuria does not follow the puncture. This experiment
was regarded by Bernard as a further proof of his theory as to the
function of glycogen in the body. The diabetic puncture fails to pro-
duce glycosuria after division of the splanchnic nerves or removal of
the suprarenal glands, and in all probability the puncture stimulates
the medulla oblongata in such a way that adrenalin is set free into the
circulation and causes glycosuria.
(4) Phloridzin Glycoswria.—Phloridzin is a glucoside, which on
hydrolysis yields glucose and phloretin. A small amount of phloridzin
or phloretin, when injected into an animal, produces glycosuria and
the disappearance of glycogen from the liver ; but phloridzin glycosuria
differs from that just described in that the percentage of sugar in the
blood is not increased, but tends rather to be diminished. Phloridzin
acts upon the cells of the renal tubules, causing the kidneys to excrete
dextrose, even when the percentage of the latter in the blood is not raised.
That it acts upon the kidneys may be shown by collecting the urine
separately from the two kidneys, and injecting a small dose of phloridzin
into one, e.g. the right, renal artery ; the urine flowing from the right
kidney is then found to contain sugar some time before it appears in
the urine from the opposite kidney. Phloridzin appears to act directly
upon the cells of the renal tubules, stimulating them to secrete sugar
from the blood, and this view is supported by the observation that the
repeated administration of phloridzin produces definite histological
changes in the cells of the renal tubules.
When repeated doses of phloridzin are given to an animal, the
glycosuria persists after the glycogen has disappeared from the liver,
: |
352 ESSENTIALS OF PHYSIOLOGY.
and even when the animal is not receiving carbohydrate food. The
sugar in this case is not derived from carbohydrate, but is formed from
protein ; this is shown by the fact that in a starving animal the ratio
of the amount of dextrose in the urine to that of nitrogen, expressed
as “ becomes constant, varying in different animals from 2:5. to 3°5,
Owing to the energy lost to the body as sugar, the tissues are compelled
to use an excessively large amount of protein as a source of energy,
and the rapid disintegration of protein increases the output of nitrogen
in the urine, the animal wastes, and its condition resembles that seen
in severe diabetes. _ 7
(5) Experimental Diabetes.—Von Mering and Minkowski were the
first to discover that the complete removal of the pancreas in dogs and
other animals is followed in a few hours by glycosuria, which soon
becomes very severe. The animals waste rapidly, the urine contains
B-oxybutyric acid and acetone, as well as dextrose, and death occurs in
one to two weeks. .During life the urine contains an excess of sugar,
and after death the liver is found to be almost free from glycogen.
These symptoms are not due to the absence of the pancreatic juice
from the digestive tract, since ligature of the pancreatic duct does not
lead to glycosuria. Nor do they occur if a small portion, one-tenth or
more, of the pancreas is left in the body, although the subsequent
removal of this fragment is followed by the train of symptoms just
described. In diabetic animals the respiratory quotient is low, and
from this it has been inferred that the muscles and other tissues are
unable to make use of and to oxidise the sugar supplied to them in the
blood, with the result that hyperglycemia occurs and sugar is excreted
by the kidneys.
Further, it is generally assumed that the pancreas furnishes an
internal secretion, that is to say, some substance which passes directly
into the blood stream, and which links the sugar to the tissue cells ;
in the absence of this link the tissues are unable to take up sugar and
therefore cannot oxidise it.
This view rests partly on the observation that while neither muscle
juice alone nor a boiled extract of pancreas alone can destroy sugar
in vitro, muscle juice to which pancreatic juice is added does destroy
sugar.
The formation of this internal secretion is attributed by many
observers to the islets of Langerhans, which are scattered throughout
the pancreas (fig. 130). The cells forming these islets differ in appear-
ance from the secretory cells of the acini of the pancreas, and contain
numerous fine granules which do not stain with eosin, as do the
a ey =r
2 METABOLISM. 353
secretory granules, but can be stained by neutral gentian. Further,
the islets do not communicate with the secretory ducts, and are clearly
not concerned with the formation of pancreatic juice.
That the islets probably take some part in carbohydrate metabolism is
shown by two observations. In the first place, ligature of the pancreatic
duct leads, eventually, to atrophy of the secreting tissue, but the islets
_ _Islet cells
~ . “egranules.
ee
zymogen granules.
Fig, 130.—Islet of Langerhans in pancreas. (From Homans, Proc. Roy. Soc.)
are unaffected and glycosuria does not occur. Secondly, it might be
expected that, if the islets produce an internal secretion essential for
normal carbohydrate metabolism, the removal of the greater part of the
pancreas would lead to over-activity of the islets still remaining in the
body, which might be demonstrated by histological or other methods ;
and this has recently been found to be the case. When the pancreas is
almost completely removed, only a small fragment being left intact, the
animal remains well for a time, but eventually developes diabetes of a
23
354 ESSENTIALS OF PHYSIOLOGY.
mild character. Subsequent examination of the small pieces of pancreas
left in the animal reveals the fact that the islet tissue has lost its
granules, and has undergone other histological changes. Jt has yet to
be proved, however, that these changes are the cause and not a result
of the diabetes. | as
The manner in which the internal secretion of the pancreas influences
carbohydrate metabolism is still obscure. Recent observations by
Starling show that the heart of a diabetic dog still possesses the power
of taking up sugar from the blood and utilising it, though probably to
a lesser degree than in the normal animal. Moreover, the respiratory
quotient in dogs, after extirpation of the pancreas, can be raised by
a liberal carbohydrate diet, an observation which suggests that their
tissues still possess some power of using carbohydrate. It is possible,
therefore, that the lessened capacity of the tissues to oxidise sugar may
not be the only factor concerned in experimental pancreatic diabetes.
(6) Diabetes in man is a progressive disease characterised by
glycosuria, and due to a gradual failure of the tissues to assimilate
and oxidise dextrose, as is evidenced by the low respiratory quotient.
At first the sugar is derived only from carbohydrate, but eventually it
is also formed from protein, which is broken down to a larger extent in
order to supply to the tissues energy, which they cannot get from carbo-
hydrate. In the later stages of the disease the urine contains B-oxy-
butyric acid and its products, often in large amount ; and the accumulation
of this acid in the body ultimately leads to poisoning of the tissue-cells,
which brings about coma and death. The blood contains an excess of
sugar, and after death the liver is almost free from glycogen ; in most
cases the pancreas shows signs of disease, but in others it appears normal.
Whether diabetes depends upon the deficiency or absence of an internal
secretion from the pancreas is not known, though from analogy with
experimental diabetes this seems very probable.
SECTION IV.
PROTEIN METABOLISM.
The products of the digestion of protein are absorbed almost entirely
as amino-acids, and there is no direct evidence that they are synthesised
into protein in the walls of the villi. On the contrary, there is no doubt
that the amino-acids enter the blood stream as such, and their presence
in the circulating blood can be demonstrated by the following means.
An artery of an anesthetised animal is connected with one end of a
series of tubes, the walls of which consist of a thin collodion membrane,
ie i fl
METABOLISM. 355
the other end being attached to a large vein. The system of tubes is
filled with saline solution, and hirudin is injected into the animal to
prevent clotting of the blood ; the blood is then allowed to flow from
the artery through the tubes and back to the vein, a continuous circu-
lation of the animal’s blood being thus maintained through the tubes.
The tubes are surrounded by normal saline solution at the body
temperature, and, as the blood flows through them, its diffusible con-
stituents, including sugar and amino-acids, pass through the collodion
wall into the saline solution and can be subsequently examined. Since
amino-acids diffuse into the salt solution, they must have been previously
present in the circulating blood
Deamination.—Immediately after their absorption amino-acids
undergo a change, which is called deamination, and which consists
in the removal of the amino-group, and its replacement by an oxygen
or hydroxyl] radicle. A simple illustration of this change is represented
in the following equation :—
CH,.CH. NH,. COOH + H,0=CH,.CHOH . COOH + NH,.
Alanine. Lactic acid.
The amino-acids are thus converted into oxy- or keto-acids, which
on reduction become ordinary fatty acids. This change is probably
brought about by an enzyme, and takes place partly in the walls of the
intestine itself and partly in the liver. Its occurrence in the intestinal
wall is shown by the fact that the amount of ammonia present in the
portal blood is increased after a protein meal; that it occurs in the
liver is proved by the observation that when amino-acids are added to
pounded liver substance under aseptic conditions, the amount of anmonia
rapidly increases. The extent to which this change in the amino-acids
takes place is not known, but a fraction of these acids enters the general
circulation without undergoing deamination. The removal of the
amino-group does not appreciably diminish the calorie value of amino-
acids or their usefulness as a source of energy to the body. Thus the
calorie value of 1 gram molecule of alanine is 389 ; if its amino group
is replaced by hydrogen, the calorie value of the propionic acid thus —
formed is 367, the difference being comparatively trivial.
FORMATION OF UREA.
The ammonia set free by the deamination of amino-acids in the
intestinal wall is carried in the portal circulation to the liver, and,
together with that similarly formed in the liver itself, is converted into
urea. If we regard the ammonia set free as entering\into combination
356 ESSENTIALS OF PHYSIOLOGY.
with carbonic acid to form ammonium carbonate, or carbamate, the
change taking place in the liver may be represented thus :—
NH NH
(1) NHL CO: _ 2H,0= COC NH
Urea.
ONH NH
(2) 00 ON! ~ Hy0 = COC NH”
The formation of urea in the liver has been proved in several ways.
In the first place, when the liver of a recently killed animal is removed
from the body and perfused with oxygenated blood to which ammonium
carbonate or carbamate is added, the ammonium salt graduaily dis- —
appears from the blood, being replaced by urea; this change does not
occur when blood containing ammonium salts is perfused through
other organs.
Secondly, it is possible to unite the portal vein with the inferior
vena cava in such a way that the blood flowing from the digestive
‘tract along the portal vein is diverted into the vena cava, and thus
into the general circulation, without going through the liver; the
fistula between the two veins is known as Eck’s fistula. The liver is
still supplied, in this case, with blood through the hepatic artery.
Dogs in which such a fistula has been made remain well when their
diet consists chiefly of carbohydrate, but when they are fed on meat or
receive ammonium salts or amino-acids by the mouth, they become
convulsed ; during the convulsions their arterial blood contains four or
five times as much ammonia as is present in a normal animal, and the
perceritage of ammonium salts in the urine rises. It may be concluded
that the liver, when supplied only with blood from the hepatic artery,
can no longer convert into urea the large amounts of ammonia entering
__ the blood after a protein meal.
7 Thirdly, in extensive disease of the liver in man the amount of
ammonia in the urine is increased, and the amount of urea is corre-
spondingly diminished. It is thus evident that the formation of urea
from ammonium salts takes place solely in the liver, probably by the
action of a synthetic ferment. 3
Urea can also be formed in other parts of the body and from other
substances than ammonium salts, the most important of these being
arginine and uric acid. Arginine occurs in the tissues, and can be
broken down by a ferment known as arginase into urea and ornithnin ;
this ferment is most abundant in the liver and kidneys.
METABOLISM. ay
ENDOGENOUS AND EXOGENOUS METABOLISM.
When the composition of the urine excreted in 24 hours by a man
taking very little protein food is compared with that of the same
person when taking an abundance of protein food,-very great differences
are observed, and are shown in the following table :— _
Abundant Percentage of Low Percentage of
Protein Diet, | Total Nitrogen.} Protein Diet. | Total Nitrogen.
Quantity of urine 1170 c.c. see 385 c.c. ile.
Total nitrogen .| 16°8 grams se 3°6 grams Ps
Urea 6 SE Oe Er + 87°5 2°2 ‘ 61°7
Ammonia ,,_ . 0°49 —Cé,, 3°0 0°42. SC, 11°3
Uric acid ,, .| O18 ,, 11 0°09, 2°5
Creatinine ,, . 0355... ,; 3°6 0°60 _—s—é,, 17°2
Total SO, . F 3°64 ,, OFS Be
Inorganic SO, .. Sad: ae, O-46i-> =}; es
Ethereal SO, . O10 sy, 6330) -+-3,
The amount of urea and sulphates in the urine is greatly
increased by a protein meal, whereas the creatinine and ammonia are
but little affected, this difference being due to the fact that the
metabolism of protein in the body is of two kinds, endogenous and
exogenous. The metabolic changes concerned in the production of
creatinine and most of the uric acid are known as endogenous
metabolism, since these substances are formed by the breaking down
of protein in the tissues as part of their ordinary wear and tear,
and are unaffected by the amount of protein food eaten, unless this
contains creatinine or uric acid., On the contrary, the amount of urea
and inorganic sulphates in the urine depends chiefly upon the quantity
of protein in the food, the urea being formed from the ammonia set _
free by the deamination of the amino-acids absorbed into the blood |
stream during digestion. It thus represents a change taking place in
the amino-acids before they reach the muscles and other tissues, and is
quite independent of the metabolic changes in the tissues themselves.
For this reason, the formation of urea and sulphates is described as
exogenous metabolism. |
The correctness of this view is proved by the rapidity with which
urea is excreted in the urine after a protein meal; the excretion of
urea begins to increase within two hours after the meal, and within
five hours half the total nitrogen taken in with the food may be
excreted as urea. It would be almost impossible for the body to
have built up the amino-acids into the living tissues, and to have
358 ESSENTIALS OF PHYSIOLOGY.
broken them down into urea within so short a time. The amount of
urea in urine, therefore, serves as an index, not of the total kata-
bolism of protein in the tissues, but of the quantity of protein taken in
the food ; and the removal of ammonia from amino-acid, and its rapid
excretion as urea, furnishes.a means by which the body rids itself of
nitrogen which is not needed, while retaining the resulting oxy-acids
as a source of energy. |
Endogenous Metabolism.—Protein, or rather the amino-acids
formed during its digestion, serves two purposes in the body.
(1) The greater part of the amino-acids, after being déaminated, is
carried to the tissues and oxidised to carbonic acid and water, thereby
serving as a source of energy.
(2) A certain proportion of the amino-acids is built up in the
tissues into living substance to replace that which is constantly being
broken down. The proteins in the tissues of different animals and of
different tissues in the same animal vary in. composition; and the
synthesis in each tissue of its characteristic proteins is made possible
by the previous disintegration of the proteins in the food into their
ultimate constituents, namely amino-acids. These acids are often
spoken of, therefore, as ‘building stones” which can be put together
in varying combinations in the building up of the tissues of the body.
For this purpose certain amino-acids, or groupings of such acids, are
essential, and cannot be manufactured in the body; among these are
tryptophane, tyrosine, and phenylalanine. Substances, such as gelatine,
which do not contain these groupings cannot act as tissue builders in
the body, and therefore cannot maintain life. Again, zein, which is
a protein in maize, contains no tryptophane ; and animals fed on this
protein, with the addition of starch and fat, rapidly waste, and die
after a short time, though life can be prolonged by the addition of
tryptophane to this diet.
The groupings essential to life probably include certain combina-
tions of amino-acids in the form of polypeptides. These groupings
are not destroyed during pancreatic digestion, and an animal can be
kept in good health when fed solely on the products of prolonged
pancreatic digestion of protein, with the addition of fat, carbohydrate,
salts, and water. When protein is broken down by prolonged boiling
with dilute mineral acid, animals fed on the amino-acids thus set free,
together with fat, carbohydrates, salts, and water, lose weight and die.
Evidently pancreatic digestion leaves intact, and hydrolysis by acid
destroys, some polypeptides which the tissues cannot form for them-
selves, although their nature cannot be determined by chemical analysis
of the products formed in the two cases.
eae Se
ee
METABOLISM. 359
Further, some of the amino-acids seem to be necessary for the
production of certain definite substances in the body, one of which |
perhaps is adrenalin.
Comparatively little is known as to the intermediate stages in the
breaking down of protein in the tissues, though the composition of
the urine during starvation shows that the end products include urea,
uric acid, and creatinine.
PURINE METABOLISM.
The nucleo-proteins of the food are broken down in the digestive
tract first into nuclein and protein, the nuclein subsequently under-
going further digestion with the setting free of nucleic acid, which is
absorbed unchanged. Nucleic acid, when hydrolysed, is found to
consist of the following bodies:—(1) phosphoric acid, (2) purine
bases, guanine and adenine, (3) pyrimidine bases, and (4) a carbo-
hydrate which is usually a pentose. The same products are yielded by
the disintegration of the nucleins present as nucleo-protein in the tissues.
The nucleic acids found in the different tissues vary in composition,
and do not necessarily contain all the constituents just mentioned.
The purine bodies are all derivatives of a substance called purine,
C.H,N,, which has the constitutional formula :—
‘N= CH
ae | C_NH
| >CH
N—O_N
Purine itself is of purely theoretical interest, but five of its de-
rivatives are found in the body, namely :—
Hypoxanthine (monoxy-purine) ; ; C,H,N,0O.
Xanthine (dioxy-purine) . ‘ epee, pth aN gos
Adenine (amino-purine) C,H,N,. NH,.
Guanine (amino-oxy-purine) . OC,H,N,O.NH,.
Uric acid (trioxy-purine) . Ԥ (CANO,
After its absorption the nucleic acid taken as nucleo-protein in the
food is broken down by a series of enzymes, called nucleases, which
are found:in many tissues, notably the liver and spleen, first into
complex groupings called nucleotides, and then into adenine, guanine,
and other bodies. Other ferments subsequently convert adenine and
guanine by a process of deamination into hypoxanthine and xanthine.
Finally, a third set of enzymes oxidise hypoxanthine to xanthine, and
~ 360 } ESSENTIALS OF PHYSIOLOGY.
the latter to uric acid. Uric acid is thus the end product of the
action of these enzymes on nucleic acid. The whole of the uric acid
formed in this way is not excreted as such in the urine, since many
tissues, more especially the liver, contain wurzeolytic enzymes, which
break down uric acid, one of the products being urea.
The amount of uric acid appearing in the urine does not necessarily
represent the whole of that formed from the nucleo-proteins of the
food, but is derived partly from them, forming exogenous uric acid,
and partly from the breaking down of the nucleins in the tissues,
endogenous uric acid. It will be seen from the table on p. 357 that, as
a rule, half the uric acid in the urine is of endogenous and half is of
exogenous origin.
When the diet is free from nucleo-protein, the excretion of endo-
* genous uric acid is extremely constant, but it is increased after severe
muscular exercise and also in fever, owing to a greater breaking down
of the nuclei in the cells of the body.
The exogenous fraction varies in amount with the character of the
diet, being absent when this contains no nucleo-protein, and increased by
food such as kidney, sweetbread, and meat, which are rich either in
nucleo-protein or in the precursors of uric acid, such as hypoxanthine.
Nucleo-proteins are not only broken down, but can also be
synthesised in the body. In the growing infant, for example, nucleo-
protein is rapidly being laid down in the body, although the food
(milk) contains hardly any nucleo-protein.
SECTION V.
The separate consideration of the changes undergone by fat, protein,
and carbohydrate, though convenient, represents very imperfectly the
complex nature of the metabolic changes in the body as a whole.
These are greatly influenced both by the nature of the diet and by the
absence of one or more of the food-stuffs from the diet.
Starvation.—The metabolism during starvation has been studied in
professional fasting men and also in the lower animals. When food is
withheld the store of glycogen in the body is rapidly used up, and after
two or three days the animal derives its energy solely from fat and
protein. The metabolism as a whole is diminished, and the consumption
of protein is reduced as far as possible, most of the energy needed by
the body being obtained by the oxidation of the fat previously present
in the fat depots. After three or four days the output of nitrogen in
the urine reaches a low level, which continues until the body fat
has been used up, and the sole source of energy is the tissue protein.
_ METABOLISM. Pee)
When this stage is reached, the output of nitrogen in the urine shows
a sudden rise for a day or two, being followed by a rapid fall in the
excretion of nitrogen and by the death of the animal.
The body loses weight, the loss falling most heavily on the less
vital organs such as the muscles, whereas the heart and central nervous
system lose little or no weight. The breaking down of protein during
starvation is probably brought about by a process of autolysis or- self-
digestion in the tissues; the amino-acids formed by the disintegra-
tion of the less important tissues are carried in the blood stream and
made use of by the vital organs. A similar process of autolysis has
been observed in the salmon ; during its stay in fresh water the salmon
takes no food, and the development of the sexual organs, which takes
place during this period, is effected at the expense of the skeletal
muscles, which undergo autolysis. In man the character of the meta-
bolism of fat and protein is modified during starvation, as is shown by
the appearance of creatine and of 6-oxybutyric acid in the urine.
The daily excretion of nitrogen in the urine during starvation
amounts in man to 10-12 grams, and it might be expected that if
this amount of nitrogen were taken in the form of protein food, it would
be used in replacing the daily disintegration of tissue in the body, and
would not appear in the urine. Experiment shows, however, that in
this case almost the whole of the nitrogen taken in the protein meal
appears in the urine in addition to that which was previously being
excreted, so that the disintegration of protein is still going on. When
protein is the sole article of diet, it is necessary, in fact, to give by the
mouth a quantity of protein containing 3 to 5 times as much nitrogen as
that which was previously being excreted during starvation in order to
obtain a balance between the intake and output of nitrogen. This
balance is called nitrogenous equilibrium. A further increase in the
amount of protein taken by the mouth does not lead to a retention of
nitrogen in the body, but the arnount of nitrogen excreted increases until
nitrogenous equilibrium is again reached at a higher level than before.
| D | Intake of Nitrogen | Output of Nitrogen
ay: as Protein. in Urine.
|
|
|
80 grams 80 grams
|
ee
2 ae 194°.
4 238, 220 ,,
6 Bagh. 998"
8
238 =, Woke acer
362 ESSENTIALS OF PHYSIOLOGY.
In the experiment recorded in the foregoing table the animal on the
first day was in nitrogenous equilibrium. When it received three times
as much protein, it again reached nitrogenous equilibrium in ‘the course
of the next seven days.
If the food does not consist solely of boasts but also contains fat
and carbohydrate, most of the energy of the body is derived from the
latter, and nitrogenous equilibrium can be maintained on a comparatively
small amount of protein; in this case the protein is used mainly to
repair tissue waste.
The Sources of Muscular Saskey: —The muscles form about 40 per
cent. of the body weight and furnish the greater part of the energy set
free as heat or work in the body. This energy is derived from all the .
food-stuffs, and the fact that the respiratory quotient remains almost
unaltered during muscular exercise shows that these food-stuffs must
be used nearly in the same proportions during exercise as during rest.
Respiratory Quotient in
Man.
Nature of Diet.
Rest. Work.
(1) Rich in carbohydrate . : 0°85 0:90
(2) Poor in carbohydrate . 0°79 0°82
That being the case, it would be expected that the breaking down of
‘protein in the muscles and the consequent excretion of nitrogen in the
urine would be increased by exercise, but observation shows that the
amount of nitrogen in the urine is practically unaffected, even by
severe exercise. The most probable explanation of this apparent
anomaly is that the nitrogenous moiety of muscle protein is resynthesised
in the muscle, and therefore does not appear as a waste product,
Although the muscles make use of all the food-stuffs, they derive
their energy, both when resting and when active, principally from the
food-stuff which is most abundantly supplied to them in the blood. If
the diet consists mainly of fat or carbohydrate, these furnish the chief
source of muscular energy, and but little energy is derived from protein ;
when protein is the principal or sole food, it does serve as a source of
energy, and the nitrogen in the urine is increased. This has been clearly
shown in dogs which were made to do work when fed entirely on lean
meat ; their bodies contained hardly any fat or carbohydrate, so that
protein must have been used as the chief source of muscular energy.
METABOLISM. 363
» >on
“* . er hts
sie 7
SECTION VI. ‘
THE LIVER.
ot oo oa. Pa
The liver consists of an enormous number of lobules, each having
a diameter of about 1 mm.; they are roughly pear-shaped, and show
facets on the surface from mutual compression of adjacent lobules.
The stalk of the pear is the point of emergence of a vein, the itra-
lobular vein, which occupies the centre of a transverse section of the
lobule. The substance of the lobule is composed of columns of cells,
i arranged radially in relation to the intralobular vein. The lobule is
} _ surrounded in the pig’s liver by a well-marked capsule of connective
: tissue, containing ¢nterlobular veins as well as branches of the hepatic
: artery and the smaller bile ducts. The connective-tissue capsule is
: continuous with a sheath containing the portal vein, hepatic artery, and
: bile duct, which enters at the hilum of the liver and is known as
Glisson’s capsule. This sheath with the contained vessels is called the
portal tract. The interlobular veins are branches of the portal vein,
and the blood passes from them to the intralobular vein in each lobule
through sinusoids, which lie between the columns of liver cells. The
sinusoids are wider than capillaries, and their walls are incomplete.
The hepatic artery also opens into the sinusoids, supplying oxygenated
blood for the nutrition of the liver cells. The intralobular vein opens
into a sublobular vein, and the sublobular veins unite to form the
tributaries of the hepatic vein.
A liver cell is roughly cubical in shape and contains a large spherical
nucleus. Its protoplasm is granular, and in the well-fed animal contains
accumulations of glycogen, which in the fresh or alcohol-hardened liver
can be stained brown with iodine. The cell contains iron in organic
combination, which can be demonstrated by treatment with dilute
hydrochloric acid and ferrocyanide of potassium ; a blue colour is pro-
duced. Small droplets of fat may also be present in the cells. Each
cell is penetrated by fine canaliculi which are continuous with.the bile
capillaries, and cavities are also described which communicate with the
sinusoids. On the side of each cell which abuts on the adjacent cell is
| a channel which, with the corresponding channel on the neighbouring
| cell, forms a bile capillary. The bile capillaries form a network, the
| contents of which flow into the small bile ducts at the periphery of the
: lobule. The ducts are lined by cubical epithelium.
Most of the functions of the liver have already been considered in
connection with digestion or metabolism, and it is only necessary at
this point to summarise these functions. ‘
ae
|
on | ae
.
-364 ESSENTIALS OF PHYSIOLOGY.
The liver plays an important part in the metabolism of all three
classes of food-stuffs, especially in the preliminary changes which they
undergo after absorption. In the first place, it serves as a store-house
for glycogen, which it forms from the carbohydrate absorbed during
digestion, and which it converts into dextrose and returns to the blood
stream in order to keep constant the percentage of sugar in the blood.
Secondly, it desaturates the fatty acids reaching it from the fat depdts,
and prepares them for the further metabolic changes which occur in
the tissues. Thirdly, it removes the amino-group from part of the
amino-acids absorbed from the digestive tract, converting them into
keto- or oxy-acids and transforming the ammonia thus set free, and
also that reaching it from the portal vein, into urea.
Interference with these functions, which sometimes occurs in
extensive disease of the liver in man, leads to the appearance of inter-
mediate metabolic products such as leucine, tyrosine, and other sub-
stances in the urine, and to disturbance of the normal course of the
metabolism of fat; similar effects are seen in animals poisoned with
phosphorus, which greatly reduces the metabolic activity of the liver.
The importance of the liver is further shown by the fact that the
complete cutting off of its blood supply in mammals is followed by
death within a few hours.
In birds, however, the portal system communicates with branches of
the renal vein, and so with the systemic venous system ; and birds may
live for three or four days after the removal of the liver. In these
circumstances uric acid, which is the normal end product of nitrogenous
metabolism in birds, is largely replaced in the urine by ammonia and
lactic acid, which in these animals are the precursors of uric acid.
Apart from its metabolic activities, the other functions of the liver
are (1) the secretion of bile, and (2) the conversion of the blood
pigment into bile pigment.
CHAPTER XII.
ANIMAL HEAT.
From the point of view of their temperature, animals fall into two
groups, namely (1) potkilothermic, or cold-blooded animals, whose
temperature varies with that of their surroundings, and (2) homoio-
thermic, or warm-blooded animals, whose temperature remains constant
except for slight daily variations, and is independent of that of their
surroundings.
To the former group belong fishes and amphibia, to the latter birds
and mammals, including man. In man the normal temperature is
37° C. (98°4° F.); it shows a daily variation of approximately 1° F.,
being highest in the afternoon and lowest in the early morning. It
is lowered by starvation or prolonged lack of sleep, and is raised by
muscular exercise. The constancy of the temperature is due to the
fact that heat production and heat loss balance each. other.
Production of Heat.—The chemical ¢hanges in the body which
constitute metabolism involve the production of heat, and this takes
place chiefly in the muscles, and to a smaller extent in the liver and
other glandular organs; the blood leaving the liver, for instance, is
warmer than that entering it. The production of heat in the glandular
_ organs depends mainly on the variations of their activity associated
with the digestion of food, and is comparatively constant from day to
day. The heat formed in the muscles, however, varies greatly, being
enormously increased by muscular exercise; and in warm-blooded
animals the amount of heat formed in the body depends largely upon
changes in the activity of the skeletal muscles. As has been pointed
out (p. 29), the greater part of the energy set free during muscular
contraction appears as heat.
The heat formed in the body can be measured by means of a
calorimeter, the most suitable form of which for man is the Atwater-
Benedict calorimeter (p. 340).
Loss of Heat.—Heat is lost from the body satus dleeogs the
365
: |
366 ESSENTIALS OF PHYSIOLOGY.
skin, and to a smaller extent in warming the expired air and the
excreta. On the average, 77 to 80 per cent. of the heat loss takes
place through the skin, 17 to 20 per cent. is lost from the lungs, and
3 per cent. in the excreta. The total daily loss varies greatly with the
conditions under which the individual is living, being as a rule from
2500 to 3500 calories.
The skin consists of two layers, a superficial layer, the epidermis,
consisting of stratified squamous epithelium, and a deeper layer, the
dermis, formed of fibrous tissue. The dermis rests upon connective
tissue of a looser texture, which is called the subcutaneous tissue and
contains a variable amount of fat. |
The epedermis consists of two principal layers, a deeper layer of cells
called the rete mucoswm, and a superficial layer known as the stratum
corneum or horny layer. The cells of the rete mucosum are mostly
irregular in shape and are connected by protoplasmic bridges, between
which are tiny channels along which lymph flows for the nourishment
of the cells. |
In the horny layer a transformation has occurred whereby the
protoplasm has been converted into keratin; at the same time the
cells have become flattened and scaly and possess no visible nuclei.
Between the stratum mucosum and the horny layer can be seen two
narrow layers, stratum granulosum and stratwm lucidum, in which the
cells are undergoing transformation. The surface cells of the epidermis
are continually being shed, and are replaced by the multiplication and
subsequent alteration of the cells of the rete mucosum.
The dermis consists of dense fibrous tissue which presents papillee
or projections on its surface. The epidermis is moulded on these
papillee, and where they are arranged in rows, as on the palmar surface
of the hand and-fingers, the epidermis shows corresponding ridges.
Blood-vessels run in the dermis and form capillary loops in the papillee.
Lying near the junction of the dermis and subcutaneous tissue over .—
the whole surface of the body are the sweat glands, consisting of coiled
tubes, the ducts of which run through the dermis and open into cork-
screw-shaped channels in the epidermis leading to the surface.
The skin is protected and kept supple by sebum, which is a fatty
material secreted by the sebaceous glands. These glands are found
wherever hairs are present, and their ducts open into the upper part
of the hair follicles. Each gland is composed of a solid mass of cells,
in the central part of which the cells are loaded with fat and the proto-
plasm has largely disappeared. The fatty material in sebum is not a
true fat, but consists chiefly of fatty acids combined with cholesterol.
The secretion of sebum is always taking place, the semi-liquid
ANIMAL HEAT. . 367
central part of the gland being squeezed on to the surface of the skin
whenever the hairs are erected by the contraction of the arrector pili
muscles. The latter are composed of unstriated muscular fibres, and —
are attached to the hair follicle and to the epidermis: The sebaceous
gland lies between the muscle and the hair follicle.
The Sweat.—Sweat is a clear, colourless fluid containing 99 per cent.
of water ; sodium chloride is the most abundant solid constituent, and
traces of proteins and of urea may also be present.
The secretion of sweat is under the control of the central nervous
system, the nerves to the sweat glands belonging entirely to the
sympathetic system. Leaving the spinal cord by the anterior roots,
they pass to the ganglia of the lateral sympathetic chain, where they
have their cell station ; from these ganglia non-medullated fibres enter
the grey rami, and run with the spinal nerves to their peripheral dis-
tribution. Sweating is usually brought about by a rise in the body
temperature, and it generally begins as soon as the temperature of
the body rises from 4 to 1° C. above the normal. In this case the
effective stimulus is the raised temperature of the blood passing
through the brain; and sweating may be produced by warming the
blood passing through the carotid artery to the brain, even though the
temperature of the rest of the body remains unchanged.
Sweating may also be produced reflexly by the local siplicatibn: of
heat to the skin, so that one arm, if warmed, may sweat, and not the
rest.of the body. It is not necessarily associated with increased vascu-
larity of the skin, and may occur, when the sympathetic fibres are
stimulated, even in an amputated and thefefore bloodless limb.
Hence the skin not only protects the delicate underlying structures
and serves as a sense organ, but by means of the secretion of sweat
plays an important part in effecting the loss of heat from the body.
The loss of heat from the skin is brought about by radiation, con-
vection, and evaporation. Heat passes by convection to the air, or to
articles of clothing in contact with the body; it is also lost from the
exposed surfaces of the body by radiation to objects at a distance. The
loss thus taking place is greater when the blood-vessels of the skin are
dilated and the skin is flushed than when the vessels are constricted.
More important than either of these is the loss of heat by the
evaporation of sweat, which is continually being formed on the surface
of the skin. When the amount of sweat is small it evaporates so
quickly as to be unnoticed, the process being called insensible per-
sptration. When the amount formed is increased, or its immediate
evaporation is prevented, it becomes visible onthe surface of the
skin as sensible perspiration. .
GO ie PA Ye
& } iF & i> ‘
3 £7 O rhs
368 ESSENTIALS OF PHYSIOLOGY.
In the process of evaporation much heat becomes latent and is lost to
the body ; and the rate at which this loss takes place may be increased
either by greater formation of sweat or by hastening the rate of
evaporation by exposing the body to a current of air.
Conversely the loss of heat in this way is checked when an indi-
vidual is surrounded by air which is already nearly saturated with
moisture. Owing to the heat taken up by water as it evaporates, heat
continues to be lost even when the temperature of the surrounding
air is higher than that of the body, provided the air is dry; and in
tropical climates the loss of heat from the skin takes place chiefly by
evaporation.
When sweating is very profuse, the amount of heat lost by the skin
relatively to that lost through the lungs is increased, whereas, when the
skin is cold and perspiration is scanty, the reverse is the case. In dogs,
in which, owing to their hairy coat and paucity of sweat glands, loss of
heat by evaporation is comparatively slight, an increase in the loss of
heat is largely effected by increased respiratory movements.
The Regulation of Temperature.—In cold-blooded animals the
amount of carbonic acid given off from the body varies directly with
the temperature of their surroundings ; and their metabolic activities,
including the production of heat, rise with the temperature, resembling
in this respect chemical reactions in the laboratory, which are
accelerated at a higher temperature. These animals possess no regula-
tive nervous mechanism by which they can counteract the effects
of heat or cold. When the surrounding temperature falls, their
metabolic activities diminish until they sink into a state of torpor.
When the temperature rises, their metabolic activities increase, and
their only means of evading the ill effects of an unduly high tempera-
ture is to hide in a stream or to burrow into moist earth.
The maintenance of a constant temperature in warm-blooded
animals is effected by an exact adjustment through the central nervous
system of the heat production and heat loss. That the production of
heat takes place mainly in the muscles and is under the control of the
central nervous system is shown by two observations. In the first
place, when the motor nerve endings are paralysed by curare so that
the muscles are cut off from nervous influences, the animal behaves
like a cold-blooded animal. Secondly, when the spinal cord is injured
in man or in the lower animals in such a way that the-lower part of
the body no longer receives impulses from the brain, this portion
becomes poikilothermic. When it is warmed, its metabolism becomes
more active, and the heat produced warms the blood passing through
it, and may be sufficient to raise the temperature of the whole body
ANIMAL HEAT. - 369
several degrees. When it is cooled, its metabolism is diminished, and
the temperature of the body may be lowered in spite of the fact that
the rest of the body still possesses its regulative mechanism. .
The loss of heat from the body depends upon the amount of blood
passing through the vessels of the skin and upon the amount of sweat
formed, both of these being under the control of the central nervous
system. The maintenance of the body temperature when the surround-
ing air becomes colder might be effected either by an increased produc-
tion of heat or by a diminished loss of heat. In many animals the
adaptation is brought about by changes in the production of heat, more
heat being evolved; in man the adjustment is made in a more
economical manner chiefly by variations in the heat loss, and to a much
smaller extent by alterations in the production of heat. On a cold day
the vessels of the skin are constricted, so as to diminish the loss of heat
by radiation and convection, and the formation of sweat is scanty.
Conversely, on a hot day the skin is flushed and moist, and the loss of
heat is more marked.
Within moderate limits of external temperature the production of
heat varies but little, though it is diminished when the surrounding
temperature becomes high. During muscular exercise both heat
production and heat loss are increased, the production exceeding the
loss, so that for a time the temperature rises above the normal level.
The intimate relation between heat production and heat loss is also
shown in the relationship between production of heat and the size of the
animal. The greater loss of heat relative to its weight which occurs in
a small animal is met by a correspondingly larger production of heat,
with the result that the animal’s temperature remains constant. In
many animals, including man, the regulative mechanism is not fully
developed at birth ; and the temperature of the new-born infant falls
unless an excessive loss of heat is prevented by keeping the child in a
warm atmosphere.
The part of the nervous system which regulates the production and
loss of heat and keeps the temperature constant is not known, though
possibly it lies in the corpus striatum, injuries to which have been
found to cause a marked rise of temperature. Wherever its seat may
-be, the mechanism is so perfect that in man the temperature remains
constant, whether he lives in the tropics or in the arctic regions,
though the adjustment fails when the heat or cold is extreme. When |
a man is exposed to excessive cold, the temperature gradually falls till
consciousness is lost and finally death supervenes. When the surround-
ing temperature is extremely high, and particularly if loss of heat by
sweating is interfered with, the temperature of the body rises, pro-
24
*
370. ESSENTIALS OF PHYSIOLOGY.
ducing the condition of heat stroke. This occurs more readily if the
heat production is also increased, for example by muscular exercise, or if
the evaporation of sweat is checked by a humid atmosphere; in these
circumstances the regulative mechanism may fail even though the
surrounding temperature is not very high.
In fever the temperature of the body is raised, the regulative
mechanism again bringing about a balance between heat production
and heat loss, but at a higher level than in the normal person. Owing
to the raised temperature of the body metabolism is more rapid, the
breaking down of the tissues is increased, the output of nitrogen in the |
urine rises, and a loss of weight generally takes place.
CHAPTER XIII.
FOOD AND DIET.
' Tue substances used as food by man and animals contain protein, fat,
carbohydrate, salts, and water; and in order to construct a suitable
diet for man, it is necessary to know first what amount of these
substances in the food best meets the needs of the body, and secondly,
the composition of the different food-stuffs.
The mere composition of the food-stuffs, however, is an uncertain
guide to their true nutritive value, since this depends upon the ease
with which they can be digested and assimilated; it is important,
therefore, that the food should be palatable and digestible.
The food is derived either directly or indirectly from vegetable
substances which are synthesised by plants from inorganic materials, the
energy of the sun’s rays being used in the process. The kinetic energy
of the sun’s rays is thus transformed into the potential energy of the
organic food-stuffs, and when these are consumed their potential energy
is again converted into kinetic energy as heat and muscular work.
DIET.
From this point of view, the body may be regarded as a machine
which converts potential energy into the kinetic energy of muscular
work and heat, the daily loss of kinetic energy being replaced by the
potential energy of food. The living tissues also undergo a constant
wear and tear, the tissue which is broken down being replaced by the
building up of fresh tissue from the digested food stuffs. A suitable
diet must thus fulfil two functions. On the one hand, it serves as a
source of energy, and, on the other, it contains the constituents
necessary to replace the breaking down of the tissues.
(1) Diet as a Source of Energy.—Calculating work in terms of
heat, it is found that the daily loss of energy in man in the form of:
heat and work is usually about 3000 large calories. It is less in those
who lead a sedentary life, and may be increased by Severe exercise to
37!
a7 °>> ESSENTIALS OF PHYSIOLOGY.
4000-5000 calories or even more. As we have seen (p. 339), the
average physiological calorie value of the food-stuffs is as follows :—
1 gram fat = 93 calories
1 ,, carbohydrate=4'1 __,,
1 ,, protein =f)
From these data it is easy to draw up a diet containing the food-stuffs
in such amount that, when oxidised in the body, they will furnish
sufficient energy to replace the daily loss. Such diets have been
constructed as the result of observations on individuals living in
institutions under similar conditions of work and surroundings. The
following represents the proportions of the different alimentary prin-
ciples which have been found most suitable :—
Protein 120 grams= 492 calories
Fat CO S55 5a OD
Carbohydrate 500 ,, =2050 _,,
3100
The calorie value of this diet is 3100, but a deduction of at least
5 per cent. must be made for food which, though taken by the mouth,
is not absorbed from the digestive tract, being lost to the body in the
excreta. The same calorie value could be obtained by combinations of
these three food-stuffs in other proportions, and, regarded merely as a
source of energy, it seems to be a matter of indifference in what form
the calorie value is supplied to the body.
_ (2) The Replacement of Wear and Tear.—In order to replace the
breaking down of the tissue proteins, the diet must contain a certain
minimum of protein, and much. discussion has arisen as to the amount
of protein in the food which is most suitable for the needs of the body.
Chittenden has put forward the view that the amount of protein
consumed by most people is excessive. He considers that it can be
largely replaced by fat and carbohydrate as a source of energy, and
that a domparatively small amount of protein is needed to repair tissue
waste and to maintain nitrogenous equilibrium. Any excess of protein
beyond this minimum is regarded by him merely as throwing additional
work on the liver and kidneys in excreting its nitrogen. Chittenden
found by observation on himself and others that it was possible to
maintain health and nitrogenous equilibrium for six to eighteen months,
and to carry out muscular work, on a diet containing much less protein
than that in the dietary mentioned above; in many cases the daily
intake of protein did not exceed 40 to 60 grams.
FOOD AND DIET. Bee
His views have not met with general acceptance. In the first place,
individuals living on such a low protein diet often suffer in general
health, and in their ability to resist infection. Secondly, the diet of
the infant contains much more protein than the minimum which would
be necessary according to Chittenden’s view, and, since nature provides
more than the minimum of protein, the minimum is probably not the
optimum. Thirdly, the protein taken in the food does not merely
replace tissue waste, but supplies certain complex chemical groupings
which the body cannot make for itself; and it may be necessary that,
in order to obtain a sufficient amount of these groups, the body should
be supplied with a quantity of protein considerably in excess of the
minimum required to replace tissue waste. Such a grouping, for
instance, might be tryptophane. Although Chittenden’s work has
been of value in showing that many people eat too much protein, yet
the actual amount needed by the body is probably much higher than
that taken in his experiments; and we may regard 110 to 120 grams
of protein daily as representing the optimum intake for most men.
The protein requirements of women are rather less, being from 90 to
100 grams. Under conditions of severe muscular stress, for example
in soldiers during war time, considerably larger amounts of protein
may be necessary to maintain health.
Although the diet in man usually conforms in a general way to the
principles just laid down, it must be remembered that wide individual
variations occur and are compatible with health; and it is impossible
to formulate any arbitrary laws as to diet.
In addition to serving as a source of energy and to supplying a
sufficiency of protein, the food normally contains certain substances,
essential to health and even to life, which have been called witamines.
Their existence was first shown in connection with the disease known
as beri-beri, which is characterised by nervous symptoms and by
wasting ; it occurs in individuals whose diet consists solely, or almost
solely, of ‘‘ polished” rice, that is rice from which the husk has been
removed. A similar condition is produced in fowls, when placed on
this diet, and can be cured by the addition to the diet of a substance
extracted from the husk of rice; this substance is not a protein, but is
probably a basic nitrogenous body, and very small amounts of it are
sufficient to relieve the symptoms in birds. In man, beri-beri is cured
or prevented by a diet of unpolished rice, or by the addition of yeast,
meat juice, or other substances to the food.
It is probable, though not proved, that other diseases, such as
-seurvy, are also brought about by the absence from the food of
vitamines which are present in fresh milk, lemon juice, and meat juice.
e
374 | ESSENTIALS OF PHYSIOLOGY.
The vitamines in milk are destroyed by prolonged boiling, and the
condition of scurvy rickets, which occurs in infants, may be due to their
being fed on a diet devoid of, or deficient in, vitamines,
Diseases which are brought about by the absence from the diet of
some essential constituent are called “deficiency diseases”; they do
not occur when the diet is sufficiently varied aad ~oaening fresh,
uncooked food.
The presence in the food of small traces of certain substances seems
also to be necessary for growth in young animals. It has been found
that when young rats are fed upon an artificial milk containing
perfectly pure caseinogen, fat, and milk sugar in the same proportions
as in milk, together with salts and water, the animals fail to grow,
although their diet is adequate both as a -source of energy and as ~
regards the amcunt of protein present. On the addition to the diet of
very small quantities of fresh milk, growth takes place in a normal
manner. Evidently the natural food contains something essential to
growth, which is removed in the purification of the constituents of the
artificial milk ; the nature and mode of action of these substances is
quite unknown, 3
We see, therefore, that in order to maintain health the diet must
fulfil the following conditions. In the first place, it must provide
sufficient potential energy to replace that lost as work and _ heat.
Secondly, it must contain sufficient protein to replace the breaking
down of the tissues, and to provide the complex chemical groupings
which the body cannot make for itself. Thirdly, it must contain the
substances known as vitamines.
Salts and water also must be present in the diet, although they
do not supply energy. Further, in young growing animals the diet,
especially protein food, must be relatively more abundant than in
adults. Not only is metabolism more active in young animals, but an
additional amount of protein is required to provide material for the
laying down of new tissue during growth.
THE COMPOSITION OF FOOD-STUFFS.
MILK,
Milk contains all the alimentary principles of a dietary, combined in
the proportions necessary for the early stages of life, The proportions
of the constituents of milk vary somewhat with the species, as will be
seen in the following table :— .
FOOD AND DIET. 375
Woman. Ass. Cow.
| Proteins — 15 1°9 3°5
| Fats 3°5 14 4:0
_ Lactose 6°5 6°3 4°5
Salts 0:2 0-4 0-7
| ‘Water . 88°3 90°0 | 87°3
|
By a comparison of these figures, it will be seen that cow’s milk
contains too large a proportion of protein and fat, and too small a
proportion of lactose, for the human infant. It is therefore necessary,
if an infant is fed on cow’s milk, to dilute the latter with water and to
add lactose in order to obtain the correct proportions.
Fresh milk has a specific gravity of 1028 to 1034, and is neutral to
litmus. Microscopically, it consists of small fat globules floating in an
almost colourless fluid, that is, it is a permanent fine emulsion. The
globules appear to be prevented from running together by the proteins
of the milk forming a fine pellicle on the surface of each globule.
The proteins of milk are caseinogen and lactalbumin. Caseinogen
is a phosphoprotein, and is insoluble in water, but soluble in dilute
alkalies. It exists in milk as a compound with calcium, and is pre-
cipitated by the addition of acetic acid, the precipitate being soluble in
excess of the acid. The precipitate of caseinogen obtained from milk
by the addition of acetic acid carries down the fats entangled with it,
and may be purified from these by washing with ether. When purified,
it is a white powder. Lactalbumin remains in the filtrate when the
caseinogen and fats have been filtered off. If the excess of acid in the
filtrate be almost neutralised so that only a trace of acidity remains,
the lactalbumin may be coagulated by heating the fluid.
The fats of milk consist mainly of tripalmitin, tristearin, and triolein.
There are, in addition, small quantities of fatty acids lower in the
scale-—myristic, caproic, caprylic, capric, and lauric. Lactose is the
carbohydrate present in milk. It is a disaccharide, C,,H,,0,,, and
reduces an alkaline solution of copper sulphate on boiling. It is
not fermented by ordinary yeast, and in this way it can be distinguished
from dextrose. It can be obtained from the filtrate from milk, after re-
- moval of the proteins and fats, by slow evaporation, when it crystallises
out. The enzyme lactase, by which lactose is converted during digestion
into dextrose and galactose, is especially abundant in young animals.
The salts of milk consist chiefly of phosphates and chlorides of
potassium, sodium, calcium, magnesium, and iron, calcium phosphate
376 ESSENTIALS OF PHYSIOLOGY.
being most abundant. Analyses have shown that these salts are
present in the milk of any one species in exactly the same proportions
in which they occur in the young animal which is nourished on that
milk. The large proportion of calcium phosphate is of especial
importance in view of the formation of bone in the growing animal.
When milk is allowed to stand it becomes sour. The acidity is due
to the formation of lactic acid from the lactose by the agency of certain
organisms, such as bactercwm lactis, present in the milk. The growth
of these germs is facilitated by warmth. The lactic acid has the same
effect on the caseinogen as the addition of acetic acid-; that is, it
precipitates the caseinogen, and the latter entangles the fats, forming a
curd. The precipitation of caseinogen in this way must not be con-
fused with the clotting of milk which is brought about by ferment
action in the stomach (p. 304). In the latter process the caseinogen
undergoes a chemical change, being converted into casein.
The suitability of the maternal milk for the needs of the growing
animal does not depend only on the fact that the various constituents
are in the correct proportions. As has already been pointed out, every
protein consists of a characteristic grouping of amino-acids, some of
these acids being present to a special extent in one protein, and others
in another. Caseinogen is remarkable in that all the amino-acids
which enter into the composition of the various proteins are represented
in its structure to a greater or less extent, so that it may form a source
from which any of the body proteins may be built up. Only glycine
is absent from the caseinogen molecule, and glycine can be formed in
the body by hydrolysis of the higher acids. 3
. The caseinogen of human milk does not forma firm clot with the
“rennet” ferment as does that of cow’s milk, but is thrown down in
the form of a flocculent precipitate. For this reason cow’s milk, even
when diluted, does not form a satisfactory substitute for maternal milk
in the case of the human infant. Other drawbacks to the “bottle-
feeding ” of infants are (1) the difficulty of obtaining sterile cow’s milk ;
(2) the fact that sterilisation can only be effected at the cost of ae ete
a proportion of the vitamines ; and (3) the loss to the child of anti-
bodies, which are present in the maternal milk, and help to protect it
from certain infective diseases.
Food-stuffs derived from Milk.—The cream of milk contains 14 to
44 per cent. of fats, and is a useful form of administering fat when an ~
extra amount is required in the dietary.
Butter is obtained by separating the fats from cream, and is ahuisiek
pure fat with a small percentage of water.
Cheese is made by the compression of clotted milk so as to express
=. Se Se. le
~s .
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FOOD AND DIET. 377
as much as possible of the water. Cheese is thus rich in protein and
fat, the protein being chiefly casein.
BREAD.
Flour contains 68 per cent. of starch, 12 per cent. of proteins, and
small quantities of cellulose, fats, and salts. When it is kneaded with
water, a change takes place in its proteins. These are two in number:
gliadin, which is soluble in alcohol, and g/utelin, which is insoluble in
alcohol. When flour is mixed with water these two substances are
converted into the sticky material, gluten. Dough is thus formed
mainly of gluten and starch. It is made spongy by the liberation of
gases in its interior, usually by the action of yeast. In the process
of baking, the starch in that portion of the loaf which is most exposed
to the high temperature, namely, the crust, is partially converted into
dextrin and dextrose.
BEEF.
The composition of lean beef is given in the table on p. 378. Fat
beef contains nearly as much protein, more fat, and less water. Mutton,
poultry, and white fish contain about the same proportions of the
various constituents, but poultry contains rather more protein and a
smaller proportion of salts, whereas white fish contains less salt and
more water.
EGGS.
The white of egg contains three proteins, egg-albumin, egg-globulin,
and ovomucoid. The yolk contains a gmall amount of a phospho-.
protein, vitellin, and a large proportion of fats, with smaller quantities
of cholesterol, lecithin, sugar, and salts.
GREEN VEGETABLES.
Green vegetables are of little value as a source of proteins, carbo-
hydrates, and fats. They have, however, three important functions in
a dietary: (1) as a source of iron, (2) as a source of vitamines, the
absence of which leads to the onset of scurvy, and (3) as stimulants
to the peristaltic movements of the digestive tract, in virtue of the
indigestible cellulose which they contain.
GELATIN.
Gelatin is a sclero-protein, and is formed by boiling collagen, the
principal solid constituent of connective tissue. Gelatin cannot replace
other proteins in a dietary, because it is deficient in three essential amino-
acid groups, phenylalanine, tyrosine, and tryptophane. Tyrosine is oxy-
phenylamino-propionic acid, tryptophane is indolamino-propionic acid ;
’
o
378 ESSENTIALS OF PHYSIOLOGY.
and these substances are essential for the building up of the body
proteins. In one experiment on a dog, it was found that nitrogenous
equilibrium could be maintained when five-sixths of the protein of the
diet was replaced by gelatin. In other experiments on animals, it has
been found possible to maintain nitrogenous equilibrium for a time on a
diet of gelatin to which tyrosine and tryptophane were added. But gelatin
alone cannot supply all the amino-acids necessary for the maintenance
of animal life.
BEVERAGES.
Tea and coffee owe their fragrance to aromatic substances and their
stimulating properties to the presence of caffeine or trimethyldi-
oxypurine. Cocoa contains about 30 per cent. of fat and 20 per cent.
of protein, and is therefore a food. It has also stimulating properties
owing to the presence of theobromine, or dimethyldioxypurine. The
methylpurines contained in these beverages do not give rise to uric
acid in the body, but are excreted unchanged.
Alcohol undergoes oxidation in the body to a limited extent, and to
that extent it acts as a food. Its value as a food is, however, counter-
balanced by its action as a poison. If taken in any quantity, it inter-
feres first with the higher mechanism for inhibition, later it disturbs
the mechanism for muscular co-ordination, and finally it paralyses the
whole nervous system. The continued use of alcohol, moreover, leads
to degenerative changes in the tissues and organs of the body, set in
that way it shortens life.
THE CONSTRUCTION OF A DIETARY.
The amount of protein, carbohydrate, fat, salts, and water required
daily being known, a dietary can be constructed with the aid of a
table showing the composition of food-stuffs, such as that given below.
Approximate Composition of some Common Food-stuffs.
Protein Carbo- Fat Salts. | Water | Cellulose
; oe , . : "
Lean beef . 21°0 a 1°5 1°0 76°5 -<
Eggs . ‘ 14°8 we | 10° 1°0 73°7 ;
Milk (cow). 35 by ie £0 0°7 87°3 :
Cheese : 33°0 aah 27°0 4°0 36°0 .
Peas (dried) 21°0 55°4 0°5 2°6 13°0 75
Oatmeal 14°6 65'1 10°1 2°1 50 3°1
Bread . 6°5 51°2 1°0 1°0 40°0 0°3
Potatoes 2°2 18°0 0'1 1:0 78°3 0°4
Carrots 0°5 10°1 0°5 0°9 86°5 1°5
Butter i bed 2 1°0 82°0 1°0 15'0 ;
FOOD AND DIET. 379
It will be observed that the animal foods are especially rich in protein
and fat, and in ordinary life most of the necessary protein is taken in
the form of beef, mutton, and eggs. Vegetable foods, on the other —
hand, are the chief source of carbohydrates, and the latter substances
in a dietary are usually derived from bread, rice, and potatoes. Some
vegetable foods contain a considerable amount of protein, but the
vegetable proteins are not so easily and completely digested and
assimilated as those contained in animal food, and therefore from a
physiological point of view it is more wasteful to obtain the necessary
protein from vegetable than from animal sources. Moreover, in order
to get the requisite amount of protein, a vegetable diet must be con-
siderably more bulky than a diet which is partly composed of animal
substances.
If the amounts of the alimentary principles required daily be
taken as:
Protein . . . ‘ . 120 grams
Carbohydrate ‘ ; Ao: (| ee
Fat 4 ie 2 ; OE) ee
Salts. aici ae res
a dietary may be constructed from the table of food-stuffs as follows :—
Protein. | Carbohydrate. | Fat. Salts,
500 grams bread ; : ‘ 31°5 256 7°5 50
220 ,, lean meat Su tiger Gk ed | i 3:8 2°2
BOG oe.) mille: ee kh a PARED 27 24°0 4-2
20 ,, butter Z 180 0-2
100 os rice j ; : ey 76 Py: axe
400 ,, potatoes . : : 8°8 72 | .0°4 4°0
100 . oatmeal . ; ; 14°6 65 | 10°0 2°0
121°9 496 | 68:2 | 17°
|
Calorie value . : ; 5 BIRT
Rice, which is included in this dietary, contains 76 per cent. of starch
and minute quantities of fat and protein. It will be observed that the
salt content of the diet is deficient, and would have to be supplemented
by means of common salt.
CHAPTER XIV.
THE URINARY SYSTEM.
| SECTION I.
THE STRUCTURE OF THE KIDNEY.
Tue kidney is a compound tubular gland having a duct, the ureter,
which connects it with the bladder and is expanded at its upper end
to form the pelvis of the kidney. On dividing the kidney lengthwise
from its outer to its inner border, it is seen to consist of two layers,
an outer reddish-brown cortex, and an inner pale layer, the medulla.
The latter is composed of a number of pyramids, the apices of which
project as papille into the pelvis of the kidney. The larger sub-
divisions of the renal artery and vein lie between the cortex and
medulla, this region being known as the boundary zone. Prolonga-
tions of the medullary tissue extend radially into the cortex, forming
the medullary rays.
The kidney consists of a mass of tubules, held together by con-
nective tissue. Each tubule begins in the cortex by a blind expanded
end (Bowman’s capsule), which may be compared with a small ball
indented so that its opposing walls almost touch; these walls consist
of a single layer of flattened epithelium. A bunch of capillaries, known
as a glomerulus, projects into the indentation, and together with
Bowman’s capsule forms a Malpighian body. At the pole opposite
the entrance of the blood-vessels Bowman’s capsule opens into the
tubule proper, which at first takes a tortuous course and is known
as the first convoluted ‘tubule; it then becomes spiral or nearly
straight (spiral tubule), and passes into the medulla, where it forms a
loop (loop of Henle) and returns into the cortex. Here it becomes
irregularly zigzag (zigzag tubule), and then convoluted (second con-
voluted tubule), and ultimately joins a straight collecting tubule
(fig. 131). The collecting tubules run into the medulla, and open at
the apices of the pyramids into the pelvis of the kidney.
‘ 380
THE URINARY SYSTEM. 381
The convoluted, spiral, and zigzag tubules are lined by columnar
or cubical cells, the lateral surfaces of which dovetail into each other ;
they are very granular, the granules tending to be arranged in rows at
right angles to the lumen, so that the cells have a rodded appearance.
The descending part of the loop of Henle is lined by clear, flattened
Bowman’s capsule. Neck. Ist convoluted tubule.
Afferent vessel.
Efferent vessel.
Intertubular capillaries. y oe
ub.
Interlobular vein. > 2nd convol.
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Interlobular artery.
Spiral tubule,
Cortical substance.
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Junctional tub.
sacoa
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Henle’s f Ascending limb.
loop | Descending limb.
Arterial arch. on
Venous arch.
Boundary zone.
Scocococceg sss
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, Medullary
Arteric rectz. substance.
e
Duct of Bellini.
Fig. 131.—Scheme of renal tubule and its vascular supply.
(From Gray’s Anatomy.)
epithelium ; the cells of the ascending limb resemble those of the con-
voluted tubules. The collecting tubules are lined by clear, cubical
cells. Throughout the whole length of the tubules the cells rest upon
a well-marked basement membrane. The tubule thus consists of (1)
Bowman’s capsule, (2) first convoluted and‘spiral tubule, (3) loop of
Henle, (4) zigzag and second convoluted tubule, and (5) collecting
tu bule.
The Blood Supply.—The renal artery enters the kidney close to the
origin of the ureter, and divides into branches, which form incomplete
>
382 ESSENTIALS OF PHYSIOLOGY.
arches lying in the boundary zone between the cortex and the medulla.
From these arches vessels pass outwards in the cortex, and give off
short afferent branches, each of which ends in a glomerulus. The
blood leaves the capillaries of the glomerulus by an efferent vessel,
which is smaller than the afferent, and after a short course breaks up
into capillaries round the convoluted tubules; from these capillaries
the blood passes into veins running to the boundary zone, and opening
into venous arches parallel with the arterial arches. The veins ulti-
mately unite to form the renal vein. Straight branches also arise from
the arterial arches to end in capillaries round the tubules in the medulla ;
from these the blood passes back to the venous arches.
THE COMPOSITION OF THE URINE.
Normal human urine is a clear, yellow fluid, acid in reaction, and
containing about 4 per cent. of total solids ; it is free from cells and
from protein, except for a small trace of nucleo-protein derived from
the bladder and urinary passages. Its specific gravity varies from
1015 to 1025, and its daily amount is about 1500 cc. Its average
composition is shown in the following table :—
Total quantity of urine. ; : 1500 c.c.
», solids : . , . 60 orams.
i, ROA , : 7, . od ie
» Uricacid. . ; : Reis tif een
», hippuricacid . Bei 4° .
», sodium chloride f - LBD =
»» Phosphoric acid ; OF 15a i ee J
», sulphuric acid . : De ee,
,) ammonia . z: : ; NG, | ares
, creatinine ; ; ; sg Pek D1, -
5 potassium ‘ : : :
» calcium . ‘ | 40) 3
4, Magnesium
Since the nitrogenous end-products of the metabolism of protein
are excreted almost entirely in the urine, its composition largely de-
pends upon the quantity of protein food consumed, and on the kata-
bolic changes in the tissue proteins. The characters of urine vary,
therefore, not only in different individuals, but even in the same
individual from day to day, and almost from hour to hour.
Amount and Specific Gravity.—The fluid taken by the mouth leaves
the body partly in the urine, and partly through the skin and lungs.
THE URINARY SYSTEM. 383
In hot weather or during exercise, when evaporation of sweat from the
skin is considerable, the urine is decreased in amount and is propor-
tionally concentrated. When the secretion of sweat is scanty, for
example on a cold day, a larger proportion of water is excreted by
the kidneys, and the urine is abundant and of low specific gravity:
copious draughts of water produce the same effect. In diabetes the
presence of sugar may raise the specific gravity to 1040 or more, while
in some forms of renal disease the specific gravity is always low (1005
to 1015).
Reaction.—The acid reaction of normal urine is due to acid sodium
phosphate (NaH,PO,); no free acid is present. The bases and acid
radicles mentioned in the foregoing table are combined to form salts, and
are derived from the food. Sulphuric and phosphoric acid are formed by
the oxidation of the sulphur and phosphorus contained in protein, and
when the food contains much protein the amount of these acids is
increased in the urine, which becomes strongly acid in reaction.
Vegetable foods contain organic salts, such as citrates and tartrates
of potassium and sodium, in abundance, and in the body these organic
acids are completely oxidised, whereas the bases are excreted in the
urine. Hence in herbivorous animals, and in man on a vegetarian diet,
the urine is neutral or alkaline, though a starving herbivorous animal
which is living on its tissue proteins, and is for the time being car-
nivorous, excretes an acid urine. .
-Colour.—The colour of urine is almost entirely due to a pigment,
wrochrome, of uncertain origin, the spectrum of which shows no
absorption bands. In addition, urine may contain three other pig-
ments, namely, (1) urobilin, (2) uroerythrin, and (3) hematopor-
phyrin.
Urobilin is formed in the digestive tract from bilirubin by bacterial
action, and after absorption into the blood is excreted into the urine
chiefly as a colourless chromogen, which can be converted into urobilin
by the addition of an acid. Urobilin itself occurs in urine in consider-
able quantity when the amount of bile pigment formed in the liver is
increased by an unusually rapid destruction of red cells in the body,
for instance in pernicious anemia. It shows an absorption band at the
junction of the green and blue part of the spectrum, and gives a green
fluorescence with zinc chloride and ammonia.
Uroerythrin occurs in combination with deposits of urates, giving
them a pink colour, which is changed to green on the addition of an
alkali; its composition is unknown,
Hematoporphyrin (p. 162) normally occurs in minute traces, but
may be present in large amount in sulphonal poisoning.
384 _ ESSENTIALS OF PHYSIOLOGY.
UREA.
The presence of urea in urine may be shown by evaporating the
urine to dryness on a water bath, and extracting the residue with
acetone (which dissolves urea). On evaporation of the acetone extract
the urea crystallises out.
Urea (CON,H,) is a solid, crystallising in colourless rhombic prisms
which are easily soluble in water, alcohol, and acetone. When heated,
the crystals decompose, giving off ammonia and yielding a body called
bruret. Urea combines with nitric or oxalic acid, forming characteristic
crystals, and is decomposed by nitrous acid with the evolution of car-
bonic acid and nitrogen, or by alkaline sodium hypobromite, according
to the following equation :—
eal
CON,H,+ 3NaBrO = CO, + N, + 2H,O + 3NaBr.
The carbonic acid is absorbed by the alkali, and the nitrogen is
given off; and by collecting and measuring in a graduated cylinder the
amount of nitrogen evolved from 5 ¢.c. of urine, the percentage of urea
can be ascertained. Theoretically, 1 gram of urea yields 371 c.c.
of nitrogen at 0° C. and 760 mm. Hg, but actually only 354 cc. are
evolved from 1 gram of urea in urine.
Another and more accurate method of estimating the amount of urea is
to treat a known volume of urine with an extract of Soya bean, which converts
urea into ammonia; the ammonia formed is passed into a known volume of
aa acid, and the amount of uncombined acid is subsequently estimated.
When urine is exposed to the air the urea soon becomes decom-
posed by micro-organisms, being converted into ammonium carbonate,
_and the urine becomes strongly alkaline.
The nitrogen present as urea usually forms about 85 per cent. of
the total nitrogen existing in one combination or other in the urine.
The. total nitrogen in urine is estimated by Kjeldahl’s method as
follows :—
A known volume of urine is boiled with pure sulphuric acid until
all its carbon is fully oxidised, the nitrogen being converted into
ammonia, which combines with the acid. The solution is then made
alkaline with caustic potash and boiled, the ammonia, which distils off,
being collected in a known volume of ra sulphuric acid. The amount
of acid neutralised by the ammonia is determined by subsequent
titration of the uncombined acid with ra caustic potash. |
Urea is the most abundant nitrogenous constituent of urine, its
.
a ee ee ee ox
THE URINARY SYSTEM. 385
amount varying, as seen in the Table on p. 357, with the quantity of
protein food consumed; in starvation urea is derived entirely from
the breaking down of the tissue proteins.
AMMONIA.
The ammonia normally found in the urine represents the small
amount which escapes conversion into urea by the liver, and is but
little affected by the amount of protein in the diet. In acidosis the
abnormal acids formed in the body combine to a large extent with
ammonia in the blood, being excreted as ammonium salts in the urine,
and the amount of urea is correspondingly diminished. The amount
of nitrogen excreted as ammonia in these circumstances is sometimes
20 per cent. or more of the total urinary nitrogen.
CREATININE AND CREATINE.
Creatinine is an anhydride of creatine, which occurs in muscle, and
it may be formed from the latter by boiling it with strong hydrochloric
NH
acid. It has the formula | | and gives a red
NH=C ~ N(CH,). CH, . CO,
colour with caustic potash and picric acid (Jaffé’s test). When creatine
is taken by the mouth, either as a pure substance or in meat, some of
it may appear in the urine as creatinine. With this exception, the
creatinine in urine ‘is derived solely from the endogenous metabolism
of protein ; and the amount excreted in the urine by the same person
from day to day is remarkably constant, and serves as an accurate
index of the extent of endogenous protein metabolism.
The origin of creatinine and its relation to the creatine present in
muscle are not fully understood, though creatinine appears in the
urine in increased amount in fever, and in other conditions in which
rapid wasting of muscular tissue is taking place. Creatine also, though
not usually present in the urine of adults, is found during starva-
tion, in diabetes, in acute fevers, and in women during involution of
the uterus. It is possible that normally the creatinine in the urine is
formed from creatine, and that in the conditions just mentioned an
increased amount of creatine is set free by protein decomposition, some
of which is converted into creatinine, while a portion is excreted in an
unaltered form.
URIC ACID.
Uric acid exists in urine in the form of biurates. On adding strong
hydrochloric acid to urine and allowing it to stand for twenty-four
hours, uric acid separates out as small pigmented*scrystals having a
a
o 5
386. ESSENTIALS OF PHYSIOLOGY.
characteristic whetstone or dumb-bell shape. It is almost completely
insoluble in water, but dissolves in weak alkalies. It slightly reduces
Fehling’s solution (p. 387), and will also reduce an alkaline solution
of silver nitrate (Schiff’s test). A solution of uric acid, when evaporated
to dryness with nitric acid at a low temperature, yields a purple
colour on the subsequent addition of an alkali (murexide test).
The uric acid is derived partly from the nuclein in food and partly
from the breaking down of the tissue nucleins. :
In addition to uric acid, small amounts of the purine bases are
found in urine.
Hippuric acid is synthesised in the kidney from benzoic acid and
glycine, the synthesis being brought about by an enzyme. It is the
only urinary constituent which is formed in the kidney itself.
Sulphates.—The sulphates are of two kinds, namely, (1) inorganic,
and (2) ethereal. The latter are compounds of sulphuric acid with
phenol, indoxyl, or skatoxyl, and potassium. Indol and skatol are
formed from tryptophane by bacterial action in the digestive tract ;
and after absorption into the blood they are converted by oxidation
into indoxyl and skatoxyl, combined with sulphuric acid, and excreted
by the kidneys. Phenol, also, is a product of protein decomposition.
As a rule the ethereal sulphates form about one-tenth of the total
sulphates, but when bacterial changes in the digestive tract become
excessive (¢.g. in constipation), the proportion of ethereal sulphates is
increased,
Some sulphur is also excreted in organic combination and is known
as “neutral” sulphur. ,
- The sulphur of the urinary sulphates is formed almost entirely by
the oxidation of the sulphur contained in protein, and the total
amount of sulphates varies therefore with the quantity of protein food
ingested. |
Urine also contains sodium chloride and phosphates, the latter
being of two kinds, namely, (1) alkaline phosphates, of sodium and
potassium, and (2) earthy phosphates, of calcium and magnesium.
URINARY DEPOSITS.
On standing, normal urine deposits a cloud of mucus (nucleo-
protein) derived from the urinary passages. When the urine is
concentrated, biurates of sodium and potassium are often deposited as
an amorphous sediment, coloured pink by uroerythrin, and dissolving
when warmed. Earthy phosphates are deposited from neutral or
alkaline urine ; they dissolve on the addition of acetic acid. Crystalline
“ : “
—— EE a oo
THE URINARY SYSTEM. 387
deposits may also occur in urine, and are usually indicative of abnormal
processes taking place either in the body or in the urine itself. The
nature of the deposits varies with the reaction of the urine. |
In acid urine those most frequently seen are, first, uric acid crystals,
which assume the form of whetstones or cylinders, and are usually,
though not invariably, pigmented; and, secondly, calcium oxalate,
occurring as small colourless octohedra, often called “envelope”
crystals from their appearance under the microscope.
Uric acid and oxalate crystals are frequently found together. Other
crystalline deposits, occasionally met with in acid urine, are cystine
(flat hexagonal colourless plates). and the acid urates of sodium or
ammonium, which form spheroidal masses with projecting spikes.
In alkaline urine, the crystals most commonly met with are (1)
earthy phosphates, star-shaped in appearance, and (2) ammonio-
magnesium phosphate, NH,MgPO,. The latter, often called “triple
phosphate,” is formed when urine becomes alkaline as a result of
the bacterial decomposition of urea; the crystals are large and very
characteristic, resembling knife-rests or coffin lids.
OTHER ABNORMAL CONSTITUENTS IN URINE.
(1) Coagulable Protein.—Except for a trace of. nucleo-protein,
normal urine contains no protein. In disease of the kidney, serum
globulin and albumin escape from the blood into the urine, and are
coagulated on boiling the urine (after the addition of a drop or two of
dilute acetic acid). Further, when urine containing protein is poured
on to the surface of strong nitric acid, a precipitate forms at the junc-
tion of the two fluids (Heller’s test).
(2) Sugar.—The conditions under which sugar occurs in urine have
already been dealt with (p. 350). In man the usual cause of glyco-
suria is diabetes, and the sugar is dextrose Lactose is sometimes
found during lactation, even in healthy women. In rare cases the
urine contains levulose or pentose. The amount of dextrose present
in the urine in diabetes may vary from mere traces up to 350 to 500
grams daily.
Dextrose reduces alkaline solutions of copper sulphate, yielding
cuprous oxide. The solutions generally used in testing for dextrose
are (1) Fehling’s solution, containing copper sulphate, caustic potash,
and Rochelle salt, which keeps the cupric hydrate in solution, or (2)
Benedict’s solution, which contains copper sulphate, sodium carbonate,
and sodium citrate. The latter is more satisfactory, since, unlike
Fehling’s solution, it is not reduced at all by uric*acid or creatinine,
e
388 ESSENTIALS OF PHYSIOLOGY.
nor does it become self-reducing when kept for some time. Other tests
for dextrose are (1) the preparation of the osazone with phenylhydrazine
and (2) the fermentation test with yeast, which converts dextrose into—
carbonic acid and alcohol. shee
- The estimation of sugar may. be effected by Benedict’s method. The
solution used contains copper sulphate, sodium carbonate and citrate,
potassium thiocyanate, and potassium ferrocyanide. 25 c¢.c. of the solution
and 3 or 4 grams of anhydrous sodium carbonate are placed. in a small
flask and boiled. The sugar solution is added from a burette until the blue
colour of the reagent disappears; this is the end point. The thiocyanate
forms a white precipitate with the cuprous hydroxide formed by the reduction
of the copper sulphate, and the end point is quite sharp. 25 c.c. of the
solution are reduced by 0°05 grams of dextrose. |
Glycuronic acid (COOH(CHOH),, CHO) sometimes occurs in urine,
either after the administration of certain drugs, such as chloral, or in
combination with indoxyl. It reduces Fehling’s and Benedict’s solutions
_ and forms an osazone, but may be distinguished from dextrose by means
of yeast, which does not ferment it.
(3) Bile is present in the urine in jaundice, giving it a greenish or
brownish colour ; its presence may be recognised by Gmelin’s or Hay’s
test (p. 320).
(4) Blood occurs in urine as the result of hemorrhage in the
kidneys or urinary passages, and may be identified by observing the
red corpuscles under the microscope, or by spectroscopic examination.
(5) Products of Abnormal Metabolism.—These include $-oxybutyric
acid, diacetic acid and acetone (p. 347), leucine and tyrosine which
are present in acute atrophy of the liver, cystine, and homogentisic
acid. |
Cystine occurs in proteins, and is set free when these break down in
the body, but is normally excreted in another form in the urine; in
rare cases the cystine derived from protein in the body is excreted as
such, and may form crystalline deposits or calculi.
OH
eS
Homogentisic acid, | CH, . COOH, is a derivative of tyrosine.
On
In certain persons the oxidation of tyrosine is incomplete, and stops with
the production of homogentisic acid, which appears in the urine. The
condition is known as alcaptonuria, and the urine darkens on standing
and reduces Fehling’s solution. In persons suffering from alcaptonuria
the amount of homogentisic acid in the urine varies from 3 to 6 grams
daily ; it is‘proportional to the quantity of phenylalanine and tyrosine
present in the proteins of the food, and is increased when these sub-
THE URINARY SYSTEM. 389
stances are given as such by the mouth. Thus the whole of the tyrosine
and phenylalanine taken into the body is excreted as homogentisic acid.
Cystinuria and alcaptonuria, when they occur, are present at birth
and persist through life ; they are due to “inborn errors” of metabolism,
probably to the lack of certain ferments, and do not lead to any
disturbance of health. . |
SECTION II.
THE FORMATION OF URINE.
Broadly speaking, the function of the kidney is to keep the com-
position of the blood constant by excreting into the urine either
abnormal constituents which enter the blood, or any excess of substances
normally present, such as water, urea, and sodium chloride. The
abolition of this function by the complete removal of the kidneys leads
to the retention of the urinary constituents in the blood, and the
animal dies in two or three days; the kidneys are, therefore, essential
to life. A very important part of their function is to regulate the
reaction, that is, the H ion concentration of the blood and _ tissues.
The respiratory mechanism prevents any accumulation of carbonic acid
in the blood, while the kidneys control the H ion concentration due to
other acids. These organs are extraordinarily sensitive to the slightest
change in the reaction of the blood, and respond by excreting in the
urine any excess of acid or alkali which is present. Recent observations
in man show that in some diseases of the kidneys the normal balance
of acid and base in the blood is no longer maintained, and the amount
of lactic and other acids in the blood is increased. This increase
stimulates the respiratory centre, as already described (p. 268),
leading to severe hyperpnoea, and to a fall in the tension of carbonic
acid in the blood.
So far as is known, there are no secretory nerves to the kidney ; its
functional activity is excited solely by any change in the chemical
composition and amount of the blood flowing through it, and is thus
largely determined by metabolic changes occurring in other parts of
the body.
The structure of the renal tubule is extremely complex, much more
so than that of most of the other glands of the body ; many views have
been held as to the function of its different parts, and even now the.
problem is not completely solved. The structure of the convoluted
tubule and of the capsule of Bowman is so different that it seems
certain that their functions must also be different; and Bowman, on
purely histological grounds, suggested that the glomeruli filtered off
:
390 ESSENTIALS OF PHYSIOLOGY.
water and salts, the remaining urinary constituents being secreted by
the tubules.
Ludwig believed that the whole of the urine was formed by filtra-
tion through the glomeruli as a fluid identical in composition with
the blood plasma minus its proteins, and that in its passage down the
lumen of the tubules much of the water and some of the salts were
reabsorbed, so that the composition of urine, as it left the kidney,
differed greatly from that of blood plasma. Since blood plasma
contains about 0°02 to 0°05 per cent. of urea, whereas urine contains
2 per cent. urea, this theory demands that at least 60 litres of fluid
should be filtered off by the glomeruli, of which all but 1°5 litres are
reabsorbed ; it is no longer accepted.
Heidenhain regarded both the tubules and glomeruli as possessing
a secretory function, the latter secreting water and salts by a selective
and vital process. This view is still accepted by some authorities,
whereas others believe that the glomeruli form by filtration a fluid
identical in composition with the blood plasma minus its proteins, and
that the tubules secrete into this fluid, as it passes along them, water
and other urinary constituents. The latter theory is really a slight
modification of Bowman’s theory
The question as to whether the formation of urine takes place by a
process of filtration or of secretion can be answered by ascertaining
whether the conditions under which it is formed conform to ‘those
known to hold for filtration or secretion‘elsewhere. In filtration, the
amount of filtrate varies directly with the difference of pressure on the
two sides of the filtering membrane, and it usually contains the same
percentage of crystalloids as the fluid undergoing filtration. When a
true secretion, such as that of saliva, takes place, the pressure of the
saliva in the ducts may rise higher than that of the blood, and the
amount of secretion is, within wide limits, independent of the blood
pressure ; moreover, the composition of the secretion differs greatly from
that of the blood. Further, during secretion, the secreting cells perform
work and take up more oxygen from the blood. |
In the discussion of this question it is convenient to consider
separately the functions of the glomeruli and of the tubules.
THE FUNCTION OF THE GLOMERULI.
In the mammalian kidney, it is impossible to obtain separately the
urine formed by the tubules and glomeruli respectively, though there
is evidence that when the flow of urine is profuse it is derived mainly
from the glomeruli. If urine is simply filtered through the walls of
— a) Pe Yo) atneed cya Y a
One
THE URINARY SYSTEM. 301
rs
the glomeruli, its amount should be increased by raising the capillary
pressure in the glomeruli and decreased by lowering that pressure,
since the pressure in the ureter is nil, and its composition should be
that of blood plasma minus proteins.
Experiment shows that such is the case. The capillary pressure in
the kidney is increased by dilatation of its arterioles, so long as the
general blood pressure remains constant, or by a rise in the general
blood pressure, if this is not accompanied by active constriction of the
renal arterioles. In either case the amount of blood flowing through
the kidney is increased, and more blood is present in it at any moment.
Conversely, the capillary pressure is diminished either by a fall in
the general arterial pressure, or by constriction of the arterioles, the
general arterial pressure remaining unchanged.
The changes in capillary pressure cannot be observed directly, but
may be measured indirectly by recording either the alterations in
volume of the kidney, or the rate of blood flow through it by one or
other of the methods already described (p. 225); an increase in the
volume of the kidney indicates a rise of pressure in the capillaries of
the glomeruli.
The capillary pressure in the glomeruli is high, partly because the
renal arteries arise directly from the aorta, and partly because the
efferent vessels of the glomeruli are smaller than the afferent vessels ;
it is probably only about 20 to 30 mm. Hg below that in the renal artery.
‘The kidneys are amply supplied with vaso-constrictor nerves from
the sympathetic system, and the calibre of the arterioles can be altered
by section or stimulation of these nerves. On section of the renal
vaso-constrictor nerves the kidney dilates, the rate of blood flow
through it is increased, and more urine is formed; stimulation of the
nerves causes shrinking of the kidney, and the flow of urine diminishes
or ceases altogether. Division of the spinal cord in the cervical
region leads to dilatation of all the arterioles, including those in the
kidneys ; but the general arterial pressure falls so low that, although
the renal arterioles are dilated, the rate of blood flow through the
kidney is much diminished and the flow of urine ceases altogether.
Stimulation of the spinal cord in the neck leads to constriction of
arterioles and a large rise of blood pressure; and the renal arterioles,
become so constricted that, in spite of the rise in blood pressure, the
volume of the kidney is lessened, and the flow of urine is small or
absent. The injection of adrenalin has the same effect as stimulation
of the spinal cord. These and other experiments make it clear that,
as seen in the following table, the amount of urine formed by the
kidney varies directly with the volume of the kidney, that is to say,
~ UTC gs ins .
2. ESSENTIALS OF PHYSIOLOGY.
39 |
with the capillary pressure in the glomeruli; and if the arterial blood
pressure falls below 40 to 50 mm, Hg, the flow of urine ceases.
General Renal Kidney | Urinary
Procedure. Blood Pressure., Vessels. | Volume. Flow.
Division of spinal cord in Falls to : .
{ 4 Relaxed. Shrinks. Ceases,
neck , ; - ; 0mm. Hg. . |
Stimulation of Ss : Rises. Constricted. | Shrinks. | Diminished.
Stimulation of cord after : Passively
section of renal nerves . safes dilated. ; Bxclis. Tnoreqeed,
Stimulation of renal nerves | Unaffected, | Constricted.| Shrinks. Diminished.
Stimulation of splanchnic
ny Le Ae eal ee Rises. Constricted. | Shrinks. Diminished.
Hydremic plethora . : Rises, Dilated. | Swells. Increased.
Hemorrhage .. . Falls. Constricted. | Shrinks. Diminished.
If the blood plasma is filtered through peritoneal membrane soaked
in gelatin, it is found that, when the difference of pressure on the
two sides of the membrané is 40 mm. Hg or more, the filtrate contains
the dissolved constituents of plasma with the exception of protein ;
with a lower difference of pressure no filtration occurs. This is due to
the fact that the proteins in plasma exert an osomotic pressure equal
to about 25 mm. Hg, and water tends to pass back by osmosis from
the filtrate into the plasma. It is for this reason that the passage of
fluid through the glomerular wall ceases when the arterial blood pressure
falls below 40 mm. Hg. If the osmotic pressure of the protein is
diminished by decreasing the amount of protein in the plasma, for
instance by diluting the plasma, urine may be formed when the blood
pressure is considerably less than 40 mm. Hg. In hydremic plethora .
(p. 240), not only is the plasma more diluted, but the renal vessels are
dilated and the pressure in the glomeruli is raised; and the formation
of urine may be extremely rapid and profuse.
The difference of pressure on the two sides of the walls of the
glomeruli may be diminished either by lowering the capillary pressure
or by raising the ureter pressure. When the escape of urine from the
ureter is prevented, the formation of urine continues until the pressure
in the ureter is 40 to 50 mm. Hg below that in the blood-vessels, after
which no more urine is formed..
We may conclude, therefore, that the amount of urine formed by
the glomeruli varies directly with the difference of pressure on the two
sides of the glomerular membrane, and that it is formed by a purely
physical process of filtration. An apparent exception is seen when the 3
renal vein is ligatured: the capillary pressure rises, but the flow of
cap “apace <=> tala etl mer meen es eat be
THE URINARY SYSTEM. 393
urine ceases entirely. The reason is that the flow of blood through the
glomeruli ceases, and their contents soon consist of little more than.
a mass of blood corpuscles, rendering filtration impossible.
The more rapidly urine is formed by the kidney, the more nearly
does its composition approximate to that of blood plasma; and when
the blood is greatly diluted, for example by repeated injections of
Ringer’s solution, the percentage of sodium chloride and urea in the
plasma and the urine may be identical.
ANALYSIS OF PLASMA AND URINE IN HypDRa@MIA. (BARCROFT.)
Chlorides as NaCl. Urea.
Plasma . : . | 0°88 per cent. | 0-04 per cent.
Drine *—.. : . | 0°88 3 0:05 BY
Similar results have been jobtained in animals, in which the renal
tubules were poisoned with corrosive sublimate, or other drugs, so as
to eliminate their functions. The fluid formed by the glomeruli appears,
therefore, to be isotonic with the blood plasma, a fact which is intelligible
if it is formed by filtration, but which does not fall into line with our
know ledge of secretion.
FUNCTIONS OF THE TUBULES.
There is no doubt that the renal tubules form urine by a process
of secretion, which in its essential features is strictly comparable with
that occurring in other secretory glands. In the first place, the com-
position of the urine formed by the tubules differs greatly from that of
the blood. Secondly, increased secretory activity of the tubules is
accompanied by a larger consumption of oxygen, and may take place
without any alteration in the rate of blood flow through the kidney.
The secretory function of the tubules has been most clearly proved
in the frog, in which, owing to the arrangement of the blood supply to
the kidneys, the functions of the tubules and glomeruli can be studied
separately. The glomeruli are supplied with blood solely through the
renal artery, whereas the tubules have a double supply. On the one
hand, the efferent vessels from the glomeruli enter the network of
vessels round the tubules; on the other hand, the tubules also receive
blood from the renal portal vein, which arises from the foxioral vein
(fig. 132).
When the renal arteries are ligatured in the frog, the circulation
394 ; ESSENTIALS OF PHYSIOLOGY.
of blood through the glomeruli is completely cut off, but the tubules are
still supplied with venous blood from the renal portal vein, The cutting
off of the supply of arterial blood to the kidneys is followed by destruc-
tion of the epithelium of the
tubules, and the frogs secrete
no urine, If, however, the frogs
are kept in an atmosphere of
oxygen after ligature of the
renal artery, sufficient oxygen
is absorbed and carried to the
tubules to replace that normally
supplied to them by the arterial
blood leaving the glomeruli;
and the nutrition of the tubules
is maintained. In these cir-
cumstances the frogs form small
quantities of urine, and, since
the glomeruli are excluded, this
must come entirely from the
renal tubules ; it is acid in re-
Fie. 132,—The blood supply to the kidney action, and contains urea and
of the frog.
salts.
A., aorta; R.V., renal vein; R.P.V. , renal-portal iS
vein; F.V., ” femoral vein; A.A.V., anterior In mammals the injection of
abdominal vein.
substances such as sodium sul-
phate or urea into the blood leads to an increased flow of urine, which
then contains a higher percentage of sodium sulphate or urea than is
present in the blood; and at the same time the consumption of oxygen
by the kidney is greatly increased.
Oxygen used by Percentage of
‘ Kidney per Gram Sodium Sulphate
per Minute. in Urine.
(1) Normal kidney. 0:04 c.c, 0-26 per cent.
(2) After the injection of
sodium sulphate 0:09 c.c. 1:25 ‘3
Since these changes may occur with little or no alteration in the
volume of, or the rate of blood flow through, the kidney, the additional
urine formed in these experiments must have been secreted by the
tubules. This conclusion is confirmed by the fact that when the renal
tubules are poisoned with drugs, such as corrosive sublimate, the
THE URINARY SYSTEM. 305
injection of sodium sulphate no longer increases the amount of oxygen
used by the kidney, and the urine may be isotonic with the blood
plasma. This experiment is not quite conclusive in mammals, since the
glomeruli, as well as the tubules, are exposed to the poisonous action
of corrosive sublimate; and it might be legitimately argued that the
glomeruli possess a secretory function, which is abolished in the poisoned
kidney. But it is possible in frogs to poison the tubules and to leave
the glomeruli intact ; and since, when this is done, the urine is isotonic
with the fluid passing through the glomeruli, the function of the latter
is not secretory, but is that of a filtering membrane.
Further evidence for the secretory function of the tubules is pro-
vided by experiments in which indigo-carmine (sulphindigotate of soda),
which is a blue pigment, is injected into the blood stream, The spinal
cord of the animal is previously divided in the neck, so as to abolish
the formation of urine by the glomeruli, and to prevent the dye from
being carried away in the urine. The animal is killed ten minutes
after the indigo-carmine has been injected, and the kidneys are fixed
in absolute alcohol. Sections of the kidneys show the presence of
blue granules of the pigment in the. lumen of the tubules, and in the
cells of the convoluted tubules, but not in Bowman’s capsule or the
cells lining it, indicating that the pigment had been secreted by the
tubules but not by the glomeruli.
Reaction of Urine.—The fluid filtered off from the glomeruli lias
the same reaction as the blood plasma, and the acid reaction of normal
urine is due to the activity of the cells of the tubules. This can be
shown by repeatedly injecting into the dorsal lymph sac of a frog acid
fuchsin, which is almost colourless in neutral or alkaline solutions and
red in acid solutions. When the kidneys are subsequently examined
microscopically, the glomeruli are seen to be colourless, whereas the
cells of the convoluted tubules are red.
Further, the more rapidly urine is formed in the mammal, that is,
the greater the amount filtered through the glomeruli, the more nearly
does its reaction approximate to that of the blood.
It has been thought that the acid reaction of urine is due to the
fact that as the glomerular filtrate passes down the tubule an absorp-
tion of bases, especially sodium, takes place, so that the urine becomes
acid, Since the urine formed by frogs after ligature of the renal
arteries is acid in reaction, it is probable that the acid reaction of
urine is due to the secretion of acid radicles by the tubules, rather
than to the absorption of bases. .
In the process of secretion the renal tubules do work which can
be approximately measured if the osmotic pressure of the blood plasma
306 2) ESSENTIALS OF PHYSIOLOGY.
and of the urine are known. The freezing point of any substance in
solution in water is lower than that of pure water, and the osmotic
pressure of such a solution is proportional to the depression of the
freezing point below 0° C. The freezing point of serum is - 0°56° C.,
and that of urine may be -—4°5° C. The osmotic pressure of urine,
therefore, is very much greater than that of blood, and the amount of
work done by the kidneys in producing urine of high osmotic pressure
from blood, of which the osmotic pressure is low, is extremely large. _
Absorption by the Tubules.—When a mixture of sodium chloride
and sodium sulphate is injected into the circulation of an animal, the
percentage of chlorides in the urine gradually falls as the experiment
continues, and may become less than that in the blood plasma, while
the percentage of sodium sulphate in the urine remains high. This is
seen in the following table :—
Chlorides. - Sulphates.
Blood serum . . | 0°49 per cent. | 0°19 per cent.
Urine . : ; . | 0°09 ms 2°0
Accepting the view that the glomeruli filter off from the blood a
fluid containing the same percentage of sodium chloride as>that in
the blood, this result can only be explained by supposing that as the
glomerular filtrate passes down the tubules they absorb sodium chloride.
Similar results have been obtained in frogs. When the kidneys
of the frog are perfused through the renal arteries with oxygenated
Ringer’s solution, the urine, which in these experiments is formed
solely by the glomeruli, is more dilute than the perfusing fluid; this
is due to the fact that, as the urine formed by the glomeruli passes
down the tubules, the latter absorb sodium chloride. When the
tubules are poisoned with corrosive sublimate, absorption no longer
takes place, and the urine is isotonic with the perfusing fluid.
This process of absorption probably serves to prevent the loss of
salts which are needed in the body, especially when they are not
being replaced in the food. That this is the case is suggested by the
observation that, when animals are fed for some days on a diet free
from chlorides, their urine is almost free from chlorides, although the
percentage of sodium chloride in the blood plasma may be unaltered.
In all probability water, and possibly other substances, are also
absorbed, The part of the tubule which carries out this process has
not been. ascertained.
|
ay —
THE URINARY SYSTEM. 397
Diuretics.—Substances which, when they enter the blood stream,
increase the amount of urine formed by the kidneys are called diuretics.
Some act by stimulating the cells of the renal tubules to increased
secretory activity; to this group belong urea and sodium sulphate.
The ground for believing that they act upon the tubules is that they
increase, not only the flow of urine, but also the amount of oxygen
consumed by the kidneys. :
The diuretics of the other group increase the formation of urine,
but do not alter the gaseous exchange of the kidney; they include
hypertonic solutions of sodium chloride, potassium nitrate, and other
salts. When hypertonic solutions of these salts are injected into the
blood, they raise its osmotic pressure and bring about the condition
of hydremic plethora (p. 240). The volume of the kidney and the
capillary pressure in the glomeruli are increased ; and as a result of
this rise of pressure, more fluid is filtered through the walls of the
glomeruli. It may be readily shown that diuretics such as sodium
chloride have no specific action, but that they increase the flow of
urine solely by raising the capillary pressure. If the usual action of
these diuretics in raising the capillary pressure is prevented, the
injection of the diuretic does not increase the amount of urine; this
can be effected by keeping the volume of the kidney constant by means
of a screw-clamp placed on the renal artery.
Some diuretics, such as sodium sulphate, produce their effect partly
by direct action on the renal tubules by which they are secreted, and
partly by producing hydremic plethora.
Summary.—From the foregoing experiments we may conclude, in
the first: place, that the glomeruli, in all probability, simply filter off
from the blood a fluid identical in composition with blood plasma,
excluding protein ; the amount of this filtrate normally depends solely
upon the capillary pressure in the glomeruli.
Secondly, the tubules secrete into this: fluid urea, uric acid,
phosphates, sulphates, and other urinary constituents, with some water,
the amount of secretion being dependent on the quantity of these
substances reaching the kidney in the blood. This will vary with the
metabolic activities of the body, with the result that the kidney
removes from the blood the waste products constantly entering it from
the tissues, and the composition of the blood is kept practically
constant. |
Thirdly, the tubules possess a selective power of absorption, whereby
certain substances such as sodium chloride, which are of importance to
the body, can be retained when required. In the normal kidney it is
probable that all these processes are taking place simultaneously, one
398 | ESSENTIALS OF PHYSIOLOGY.
or other predominating according to circumstances. The drinking of a
large quantity of water, for instance, is followed by its rapid excretion
through the glomeruli, by which most of the water of the urine is
normally excreted.
SECTION III.
| MICTURITION.
The urine formed in the kidneys passes along the ureters to the
bladder, where it accumulates, the bladder being emptied.from time to
time by the process of micturition. The flow of urine along the ureters
is assisted by rhythmic waves of contraction, passing down from the
pelvis of the kidney to the bladder at intervals of a few seconds ; they —
can still be observed in the ureter when it is isolated from the central
nervous system.
The wall of the bladder consists of unstriated muscle fibres arranged
in three layers, an outer and an inner longitudinal layer, and a middle
layer of fibres running circularly ; it is lined by transitional epithelium.
When the muscular walls contract they lessen the size of the cavity.
The escape of urine from the relaxed bladder is prevented by two
sphincters, namely, first, circular unstriped muscular fibres, forming
a loop round the orifice of the bladder and called the trigonal sphincter,
and, secondly, the sphincter wrogenitalis, or compressor urethree, which
encloses the second part of the urethra, and is composed of striated
muscular fibres. The bladder receives its nerve supply from (1) the
nervi erigentes, stimulation of which causes it to contract, and (2)
sympathetic fibres from the hypogastric plexus, stimulation of which
is followed in some animals by inhibition and in others by contraction
of the bladder wall; afferent fibres also pass from the’ bladder in the
nervi erigentes to the spinal cord,
Micturition is normally carried out as a reflex action which, in the
adult, is controlled and can be inhibited by impulses from the higher
parts of the brain. Distension of the bladder wall gives rise to impulses
which, travelling to the spinal cord, reflexly bring about emptying
of the bladder by the contraction of its muscular coat, the nerve cells
concerned in the reflex lying in the sacral region of the cord; the
escape of urine is made possible by the simultaneous relaxation of the
sphincter muscles. The intensity of the afferent impulses varies with
the rate of filling of the bladder. When the bladder fills slowly, its
muscular wall relaxes, and it may contain a considerable amount of
urine before any appreciable tension is placed upon its muscular fibres.
On the contrary, when urine is being formed rapidly, the tension within
Enase 6
THE URINARY SYSTEM. — 399
the bladder may be quickly and greatly increased by the presence of
a comparatively small quantity of urine. In man, micturition usually
occurs when the pressure within the bladder is about 150 mm. of
water ; and if the pressure is suddenly raised to this level by injecting
fluid through the urethra into the bladder the desire for micturition
is experienced. | |
Transection of the spinal cord in the upper lumbar region does not
destroy the -reflex mechanism, though it severs the path by which
sensory impulses reach the brain and voluntary impulses pass to the
sacral centre. By means of voluntary impulses the emptying of the
bladder can be inhibited in the adult, but in the infant such im-
pulses are lacking and the act of micturition is purely reflex. The
emptying of the bladder is usually assisted by voluntary contraction of
the abdominal muscles, and in man such a contraction, by increasing
the pressure within the bladder, frequently initiates the reflex action.
When the centre in the sacral region is destroyed, for instance as
the result of injury in man, the bladder still reacts like plain muscle
elsewhere, and increased tension causes it to contract. As soon as the
pressure within the bladder falls below a certain level, however, it fails
to overcome the resistance offered by the sphincter muscles, and the
bladder is not completely emptied.
CHAPTER XV.
THE DUCTLESS GLANDS.
THE ductless glands include a group of organs of very varied functions,
the only feature which they have in common being the absence of any
secretion passing either to the surface of the body or into the digestive
tract. Many of them, however, such as the thyroid, suprarenal, and
pituitary glands, do form substances which pass either directly into the
blood stream or into the lymph channels and are described as internal
secretions,
The internal secretions belong to the class of bodies known as
hormones, whose general characters have already been dealt with
(p. 306), and which are also formed by organs, e.g. the pancreas, which
possess an external secretion. The presence of these hormones in the
body is in many cases essential to health and even to life; and the
activity of the internally secreting glands is correlated with and
regulates the functions of distant organs, the only link being the blood
by which the hormone is carried from its place of origin to its place of
action, ;
Much of our knowledge of the functions of the ductless glands is
derived from the study of the symptoms observed in human beings, in
whom one or other of them is diseased. Hence it is usual in studying
their functions to consider (1) the effects of disease in these glands in
man, (2) the effect of their extirpation in animals, and (3) the effects of
extracts of the glands, either upon normal animals or as therapeutic
agents in man.
THE SUPRARENAL GLANDS.
One suprarenal gland lies at the upper end of each kidney. Each
consists of an outer yellowish cortex partially or completely eas.
a darker central portion, the medulla.
The cortex is composed of cells arranged in radial columns and
400
—
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A
THE DUCTLESS GLANDS. | 401
forming three zones: an outer or zona glomerulosa, middle or zona
fasciculata, and inner or zona reticularis. The columns are supported
by strands of connective tissue-in which lie numerous capillaries; the
cells are polyhedral, the cell substance being clear and often containing
lipoid globules.
The cells of the medulla are arranged in an irregular network, their
protoplasm being granular and often pigmented. The blood supply of
the gland is extremely abundant, particularly in the medulla, in which
the interstices of the network of cells are occupied by large sinusoids
in intimate relation with the medullary cells. The glands are supplied
with nerve fibres from the semilunar ganglia, and a few scattered herve
cells are present.
The medullary cells contain a substance which stains brown: with
chromates, and which, on account of its affinity for chromates, has been
described as chromaffine material. This is also found, apart from the
suprarenal glands, as small masses of tissue (paraganglia) lying along
the large abdominal blood-vessels, and in or close to the sympathetic
ganglia. The amount of this accessory chromaffine substance varies
greatly in different groups of animals.
The cortical and medullary parts of the glands have a different
origin, the cortex being developed from mesodermic tissue (the Wolffian
body), whereas the medulla is ectodermic, forming part of the primitive
sympathetic system, from which it finally becomes separated, and
differentiated. In some fishes the cortical and medullary tissue persist
as anatomically separate organs, and it is not known whether their
coalescence into a single organ in mammals implies any physiological
relationship between them.
In 1855 Addison pointed out that, in man, a disease of which the
chief symptoms are prostration, muscuiar wasting, vomiting, and
pigmentation of the skin, and which ends fatally, is associated with
disease or atrophy of the suprarenal glands. This observation was soon
followed by the study of the effects of their removal in animals; and it
was found that, in mammals, removal of the glands was followed by
death in two or three days.
Subsequent investigation has thrown but little light on the functions
of the cortical part of the gland, beyond the fact that tumours of the
cortex are sometimes associated with abnormally precocious sexual
development. In fishes the removal of the interrenal body, which
corresponds in structure and origin with the cortex of the mammalian
suprarenal gland, is said by some observers to cause death, though
others deny this.
The medulla contains a substance, adrenalin, Which can be ex-
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THE DUCTLESS GLANDS. 403
tracted from it in the pure condition, and which has the constitutional
formula
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Adrenalin has also been prepared synthetically.
The brown staining of the medulla when the gland is hardened in
a chromate solution is due to the combination of the chromate’ with
adrenalin, and the depth of the colour is roughly proportional to the
amount of adrenalin present. The accessory chromaftine material, which
Adr.
Vagt Intact.
Fic. 134.—Blood-pressure tracing. Effect of injecting 0°05 mgr. adrenalin
into a vein.
Note the marked slowing of the heart.
also stains with chromate, contains adrenalin. Adrenalin is completely
absent from the cortex of the suprarenal glands. —
When a small amount of adrenalin is injected into a blood-vessel
it produces constriction of almost all the arterioles of the body, and, if
the vagus nerves have been divided, an enormous rise of blood pressure
is produced (fig. 133). When the vagus nerves are intact the rise of
blood pressure is less (fig. 134), because, in accordance with Marey’s
law (p. 223), slowing of the heart takes place.
The action of adrenalin is not confined, however, to the blood-
vessels, but extends to every structure in the body which is normally
supplied with nerve fibres from the sympathetic system. It stimulates
the nerve endings of these fibres in the structures which they supply,
and the results of the injection of adrenalin are identical with those
of stimulation of the entire sympathetic system. Thus it increases
404 ESSENTIALS OF PHYSIOLOGY.
the force and (if the vagi are divided) the rate of the heart, and at
the same time dilates the coronary vessels, so that, in spite of the rise
in blood pressure, the efficiency of the heart is maintained and its
output may even become larger. It inhibits the movements of the
digestive tract and (in many animals) of the bladder, but causes
constriction of the ileo-colic sphincter ; it may also produce sweating,
erection of hairs, and dilatation of the pupil.
Adrenalin is an extremely active substance, and even 0:0025
milligram per kilo of body weight, when injected into the circula-
tion, produces a definite rise of blood pressure.
The exact point of action of adrenalin is not on the sympathetic
endings proper, but probably on some receptive substance (neuro-
muscular junction), which is believed to lie between the actual nerve
ending and the muscular fibre which it supplies, and which is un-
affected by degeneration of the nerve fibre. This can be shown by the
following experiment. When the cervical sympathetic nerve, which
supplies the pupil, is stimulated, or when adrenalin is injected into a
vein, the pupil dilates. 1f the superior cervical ganglion on one side
is removed, the post-ganglionic fibres degenerate, and when time has
been allowed for their degeneration electrical stimulation of these fibres
produces no effect on the pupil, whereas on the injection of adrenalin
the pupil dilates even more fully than in the normal animal.
Adrenalin is constantly being formed by the suprarenal glands,
from which it passes into the blood stream, and is thus a true internal
secretion. This secretion can be increased in amount by stimulation
of the splanchnic nerves, which contain secretory fibres for the supra-
‘renal glands. The nerves may be stimulated either directly, e.g. by
stimulation of the peripheral end of a divided splanchnic nerve, or
reflexly, the centre for this reflex being either the vaso-motor centre
or an adrenalin centre lying very near the vaso-motor centre. The
occurrence of reflex secretion of adrenalin by the suprarenal glands can
be demonstrated in the following manner: one splanchnic nerve, e.g.
the left, is divided in the cat, so as to cut off the efferent path to one
gland; and it is found that, as the result either (1) of stimulation of
sensory nerves, or (2) of violent emotion, such as fear, that the
adrenalin is discharged more or less completely from the right supra-
renal gland, while the left gland remains unaffected. Evidently
division of the splanchnic nerve, by breaking the efferent side of the
reflex arc, prevents any reflex secretion of adrenalin from that gland
into the blood stream. The effects of stimulation of the splanchnic
nerve, and the consequent setting free of adrenalin, on the arterial
blood pressure and on the constriction of arterioles outside the
y
, |
2
g
THE DUCTLESS GLANDS. 405
splanchnic area have already been described (p. 234). Anzesthetics,
such as chloroform, also excite the centre and bring about a discharge |
of adrenalin. .
In all probability, the varying activity of the suprarenal glands,
brought about by impulses reaching them along the splanchnic nerves,
plays an important part in the adjustment of the vascular system to
the changes constantly taking place in the body. A striking instance
of this adjustment is seen, as Cannon has pointed out, in states of
violent emotion, such as rage, pain, or fear. The additional adrenalin
sent into the blood stream in these circumstances increases the sugar
in the blood, thereby providing a further supply of sugar to the
skeletal muscles, and also improves the nutrition and efficiency of
the heart and the blood supply to the brain. In this way the reaction
of the animal to these emotional states, by movements of attack or
defence, is rendered more effective.
Owing to its action on the blood-vessels, adrenalin has proved of
great value, both in checking hemorrhage when ‘applied locally, and in
raising the arterial blood pressure in the condition of shock. In
Addison’s disease the repeated injection of adrenalin is stated in some
cases to produce improvement, though the disease cannot be cured.
THE PITUITARY BODY (HYPOPHYSIS CEREBRI).
The pituitary body, which lies in the sella turcica and is connected
by the infundibulum with the third ventricle, is a small mass consisting
of two lobes, anterior and posterior. The posterior lobe originates as
an outgrowth from the under surface of the brain, and at first is hollow ;
in man the cavity ultimately disappears and it is composed entirely of
neuroglia. The anterior lobe arises as a hollow projection from the
buccal epithelium, and in the adult consists of a solid mass of cells,
many of which are clear-and stain deeply with eosin, whereas others
are very granular. Between the columns of cells are numerous
capillaries.
The two lobes are separated by a narrow cleft lined by clear cells,
which form the pars intermedia and are most abundant on the posterior
wall of the cleft, partially surrounding and extending into the posterior
lobe. The cells tend to become transformed into colloid material,
which ultimately reaches the cavity of the third ventricle.
Complete removal of the whole gland or of the anterior lobe is
followed by death in a few days, though the animals survive when
minute fragments of the anterior lobe are left. After partial removal
of the anterior lobe, young animals often show abnormal metabolic
406 ESSENTIALS OF PHYSIOLOGY.
changes, ¢.g. obesity, and failure of sexual development ; in the adult
degenerative changes take place in the sexual organs.
In man two abnormal conditions have been found in association
with hypertrophy of the pituitary body. In the first place, hyper-
trophy has been observed in unusually tall people, and, secondly, over-
growth or tumours of the gland are found in the disease known as
acromegaly. This occurs in adults, and is characterised by progressive
enlargement of the bones of the face and extremities.
Both these changes are probably the result of increased functional
activity of the anterior lobe only, since young animals to whose food
the anterior lobe is added grow more rapidly than control animals
which receive no such addition ; extracts of the posterior lobe or pars
intermedia have no influence on growth.
We may conclude, therefore, that the anterior lobe is concerned
with the process of growth, hypertrophy leading to overgrowth of the
skeleton, and partial removal to failure of development of the body as
a whole and of the sexual glands. |
Extracts of the posterior lobe, when injected into an animal, have a
direct action on plain muscle all over the body ; they cause constriction —
of the arterioles and a rise of blood pressure, contraction of the
muscular coats of the digestive tract and of the bronchioles, and
contraction of the uterus.
The extracts also produce an increased flow of urine, which was at
first attributed to the presence of a substance having a specific effect
upon the renal cells ; it is probable, however, that the diuretic effect
is merely an indirect result of the more rapid flow of blood through the
‘kidney which follows the injection of the extract.
Extracts of the posterior lobe increase the secretion of milk, and
after the injection a larger amount of milk is formed by the animal
in the course of twenty-four hours. The active principle of the extract
has not been isolated, but it is not destroyed by boiling, and is compara-
tively stable. It is formed entirely by the pars intermedia, extracts of
the posterior lobe proper being quite inert.
The secretion of the pars intermedia is believed by some writers to
pass into the cerebro-spinal fluid, but the extent to which the effects
produced by extracts of the posterior lobe of the gland are normally
brought about in the body is unknown.
THE THYROID AND PARATHYROID GLANDS.
The thyroid gland consists of two lobes, one on each side of the
trachea, united by an isthmus, and is composed of closed spherical
vesicles containing a viscid colloid material; the walls of the vesicles
ee ee a er
THE DUCTLESS GLANDS. 407
consist of a single layer of cubical epithelial cells. The colloid material
contains iodine in organic combination as a substance, codothyrine, and
the vesicles are bound together by connective tissue in which lie
numerous blood-vessels and nerves.
The parathyroid glands are four.in number and lie close to, or
embedded in, the thyroid gland; they are quite small and consist of
columns of granular cells, some of which are pigmented. _
Attention was called to the importance of the thyroid gland first
by the observation that the gland is atrophied in the disease known as
myxedema, and later by the serious and even fatal effects of its
complete removal in man; this may be followed either by acute nervous
symptoms accompanied by muscular spasms (tetany), or by chronic
changes resembling myxedema. The symptoms of myxodema are
obesity, dryness and thickening of the skin, falling out of the hair,
slowness of mental processes and of speech ; indeed, all the metabolic
processes in the body become more rag ini and the respiratory
exchange is diminished.
Deficiency or absence of the gland at birth gives rise to the disease
known as eretinism, in which growth, both mental and physical, is
extremely retarded; a cretin. aged fifteen to eighteen years may
resemble a child of two or three years of age in its size and mental
development. The symptoms, both of cretinism ‘and of myxcedema, are
due to the absence of some substance normally formed by the thyroid
gland, from which it passes into the lymphatics and so into the blood
stream. This is shown by the fact that extracts of the gland, or the
gland itself, when given by the mouth, lead to complete recovery in
myxcedema and to very marked improvement in cretinism.
The action of the gland seems to depend partly or wholly upon the
presence of iodothyrine, since the activity of the extracts is greater
when they contain much iodine. It is evident that the thyroid gland
exerts an important influence on the metabolism of the body, including
the nervous system. ‘This is further shown by the observation that in
myxcedema much larger quantities of sugar can be taken by the
mouth without producing alimentary glycosuria than is the case in
normal persons.
The effects of removal of the at in' animals are of two kinds.
Frequently, especially in carnivora, its removal is rapidly followed by
acute symptoms, of which the most striking are spasms of the skeletal
muscles known as tetany, and in young animals death may occur in a
few days. The acute stage may be followed or may be replaced by
chronic disturbance of nutrition, and in monkeys typical myxedema
similar to that seen in man has been observed. Opinion is divided as
408 ESSENTIALS OF PHYSIOLOGY.
to the part played by the absence of thyroid and parathyroid tissues
respectively in the production of the symptoms which follow the
removal of the entire gland. Some consider that parathyroid tissue
has no distinct function of its own, but is merely undeveloped thyroid
tissue. They find that when the greater part of the thyroid is removed -
the parathyroids increase in size and contain colloid material; and they
regard all the symptoms which fellow the removal of the thyroid and
parathyroid glands as the result of the removal of thyroid tissue
proper.
Others believe that removal of the parathyroid glands: produces the
acute nervous symptoms, and more especially tetany, whereas removal
of the thyroid gland alone brings about chronic changes in metabolism
and nutrition. The difficulty of determining the function of the para- |
thyroid tissue is due to the fact that in many animals it is deeply
embedded in the substance of the thyroid gland. When this is the
case, it is difhcult either to remove the thyroid gland without also
damaging the parathyroid tissue, or to remove the parathyroid glands
without serious injury to the thyroid glands.
The balance of evidence, however, favours the view that the
functions of the thyroid and parathyroid tissue are distinct, and that
the former is concerned solely with metabolism, tetany being the result
of removal of the parathyroids.
t
THE SPLEEN.
The spleen is a solid organ enclosed in a capsule, which is partly
_ fibrous and partly consists of plain muscular tissue. The capsule sends
trabecule, also containing unstriated muscle, into the interior of the
organ; these branch to form a framework, in the interstices of which
lies the spleen pulp. This consists of a fine network of connective-
tissue fibrils, covered by endothelial cells, and containing in its meshes
lymphocytes, red blood corpuscles, and large cells which are amoeboid
and often contain partly broken-down red corpuscles. Multinucleated
giant cells are also sometimes present.
The outer coat of the arteries in the spleen consists of lymphoid
tissue, an enlargement of which is present on each arteriole and forms
a Malpighian corpuscle. Some capillaries are found in the Malpighian
bodies, but, with this exception, the arterioles open directly into the
spleen pulp, from which the blood is again gathered up to leave the
spleen along the splenic vein. The blood thus comes into direct contact
with the tissue elements of the spleen, whereas in almost every other
organ of the body it is separated from the tissues by a capillary wall.
The flow of blood through the spleen is assisted by the alternate
THE DUCTLESS GLANDS. 409
contraction and relaxation of the muscular tissue in its capsule and
trabecule ; this rhythmic contraction, which takes place about once a
minute, can be recorded by enclosing the spleen in a plethysmograph
connected. with a tambour. The muscular fibres are supplied with
nerves from the sympathetic system, and the direct or reflex stimulation
of these nerves, or the injection of adrenalin, produces contraction of
the muscle and diminution of the volume of the spleen.
The functions of the spleen are not fully known, though it is not
essential to life and can be removed without serious after-effects. The
presence of partially disintegrated red blood corpuscles in the phagocytic
cells of the pulp indicates that it is concerned in the destruction of
red cells, but the extent to which this takes place is not known. The
spleen normally contains a relatively large amount of iron, and when
the destruction of red cells in the body is excessive this amount is in-
creased. Further, the Malpighian bodies undoubtedly form lympho-
cytes. In all probability the spleen also takes part in the production
of uric acid, since it contains enzymes which can convert xanthine
and hypoxanthine into uric acid. |
In many infective diseases the spleen is enlarged, and it seems to
play a part in the protection of the body against disease by removing
micro-organisms from the blood, possibly also by destroying the poisons
formed by such organisms.
THE THYMUS.
The thymus is composed of lobules united by connective tissue ;
each lobule consists of an outer, denser cortex and an inner medullary
part, the cortex being subdivided by strands of connective tissue into a
number of compartments. Both the cortex and medulla are composed
of a network of fibrils covered with endothelial cells, the meshes being
_ occupied by lymphocytes. In the medulla are found a number of
small masses of flattened epithelial cells arranged concentrically ; they
are called Hassall’s corpuscles, and represent the remains of part of
the epithelium of the third visceral pouch. The gland is abundantly
supplied with blood-vessels. In man it reaches its maximum size
during the first two or three years of life, and then becomes smaller,
being almost completely absent in the ‘adult. After its removal in
animals, the testes develop more rapidly, and conversely castration
delays the atrophy of the thymus. When young animals receive fresh
thymus with their food sexual maturity is delayed, and, in the male,
degeneration of the testis takes place.
CHAPTER XVI.
REPRODUCTION.
In all except the lowest forms of life, the continuance of the species is
effected by means of certain tissues set apart for this purpose. These
form cells which develop into a new animal of the same species,
the process constituting reproduction. In most animals these cells are
of two kinds, namely, spermatozoa and ova, formed by the reproductive
organs of the male and female respectively ; a spermatozoon and ovum
fuse to form,a new cell which develops into an animal resembling its
parents in its general characters. This, again, is capable of reproducing
itself, and the cycle of life is completed.
THE MALE REPRODUCTIVE ORGANS.
These consist of the testes, which form spermatozoa, and of accessory
organs, namely the vesicule seminales, the prostate gland, the glands.
of Cowper, and the penis. Each testis is covered by a strong fibrous
capsule, the twnica albuginea, from which trabecule pass into the gland,
dividing it into lobules which contain the seminal tubules. Each
tubule is convoluted, and consists of a lining epithelium several layers
thick, resting upon a laminated basement membrane. The cells
nearest the basement membrane are called spermatogonia; these
divide, giving rise to the spermatocytes, which lie more internally.
Within the layer of spermatocytes, and formed from them by division,
are the spermatids, which develop into spermatozoa, Lying in the
connective tissue between the tubules are groups of polyhedral cells,
called interstitial cells. The seminal tubules lead into straight tubules
(rete testis) which open into the epididymis ; this is a convoluted tube
lined by ciliated cells, and is continued as the vas deferens, a thick
muscular tube which opens into the prostatic part of the urethra. The
vesicule seminales are “branched sacculated outgrowths from the
vas deferens. The prostate gland surrounds the first part of the
410
ee ie ae Ete de® Be Beith ee
F 4
REPRODUCTION. AII
urethra, and is made up of numerous branched, glandular tubes
supported by connective tissue and unstriated muscular tissue.
The penis consists of erectile tissue, which forms the corpus
spongiosum and the two corpora cavernosa, and it contains the
urethra; the erectile tissue is made up of a meshwork of elastic and
muscular tissue into which arterioles open directly, the blood escaping
into veins. When the muscle fibres are relaxed the spaces become
distended with blood, causing erection of the organ.
_ The formation of spermatozoa begins at puberty, and each sperma-
tozoon consists of a head, body, and tail, and is actively motile. The
fully formed spermatozoa pass from the testis into the epididymis and
vas deferens, and so to the seminal vesicles.
FEMALE REPRODUCTIVE ORGANS.
The female generative organs are the abit Fallopian (uterine)
tubes, uterus, and vagina.
The Ovary is a solid organ composed of fibrous tissue (stroma), with
many spindle-shaped cells, and is covered by a layer of cubical cells
called the germinal epithelium. Groups of interstitial cells are found
in the stroma, similar to those which occur in the testis. The ovary
contains many vesicles of varying size (Graafian follicles), and a large
number of primordial follicles ; the latter are formed during foetal life
from down-growths of the germinal epithelium into the stroma, and
each consists of an ovum surrounded by a layer of flattened cells.
The ovwm is a large spherical cell enclosed in a striated envelope
called the zona pellucida (striata ); its protoplasm, which is abundant, is
filled with fatty and albuminous granules, sa contains a spherical
nucleus (germinal vesicle) and nucleolus.
At puberty some of the primordial follicles develop into Graafian
follicles (vesicular ovarian follicles). The epithelial cells covering the
ovum multiply to form a mass in which fluid appears, separating the
epithelium into two parts, an outer layer, the membrana granulosa, form-
ing the wall of the follicle, and an inner layer, the diseus proligerus, sur-
rounding the ovum. At this stage the whole follicle is enclosed in a
fibrous capsule derived from the stroma. As the amount of fluid in-
creases, the follicle approaches the surface of the ovary, and eventually
bursts, the ovum being set free and passing into the Fallopian tube.
The process just described is called ovulation. The space left by the
escape of the ovum and fluid is filled up by the ingrowth of vascular
processes fromm the surrounding tissue, forming the corpus luteum, so
called because its cells are yellowish in colour owing to the presence of
412 ' ESSENTIALS OF PHYSIOLOGY.
a fatty pigment, /wtein. It gradually undergoes fibrous changes and
disappears within two months. If pregnancy occurs, the corpus luteum
becomes much larger and does not disappear until after parturition.
The primordial ova are extremely numerous in the ovary, but only a
small proportion of them develop into Graafian follicles, and only a few
of the latter reach maturity and burst, the others, after developing to
a certain extent, undergoing atrophy. During sexual life ovulation
usually occurs once a month, a single ovum being discharged on each
occasion. The process is intimately bound up with menstruation.
The Uterus.—The uterus consists of two parts, the body and the
cervix. Its cavity is lined by a thick mucous membrane, composed of
soft connective tissue covered by ciliated epithelium which dips down
into the membrane to form long tubular glands,
The mucous membrane rests on a thick muscular coat arranged in
two layers. The fibres of the outer layer run chiefly longitudinally,
but some run circularly ; the fibres of the inner layer, which is much
thicker than the outer one, run circularly, and are really a greatly
hypertrophied muscularis mucose.
The Fallopian (uterine) tube consists of a mucous membrane thrown
into numerous longitudinal folds and lined by ciliated epithelium.
- The mucous membrane rests upon a muscular coat, the outer fibres
being longitudinal and the inner circular.
Both the uterus and Fallopian tubes are covered by a serous
membrane derived from the peritoneum.
Menstruation.—This marks the onset of puberty, and occurs first
between the ages of 13 and 16; as a rule, it recurs once a month until
about the age of 45, its cessation at this age being called the menopause,
Each month the mucous membrane of the uterus becomes con-
gested and thickened, and eventually some of the blood-vessels of the
membrane rupture; the escaping blood, together with the superficial
epithelium of the uterus and the secretion of the uterine glands, form
the menstrual flow, which lasts four or five days, the loss of blood
varying from 100 to 300 c.c. When it ceases, the mucous membrane
of the uterus is gradually regenerated, and returns to its original
condition.
Menstruation is associated with feelings of malaise and often with a
slight rise of temperature. It is absent during pregnancy, and usually
during lactation, and is undoubtedly related to and dependent upon
ovulation, though the latter may either precede or follow the menstrual
flow. Menstruation ceases after the removal of the ovaries, and also
at the menopause, when ovulation no longer takes place; its object
appears to be to render the condition of the uterus, at the end of the
;
i
REPRODUCTION. mee 413
menstrual period, suitable for the reception and development of the
ovum if fertilisation takes place. In many of the lower animals a
somewhat similar change in the uterus, known as the estrus, occurs at
certain seasons of the year, and is ‘accompanied by ovulation and sexual
activity.
Maturation of the Ovum.—After being discharged from the ovary
the ovum undergoes certain changes known as maturation. When a
cell divides in other (somatic) tissues the process is initiated by changes
in the nucleus and is known as mitosis. The network of chromatin in
the nucleus breaks up into a number of segments called chromosomes,
each composed of rows of granules; the number of chromosomes thus
formed is constant for every somatic cell for a given species of animal,
and in man the number is sixteen. The next stage consists in the
splitting of each chromosome longitudinally into two halves, which
travel to opposite ends of the cell. While this is taking place the cell
protoplasm constricts and then divides between the nuclei to form two
cells, each of which contains the same number of chromosomes as the
original cell. Finally, the chromosomes of each daughter cell fuse into
a single chromatin filament. :
In the germ celis a different form of cell division takes place, and is
known as heterotype mitosis. The ovum divides twice. The daughter
cells from the first division differ greatly in size, and the smaller
one, called the first polar body, breaks up and disappears. The
characteristic feature of this division is that the number of chromosomes
formed is only half that occurring in a somatic cell in the same animal. —
The larger cell, which contains only half the normal number of chromo-
somes, divides again to form two daughter cells, one of which is small,
the second polar body, whereas the other is large and constitutes the
mature ovum.
A similar process takes place in the formation of spermatozoa,
except that the four davghter cells, spermatids, formed from the sper-
matocytes, all develop into spermatozoa ; and in man each spermatozoon
and mature ovum contains eight chromosomes, whereas. the somatic
cells contain sixteen,
Fertilisation takes place as a result of the introduction of sperma-
tozoa into the vagina during the act of coitus. The motile spermatozoa
travel into the uterus and Fallopian tubes, where they may live for
some days. If a spermatozoon penetrates into an ovum, it loses its
tail, changes take place in its head and neck, and it is converted into a
male pronucleus, which fuses with the nucleus of the ovum (female
pronucleus) to form a new cell containing the normal number of
chromosomes; this process constitutes fertilisation.
414 * ESSENTIALS OF PHYSIOLOGY.
The cell thus formed at once divides, and when the ovum which has
been fertilised in the Fallopian tube reaches the uterus, it has already
divided and subdivided to form a small mass of cells called a morula.
The further development of the’ morula takes place in the uterus
and constitutes pregnancy. The morula makes its way into the uterine
mucous membrane, which then consists of three parts: namely, (1) the
decidua basalis, lying between the embryo and the muscular wall of the
uterus ; (2) the decidua capsularis (reflexa), between the embryo and the
cavity of the uterus ; and (3) the decidua vera, lining the remainder of
the uterus. As the embryo grows, it becomes enclosed in a sac filled
with fluid and called the amnion ; surrounding the amnion is a vascular
membrane, the chorion, from which blood-vessels pass to and from the |
foetus in the umbilical cord. After the third month of pregnancy the
foetus receives its nutrition from the placenta, which is formed partly
from maternal and partly from foetal tissue. It consists essentially of
large blood spaces in the decidua basalis, into which open the uterine
arteries, and from which the blood of the mother is carried into the .
uterine veins. Projections from the chorion (chorionic villi) containing
foetal blood-vessels lie in these spaces and are bathed by the maternal
blood ; the blood reaches the villi along the umbilical arteries, and is
. returned to the foetus in the umbilical vein. The maternal and foetal
blood are thus separated by a double layer of epithelium, and the
nutrition of the foetus is effected by the diffusion of oxygen and
nutritive material through this epithelium. °
Parturition.—The average duration of pregnancy is 280 days,
during which the muscular wall of the uterus not only increases in
size, but becomes greatly thickened. Parturition is brought about by
rhythmic contraction of the uterine muscle, and the foetal membranes
and their contained fluid are forced through the os uteri, which becomes
fully distended. This, the first stage of labour, ends when the os uteri
is fully dilated ; and the membranes rupture about this time.
The foetal head then enters the pelvis, and the uterine contractions
become more prolonged and frequent, being accompanied by voluntary
contractions of the abdominal muscles. The foetus is gradually forced
through the pelvic canal and vulva, the head normally being born first.
The second stage of labour ends when the child is born. The whole
process of parturition varies greatly in duration, and may last twenty-
four hours. Shortly after the birth of the child the uterus contracts
further and expels the placenta.
After parturition the uterus rapidly decreases in size, this being
known as involution.
The Mammary Gland.—The mammary gland consists of a number
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REPRODUCTION. 415
of lobules embedded in fat and areolar tissue. Each lobule is composed
of alveoli lined by columnar epithelium, resting on a basement |
membrane. The ducts open on to the nipple, and are lined by cubical
epithelium ; their walls are said to contain unstriated muscular fibres.
During the secretion of milk the superficial part of the cells, which
contains fat globules and secretory granules, disintegrates and is cast
off to form part of the secretion ; during the periods of rest the cell
substance is re-formed. ;
~The development of the mammary gland during pregnancy is
brought about by the influence of hormones derived from the re-
productive organs. In the earliest stage of pregnancy a hormone
appears to be formed in the corpus luteum, and if a Graafian follicle
is artificially ruptured in an animal the mammary glands develop for
a short time apart from pregnancy. Later, another hormone is possibly
formed by the foetus itself, since it has been shown that injection of
an extract of foetal tissue into a virgin rabbit leads to growth of the
mammary glands... That their development: is due to chemical and
not to nervous influence is further indicated by the fact that, even after
the severance of all nerves to the mammary gland, it undergoes normal
development during pregnancy. _
The Secretion of Milk.—Very little is known as to the mechanism
by which the secretion of milk is brought about, although it can un-
doubtedly be influenced through the nervous system. As already
mentioned, the injection of extracts of the hypophysis increases the
secretion of milk, but it is not known whether the hypophysis normally
plays any part in the process.
SUBSIDIARY FUNCTIONS OF THE REPRODUCTIVE ORGANS.
The reproductive glands not only form the essential reproductive
elements, namely, spermatozoa and ova, but influence very markedly
the growth and development of the rest of the organism.
In the male the onset of puberty, z.e. the formation of spermatozoa
in the testis, is associated with the development of secondary sexual
characteristics, such as changes in the larynx, deepening of the voice,
and the growth of hair on the face and pubes. If the testes are
removed before puberty these characters do not develop, and the body
remains infantile. After puberty castration leads to atrophy of the
accessory genital organs. In the lower animals castration also prevents
the appearance of secondary sexual characteristics, such as the antlers
of the stag, or the comb of the cock.
In the female extirpation of the ovaries iaente the occurrence
co 8 @ ieee ESSENTIALS OF PHYSIOLOGY.
of menstruation and the development of the mammary gland, which
normally take place at puberty; their removal after puberty brings
about the cessation of menstruation.
The normal changes accompanying sexual development appear to
depend for their occurrence upon an internal secretion formed by the
testes or ovaries, and they furnish a striking illustration of the chemical
interrelation between the different parts of the body. The hormones
concerned in the growth of secondary sexual characteristics have not
been isolated, anc the exact site of their formation is doubtful; some
observers believe that they are formed by the interstitial cells of
the testis and ovary, since ligature of the vas deferens, while causing
atrophy of the seminiferous tubules, does not affect the interstitial cells
or the development of secondary sexual characteristics.
HEREDITY.
When reproduction takes place the offspring bear a general resem-
blance to the parents, though differing in detail from both of them.
The transmission of the qualities of the parent is carried out solely by
the germ plasm, and the hereditary qualities are present in the nuclei
of the spermatozoon and ovum respectively. The spermatozoon, how-
ever, appears to have a double function; in addition to containing
potentially the characters of the male parent, it acts as a chemical or
physical stimulus to segmentation and division of the ovum. This
latter function can, to some extent, be replaced in certain invertebrates
(e.g. sea urchins) by producing physical changes in the environment of |
the ovum ; and under such conditions unfertilised ova have been made
to develop partially or completely into normal larvee.
The formation of polar bodies, and the analogous process taking
place in the formation of spermatids prior to fertilisation, involves
the loss of part of the nuclear substance, and therefore, probably, of
part of the parental qualities transmitted to the offspring. On this
account the characters of the offspring show variations from those of
the parents. In many cases these variations have proved advantageous
in the struggle for existence by adapting the animal more closely to its
environment ; and the survival of those most fitted for their surroundings
has led to the evolution of the higher forms of life.
An important problem in heredity is whether the characters of the
offspring represent a mean between those of the parents, or whether a
parental character can be transmitted completely or not at all. There is
evidence that in many cases the latter is true, and the conditions under —
which this transmission occurs are known as Wendel’s law, When tall
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REPRODUCTION. 417
and dwarf peas, for example, are crossed, all the seeds produce. tall
plants. If the seeds from these tall plants are crossed with each other,
three-quarters become tall and one-quarter are short. In this case the
qualities of tallness and shortness have not fused, and the quality of
tallness is either present or absent. When it is present the plant is
tall, even though the quality of shortness is also present; tallness is
said, therefore, to be a domznunt, and shortness a recessive character.
If the recessive character alone is present the plant is short.
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INDEX
Abdominal muscles, 247,
action in respiration, 247.
Abducens nerve, centre, 67, 69.
Aberration, chromatic, 127.
spherical, 127.
Absorption from tissues, 242.
of carbohydrates, 331.
fats, 331,
proteins, 330.
Acapnia, 268.
Accelerator nerves, 221.
Accessory auditory nucleus, 70, 91.
Accommodation of eye, 123.
Acetone, 347.
Achroo-dextrin, 7, 293.
Acid metaprotein, 9, 303.
Acidosis, 347, 385.
Acoustic tubercle, 70, 91.
Acromegaly, 406.
Adaptation, 106, 133.
Addison’s disease, 401.
Adenine, 11, 359.
Adrenalin, 218, 224, 227, 234, 401.
action on coronary arteries, 218.
_effect on heart, 224.
production, nervous control of, 234.
Adsorption, 16.
Aérotonometer, 255, 261.
ZEsthesiometer, 110.
. Afferent nerves, 34.
projection system, 83.
After-images, 135, 136.
Air, alveolar, 251.
atmospheric, 250.
changes by breathing, 250.
complemental, 249.
quantity breathed, 249.
reserve, 249.
residual, 249.
tidal, 249.
transmission. of sound-waves through,
145.
Air-sacs, 245.
Alanine, 12, 355.
Albumin, 10.
characters of, 10.
egg, 13.
serum, 13, 166.
418
—
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j
3
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)
|
Alcaptonuria, 388.
Alcohol as a beverage, 378.
Aldehyde, 5.
Aldoses, 6.
Alimentary glycosuria, 350, 407.
Alkali metaprotein, 9, 314.
Alveolar air apparatus, 251.
composition of, 251.
Amacrine cells, 118.
Ametropia, 126.
Amino-acids, 9,
355.
in blood, 855,
Ammonia in urine, 348, 385.
Amnion, 414,
Ameeba, 1.
Ameeboid movement, 164.
Ampulla, 151.
Amylolytic enzymes, 293, 315.
Anabolism, 338,
Anacrotic pulse, 206.
Anaphylaxis, 168.
Anarthria, 93.
Anelectrotonus, 35.
Animal cell, 2.
heat, 365.
Anode, 23.
Antagonistic colours, 136.
muscles, reciprocal action of, 61.
Anterior cerebro-spinal fasciculus, 52,
Antero-lateral ascending tract, 54.
descending tract, 53.
Antibodies, 167.
Antidromic impulses, 229.
Antigens, 167.
Antithrombin, 172.
Antitoxin, 168.
Apex beat, 193.
Aphasia, 93.
Apnoea, 249, 267, 276.
in ducks, 277.
Aqueduct, cerebral, 76.
Aqueous humor, 119.
Arcuate fibres, 68.
Area, association, 92.
audito-psychic, 91.
audito-sensory, 91.
gustatory, 92.
12, 314, 325, 331,
Area, kinesthetic, 86, 89.
motor, 86.
olfactory, 92.
visuo-psychic, 90.
visuo-sensory, 90.
Arginase, 356.
Arginine, 13, 356.
Aristotle’s experiment, 111.
Arteria centralis retin, 119.
Arterial blood pressure, 181, 185.
in asphyxia, 270,
Arteries, 178.
bronchial, 246.
coronary, 218.
elasticity of, 208.
in kidney, 381.
nerves of, 226.
pressure of blood in, 181, 185.
rate of blood flow in, 187.
structure of, 178.
umbilical, 414.
Arterioles, 178.
local changes in, 236.
rhythmic contraction of, 231.
Ary-epiglottic fold, 148.
Arytenoid cartilages, 148.
muscle, 149.
Aspartic acid, 12.
Asphyxia, 231, 269.
blood pressure in, 270.
stages of, 269.
Association areas, 82.
fibres, 80, 83, 84.
tracts, 84. —
Astasia, 74.
Asthenia, 74.
Astigmatism, 126.
Atmospheric air, 250.
Atonia, 74.
Atrium, 176, 245.
Atropine, effect on heart, 224,
on pupil, 130.
on salivary gland, 296.
Atwater-Benedict calorimeter, 340.
Auditory area, 90,
nerve, 70.
radiation, 83.
tract, 91.
tube, 141, 143.
Auerbach’s plexus, 326.
Auricles, 176.
Auricular diastole, 179.
systole, 179.
Auriculo-ventricular bundle, 177.
node, 214.
valves, 176, 177.
Autogenetic nerve theory, 46.
Autolysis, 361. —
Autonomic ganglia, functions of, 103.
nervous system, 99,
Axis cylinder, 33.
Axon, 33, 41.
reflex, 103, 230.
INDEX. — 41g
Bacterial action in intestinl digestion,
334, 386.
Bacterium ‘lactis, 376.
Barcroft’s blood-gas apparatus, 252,
tonometer, 256.
Basilar membrane, 1438, 144.
Basket cells, 73.
' Basophile cells, 156.
Benedict’s test, 6, 387.
Beri-beri, 373.
Betz cells, 81.
Bidder’s panigeien: 210.
Bile, 319.
absorption of bile salts, 321,
analysis of, 319.
capillaries, 363.
digestive properties, 322.
influence on fat absorption, 331.
mucin, 320.
pigments, 319, 383.
quantity of, 323.
salts, 320.
secretion of, 323.
uses of, 322.
Bilirubin, 162, 320,
Biliverdin, 320.
Binocular vision, 138, 140.
Bipolar nerve cells, 40.
Biuret reaction, 384,
Bladder, urinary, 398.
Blind spot, 131.
Block, heart, 215, 272,
Blood, 155.
amino-acids in, 354.
arterial and venous, 180.
carbonic acid in, 259.
circulation of, 179.
in foetus, 414.
coagulation of, 169.
defibrinated, 169.
fibrin, 166, 169.
flow, rate of, 187.
gas analysis, 252.
gases of, 252.
nitrogen in, 282.
oxalated, 171.
oxygen in, 256.
plasma, 165.
platelets, 156.
proteins of, 166.
quantity of, 175.
reaction of, 165, 168, 389.
salts of, 166.
serum, 166.
specific gravity of, 155.
venous, 180.
Blood corpuscles, red, 155.
chemistry of, 158.
development of, 163.
fate of, 164.
function of; 159.
number of, 156.
origin of, 163.
420°
Blood corpuscles, specific gravity of, 165,
stroma of, 157.
Blood corpuscles, white, 156, 164.
amoeboid movements of, 164.
chemistry of, 165.
functions of, 164.
origin of, 165. -
varieties of, 156.
Blood flow through an organ, 225.
Blood platelets, 156.
Biood pressure, 180.
in arteries, 182.
in capillaries, 186.
in veins, 182, 185.
influence of gravity on, 232.
influence of respiratory movements on,
283.
measurement of, in man, 185.
Blood vessels, structure of, 178.
Bone marrow, 164,
Boundary zone, 380.
Bowman’s capsule, 380.
Bowman’s glands, 113.
B-oxybutyric acid, 347, 354, 388.
Brachium conjunctivum, 71, 73.
inferior, 77.
pontis, 71, 73.
superior, 78,
Brain, 66,
fore-brain, 79.
hind-brain, 66.
mid-brain, 76.
projection fibres in, 83.
stem, 79.
Bread, 377.
Broca’s convolution, 93.
Bronchi, 245.
Bundle of His, 178, 214.
Calcium salts, 4.
effect on heart, 218.
excretion in large intestine, 333.
in coagulation of blood, 170.
of milk, 304.
hosphate in urine, 386.
Calorie value of diet, 379.
of food-stuffs, 339, 372.
Calorimeter, 339, 340,
Canal of cochlea, 143.
of Schlemm, 115.
eee sugar, 6
igestion of, 324,
Cap; illary circulation, 189.
electrometer, 24,
pressure, 190.
wall, diffusion through, 190.
Carbohydrates, 5
absorption of, 331.
action of gastric juice on, 302,
of pancreatic juice on, 315.
of saliva on, 293..
metabolism, 348.
ESSENTIALS OF PHYSIOLOGY.
Carbonic acid :
excretion of, 244,
in atmosphere, 250,
in alveolar air, 251,
in blood, 259, 265.
tension in alveoli, 262,
in blood, 259.
in tissues, 279.
Carbon monoxide poisoning, 283.
Carboxyhemoglobin, 160.
Cardiac cycle, 192.
impulse, 192. _
murmurs, 195,
muscle, 177.
influence of salts on, 218.
of temperature on, 219.
of tension on, 202.
physiological properties of, 212.
nerves, 219.
output, 198, 202.
reflexes, 221.
sounds, 194.
Cardiogram, 193.
Cardiograph, 193.
Cardio-inhibitory centre, ei 221,
Cardiometer, 199.
Casein, 304.
Caseinogen, 10, 18.
action of gastric juice on, 304.
in milk, 375.
Catalysts, 290.
Cells, 2.
Central nervous system, 39,
Centres in medulla oblongata, 69.
Cephaline, 8.
Cerebellar ataxy, 74.
Cerebello - spinal tract of Lowenthal,
53
Cerebellum, 71.
ablation of, 74,
afferent tracts of, 73.
efferent tracts of, 74,
functions of, 74.
inferior peduncle, 68, 71.
localised lesions of, 75,
middle peduncle, 69, 71.
roof nuclei, 71.
stimulation of, 75.
structure of, 71.
superior peduncle, 71,
vermis, 71.
Cerebral circulation, 236.
hemispheres, ablation of, 84, 85.
association fibres in, 84.
associative functions, 85,
commissural fibres in, 84.
cortex of, 80.
excitability of, 86.
function of layers, 81.
motor and sensory areas (sce Areas).
projection fibres in, 83.
structure of, 80.
tracts in, 82.
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INDEX. 421
Cerebral peduncles, 76.
Cerebro-cerebellar fibres, 83.
Cerebro-spinal fasciculi, 52, 88, 94.
fluid, 97.
Chemiotaxis, 164. —
Cheyne- Stokes breathing, 277.
Cholesterol; 8, 319, 321.
Choline, 8
Chorda tympani, 294.
vaso-dilator fibres of, 228, 295.
Chorde tendinexw, 177, 191.
Choroid coat, 115.
Choroid plexus, 97.
Chromaffine material, 401, 403.
Chromatic aberration, 127.
Chromatin, 11.
Chromatolysis, 42.
Chromoproteins, 12.
Chromosomes, 413.
Chyle, 331.
Ciliary ganglion, 102.
movement, 32.
muscle, 115, 124.
nerves, 128.
processes, 115.
Circulation of blood, 179.
cerebral, 236.
general course of, 179.
schema, 183.
time, 190.
Clarke’s column, 48.
Coagulation of blood, 169.
of colloids, 17.
of milk, 304,
. of proteins, 10.
time of, 174.
Coagulometer, 174.
Cochlea, 143.
Cochlear nerve nucleus, 70,
Coefficient of solubility of a gas, 254.
Cold, effect of, on muscular contraction,
22.
sensations, 111.
spots, 108.
Collagen, 11, 304.
Collaterals in spinal cord, 47.
Collecting tubules of kidney, 380.
Colloids, 16.
adsorption by, 16.
aggregation of, 17.
metals, 16.
osmotic pressure of, 16.
precipitation of, 17,
solutions of, 16.
Colon, movements of, 335.
Colour-blindness, 135.
Colour-contrast, 136.
Colour-vision, 135.
Comma tract, 53.
Commissural fibres of cerebrum, 84.
Compensatory pause, 213.
Complement, 158, 168.
Complemental air, 249.
Complementary colours, 136.
Compressor urethre, 398.
Conduction in nerve, 34.
Cones, 117.
functions of, 131.
Conjugate deviation, 137.
- Conjugated proteins, 11.
Conjunctival reflex, 2, 59.
Consonance, 146. :
Consonants, 150.
Constrictor fibres to arteries of limbs,
101, 228.
Contraction of muscle, 20..
of pupil, 129.
voluntary, 31.
wave, 22,
Contrast in visual sensation, 136.
Convection, loss of heat by, 367.
Convoluted tubules, 380.
secretion by, 393,
Co-ordinated movements, 74, 153.
Cord, spinal, 47.
Cornea, 114.
nerve-endings in, 107.
Corniculate cartilages, 148.
Coronary arteries, 218.
Corpora quadrigemina, 76, 77.
connection with auditory tract, 77.
with optic tract, 78.
Corpus Arantii, 177.
callosum, 84.
luteum, 411, 415.
striatum, 80, 369.
Corpuscles of blood, 155.
of Golgi and Mazzoni, 108.
of Meissner, 108.
of Pacini, 108.
of Ruffini, 108.
Corresponding points, 138.
Cortex cerebri (see Cerebral hemispheres).
Corti’s organ, 144.
Costal respiration, 247.
Cranial autonomic fibres, 102.
Creatine, 13, 19, 385.
Creatinine, 385.
Cretinism, 407.
Crico-arytenoid muscles, 149.
Cricoid cartilage, 148.
Crico-thyroid muscle, 149.
Crista acustica, 152.
Crura cerebri (see Cerebral vodtealan}
Crystalline lens, 119.
Crystalloids, 16.
Cuneiform cartilages, 148.
Curare, action of, 20.
Current, demareation, 37.
electric, in eye, 131.
heart, 216.
muscle, 24.
salivary gland, 299.
of action, 26, 36.
of injury, 26, 3%.
of rest, 26.
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422
Cutaneous end-organs, 107.
impulses, conduction of, 108. :
sensations, 107.
Cystine, 12, 13, 387, 388.
Cyton, 40.
D:N ratio, 352.
Dark-adapted eye, 106.
Dead space, 251.
Deamination, 355, 359, 364.
Decerebrate dog, 85.
frog, 84,
pigeon, 85. '
rigidity, 75.
Decidua, formation of, 414.
Decussation of fillet, 67.
of pyramids, 66,
Deep reflexes, 63.
Deep sensibility, 111,
Defects of eye, 126.
Defibrinated blood, 169,
Degeneration of nerve, 44.
Deglutition, 299.
nervous mechanism of, 301.
Deiters’ nucleus, 70.
Denaturation of proteins, 10.
Dendrons, 41.
Dental consonants, 150.
Depressor nerve, 222, 230, 238.
Descending tracts, 52.
Dextrin, 7, 349.
Dextrose, 5.
in blood, 166, 350.
in urine, 350, 387.
Diabetes, experimental, 352.
in man, 354. .
sugar consumption in, 354,
Diabetic puncture, 351.
Diacetic acid, 347.
Diamino-acids, 13.
Diamino-nitrogen of proteins, 14.
Diapedesis, 190.
Diaphragm, 246.
Diastolic arterial pressure, 185.
Dicrotic wave, 205, 207.
Diet, 371.
Dietaries, 378.
Differential blood-gas manometer, 253.
Diffusion, 14,
Digestion, 288.
course of, 291.
gastric, 302,
intestinal, 312.
pancreatic, 313.
salivary, 291.
Dilatation of pupil, 130.
Dilator pupille, 116.
Dioptre, 127.
Diphasic variation, 26.
Direct cerebellar tract, 54.
Disaccharides, 5
Discord, 146.
Discus proligerus, 411.
Diisee i Cy
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ESSENTIALS OF PHYSIOLOGY.
es
Dissociation of oxyhemoglobin, 256. »
Dissonance, 146.
Diuretics, 397.
Dominant characters, 417.
Dorsal nucleus, 48.
spino-cerebellar fasciculus, 54.
Ductless glands, 400.
Ductus cochlearis, 143.
endolymphaticus, 151.
Dudgeon’s sphygmograph, 50h.
Dyspnea, 249.
TS SO
-
7
Ear, external, 141,
internal, 143.
middle, 141.
Eck’s fistula, 356.
Effector organ, 58.
Efferent nerves, 34,
path of reflex, 58.
projection fibres, 83.
tracts of cerebellum, 74.
Egg-albumin, 9, 13.
Eighth nerve, nucleus, 69.
Elasticity of arteries, 208.
of muscle, 20, 23.
Elastin, 11, 3038. °
Electrical changes in eye, 131.
in heart, 216,
in muscle, 24.
in salivary gland, 299.
Electrical stimulation, 35.
variations, 26.
Electrocardiogram, 216.
Electrolytes, 14.
Electrolytic dissociation, 14.
Electrotonic current, 37.
Electrotonus, 35.
Embryo, 414.
Emmetropic eye, 126.
Emulsion, 7.
End-bulbs, 108,
Endocardiac pressure, 195.
Endolymph, 148.
End-organs in skin, 107.
End-plate, 19.
fatigue in, 31, 96,
End -produets, effect on enzymes, 290,
315.
Energy, sources of muscular, 362.
total daily output of, 366.
Enteroceptive nervous system, 105.
Enterokinase, 314.
Enzymes, 288.
characteristics of, 288,
reversibility of action, 289,
Eosinophile leucocytes, 156.
Epicritic sensibility, 111.
Epididymis, 410.
Epiglottis, 148.
Equilibration, 154.
functions of labyrinth in, 153.
Erectile tissue, 411.
Erepsin, 314, 324.
INDEX. 423
Ergastoplasm, 299, |
Ergotoxin, action of, 230.
Erythroblasts, 163.
Erythrocytes, 155.
Erythrodextrin, 7, 293.
Ethereal sulphates, excretion of, 386.
Eupnea, 249.
Eustachian tube, 141, 143.
Excitatory process in nerve, 34.
Expired air, 250.
Extensibility of muscle, 20.
Extensor reflex, 60.
External auditory meatus, 141.
geniculate body, 78, 79, 134.
/
Exteroceptive nervous system, 76, 105.
Extra systole, 213.
Eye, dark-adapted, 106, 133.
electrical changes in,- 131.
filtration angle of, 115.
formation of image in, 122,
movements of, 137.
nutrition of, 119.
optical defects of, 126.
structure of, 114.
Eye-ball, 114.
Eye-muscles, extrinsic, 137. .
Facial nerve, nucleus of, 67, 69.
Facilitation of reflexes, 62.
Feces, 334.
False vocal cords, 148.
Far point of vision, 125.
Fasciculus cuneatus, 54.
gracilis, 53.
Fasting, metabolism in, 360.
Fat, absorption of, 331.
composition of, 7.
depots, 344,
formation from carbohydrates, 345.
functions of, 346.
in intestinal epithelium, 332.
metabolism in starvation, 360,
oxidation of, 347.
Fatigue, cause of, 29, 30, 96.
of visual mechanism, 136.
Fats, 7.
digestion in stomach, 305.
in intestine, 316.
Fatty acids, 7.
heat equivalents, 339.
Fatty degeneration, 347,
infiltration, 346.
Fehling’s test, 5, 387.
Fenestra ovalis or vestibuli, 141.
rotunda or cochlex, 141.
Fermentation test, 6.
Ferments (see Enzymes).
Fertilisation, 413.
Fibrin, 166, 169.
Fibrinogen, 166, 170.
Field of vision, 138.
Fifth nerve, nucleus of, 67, 69, 79.
Fillet, decussation of, 67.
Fillet, lateral, 77, 91.
Final common path, 61,
Flexion reflex, 59,
Flicker, 135.
Fluid, cerebro- -spinal, 97,
Fluoride, effect on clotting of blood, 174,
Focus of lens, 121.
Food, amount necessary, 372,
Food- stuffs, digestibility of, 379.
calorie values of, 339, 372,
Foramen of Majendie, 97,
Forced movements, 75.
Fore-brain, 79.
Fountain decussation of Meynert, 78.
Fourth nerve, nucleus of, 67, 79.
Fovea centralis, 118.
Free nerve- endings, 107.
Freezing-point, depression of, 15.
Frog, heart of, 209.
muscle-nerve reparation, 20..
Fronto-pontine fibres, 76, 83.
Fructose, 6, 331.
Fundamental tone, 146:
Funiculus cuneatus, 54.
gracilis,.53.
Galactose, 6, 324, 331.
Gall-bladder, 323.
Galvanometer, 24.
Ganglia, spinal, 42, 47, 49.
sympathetic, 99.
Gaseous exchange i in lungs, 260.
metabolism of organs, 279.
Gases, tension of, 255.
Gastric digestion, 301,
fistula, 301.
hormone, 306.
juice, 301.
action on barbideydretea, 302,
fats, 305.
proteins, 303.
appetite secretion, 306,
composition of, 302.
secretion of, 305. .
secretin, 306.
Gastro-colic reflex, 335.
Gastro-ileac reflex, 333.
Gelatin, composition of, 12, 13.
food value of, 377.
Gels, 17.
Geniculate sete external or lateral, 78,
79,1
internal fe Taedial, 77, 79.
Germ cells, 413.
chromosomes in, 413. ‘as yea
division of, 413.
Germinal vesicle, 411.
Glaucoma, 120.
Gliadin, 13.
Globin, 13.
Globulins, 10.
Glomerulus, filtration in, 392,
pressure of blood in, 391.
‘424 | ESSENTIALS OF PHYSIOLOGY.
Glosso-pharyngeal nerve, 277, 301.
nucleus of, 67, 68.
Glottis, 148.
Glucoproteins, ll.
Glucosamine, 11, 304.
Glucosazone, 6.
Glucose (see Dextrose).
Glutamic acid, 12.
Glycerol, 7.
Glycine, 12.
Glycogen, 6, 19.
formation of, 349.
preparation of, 348.
Glycosuria, 350.
adrenalin, 351.
alimentary, 350.
phloridzin, 351.
Glycuronic acid, 388.
Golgi and Mazzoni, corpuscles of, 108.
Gowers’ tract, 54.
Graafian follicles, 411.
Gram molecule, 15.
Gravity, effect of, on ehedidiation, 282.
Green blindness, 135.
Grey matter of cerebral cortex, 80.
rami communicantes, 101.
Guanine, 11, 359.
Gustatory area, 92.
- sensations, 112.
Guttural consonants, 150.
Hematin, 160, 161.
Hematoidin, 162.
Hematoporphyrin, 162.
in urine, 162, 383.
Hemin, 161.
Hemochromogen, 161.
Hemocytometer, 156.
Hemoglobin, 12, 158.
absorption spectrum of, 159.
affinity for carbon monoxide, 160, 283.
composition of, 159,
erystals of, 159.
derivatives of, 161.
dissociation curve of, 256.
iron in, 160,
oxygen capacity of, 256.
union with oxygen, 159, 256.
Hemoglobinometer, 162.
Hemolysis, 157.
Hemolytic sera, 157.
Hair follicles, 367.
Harmony, 146,
Hay’s test, 320.
Head’s diaphragm slip, 265.
Head’s experiment on cutaneous sensa-
tion, 111.
Hearing, 141, 145.
end-organ of, 144.
Heart, 176.
accelerator nerves to, 221,
changes in form of, 193.
Heart, diastolic filling of, 191, 202.
of frog, 209.
‘inhibitory nerve to, 220.
nerve ganglia in, 210,
nutrition of, 217.
output of, 198, 202.
sequence of events in, 191,
sounds, 194.
work of, 198, 204.
Heart beat, 191.
causation of, 209.
effect of calcium on, 218.
of hydrogen ion concentration on, .
218.
of muscarine on, 224,
of nicotine on, 224.
of potassium on, 218.
electrical variations, 216.
influence of temperature on rate of,
myogenic hypothesis, 211, 214.
neurogenic hypothesis, 210.
propagation of, 211.
rate in exercise, 286.
reversal of rhythm of, 211.
significance of CO, for, 219.
Heart block, 215, 272.
Heart lung preparation, 199.
Heart muscle, 177.
influence of tension on, 202, 213.
physiological properties of, 212.
refractory period of. 213.
structure of, 177.
Heat loss, 365, 367, 369.
production, 365, 368.
in muscle, 28, 386.
regulation, 368.
sensations, 111.
spots, 108.
Helicotrema, 143.
Heller’s test, 387.
Hemianesthesia, 55,
Henle’s loop, 380.
Hensen’s line,. 18,
Heredity, 416.
Hering’s theory of colour vision, 136.
Heterotype mitosis, 413.
Hexone bases, 13, 325.
Hexoses, 5.
Hind-brain, 66.
Hippocampus, 82.
Hippuric acid, 386.
Hirudin, 174.
His’ bundle, 178, 214.
Histidine, 12, 13.
Histones, 10, 158,
Homogentisic acid, 388,
Homoiothermic animals, 365,
Hopkins’ test for lactic acid, 27.
for tryptophane, 9.
Hormones, 2.
gastric, 306.
genital, 416.
Se a
Ve Gene ae) ee ee wait 3 gd .
INDEX. 425
Hormones, intestinal, 325. Intra-thoracic pressure, 248.
mammary, 414. Intra-vascular coagulation, 173.
pancreatic, 318. : Inversion, 324.
Hiirthle’s manometer, 196. Invertase, 324.
Hyaloid membrane, 119. Involuntary muscle, 31.
Hydremic plethora, effect: on volume of | Iodine in thyroid gland, 407.
urine, 392. Iodo-thyrin, 407,
Hydrated proteins, 9. Ionisation, 10,
Hydrogen ion concentration, 168. Ions, 10, 14.
Hydrolysis of starch, 6. Iris, 115.
of proteins, 9. functions of, 128.
Hyperglycemia, 350. local stimulation of, 129.
Hypermetropia, 126. nerve supply of, 128.
Hyperpneea, 249, structure of, 115.
Hypertonic solutions, 16, 397. Tron, 4, 12, 160.
_ Hypogastric nerves, 101, 335. Irradiation, 59, 231.
Hypoglossal nucleus, 67, 68. Island of Reil, 82.
Hypophysis cerebri (see Pituitary body). Islets of Langerhans, 318, 352.
Hypotonic solutions, 16. Isoleucine, 12.
Hypoxanthine, 19, 359. Isometric contractions, 23.
Isotonic contractions, 23.
solutions, 16.
Ileo-colic sphincter, 328, 402. Isotropous substance, 19.
Illusions, optical, 140.
Image, retinal, 121, 130.
tae ol, 12. : Tegee bee epilepey, 88.
Immunity, 168. alfe’s test, 38
Incus, 142,
Indigo-carmine. experiment, 395. Katacrotic pulse, 206.
Indirect vision, 132. Kathelectrotonus, 35.
Indol, 334, 386. Kathode, 23,
Induction, retinal, 137. Keratin, 11, 13.
Inferior cervical ganglion, 101. Ketoses, 6.
colliculi, 76. Kidney, functions of glomeruli of, 390.
mesenteric ganglion, 101. nerves of, 389, 391.
peduncles of cerebellum, 68. secretion in frog, 393.
Inflammation, 190. mammal, 394.
Infundibula of lung, 245. structure of, 380.
Infundibulum of brain, 405. tubules, 381.
Inhibition of heart, 220, absorption in, 396.
of reflexes, 60, secretion in, 393,
Inner cell lamina, 81. work of, 395.
Inner fibre lamina, 81. Kinesthetic area, 86, 89.
Inorganic constituents of the body, 4. Knee jerk, 62.
food-stuffs, 374. Krause’s membrane, 18.
Inosite, 7, 19. 4 Krogh’s microtonometer, 261.
Intercostal muscles, action of, 247. Kymograph, 181.
Internal capsule, 80, 93. :
ear, 143.
geniculate body, 77, 79. _ Labial consonants, 150.
respiration, 244. Labyrinth, functions of, 151.
secretions, 400. ; Labyrinthine sensation, 152.
Intestine, large, 332. Lachrymal secretion, 114.
small, absorption i in, 328, Lactalbumin, 375.
movements of small, 326. Lactase, 324.
of large, 335. Lactation, 415.
Intestinal digestion, 312. ‘Lacteals, 239.
juice, 324. Lactic acid, 27.
composition of, 324, effect on respiratory centre, 268.
functions of, 324. on vaso-motor centre, 231.
secretion of, 325. in blood, 231, 286, 389.
Intra-ocular fluid, 120. in muscle, 27. .
tension, 120. “Pus in urine, 28, 286.
426
Lactose, 6, 375, 387.
Levulose, 6,
Langerhans’ islets in pancreas, 318, 352.
Langley’s ganglion, 297. .
Large intestine, 332.
absorption in, 334,
excretion in, 333.
functions of, 333.
movements of, 335.
Laryngoscope, 149,
Larynx, 148.
movements.of, in deglutition, 300.
Lateral cerebro- -spinal fasciculus, 52.
Lateral columns of cord, 48
fillet, 77, 91.
geniculate body, 78, 79.
Law of forward direction, 43.
of specific irritability, 106.
Lecithin, 8.
Lens, crystalline, 119.
elasticity of, 124, :
variations with age, 125.
Lenses, formation of images by, 121.
Leucine, 12, 314.
Leucocytes, 156.
number of, 156,
origin of, 165,
varieties of, 156.
Leucocytosis, 165.
Ligamentum pectinatum iridis, 115,
Light, nature of, 133.
reflex, 129.
Lipase, gastric, 305.
pancreatic, 316.
Lipoids, 7.
Lissauer’s tract, 54.
Liver, 363.
bile formation in, 323,
glycogen of, 349.
- hemolysis in, 164.
lymph production i in, 241.
_ urea formation in, 355, 364,
uric acid formation in, 364,
Load, effect on contraction, 28,
Local reflexes, 327.
sign, 110, 139.
Localisation of function in cerebral hemi-
sphere, -86.
Locke’s fluid, 218.
Locomotion, co-ordination of, 74.
Locomotor ataxia, 64.
Long ciliary nerves, 128.
Longitudinal inferior fasciculus, 84.
. superior fasciculus, 84.
Loudness of sound, 146.
Ludwig’s stromuhr, 188.
Lungs, 244.
exchange of gases in, 260.
nerves of, 227.
structure of, 245.
Lutein, 412,
Lymph, composition of, 239.
flow of, 242.
ESSENTIALS OF PHYSIOLOGY.
Lymph, formation of, 240.
function of, 239.
Lymphagogues, 241,
Lymphocytes, 156.
Lysine, 13.
Lysins, 168..
Macula acustica, 152.
lutea, 118.
Malleus, 141.
Malpighian bodies of spleen, 408.
Maltase, 289, 315.
Maltose, 6, 293.
Mammary gland, 414.
hormone, 415.
Manometer, 181.
Hiirthle’s, 196.
Piper’s, 196.
Marchi’s ftuid, 45.
Marey’s law, 228, 238.
tambour, 196.
Marrow, 164.
Mayer curves, 231.
Mean systemic pressure, 184.
Mean systolic pressure, 185.
Medial fillet, 77.
geniculate body, 77, 79.
longitudinal fasciculus, 69, 77.
Medulla oblongata, 66.
spinalis, 47.
Meissner’s corpuscles, 108,
Membrana granulosa, 411.
’ reticularis, 144.
tectoria, 144.
tympani, 141.
Membranes, permeability of, 14.
semipermeable, 14...
Membranous labyrinths 143, 151.
Mendel’s law, 416.
Menstruation, 412,
Mercurial manometer, 181.
Metabolism, 338.
during starvation, 360.
endogenous, 357.
exogenous, 357.
methods, 338.
of carbohydrates, 348.
of fats, 344.
of muscle, 27.
of muscular work, 362.
of nucleo-proteins, 359.
of protein, 354.
relation of, to body surface, 348, 369.
Metabolites, action on blood- vessels, 236.
Metaproteins, 9, 303.
Metathrombin, 172,
Methemoglobin, 160.
Microtonometer, 261.
Micturition, 398.
Mid-brain, 76.
functions of, 79.
Middle cell lamina, 81.
peduncles of cerebellum, 69.
INDEX. — | 427
Milk, 374.
as a diet for infants, 376.
composition of, 375.
fats of, 375.
proteins of, 375.
salts of, 375. —
secretion of, 415.
Milk sugar, 6, 375.
Millon’s reaction, 9.
Mitosis, 413.
Mitral cells, 92.
valve, 177.
Modality, 106.
Molecular layer, 73, 81.
of retina, 117.
Monoamino-acids, 12.
Monoamino-nitrogen in proteins, 14.
Monophasic variation, 26.
Monosaccharides, 5.
Moss fibres, 73.
Motor aphasia, 93.
area, 82,
Mountain sickness, 281.
Movement, co-ordination of, 74.
sensation of, 153.
Movements of deglutition, 299.
of large intestine, 335.
of small intestine, 326.
of stomach, 309.
Mucins, 11, 304.
Miiller’s law of specific irritability, 106.
Murexide test, 386.
Muscarine, action on heart, 224.
Muscle, 18.
afferent nerves of, 19.
anisotropous substance of, 19,
chemical changes in, 27.
chemical stimulation of, 20.
clotting of, 19.
composition of, 19.
contractile stress of, 23.
elasticity of, 20, 23.
electrical current in, 24,
excitation of, 20.
extensibility of, 20.
heart, 177.
heat production in, 28.
independent excitability of, 20.
injury current of, 26.
involuntary, 31.
isotropous substance of, 19.
metabolism of, 27.
production of carbonic acid in, 29.
products of activity of, 29.
reciprocal innervation of, 60.
red, 19.
source of energy of, 27.
stimulation by constant current, 23.
tone, 64, 153.
voluntary, 18,
structure of, 18.
Muscle-current, 24.
-fibre, 18.
Muscle-plasma, 20,
-sound, 195,
-spindle, 63, 154,
-twitch, 21,
-wave, 22.
Muscular contraction, 22.
. fall or none” phenomenon, 22, 212,
effect of fatigue, 22.
of load, 23.
of temperature, 22,
of tension, 29, 213.
heat production “during, 28.
latent period of, 21,
mechanical changes in, 21.
respiratory quotient during, 342, 362.
strength of stimulus, 22.
time relations of, 21.
voluntary, 31.
electrical changes in, 31.
Muscular contractions, summation of, 23.
source of energy for, 27.
exercise, 285.
movements, co-ordination of, 74.
relaxation, 21.
sense, 89, 153.
paths for impulses of,. 55.
tone, 64, 153.
work, 29.
- effect on metabolism, 362.
Musculi papillares, 177.
Musical sounds, 145. .
Myelin sheath, 33.
Myelination, 51.
Myelocytes, 165.
Myenteric plexus, 326, 336.
Myogenic theory, 211, 214.
Myopia, 126.
Myosin, 20.
Myosinogen, 19.
Myxcedema, 407.
Near point, 125.
Negative after image, 136, 137.
variation, 36.
ventilation, 273.
Nerve, 33.
double conduction in, 36.
electrotonic currents in, 37.
electrotonus, 35,
medullated, 33.
nodes of Ranvier, 33.
non-fatiguability of, 30.
non-medullated, 33.
structure of, 33.
Nerve cell, 40.
function of, 43.
Nissl’s granules in, 40.
pericellular network of, 41.
structure of, 40.
Nerve centres in medulla, 69.
Nerve endings in skin, 107.
Nerve fibres, 33.
425°
Nerve impulse, 35.
electrical changes, 35.
excitatory condition, 34.
influence of drugs on, 37.
of temperature on, 37.
propagated disturbance, 35.
velocity of, 36,
Nervi erigentes, 99, 228, 335, 398.
Nervous system, 39.
Neurolemma, 33.
Neurofibrillar network, 41,
continuity of, 41.
Neurogenic theory, 210.
_ Neuroglia, 40.
Neuron, 40.
N eutrophile leucocytes, 156.
Nicotine, effect on heart, 224.
on synapse, 101.
Ninth nerve, nucleus of, 67.
Nissl’s granules, 40.
Nitrogen, endogenous, in urine, 357.
estimation, 384.
excretion in starvation, 361.
exogenous, in urine, 357.
in protein, 8.
output, 357.
requirements of body, 373.
Nitrogenous constituents of urine, 382.
equilibrium, 361.
Nociceptive stimuli, 59,
Nodal point, 121.
Nuclease, 359.
Nuclei pontis, 70.
Nucleic acid, 3, 11, 359.
Nuclein, 11, "359.
metabolism of, 359.
Nucleoprotein, 11.
digestion of, 11, 304, 359.
Nucleus, 2.
ambiguus, 67, 68.
caudatus, 94.
cuneatus, 68.
Deiters’, 70.
dentatus, 71.
emboliformis, 71.
fastigii, 71.
globosus, 71.
gracilis, 68.
lenticularis, 94.
Occipital lobe, 90.
Occipito-frontal fasciculus, 84,
Oculo-motor nucleus, 67, 79, 126.
Odours, 113.
(Esophagus, 300.
in deglutition, 300.
Cstrus, 413.
Oleic acid, 7.
Olfactometer, 113.
Olfactory apparatus, 92.
area, 92,
glomeruli, 92.
ESSENTIALS OF PHYSIOLOGY.
Olfactory lobe, 92.
mucous membrane, 118.
sensations, 113.
tract, 92.
Olive, 68.
Olivo-cerebellar fibres, 73.
Olivo-spinal tract, 58,
Oncometer, 225.
Optic chiasma, 90,
decussation, 90.
disc, 118.
nerve, efferent fibres in, 18.
radiation, 83, 90.
tract, 90.
Optical axis of lens, 121.
centre of lens, 121.
defects of eye, 126.
illusions, 140.
Optimum temperature for
288.
Optograms, 130.
Ora serrata, 116, 118,
Organ of Corti, 144.
Organic compounds i in body, 4
Osazones, 6.
Osmometer, 15.
Osmosis, 14, 241,
Osmotic pressure, 14,
freezing-point method, 15.
measurement of, 15.
of colloids, 16.
of electrolytes, 15.
of plasma, 167.
of proteins, 242, 392.
Osseous labyrinth, 143.
Otic ganglion, 297.
Otoliths, 152,
function. of, 153.
Outer cell lamina, 81.
fibre lamina, 81.
Overtones, 146.
Ovulation, 411.
Ovum, 411.
maturation of, 413.
Oxalate crystals, 387.
plasma, 171.
Oxidases, 290.
Oxyacids, 355, 364.
Oxygen, avidity of tissues for, 281.
capacity of blood, 252.
effect of changes in tension of, 268.
exchange in lungs, 261.
lack, in asphyxia, 270.
production of lactic acid in, 268.
tension in alveoli, 261.
in blood, 255, 268.
Oxyhemoglobin, 159, 256,
absorption spectrum of, 159.
dissociation of, 256.
influence of acids on, 258.
of temperature on, 258,
reduction of, 159.
Oxyproline, 12, 13,
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INDEX.
Pacinian corpuscles, 108.
Pain impulses, path of, in cord, 56.
sense, 111.
spots, 108.
Palmitic acid, 7.
Pancreas, changes during secretion, 318.
internal secretion of, 352.
islets of, 318, 352.
Pancreatic diabetes, 354.
fistula, 313.
juice, 313.
action on carbohydrates, 315.
' on fats, 315.
on milk, 316.
on proteins, 313.
activation of, 314.
_ by calcium salts, 314.
composition of, 313.
secretion of, 316.
effect of acids in duodenum, 317.
secretin, 317.
Paramyosinogen, 19.
Parathyroids, 407.
functions of, 408.
Parotid gland, 297.
nerves of, 297.
Pars ciliaris retin, 115.
Parturition, 414.
Pawlow’s gastric fistula, 301.
Pelvic visceral nerves, 102.
to bladder, 398.
to colon and rectum, 335.
-Pendular movements of intestine, 327.
Pentoses, 5, 359, 387.
Pepsin, action of, 303.
Peptones, 9, 303, 314.
effect on coagulation of blood, 174.
Pericardium, 178.
Perilymph, 143.
Perimeter, 138.
Peripheral ganglia, effect of nicotine on,
101,
Peristalsis, 326.
Permeability of membranes, 14. .
Peroxidases, 290.
Pes, 76.
Phagocytes, 165.
Phakoscope, 123.
Phenylalanine, 12, 13, 358, 377, 388.
Phenylglucosazone, 6.
' Phenylhydrazine test, 6, 388.
Phloridzin glycosuria, 351.
Phosphates, excretion by large intestine,
333. |
in urine, 386.
Phospholipines, 8.
Phosphoproteins, 10.
Photochemical substance in retina, 135.
Photohematachometer, 189.
Pilocarpine, effect on heart, 224.
Pineal gland, 79.
Piotrowski’s test, 9.
Piper’s manometer, 196.
429
Piston recorder, 199.
Pitch, 145.
Pituitary body, 405,
Placenta, 414.
Plain muscle, 31.
Plasma, blood, 165.
osmotic pressure of, 167.
* muscle, 20.
Plasma skin, 8.
Plethora, hydraemic, 240, 392, 397,
Plethysmograph for kidney, 225,
for limb, 229.
Pleura, 246.
Pleural cavity, pressure in, 248,
Poikilothermic animals, 365.
Polar bodies, 413.
Polarisation, 24.
Polymorphic layer, 81.
Polymorphonuclear leucocytes, 156.
Polypeptides, 9, 358.
formed in digestion, 314,
Polysaccharides, 5.
Pons, 69.
functions of, 71.
structure of, 69.
Portal vein, 179, 356, 363.
Position, sensations of, 151.
Positive after-images, 135.
Posterior columns of cord, 48,
funiculi, 48,
longitudinal bundle, 69, 77, 79.
root ganglia, 42, 47.
Post-ganglionic fibre, 99.
Postural reflexes, 153.
Precipitins, 167.
Prefrontal region, 82.
Preganglionic fibre, 99.
Pregnancy, 414.
Presbyopia, 125.
Pressure sense, 107.
paths in cord, 55.
spots, 108.
Primary colour sensations, 135.
Primordial follicle, 411.
Principal focus of lens, 121.
Projection fibres, 80, 83.
Proline, 12, 13.
Propagated disturbance in nerve, 35.
Proprioceptive system, 76, 105, 151.
Prosecretin, 318.
Prostate, 410.
Prosthetic group, 11.
Protamines, 10.
Protected nerve-endings, 107.
Proteins, 8.
absorption of, 330.
ammonia nitrogen in, 14.
composition of, 8.
conjugated, 11.
crystallisable, 8.
digmino-nitrogen in, 14.
gastric digestion of, 303.
hydrated, 9. ~
430
Proteins, hydrolysis of, 9.
molecule, 8, .
monoamino-nitrogen in, 14.
pancreatic digestion of, 314.
Proteose, 9, 308.
Prothrombin (see Thrombogen).
Protopathic sensibility, 111.
Protoplasm, 2.
' Pseudo-solution of colloids, 16.
Psychical secretion of gastric juice, 306,
Ptyalin, 292.
Puberty, changes at, 415.
Pulmonary circulation, 180.
ventilation, 266.
Pulse, 204.
anacrotic, 206.
dicrotic wave, 205, 207.
katacrotic, 206.
percussion wave, 205,
predicrotic wave, 205.
pressure, 185,
primary wave, 205.
rate of, in man, 179,
reflected wave, 206.
velocity of wave, 205, 206.
venous, 208.
Pupil, 115.
contraction of, 128, 129.
dilatation of, 128, 139.
Purine bases, 11, 359.
in urine, 386.
metabolism of, 359.
ring, 11, 359.
Purkinje cells, 72.
fibres, 214.
images, 131.
Pyloric canal, 309.
sphincter, 309.
vestibule, 309.
Pylorus, 312,
opening of, 312.
Pyramidal cells, 81.
decussation, 66.
tracts, 52, 88,94.
Pyrrol ring, 12.
Pyruvic acid, 345.
Rami communicantes, 101.
Reaction time, 134.
Reactions, reversible, 289.
Receptive substance, 404.
action of adrenalin on, 404,
Receptor organ, 58, 106.
Reciprocal innervation, 60.
feed Liindaces, 135.
Red corpuscles, 155.
marrow, 164.
nucleus, 77.
Reduced eye, 122.
reflex time, 59, .
Reflex action; 58.
facilitation of, 62.
ESSENTIALS. OF PHYSIOLOGY. ;
inhibition of, 60.
in spinal animal, 58, 60.
reinforcement of, 64.
Reflex arc, 58,
scratch, 61.
time, 58. —
tone, 65. e
Reflexes, 58.
axon, 103, 230.
prepotent, 61.
Refraction, 121.
in reduced eye, 122..
Refractory period, 34, 213.
Regeneration of nerve, 46:
Reinforcement, 64.
Reissner’s membrane, 143.
Remak’s ganglion, 210. ;
Renal excretion (see Kidney). ~
Rennin, 304.
Reproduction, 410.
female organs of, 411.
male organs of, 410.
Reservoir action of lungs, 203.
Residual air, 249.
Resonance, 147.
Respiration, 244.
apparatus, Benedict’s, 340.
Haldane and Pembrey, 342,
chemical regulation of, 265.
chemistry of, 250.
Cheyne-Stokes, 277.
effect.of changes in air on, 265.
of changes in barometric pressure on,
281.
on circulation, 283.
of division of vagi on, 272.
of exercise on, 275, 285,
nervous regulation of, 272.
rate of, 249,
Respiratory centre, 263.
action of vagus on, 272.
automaticity of, 264.
chemical excitants of, 265.
exchange, 260.
movements, 246.
Head’s method, 265.
muscles, 246, 247.
passages, 244.
quotient, 250, 341.
effect of foods on, 343.
in diabetes, 354,
in hibernation, 348.
in pues work, 343, 362.
sounds, 249, ,
Restiform body, 68, 73.
Retina, ‘116.
chemical changes in, 180.
electrical changes in, 131.
histological changes in, 130.
Retinal image, 121, 130.
induction, 137. ,
Reversibility of ferment action, 289.
Reflex action, fatigue of, 59. |
;
dl
-
Rhodopsin, 116, 130.
Rhythm of heart, 210, 214.
Ribs, movement of, 246,
Rigor mortis, 19, 29.
Ringer’s fluid, 218.
Riva-Rocci sphygmomanometer, 185.
Rods of Corti, 144.
of retina, 116.
functions of, 131.
Roof nuclei of cerebellum, tis
Rubro-spinal tract, 53, 69, Vii
Ruffini, corpuscles of, 108.
Saceule, 151.
Saccus endolymphaticus, 151.
Sacral autonomic fibres, 102.
Saliva, composition of, 292.
functions of, 292.
secretion of, 294.
effect of nerves on, 294, 297.
effect of metabolites on, 297.
histological a in, 298.
pressure of, 296.- —
Salivary digestion, 293,
in stomach, 294.
Salivary glands, 292.
nerves of, 294.
Salmine, 13, 14.
Salmon, 361.
Salts, absorption of, 330.
Sanson’s images, 123.
Saponification, 7.
Sarcolemma, 18.
Sarcomere, 18.
Sarcoplasm, 18. —
Sarcostyle, 18.
Scala tympani, 143.
vestibuli, 143.
Schiifer’s theory of contraction, 19.
Schematic eye, 122.
Schlemm, canal of, 115.
Sclera, 114,
Scleroproteins, 11.
Seratch reflex, 61.
Sebaceous glands, 366.
Second wind, 287.
Secretin, 317.
effect on intestine, 325.
on liver, 323.
on pancreas, 317.
Semicircular canals (ducts), 151.
destruction of, 152.
functions of, 152.
Semilunar ganglion, 101.
‘valves, 177.
Semipermeable cell, 14.
Sensations, 105.
Sensory adaptation, 106.
aphasia, 92, 93.
areas of cortex, 89-92.
path, 69, 94.
Septum transversum, 152.
e°
INDEX.
Serine, 12...
Serous salivary glands, 298.
Serum albumin, 13, 166.
globulin, 166.
osmotic pressure of, 167.
protein, 166.
Seventh nerve, nucleus of, 67, 69.
Sexual organs, relation +o ductless
glands, 406, 409.
Sham feeding, 305.
Shock, 235.
spinal, 65.
Short ciliary nerves, 128.
Sight-testing, 122.
Simultaneous contrast, 136.
Sino-auricular node, 214.
Sinus venosus sclere, 115,
Sixth nerve, nucleus of, 67, 69.
Skin, 366.
functions of, 367.
Sleep, 96.
Small intestine, 312.
absorption from, 328.
movements of, 326.
nerves of, 327.
Smell, sense of, 111.
sensations, 113,
Solar plexus, 101.
Sols, 17.
Solubility of gas, 254.
Sound, nature of, 145.
waves, 145.
Specific irritability, 106.
Speech, 93, 150.
Spermatids, 410.
Spermatocytes, 410,
Spermatozoa, 410,
Spherical aberration, 127.
Sphincter, ileo- -colic, 328.
pupille, 116.
pyloric, 312.
trigoni, 398.
Sphingo-myeline, 8.
Sphygmograph, 205.
Spinal accessory nerve, nucleus of, 67.
Spinal animal, 58.
Spinal cord, 47.
ascending tracts, 53.
cells of, 48.
collaterals of, 49.
conduction in, 50,
descending tracts, 52,
effect of strychnine, 60.
endogenous fibres of, 53.
hemisection of, 55,
path of impulses in, 55.
reflex centres in, 65.
structure of, 47.
tracts, methods of tracing, 50.
Spinal dog, 5
m@dulla ‘abs ‘Spinal’ cord).
shock, 65.
Spino-cerebellar travts, 54.
431
«Ys aires ESSENTIALS OF PHYSIOLOGY.
Spino-tectal tract, 54, 69.
Spino-thalamic tract, 54.
Spiral ganglion, 91. .
lamina, 148.
ligament, 143.
Spirometer, 249,
Splanchnic nerve, 101, 231,
Spleen, 408.
Staircase phenomenon, 212.
Stannius ligature, 212.
Stapedius muscle, 143.
Stapes, 142,
Starch, 6.
digestion by saliva, 293.
hydrolysis of, 6.
molecule of, 5.
soluble, 293.
Starvation, 360.
carbohydrate metabolism in, 360,
fat metabolism in, 361.
loss in various organs, 361.
protein metabolism in, 361.
Stearic acid, 7.
Stellate ganglion, 101.
Stercobilin, 322.
Stethograph, 265.
Stimuli, summation of, 23, -
Stimulus, adequate, 105.
strength required for sensation, 105.
threshold, 106.
Stomach, 301. —
digestion in, 301.
movements of, 309,
nerves to, 306, 311.
sphincter, pyloric, 312.
Stratum granulosum, 366.
lucidum, 366.
opticum, 118.
Strie acustice, 91.
_ medullares, 91.
Striated muscle, 18.
String galvanometer, 24, 216, 273.
Stromuhr, 188.
Structural basis of body, 2.
Strychnine, effect on cord, 60.
Subliminal stimuli, 106.
Sublingual gland, 298.
Submaxillary ganglion, 297,
gland, 294, 298.
Substantia nigra, 76.
Successive contrast, 137.
Suceus entericus (see Intestinal] juice),
Sugar in blood, 166.
in urine, 387.
utilisation of, in body, 350,
Sugars, 5,
Sulphocyanate in saliva, 292.
Summation of contractions, 23,
of stimuli in reflex action, 62.
in sensation, 106.
Superior cerebellar peduncles, 71. #
cervical ganglion, 101.
colliculi of corpora quadrigemina, 76,
Superior mesenteric ganglion, 102.
oblique muscle, 137.
Supplemental air, 249,
Suprarenal glands, 400.
function of, 401,
nerve supply of, 234, 404.
Surface tension, 2, ay
Suspensory ligament, 119.
Swallowing (see Deglutition),
Sweat, 367.
composition of, 367.
glands, 366.
loss of heat by, 367.
nerves, 367.
secretion of, 367.
Sympathetic ganglia, 99.
nerves of heart, 101, 221.
synapse in, 99. °
Synapse, 41, 99.
Tactile discrimination, 109.
localisation, 107.
sensations, 107,
path of impulses, 55, 56.
Taste, 111.
area, 92.
bulbs, 112.
nerves, 92, 112,
sensations, 112.
Tears, 114.
Tecto-spinal tract, 78.
Tegmentum, 76.
Temperature, effect on heart, 219,
on metabolism, 370.
on muscle, 22,
sense, 107.
Temporal lobe, 91.
Temporo-pontine fibres, 76, 83.
Tendon reflexes, 62.
Tendril fibres, 738.
Tension, effect of, on bladder, 399.
on heart, 202, 213.
on muscle, 29.
of gases in liquids, 255.
Tensor tympani muscle, 143.
Test types, 122,
Testis, 410.
Tetanus, 23.
Thalamo-cortical tract, 83.
-frontal fibres, 83.
-parietal fibres, 83,
Thalamus, 79, 98,
Thermopile, 28.
Third nerve, functions of, 129, 137.
nucleus of, 67, 79, 126.
Threshold stimulus, 106.
Thrombin, 170, 172.
Thrombogen, 170.
haombokrinsse, 171,
Thymus, 409,
Thyroid cartilage, 148.
INDEX.
Thyroid gland, 406.
extirpation of, 407.
extract, effects of, 407.
structure of, 406.
Thyro-arytenoid muscle, 149.
Tidal air, 249.
Timbre, 146.
Tissue proteins, 357.
respiration, 278.
Tone in muscle, 64, 153.
Tonometer, 256.
Tortoise heart, 211.
Touch (see Tactile).
Touch corpuscles, 108.
Toxins, 168,
Trachea, 244,
Trapezium, 91.
Traube-Hering curves, 231.
Tricuspid valve, 177.
Trigeminal nerve, nucleus of, 67, 69, 79.
Triolein, 7.
Tripalmitin, 7.
Triple phosphate, 387.
Tristearin, 7.
Trommer’s test, 5.
Trypsin, 314.
action on proteins, 314.
destruction of, 315.
Trypsinogen, 314,
Tryptophane, 9, 12, 13, 358, 373, 377.
Tuberculum acusticum, 70, 91.
Tympanic membrane, 141.
movements of, 145.
Tyrosine, 9, 12, 314, 358, 364, 377.
crystals, 314,
in urine, 388.
test for, 314.
Umbilical cord, 414.
Unstriated muscle, 31.
Urate deposit, 386.
Urea, 384.
bacterial action on, 384.
effect of hypobromite on, 384.
nitrate, 384.
origin of, 355.
output on protein diet, 357.
oxalate, 384,
preparation from urine, 384.
Ureter, contractions of, 398.
Uric acid, 11, 385.
excretion of, 357, 360, 364, 386.
origin of, 359.
oxidation of, 360.
production in birds, 364.
tests for, 386.
Urinary constituents, 382.
daily amount of, 382.
deposits, 386.
Urine, 382.
abnormal constituents in, 387,
acetone in, 388.
433
Urine, ammonia in, 385.
chlorides of, 386.
composition of, 382.
freezing- point of, 396.
lactose in, 387.
neutral sulphur i in, 386.
organic constituents of, 384.
phosphates in, 386,
pigments of, 383.
pressure of, in ureter, 391, 392.
reaction of, 383, 395.
salts of, 386.
secretion of, 393,
specific gravity of, 382.
sugar in, 387.
sulphates of, 386.
Uriniferous tubule, 380.
functions of, 393.
Urobilin, 322, 3838.
Urochrome, 383.
Uroerythrin, 383.
Uterus, 412.
changes in, during pregnancy, 414.
Utricle, 151.
Vagus nerve, 102.
action on heart, 220.
on intestines, 327.
on cesophagus, 301.
on respiration, 272.
on stomach, 310, 311.
apnea, 273, 276.
escape, 220.
expiratory fibres in, 273.
inspiratory fibres in, 273.
nucleus of, 67, 68.
tonic action on heart, 222.
Valves of heart, 176, 177, 191.
Variation, diphasic, 26.
monophasic, 26.
Vascular mechanism, 176.
tone, 224.
Vaso-constrictor fibres, course of, 226.
Vaso-dilator fibres, 228.
in mixed nerve, 228.
in splanchnic nerve, 230.
Vasomotor centre, 226.
effect of acids on, 231.
centres in cord, 227,
impulses, path of, 101,
nerves, 226.
reflexes, 230.
Vegetable food, utilisation of, 379.
Veins, 179.
rate of flow in, 187.
valves in, 179.
Velocity in vascular system, 187,
Venous blood pressure, 182, 185.
outflow, determination of, 225.
pulse, 208.
Ventral spino - cerebellar
54, 5
fasciculus,
28
434 ESSENTIALS OF PHYSIOLOGY.
Ventricles, capacity of, 178.
pressure in, 195.
Ventricular diastole, 179.
folds in larynx, 148.
systole, 179.
Vesicular murmur, 249,
Vestibular nerve, 70, 152.
functions of, 153.
membrane, 148.
nucleus of, 70.
Vestibule, 151..
Vestibulo-spinal tract, 53.
Villus, 329.
changes in, during fat absorption, 332.
during protein absorption, 330,
structure of, 329.
Visceral nerves, 102.
Vision, 120.
binocular, 138, 140.
colour, 135.
direct, 138.
field of, 138.
indirect, 132, 138.
Visual acuity, 123.
adaptation, 133.
angle, 122.
area, 90.
fatigue, 136.
judgments, 134, 139.
path, 90.
purple, 116, 130,
sensations, 133.
Weber’s law, 107.
stimuli, time relations of, 134.
Visuo-psychic area, 86, 90.
Visuo-sensory area, 82, 86, 90.
Vital capacity, 249.
Vitreous humor, 119.
Vocal cords (folds), 148.
Voice, 148.
pitch of, 150. -
production of, 148,
Volume of an organ, 225.
Voluntary contraction, 31,
electrical changes i in, 31.
muscle, 18,
Vowel sounds, 150.
Wallerian degeneration, 44.
method, 51.
Warm points, 108,
Water, absorption of, 328, 334,
Weber’s law, 107.
in vision, 107.
White blood corpuscles (sce Leucocytes),
White rami communicantes, 101,
White, sensation of, 133.
Word-blindness, 93.
Work done by heart, 198, 204,
by muscle, 29.
Xanthine, 19, 359.
Xanthoproteic reaction, 9.
Xylose, 5
Yeast, action on dextrose, 6.
Yellow spot, 118.
Young-Helmholtz theory, 136.
Zein, food value of, 358.
Zona fasciculata, 401.
glomerulosa, 401.
pellucida, 411.
reticularis, 401.
Zonula ciliaris, 119.
Zymogen granules, 299, 309,
in pancreas, 319.
in salivary glands, 299.
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