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Wes THE BODY AT WORK
few drops of the almost colourless, transparent blood of a —
lobster afford an excellent opportunity of studying the forma-
tion of the clot—innumerable filaments of the most delicate
description are seen to shoot out from many centres. They
multiply until they constitute a felt-work. In the case of blood
obtained from a vertebrate animal, this felt-work holds the
corpuscles in its meshes. Its filaments exhibit a remarkable
tendency to ccntract. They shorten as much as the enclosed
corpuscles allow.
The filaments may be prevented from entangling the cor-
puscles by whipping the blood, from the instant that it is shed,
with a bundle of twigs or wires. The fibrin collects on the
wires, while the corpuscles remain in the serum. If this fibrin
is washed in running water until all adherent serum and cor-
puscles are removed, it appears as a soft white stringy sub-
stance which, when dried, resembles isinglass.
Clotting is a protection against hemorrhage. As it oozes
from a scratch or tiny wound, blood clots, forming a natural
plaster which prevents continued bleeding. It has little if any
influence in resisting a strongly flowing stream of blood. But
a clean cut through a large vessel is an accident which rarely
happens as the result of natural causes. It is not the kind of
injury to which animals are liable. When an artery is severed
by a blunt instrument, the muscle-fibres of its wall contract.
They occlude the vessel. The blood clots at the place where
the vessel is injured, and plugs it. This happens also when a
surgeon ties an artery. He is careful to pull the ligature suffi-
ciently tight to crush its wall. His sensitive fingers feel it give.
He stops before the thread has cut it through. As will be ex-
plained later, the clotting of blood is promoted by contact with
injured tissue. If in tying an artery its wall be not crushed,
the blood in it may remain liquid. When it is skilfully tied,
the blood clots, forming a firm plug which is practically a part
of the artery, by the time that the silk thread used in tying it
is thrown out, owing to the death of the ring of tissue which it
compressed. After a tooth has been extracted, the cavity is
closed and further bleeding stopped by clotted blood.
When large vessels have been severed, the copious heemor-
rhage which follows induces fainting. For a short time the
heart stops, or beats very feebly. The blood-pressure falls.
Ns a i
a “of coming into cs It is a useful reflex action, always sup-
posing that the person who is liable to it faints at the sight of
_ his own blood. Amongst other reasons for the greater fortitude
_ of women—they are far less subject to this emotional reflex
than men—might be alleged the circumstances of life of primi-
tive people. It was the part of their women-folk to dress
wounds, not to receive them.
The phenomenon of coagulation has attracted attention
from the earliest times. It was a phenomenon that needed
explanation, and culinary experience suggested analogies close
at hand. Hippocrates attributed the clotting of blood to its
coming to rest and growing cold. The blood which gushed
from a warrior’s wound formed a still pool by his side. It set
into a jelly as it cooled. Until the second quarter of the nine-
teenth century this theory was deemed sufficient. It then
occurred to two men of inquiring mind to institute control
experiments. John Davy placed a dish of blood upon the hob.
William Hunter kept one shaking. In both experiments the
blood clotted more quickly than it did in vessels of the same
size, containing the same amount of the same blood, left upon
the table.
Even before this date an observation had been made regard-
ing the circumstances in which clotting occurs, which has thrown
much light upon the causes of the phenomenon. In 1772
Hewson gently tied a vein in two places. At the end of a couple
of hours he opened the vein. The blood was still liquid, but
clotted in a normal manner after it was shed. Scudamore
showed that blood clots more slowly in a closed than in an
open flask. A new theory, as little trustworthy as Hippo-
crates’, was based upon these observations. Blood clotted
because it was exposed to air. A record of all observations of the
circumstances of coagulation, and of all the theories to which
they have given rise, would make an exceptionally interesting
chapter in the history of human thought. It would bring into
singular prominence stages in the development of what is now
known as the “scientific method.’ Not that Science has a
method of her own. Philosophers of all classes would follow
ait es can be brought to a test of HGR. the ‘
political, or economic theory are not susceptit
confronted with control experiments. The control ext
is the alphabet and the syntax of the scientific method.
hypothesis is admissible into the pyramid of theory until it has _
passed this test. A natural phenomenon is observed. Every
measurement which is applicable is taken and recorded—time, _
weight, temperature, colour. Scientific observation implies
the tabulation of all particulars which are capable of statistical
expression. Reflecting upon the relation of the phenomenonto
other phenomena of a like nature, the philosopher—it is the
philosophy of physiologists which interests us—formulates an
hypothesis as to its cause. At this point the real difficulty of —_
applying the scientific method begins. It is easy to formulate
hypotheses. It is very difficult to devise control experiments.
An experiment must be arranged which will provide that,
while all other conditions in which the phenomenon has been
observed to occur are reproduced, the condition which was ez
hypothes: its cause shall be omitted. This digression into the
philosophy of science may seem to be somewhat remote from
our line of march,. but it may perhaps hasten our progress in
the comprehension of the story of physiology. There is no
other science in which the control experiment plays an equally
important part. Unless this is realized, the whole trend of
experimental work will be misunderstood. Scudamore ex-
plained coagulation as due to contact with air. Based on the
observations we have cited, no hypothesis could have seemed
more reasonable. With a view to checking this hypothesis,
blood was received into a tube of mercury. It coagulated in
the Torricellian vacuum. Scudamore’s hypothesis, like many
earlier and later, when confronted with a control experiment,
was turned away, ashamed.
Clotting is a property of plasma. Red corpuscles play no
part in the process. Coagulation does not occur in a living
healthy vessel. It occurs when the vessel, and especially when
its inner coat, is injured. It is hastened by contact with
wounded tissues, especially with wounded skin. Contact with
a foreign body also starts coagulation. If a silk thread is
ee
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Plasma contains a substance which sets into fibrin. It has
been termed “fibrinogen.” It is present in lymph, and in
almost all forms of exuded lymph. If sodium chloride (common
salt) is added to plasma until it is half saturated—until it has
dissolved half as much as the maximum quantity which it
can dissolve—fibrinogen is thrown down as a flocculent pre-
cipitate. It can be redissolved and reprecipitated until it
is pure. When fibrinogen was separated from plasma a step
was taken towards the explanation of coagulation. Under
certain conditions fibrinogen sets into fibrin. The question
which then presented itself for solution was as follows : What
is the substance which, by acting upon or combining with
fibrinogen, converts it into fibrin? The clue to the solution of
this question was obtained from the consideration of certain
observations made by Andrew Buchanan in 1830, but long
neglected, because their significance was not understood.
Buchanan had observed that some specimens of lymph exuded
into a lymph-space—the peritoneal cavity, for example—will
clot ; others will not. He noticed that they clot when, owing
to puncture of a small bloodvessel during the process of drawing
them off, they are tinged with blood. Determined to ascertain
which of the constituents of blood is effective in rendering non-
coagulable effusions capable of clotting, he added to them in
turn red blood-corpuscles, serum, and the washings of blood-
clot. Either of the two latter was found to contain the clot-
provoking substance. Thirty years later a German physiolo-
gist prepared fibrinogen from effused lymph by precipitating it
with salt. He also treated serum in a similar way, precipitating
a protein which he termed fibrinoplastin. When these two sub-
stances were dissolved and the solutions mixed, he obtained a
clot, which he regarded as a compound of fibrinogen and fibrino-
plastin. Subsequently he found that the mixture did not
always clot, but he discovered that if he coagulated blood with
alcohol, and washed this residue, the washings added to the
mixed solution just referred to invariably produced a clot.
Thinking that the substance which he obtained from his
14
“‘ fibrin-ferment.” He neglected the control experiment. He
failed to ascertain whether or not all three substances were
needed. Had he tried adding fibrin-ferment to fibrinogen, he
would havediscovered that the further addition of fibrinoplastin
was unnecessary. He did not ascertain, as he might have done,
that the weight of fibrin formed is somewhat less, not greater,
than the weight of fibrinogen used. (Fibrinogen gives off a
certain quantity of globulin when it changes into fibrin.) He
was also wrong in supposing that the water which he added to
alcohol-coagulated blood dissolved no protein. His “ fibrin-
ferment ”’ is always associated with a protein. Since it may
also be obtained from lymphatic glands, thymus gland, and
other tissues which contain lymphocytes, it has been inferred
that it is itself a protein, of the class known as nucleo-proteins.
The fact that it is destroyed at so low a temperature as 55° C.
has been supposed to confirm the theory that it is a protein.
But with regard to the chemical nature of fibrin-ferment, as of
all other ferments, we are at present in the dark. Under
ordinary circumstances, when blood clots, the fibrin-ferment,
or plasmase, or thrombin—it has received various names—is
set free by leucocytes. Fluids which contain fibrinogen clot on
the addition of a “‘ ferment ” which is either secreted by leuco-
cytes or set free from leucocytes when they break up—as they
are very apt to do, as soon as the conditions upon which their
health depends are interfered with.
Freshly shed blood contains minute particles, termed “ plate-
lets,’ in diameter measuring about a quarter that of a red blood-
corpuscle. When the inner coat of a vessel is injured, platelets
accumulate at the injured spot. They form a little white heap,
from which coagulation starts. Evidently they supply the
ferment, or a precursor of the ferment. As yet their origin has
not been traced. They are too large to be the unchanged
granules of granular leucocytes, but that they are in some way
derived from leucocytes seems probable.
The further study of coagulation has shown that the con-
ditions under which it occurs are more complicated than the
simple explanation just given would seem to imply. This
explanation holds good, so far as it goes, but facts connected
with the details of the process have recently been brought to
- aleohol-coagulated blood could not be proteid, he termed it
F ‘THE FLUIDS OF THE BODY 15
i light which warn the physiologist that as yet his theory of
coagulation is incomplete.
The presence of salts of lime has an important relation to
coagulation. If blood is received into a vessel in which has
been placed some powdered oxalate of potash, or soap, or any
other chemical which fixes lime, the blood does not coagulate.
All other conditions are as usual, but lime is withdrawn from
the plasma. The non-coagulation of oxalated plasma was
interpreted as indicating that lime, under the influence of
fibrin-ferment, combines with fibrinogen to form fibrin; that
fibrinogen altered by fibrin-ferment combines with lime. This
hypothesis was based upon the analogy of the curdling of
milk. Milk cannot curdle if lime be absent. If rennin (milk-
ferment), prepared from milk from which lime has been re-
moved, be added to a solution of caseinogen (the coagulable
protein of milk), also prepared from lime-free milk, no curd is
produced. The addition of a few drops of a solution of chloride
of lime results in the immediate curdling of the mixture.
Evidently rennin so alters caseinogen as to bring it into a
condition to combine with lime. But the analogy does not
hold good for blood. In the case of plasma, lime acts, not upon
fibrinogen, but upon the fibrin-ferment—or rather upon a
precursor of fibrin-ferment—in such a way as to render it
effective. Leucocytes produce a prothrombin, which in contact
with lime-salts is converted into thrombin, which coagulates
fibrinogen.
Fibrinogen is the substance which fibrin-ferment combined
with salts of lime changes into fibrin. Yet even now the
story is not complete, if the theory of coagulation is to be
brought up to date. A perfectly clean cannula is passed into
an artery of a bird. If it be thrust well beyond the place
where the vessel has been cut, if the vessel be tied so gently as
to avoid injury to its inner coat, and if the blood which first
passes through the cannula be allowed to escape, the blood
subsequently collected will not clot. It contains fibrinogen,
lime salts, and fibrin-ferment, ordinarily so called; but the
ferment is ineffective. The addition to the blood of a frag-
ment of injured tissue, or of a watery extract of almost any
tissue, immediately sets up coagulation. This observation
brings fibrin-ferment into line with other ferments. Digestive
dacneed by a kinase before ere acquire Sechaniative ys _
So, too, must thrombogen be changed into thrombin, under —
the influence of thrombokinase, before it can act upon
fibrinogen. Almost all tissues yield the kinase which actuates
fibrin-ferment. The utility of this provision is manifest. A
bird’s blood contains everything necessary to form a clot with
the exception of thrombokinase. The injury which brings the
blood into contact with a broken surface supplies this ferment
of the ferment. Fibrin-ferment, rendered active, at once —
changes fibrinogen into fibrin. The same interaction is neces-
sary before the blood of a mammal is susceptible of clotting.
But a mammal’s blood is even readier to clot than is the blood
of a bird ; for not only will a broken surface provide it with
thrombokinase, but the leucocytes contained within the blood,
when injured, also yield it. And the leucocytes are exceed-
ingly sensitive of any change of circumstance ; on the slightest
indication that conditions are not normal they set free, perhaps
owing to their own disintegration, the kinase which turns
thrombogen into thrombin.
There is a constitutional condition, fortunately rare, in
which blood does not coagulate. A person subject to this
abnormality is said to suffer from hemophilia. It is alleged
that this condition is due to deficiency of lime in the blood ;
and the deficiency of lime is said to be due to excess of phos-
phates. The subject suffers from phosphaturia. His kidneys
get rid of the superabundance of phosphates by excreting
them in combination with lime. If this explanation be correct,
there is a chronic insufficiency of lime in the blood, because it
is being constantly withdrawn in the process of removing
phosphates.
The difficulty in the way of establishing a complete theory
of the coagulation of blood increases when the phenomena of
incoagulability are considered. Blood may be rendered in-
capable of clotting in a variety of ways. Leeches and other
animals which suck blood have the capacity of rendering it
incoagulable. If the heads are removed from a score of
leeches, thrown into absolute alcohol, dried, ground in a
pepper mill, extracted with normal saline solution, a dark
turbid liquor is obtained. This liquor, after filtration and
ss THE FLUIDS OF THE BODY 77
sterilization at a temperature of 120° C., injected into the
veins of an animal, renders its blood incoagulable.
The preparation sold by druggists under the name “ pep-
tone,” when injected into the veins of a dog, renders its blood
incoagulable. Commercial “‘ peptone”’ is a mixture of many
substances. Its anticoagulation-effect is not due to the
peptone which it contains. It has been supposed to be due
to imperfectly digested albumin and gelatin (proteoses), but
products of bacteric fermentation (toxins and ptomaines) are
more probably the active bodies. Not only is the peptonized
blood of a dog incoagulable, but if this blood be injected into
the veins of a rabbit (an animal upon which the direct injec-
tion of peptone has no effect), it diminishes the coagula-
bility of the rabbit’s blood. If peptonized blood be mixed
in a beaker with non-peptonized blood, it prevents the coagula-
tion of the latter. There is little doubt but that the poison,
whatever it may be, acts upon the leucocytes ; and there are
some reasons for thinking that the poison is not contained
in the “ peptone,” but is secreted by the liver of the animal
into which the “ peptone ”’ has been injected.
A still more remarkable property in relation to coagula-
tion must be assigned to leucocytes. The blood of a dog
which has been rendered incoagulable by injection of peptone
recovers its coagulability after a time. If a further injection
of “ peptone’ be made, the animal is found to be immune.
Injection of “ peptone ” no longer renders its blood incoagu-
lable. In a similar manner the blood develops a power of
resisting the action of agents which induce its coagulation
whilst circulating in the vascular system. Nucleo-proteins
contained in extracts of lymphatic glands and other organs
when injected into the veins of living animals cause their
blood to clot, provided they are injected in sufficient quantity.
If they are injected in quantity less than sufficient to induce
coagulation, they render the animal immune to their influence.
A larger quantity given to an animal thus prepared fails to
take effect. This brings the phenomena of coagulation and
resistance to coagulation to the verge of chemistry. They
extend into the domain in which pathology reigns. Tempting
though it be to record other facts with regard to these pheno-
mena which recent investigation has brought to light, it is
THE BODY. aT WORK ne
probably judicious to leave the problem at the frontier. re ross
the frontier lies a fascinating land, rich with unimaginable — a 4
possibilities for the human race. Settlement is rapidly pro-
ceeding in this country, which is charted, like other border-
lands, with barbarous names: “antibodies,” “‘haptors,” —
“amboceptors,” ‘“‘ toxins,” “ antitoxins,” and the like—
finger-posts to hypotheses which show every sign of hasty and
provisional construction. But certain facts stand out, in
whatever way theory may, in the future, link them up. The
virus of hydrophobia, modified by passing through a rabbit,
develops in human beings, even when injected after they have
been infected, the power of resisting hydrophobia. The serum
of a horse which has acquired immunity to diphtheria aids the
blood of a child, which has not had time to become immune,
in destroying the germs of this disease. It is a contest between
the blood and offensive bodies of all kinds which find en-
trance to it, whether living germs or poisons in solution ; with
victory always, in the long-run, on the side of the blood, pro-
vided its owner does not die in the meantime. And not only
is the blood victorious in the struggle with any given invader,
but having repulsed him, it retains for a long while a
property which neutralizes all further attempts at aggression
on his part. In the past, physicians have fought disease with
such clumsy weapons as mercury, arsenic, and quinine. Now
they anticipate disease. In mimic warfare with an attenuated
virus the blood is trained to combat. Smallpox which has
been passed through the body of a cow is suppressed by the
blood’s native strength. The exercise develops skill to deal
with the most virulent germs of the same kind. In cases in
which physicians cannot anticipate disease in human beings,
they train the blood of animals to meet it ; and, keeping their
serum in stock, they can, when the critical moment arrives,
reinforce the fighting strength of the patient with this mer-
cenary aid.
The Spleen.—The spleen is placed on the left side of the
body, and rather towards the back. It rests between the
stomach and the inner surface of the eighth, ninth, tenth, and
eleventh ribs. It is quickly distinguished from other organs by
its brown-purple colour, a sombre hue to which it owed its
evil reputation with the humoralists. The liver’s yellow bile
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Me, yr “Se a le 5 ee BS pg ll nt I Oe f
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‘THE FLUIDS OF THE BODY 79
tin yed man’s mental outlook, preventing him from seeing
Zz piece | in their natural brightness ; but the spleen made black
bile, which, mounting to the brain, displayed its malign
influence upon the action of that organ, as, or in, the worst
of humours.
The spleen is invested with a capsule of no great toughness.
Inside the capsule is “‘ spleen pulp.””’ When the fresh organ
is cut across, it is seen that, although most of the pulp is of
the colour of dark venous blood, it is mottled with light
patches. In some -animals—the cat, for example—these
whitish patches are small round spots, regularly arranged at a
certain distance from the capsule. The distinction into “ red
pulp ” and “white pulp” marks a division into two kinds
of tissue with entirely different functions. The white pulp is
lymphoid tissue, lymph-follicles developed in the outer or
connective-tissue coat of the branches of the splenic artery.
Its function is to make lymphocytes, of which, for reasons
which will shortly appear, the spleen needs an abundant
supply. The constitution of the red pulp is entirely different,
and peculiar to the spleen. The branches of the splenic artery
divide in the usual way into smaller and still smaller twigs
until the finest arterioles are reached ; but these arterioles do
not give rise to capillary vessels. At the point at which in
any other organ their branches would attain the calibre of
capillaries, the connective-tissue cells which make their walls
scatter into a reticulum. They are no longer tiles with
closely fitting, sinuous, dovetailed borders, but stellate cells
with long delicate processes uniting to constitute a network.
The blood which the arterioles bring to the pulp is not con-
ducted by closed capillary vessels across the pulp to the
commencing splenic veins. It falls into the general sponge-
work. The venules commence exactly in the same way as the
arterioles end. Stellate connective-tissue cells become flat
tiles placed edge to edge. The endothelium of an arteriole
might be likened to a column of men marching shoulder to
shoulder, three or four abreast ; the connective tissue of the
pulp, to a crowd in an open place. The column breaks up into
a crowd. On the other side the crowd falls into rank as
the endothelium of veins. The capsule and the red pulp are
largely composed of muscle-fibres. These relax and contract
about once a minute. By their contraction ae bloc 00
squeezed out of the sponge.
If the spleen be enclosed in an air-tight box (an oncometer), - )
from which a tube leads to a pressure-gauge—a drum covered a
with thin membrane on which the end of a lever rests, or w a
bent column of mercury on which it floats—the pressure-
gauge shows the changes in volume of the spleen. The long —
end of the lever, which records the variations of pressure in
the gauge, may be made to scratch a line on a soot-blackened
surface of travelling paper. A record of the variations in
volume of the organ, which can be studied at leisure, is thus
obtained. It shows that the spleen is sensitive to every change
of pressure in the splenic artery. Small notches on the tracing
correspond to the beats of the heart. Larger curves record
the changes of blood-pressure due to respiration. A long slow
rise and fall marks the rhythmic dilation and contraction of
the spleen itself.
One of the three large arteries into which the cceliac axis
divides delivers blood to the spleen direct from the aorta.
The splenic vein joins the portal vein shortly before it enters
the liver. Thus the spleen is placed on a big vascular loop
which directs blood, not long after it has left the heart, from
the aorta, through the spleen, to the liver.
The peculiar construction of the splenic pulp which brings
the blood more or less to rest within its sponge-work, and the
transmission to the liver of the blood which leaves the spleen,
indicate that it is an organ in which blood itself receives
some kind of treatment. It is not passed through it, as it is
through all other parts of the body, in closed pipes. The
spleen is a reservoir, or a filter- bed, into which blood is
received.
The red blood-corpuscles of mammals are cells without
nuclei, and with little, if any, body-protoplasm. They are
merely vehicles for carrying hemoglobin. We should deny to
them the status of cell, if it were possible to prescribe the
limit at which a structural unit ceases to be entitled to rank
as a cell. They are helpless creatures, incapable of renewing
their substance or of making good any of the damage to which
the vicissitudes of their ceaseless circulation render them
peculiarly liable. It is impossible to say with any approach
ary
_ THE FLUIDS OF THE BODY 81
J Ete aie aie
to accuracy how long they last, but probably their average
duration is comparatively short. The spleen is a labyrinth
of tissue-spaces through which at frequent intervals all red
corpuscles float. If they are clean, firm, resilient, they pass
through without interference. If obsolete they are broken
up. In the recesses of the spleen-pulp, leucocytes overtake
the laggards of the blood-fleet, attach their pseudopodia to
Fia. 5.—A MINUTE PORTION OF THE PULP OF THE SPLEEN, VERY HIGHLY MAGNIFIED.
Stellate connective-tissue cells form spaces containing red blood-corpuscles and leucocytes.
In the centre of the diagram is shown the mode of origin of a venule. It contains two
phagocytes—the upper with a nucleus, two blood-corpuscles just ingested, and one
partially digested in its body-substance ; the lower with two blood-corpuscles.
them, draw them into their body-substance, digest them.
The albuminous constituent of hzmoglobin they use, pre-
sumably, for their own nutrition. The iron-containing colour-
ing matter they decompose, and excrete in two parts; the
iron (perhaps combined with protein) ; the colouring matter,
without iron, as the pigment, or an antecedent of the pigment,
which the liver will excrete in bile. Hemoglobin is un-
6
=e THE BODY AT WORK
doubtedly the source of bilirubin, and general considerations
lead to the conclusion that it is split into protein, iron, and
iron-free pigment in the spleen ; but the details of this process
have never been checked by chemical analysis. Neither bile-
pigment nor an iron compound can be detected in the blood
of the splenic vein. The only evidence of the setting free of
iron in the spleen is to be found in the fact that the spleen
yields on analysis an exceptionally large quantity of this metal
(the liver also yields iron), and that the quantity is greatest
- when red corpuscles are being rapidly destroyed.
As a rule, it is very difficult to detect leucocytes in the act
of eating red corpuscles; but under various circumstances
their activity in this respect may be stimulated to such a
degree as to show them, in a microscopic preparation, busily
engaged in this operation. The writer had the good fortune to
prepare a spleen which proved to be peculiarly suitable for this
observation (Fig. 5). His method was an example of the way
in which a physiological experiment ought not to be conducted.
Having placed a cannula in the aorta of a rabbit, just killed
with chloroform, he was proceeding to wash the blood out of
its bloodvessels with a stream of warm normal saline solution,
when the bottle from which the salt-solution was flowing over-
turned. Fearing lest an air-bubble should enter the cannula,
he hastily poured warm water into the pressure bottle, and
threw in some salt, in the hope that it would make a solution
of about 0-9 per cent. The salt-solution was allowed to run
through the bloodvessel for rather more than an hour. When
sections of the spleen were cut, after suitable hardening,
every section was found to be packed with leucocytes gorged
with red corpuscles. Some of the corpuscles had just been
ingested ; from others the hemoglobin had already been
removed. It may be that, for some unknown reason, the
destruction of red corpuscles was occurring in this particular
rabbit with unusual rapidity at the time when it was killed ;
but it seems more probable that the animal’s leucocytes were
provoked to excessive activity by changes in the red cor-
puscles brought about by salt-solution which was either more
or less than “‘ tonic.”” Asa score of attempts to reproduce the
experiment, with solutions of different strengths, have failed,
it is impossible to be sure that this is a valid explanation.
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_ ‘THE FLUIDS OF THE BODY $3.
There must be something in the condition of worn-out red
corpuscles which either makes them peculiarly attractive to
predatory leucocytes or renders them an exceptionally easy
prey. It does not require much imagination to picture the
drama which is enacted in the spleen. Slow-moving leuco-
cytes are feeling for their food. The majority of red cor-
puscles pass by them ; a few are held back. The leucocytes,
like children in a cake-shop, cannot consume all the buns.
A selection must be made, and preference is given to the
sticky, sugary ones. Red corpuscles when out of order show
a tendency to stick together. When blood is stagnating in
a vein, or lying on a glass slide in a layer thin enough for
microscopic examination, its red discs are seen after a time to
adhere together in rouleaux. The parable of a child in a cake-
shop is not so fanciful as it may appear.
The differentiation of function of organs is not as sharp as
was formerly supposed. Evidence of their interdependence
is rapidly accumulating. The activity of various organs is
known to result in the formation of by-products termed
‘internal secretions,’ which influence the activity of other
organs, or even of the body as a whole. The spleen enlarges
after meals. This may be merely connected with the en-
gorgement of the abdominal viscera which occurs during active
digestion, or it may indicate, as some physiologists hold, that
an internal secretion of the spleen aids the pancreas in pre-
paring its ferments. The spleen enlarges greatly in ague and
in some other diseases of microbial origin. This has been
regarded as evidence that it takes some part in protecting the
body against microbes. But whatever may be the accessory
functions which it exercises, they are not of material import-
ance to the organism as a whole, seeing that removal of. the
spleen causes no permanent inconvenience either to men or
animals. Its blood-destroying functions are taken on by
accessory spleens, if there be any, and by lymphatic glands.
The marrow of bone also becomes redder and more active.
Under certain circumstances, red corpuscles, or fragments of
red corpuscles, are to be seen within liver-cells; but it is
uncertain whether blood-destruction is a standing function of
the liver.
6—2
CHAPTER V
INTERNAL SECRETIONS
Thyroid Gland.—On either side of the windpipe, rather below
the thyroid cartilage (Adam’s apple), lies a somewhat conical
mass of tissue. The two masses are connected by an isthmus ;
lobes and isthmus make up the thyroid gland. The whole
weighs about an ounce. In health it is so soft that only the
finger of an anatomist could detect it through the skin and the
thin flat muscles which connect the hyoid bone and the thyroid
cartilage with the breast-bone. It makes no visible prominence
on the front of the neck. The thyroid gland is, however,
liable to enlargement, especially amongst the people who live
in certain districts. In the Valais, “ goitre,” as it is termed,
is so frequent that anyone walking up the Rhone Valley is
sure to meet a number of persons—for the most part women—
whose swollen necks overhang their collar-bones, like half-
filled sacks. Goitre is even more common in the Valle d’Aosta,
on the Italian side of the Alps. In England this condition,
comparatively rare, is known as “ Derbyshire ”’ or “ Hunting-
donshire ”’ neck.
In the majority of cases the tumour in the neck develops
slowly, and does not reach its full dimensions until after
middle life. Goitre in this form, although inconvenient,
causes no serious discomfort. But when it appears in early life,
it is associated with an extraordinary complex of malforma-
tions and ill-performed functions. The condition into which
a goitrous child sinks is known as cretinism. With the
exception of the skull-case, its skeleton does not attain to its
proper proportions ; and, since the soft parts do not equally
submit to arrest of growth, the dwarf is heavy and ungainly,
with large jowl and protuberant abdomen. The appearance
84,
~ INTERNAL SECRETIONS 85
of distortion is extraordinarily heightened by hypertrophy of
the skin and the subcutaneous connective tissue. Ears, eye-
: lids, nose, lips, fingers, are thick and heavy. The hair and nails
F are coarse. The skin is folded, wrinkled, rough.
The bodily ungainliness of a cretin has its counterpart in
the deformity of his mind. He is an idiot whose deficiency is
chiefly marked by apathy.
Cretinism exhibits itself in varying degrees. The descrip-
tion that we have just given would not be accurate for all.
For the sake of brevity, we have chosen a case which might be
that of a goitrous cretin of a certain type, or that of a cretin
whose thyroid gland, in lieu of showing what looks like over-
growth, has failed to properly develop. Nothing is more
remarkable with regard to this organ than the fact that the
condition associated with its overgrowth and the effects of its
atrophy, or inadequate growth, are the same. A considera-
tion of the function of the gland will suggest an explanation
of this seeming paradox.
The inconvenience caused by goitre induced surgeons, about
twenty-five years ago, to remove the tumour in simple un-
complicated cases. Owing to the accessibility of the gland,
the operation is both safe and easy; but its removal was
found to be followed by symptoms of a very serious nature,
especially overgrowth and cedema of subcutaneous tissue,
muscular twitchings and convulsions, mental dulness. About
the same date, physicians recognized that the disease myx-
cedema—so called because the cedema is not watery, as in
dropsy, but firm and jelly-like—is due to deficiency of the
thyroid gland.
No other organ of the body has so weird an influence upon
the well-being of the whole. No other organ has an equally
mysterious ancestral history. Assuredly the thyroid gland
was not always such as we see it now. In prevertebrate
animals it must have been quite different, both in structure
and in function. From fishes upwards, however, its struc-
ture is always the same. It is composed of spherical vesicles
or globes. Every globe is lined by a single layer of cubical
epithelial cells. Its cavity is filled with a homogeneous semi-
solid substance known as “‘colloid.”’ The globes are asso-
ciated into groups or lobules. They are in contact with large
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The stomach has been cut across a short distance from the pyloric valve, and removed, to show
the viscera which lie behind it. The descending aorta and the vena cava rest upon the ver-
tebral column. They are crossed by the pancreas and the transverse portion of the
duodenum. The head of the pancreas is enclosed by the curvatures of the duodenum.
The ducts of the liver and pancreas are seen entering the descending duodenum side by
side.
cesophagus and stomach is closed by a muscular ring, or
sphincter muscle —the cardiac sphincter; the junction of
stomach and intestine is guarded by a much stronger pyloric
sphincter The average diameter of the small intestine is
about 14 inches. It is wide enough, therefore, to admit two
fingers. The length of the tube is about 22 feet. Its first
part is termed the “ duodenum,” because its length equals
the breadth of twelve fingers—7.e., about 9 inches. The
remainder is divided aghiteanily into jejunum and mages The
100 = THE BODY AT WORK
duodenum makes three sharp curves. First it inclines up-
wards and to the right, then vertically downwards, then hori-
zontally to the left, and finally forwards. The ducts of the liver
and pancreas open by a common orifice into the descending ~
portion. Its horizontal portion is bound firmly to the ver-
tebral column. After this the whole of the small intestine is
supported by the mesentery, a double fold of peritoneum which
allows it to hang freely in the abdominal cavity. The mesen-
tery is attached to the back of the body-wall. Commencing
on the left side of the second lumbar vertebra, its line of attach-
ment inclines obliquely downwards and to the right, across
the vertebral column, for about 6 inches. Measured from
its attached edge to the edge which bears the intestine,
it has a width of about 8inches. Its free border has, as already
said, a length of 22feet. Its measurements being as just stated,
it is clear that it must be folded backwards and forwards upon
itself, like a goffered frill. In the right groin the small intestine
joins the large intestine, or colon. It does not, as might have
been expected, simply dilate into the large intestine, but
enters it on its mesial side, its orifice being guarded by the ileo-
colic valve. In other words, the large intestine projects down-
wards beyond this orifice, as the czecum coli. In many animals
the cecum is of great length and capacity. In the human
embryo it begins to assume a similar form ; but a very small
portion only (the so-called “cecum” of human anatomy)
dilates to the calibre of the colon. The real cecum retains
throughout life its embryonic calibre. It has a length of about
34 inches, and a diameter of not more than } inch. This is
the “ vermiform appendix,” of ill fame, which must be looked
upon as one of Nature’s misfits. Its great liability to become
inflamed is commonly explained as due to the tendency of such
articles of food as pips, the fibre of ginger, flakes from the
inside of enamelled saucepans, etc., to become lodged in its
cavity. But whether this explanation be correct or no—and
there are reasons for thinking it somewhat fanciful—it is much
to be wished that the process of evolution would hasten the dis-
appearance of this functionless vestige of a cecum. As there
is no tendency towards the inheritance of characters due to
mutilation, and since the surgeon’s knife now prevents this
death-trap from claiming its toll of possible parents, we must
a
, DIGESTION: . 101
k ‘upon the paliideaitary cecum, with its liability to inflam- |
x ‘mation, as a permanent burden on the human race. In justice
to the appendix, however, it must be pointed out that it has
acquired its criminal reputation during the past twenty years.
The frequency of appendicitis has increased so enormously
during this period that it ought to be possible to correlate its
prevalence with the introduction of the cause upon which it
chiefly depends.
The colon has a length of about 5 feet. Its greatest width,
about 3 inches, is at its commencement, but it is everywhere
much wider than the small intestine. Whereas the wall of the
small intestine is smooth externally, the wall of the colon is
_ gacculated. Three muscular bands constrict it longitudinally ;
circular bands at intervals of about 1 inch or 14 inch throw
it into pouches. It ascends on the right side, lying far back
against the body-wall, to which it is bound by peritoneum,
which in this part of its course covers only its anterior surface.
Having touched the under side of the liver, it loops forwards
and to the left side, crossing the middle line just above the
umbilicus. On the extreme left side it touches the spleen,
getting very near to the back of the abdominal cavity. It
then descends on the left side, again bound to the body-wall
by peritoneum, although not so closely as on the right side,
until it reaches the inner lip of the crest of the hip-bone. From
here onwards the fold of peritoneum which attaches it allows
it a free movement. This portion of the large intestine, the
sigmoid flexure, may even fall over into the right groin.
Lastly it curls backwards into the pelvis, as the rectum.
Movement of the contents of the alimentary canal may be
favoured by judicious pressure, or massage. From the descrip-
tion of the situation of its several parts given above, it will be
understood that if the right hand be placed on the abdomen
immediately beneath the ribs, with the fingers well round to the
left side, the stomach will be covered. Pressure from left to
right will tend to drive its contents towards the pyloric valve.
The small intestine is so irregular in its course as to preclude
the possibility of following it with the hand. Pressure first
on one side and then on the other, with a general tendency to
work from above downwards, tends to press forward its
contents ; but, owing to its circular form and strong muscular
102 «=53©)©) THE BODY AT WORK _
walls, it is not in much need of help. Very different is the
position of the large intestine in this respect. Its calibre is
much greater, its wall is sacculated, its contents comparatively
firm. If the palm of the hand be placed above the right groin
and ‘pressure directed upwards, the czecum coli and ascending
colon are emptied. If pressure be directed from the extreme
right side just below the ribs, across the middle line to the left
side, the transverse colon is emptied. The descending colon
needs pressure from above downwards on the left side; the
sigmoid flexure, pressure above the left groin, downwards, and
towards the middle line. :
The inner wall of the cesophagus is smooth, save for the —
wrinkles into which it is thrown when not distended ; but from
the cardiac orifice of the stomach onwards the mucous mem-
brane of the alimentary canal exhibits folds and other pro-
jections which serve many purposes. They serve to delay the
food, keeping it longer in contact with the secreting surface.
They increase the area pitted with tubular glands ; they increase
also the area through which absorption of the products of
digestion occurs. On the inner surface of the stomach the
folds produce a reticulated pattern. In the upper portion of
the small intestine, especially the duodenum, there are promi-
nent transverse shelves (valvule conniventes). No definite folds
occur below the upper three-fourths of the small intestine, with
the exception of the constrictions of the transverse colon
already referred to, which affect the whole thickness of its wall.
Throughout the whole of the small intestine the mucous mem-
brane projects in finger-like processes, or villi, which give it a
characteristic velvety appearance. The villi are longest in the
duodenum.
Lympbh-follicles occur at intervals in the intestine. In the
ileum they are collected into patches (Peyer’s patches), on the
side opposite to the line of attachment of the mesentery. They
serve both for the supply of phagocytes, which hunt any germs
that have penetrated the mucous membrane, and also as
stations to which germ-laden phagocytes retreat.
The wall of the intestine is composed of mucous membrane,
submucous tissue, and muscle. The mucous membrane is
everywhere pitted with tubular glands, termed in the stomach
‘* gastric glands,” and in the intestines, both small and large,
erypts of Lieberkiihn.” Their relation to the wall might be
exemplified by taking a block of dough about 6 inches thick
and pushing a pencil vertically into it almost down to the table
on which it rests. The holes should be made as close together
as possible, since, especially in the stomach, extremely little
tissue intervenes between the tubes of gland-cells. If the
piece of dough were placed upon a folded cloth, the cloth
would represent the muscularis mucose, a layer properly
regarded as a constituent of the mucous membrane. The
fibres of this coat are disposed in two or three sheets, the
fibres of one sheet crossing those of the next. By their con-
tractions they squeeze the ends of the crypts, and probably
wobble them about, expelling their secretion. Beneath the
muscularis mucose is a layer of connective tissue, the cub-
mucosa, which contains abundant lymphatic channels, blood-
vessels, and nerves. At the pyloric end of the stomach, the
tubes of gland-cells tend to pierce the muscularis mucose. In
the first part of the duodenum, certain tubes, having pierced this
layer, branch in the submucosa. A layer of racemose glands
is thus formed—the glands of Brunner. Outside the sub-
mucosa is the muscular coat proper, composed of plain muscle-
fibres, except in the upper part of the cesophagus, where the
fibres are striated. It consists of an inner and an outer sheet,
the fibres being disposed circularly in the inner, longitudinally
in the outer sheet, with a slight departure from this regular
arrangement in the wall of the stomach. On its outside the
canal is invested by peritoneum, a layer of flattened epithelial
cells supported by connective tissue. The abdominal wall also
is lined with peritoneum. The smooth moist surface of the
peritoneum covering the intestines glides on the peritoneum
lining the abdominal wall. Between the two is a “ potential ”’
space. In dropsy, fluid accumulates within this space. In a
healthy condition the apposed surfaces are merely moist.
The movements of the intestines are of two kinds. At all
times they exhibit swaying movements, in the production of
which the longitudinal fibres play the chief part, although the
circular fibres also contract. The object of this undulation is
to thoroughly mix the contents of the gut with its secretions.
If pills of subnitrate of bismuth are administered, and their
progress observed by the aid of Rontgen rays, they are seen to
5
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104 THE BODY AT WORK
oscillate backwards and forwards on their way down the
canal. The slower vermicular movement which squeezes the
contents forwards is called “ peristalsis.”’ It resembles the pro-
gressive contraction of an elastic tube which may be effected
by drawing it through a ring, but is rather more complicated.
At the point at which it is occurring the circular coat is sharply
contracted. Above this it is also somewhat contracted ;
below it is relaxed. The longitudinal fibres, using the con-
stricted portion as a point d’apput, pull up the segment of the
intestine which lies immediately below it, drawing it off the
contents of the tube as a glove from a finger.
When food is swallowed, it falls down the esophagus, aided
by slight peristalsis. As soon as sufficient has accumulated on
the upper. surface of the cardiac valve of the stomach, the
valve relaxes; at the same time a stronger peristalsis of
the lower portion of the cesophagus squeezes its contents into
the stomach. Food remains in the stomach until it has reached
a certain stage of digestion, the chief object of which is its
subdivision into small particles. Until this stage is reached,
the pyloric valve is firmly closed. The contractions of the wall
of the stomach drive its contents round and round—down the
greater and up the lesser curvature—mixing them thoroughly
with the gastric juice (cf. p. 124). As the acidity of the mixture
increases, the peristaltic contractions of the stomach become
more vigorous, until, the pyloric valve relaxing, the food is
little by little driven into the duodenum.
The alimentary canal has an abundant supply of nerves from
the vagus and the sympathetic systems. It contains also within
its own wall an enormous quantity of nerve-fibres and nerve-
cells. They are disposed as two plexuses, one in the sub-
mucosa, the other between the circular and longitudinal
muscular coats. In a specimen successfully stained with
methylene blue, they are so abundant as to give the impression
that every plain muscle-cell may have its own separate nerve-
twig. Nevertheless, the contraction of the muscle-cells may
take place independently of all nerve-influence—independently,
even, of the local mechanism, the plexus referred to above.
Nicotin applied to the wall of the intestine paralyses the local
nerves ; yet rhythmic contractions still occur. They are, how-
ever, no longer progressive. They do not drive the contents
Sr, =
‘DIGESTION eT as 105
F ; of the intestine forwards. Co-ordinated contraction is observed
so long as the local mechanism is intact, even though all
ae.
external nerves have been cut. The intestines have their own
nerve cells and fibres, which, acting as a linked system of reflex
centres, provide for the harmonious contraction of their walls.
External nerves, sympathetic and splanchnic, convey impulses
which either intensify the movements or inhibit them, as need
may be.
In the matter ofiits nerve-supply, the alimentary canal stands
apart from the other organs of the body. It may be supposed
that it presents a more primitive condition. Its muscular
fibres have the power of contracting spontaneously. The
pressure of the contents of the tube acts as a stimulus. When
the fibres are stretched, they contract. When the tube is
dilated, its muscles endeavour to restore it to its normal calibre.
Such direct action would not, however, provide for the forward
passage of its contents. To bring about peristalsis, a nervous
mechanism is needed, as abundant and complicated as that
which ensures the progress of a slug ora worm. ‘To deal satis-
factorily with the various contents of the tube—liquid, solid,
gaseous—the mechanism must be capable of complicated ad-
justments. The dilated portions of the tube—stomach, cecum
coli, rectum—require special arrangements of muscle and nerve.
Nor is the canal altogether independent of the rest of the body.
To a large extent its work is carried on without regard to the
activities of other organs, yet it is not wholly free from the
control of the central nervous system. It is regulated by means
of both afferent and efferent nerves of the vagus and sympa-
thetic. Even the brain has something to say with regard to
the way in which it shall contract. It is a matter of common
experience that emotional influences may affect the movements
of the stomach and intestines—‘“ His bowels yearned.”
Normally, vomiting is due to irritation of the endings of the
vagus nerve in the stomach, although the afferent impulses
may have other sources. Touching the upper surface of the
epiglottis with the finger will provoke the reflex. So also will
stimulation of the olfactory nerves by a foul smell. In this
latter case the emotion of disgust to which the odour gives
rise brings about the reflex action. A flow of saliva precedes
the act of vomiting. A deep inspiration is then taken, in order
that for a time the en may be independent ofa fres!
ofair. The glottis is closed, the diaphragm fixed. Contracti
of the abdominal wall presses the stomach against the di
phragm ; its cardiac sphincter relaxes, and its contents are
squirted into the oesophagus, which undergoes a forcible
retrogressive peristalsis.
It is interesting to note the difference between carnivora and — “a
herbivora in regard to vomiting. Carnivora swallow fur and
other indigestible materials, as well as many unwholesome
things which they need to be able to return. A dog can,
apparently, vomit at will. Never, while in a state of nature,
do herbivora need to return the contents of the stomach. No
provision is made for vomiting. A heifer which has strayed
into a dewy clover-field is not unlikely to die from the effects
of distension of its paunch, if relief be not given by opening
it with a knife. In a horse the cardiac sphincter is strong,
the pyloric weak. Pressure on the stomach tends to drive its
contents through the pyloric valve into the duodenum, not
backwards into the esophagus. 'The stomach is not so placed
as to allow of its being compressed between the wall of the
abdomen and the diaphragm. Horses cannot vomit. It is a
mistake to suppose that they suffer from sea-sickness. In
rough weather they sweat, their limbs tremble, they go off
their feed; but these symptoms are probably due to the
fatigue which results from excessive anxiety to maintain their
balance, and to fear. We can never know their feelings, but
there is no reason for supposing that they experience the
sensation of nausea.
Vomiting is a frequent symptom of cerebral disturbance. The
fluctuations of pressure which the brain experiences as it rocks
~ about on its ‘“‘ water-bed ” within the skull is the cause of sea-
sickness. Yet the motion of a ship may produce violent
headache without nausea, the brain only, not the stomach,
appearing to be troubled by the motion. Not that headache
is a pain “inside the head.” Nor is it properly described as
a pain in the scalp, although the messages which are felt in
consciousness as headache originate in the endings of the
nerves of the skin which covers the skull. The excessive
sensitiveness of these nerves is due to vaso-motor conditions,
usually the dilation, occasionally the constriction, of the
oe a © oe
DIGESTION 107
: ploodvessels of the scalp. But the vaso-motor condition is
_ sympathetic with the disturbance of the brain ; and the special
urgency or efficiency of the messages from the skin results
from their being delivered into excited brain-tissue. Nausea
and headache are equally symptoms of the irritability of the
brain caused by the motion of the ship. In one case messages
from the stomach, in the other case messages from the scalp,
acquire undue importance, owing to the agitated condition of
the brain-tissue through which they pass. Not uncommonly
the voyager, who wakes in the morning reconciled to the
changes of pressure which he has experienced while recumbent,
finds, when he stands upright, that the base of his brain is
as sensitive as ever. Visual sensations also contribute to the
brain-disturbance. So, too, do the movements of endolymph in
the semicircular canals (cf. p. 335). It is, indeed, possible that
this last factor is more important than the variations in
pressure on the surface of the brain. Probably it accounts
for the after-image of rolling which almost everyone experiences
for at least a day after leaving the ship. Its cause being
cerebral, the tendency to sea-sickness can be controlled by
drugs which, like the bromides, chloral, alcohol, etc., deaden
the brain.
Salivary Glands.—The secretion which accumulates in
the mouth is the combined product of the sublingual, sub-
maxillary, and parotid glands. It is a very thin, watery
solution containing not more than 0-5 per cent. of solid sub-
stance. If red litmus-paper is moistened with saliva, it becomes
blue, showing that the secretion is alkaline. It contains a
ferment, ptyalin, which digests starch. The action of this
ferment can be demonstrated by holding in the mouth for half
a minute some warm starch mucilage—boiled arrowroot, for
example. It quickly loses its viscidity owing to the conversion
of starch into sugar. Chemically this change may be demon-
strated by adding iodine-water to a specimen of the starch
before and after action. Before the starch is taken into the
mouth the iodine turns it blue (a characteristic reaction for
starch). After it has been exposed to the digestive action of
the saliva, iodine fails to colour the mixture, which now contains
no starch. All the starch has been converted into dextrin
and sugar. If unboiled arrowroot is placed in the mouth,
some sugar is eroded: but the process er convantol i
slow. It is almost impossible to digest raw starch in t!
mouth sufficiently to render it insusceptible to the colouri
action of iodine. The sugar produced by the action of ptyalin Be?
is of the same nature as that which appears during the malting —
of barley. It is therefore termed “maltose.” It closely re-
sembles grape-sugar, but is not identical with it.
_ The Secretion of Saliva.—The accessibility of the salivary
glands, and especially of the submaxillary, has led to their
being used for a very large number of experiments. They have
been studied with the aim of coming to an understanding of the
mechanism of secretion in general. The glands consist of tubes
of gland-cells, each tube suspended in a basket of connective
tissue, in a bath of lymph (cf. Fig. 3). Innumerable capillary
bloodvessels traverse the lymph-bath. The arteries which carry
blood to the gland are supplied with nerves, which regulate
their calibre, and therefore determine the amount of blood
which passes through the capillaries into which they break up.
The glands also are supplied with nerves which influence their
functional activity. Nutrient substances and oxygen pass out
of the blood into the lymph. Carbonic acid passes into the blood
from the lymph. Waste products are either carried away in
the lymph-stream, or make their way through the walls of the
capillaries into the blood. Many problems present themselves
for solution. How does the amount of work done by the
gland affect its supply of blood ? Does the quantity of saliva
secreted vary directly with the pressure of lymph in the spaces
by which the gland is surrounded ? Is this pressure wholly
dependent upon the pressure of the blood? Are the sub-
stances secreted by the gland supplied as such by the blood,
or does the gland make the ptyalin and mucus which it secretes?
If it makes its secernable products, what materials does it
abstract from the blood for the purpose of their manufacture ?
Does it use the whole of these materials, whatever they may
be, or does it use part only and return the residue to the
lymph ? Does it make its products only when it is actively
secreting, or is it always making them, and storing them in its
cells in order that it may have a supply to discharge when
called upon by the stimulation which results from the presence
of food in the mouth ? Is their discharge merely a washing-out
Ce ee
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_ - DIGESTION | 109
> to the rush of fluid which occurs when the bloodvessels are
dilated, or can the gland-cells expel their products in response
- to nervous action ? In what way do the nerves of the gland
influence secretion ? Do they call for increased production, or
increased output, or both ? These are some of the problems
which the exposed situation of the submaxillary gland allows
physiologists to tackle.
By means of a very simple operation, the ducts of one or both
parotid or submaxillary glands can be brought to the skin,
and made to pour their secretions on to the surface instead of
into the mouth. The flow under various circumstances can
be watched. The saliva can be collected and measured.
The nerves of the submaxillary gland are easily isolated. A
nerve leaves the seventh (or facial), crosses the drum of the
ear, comes out through a minute crevice in the skull, and
runs for some little distance as a separate nerve before it
applies itself to the lingual branch of the fifth, which runs
along the side of the tongue. Owing to its passage across
the tympanic cavity (drum of the ear), it is termed “‘ chorda
tympani.” As its fibres are very small, they can be recognized
wherever they form a part of the lingual nerve. They leave
the lingual to go to a ganglion, the submaxillary ganglion, from
which the gland is supplied. The gland also receives branches
from the sympathetic nerve which ascends the neck. The last-
named branches accompany the facial artery. Stimulation of
either of these nerves causes the gland to secrete. The flow of
saliva which follows stimulation of the chorda tympani is much
more copious than that which follows stimulation of the sym-
pathetic, and as a rule it contains far less organic matter,
although about the same amount of mineral salts. Under
normal conditions the activity of the chorda tympani is
brought into play in a reflex manner by impulses which travel
up the nerves of taste (the lingual and glosso-pharyngeal) to
the cerebro-spinal axis ; but almost any other nerve will serve
as an afferent path. The gland may also, as we shall presently
explain, be called into activity by the cortex of the brain.
It is certain that in the case of the submaxillary gland
secretion is not the direct result of increased blood-pressure.
It is not a case of filtration from the blood through certain
membranes and cells into the salivary duct. Atropin (bella-
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donna) dilates the blaodveusele increasing blond presente but a
it stops secretion. After belladonna-poisoning, the mouth, like __
the skin, is hot and dry. Other drugs there are which provoke
a certain amount of secretion, even after the bloodvessels
going to the gland have been tied. It is possible, by stimulating
the chorda tympani, to obtain a pressure in the fluid in the
duct very much greater than that in the bloodvessels which
supply the gland. Here we have clear proof that secretion is
not filtration. Filtration is the passage of fluid through a
filter-bed from a higher to a lower pressure. In filtration,
moreover, soluble diffusible salts accompany the water. The
saliva contains only half as much of these diffusible salts as the
blood. Therefore the gland tissue stops half the salts. Secre-
tion is an active process carried out by the gland-cells, under
the influence of nerves, in opposition to the laws of filtration.
The gland-cells determine how much water shall pass through
them and what percentage of salts shall accompany the water.
How does a gland-cell make the substance which it secretes ?
There is no reason for supposing that the ptyalin or the mucus
which the salivary glands secrete is present in the blood,
either ready formed, or, as it were, half formed, in combinations
which can be easily broken up. All the evidence obtainable
points to the conclusion that the gland-cells take out of the
lymph proteid materials from which they manufacture the
peculiar substances which they secrete. During rest, granules
accumulate in the cells. During activity they disappear. It
has been shown in the case of the gastric glands that these
granules consist of the special ferment which the gland secretes,
in an inactive form. It may be that it is combined with a sub-
stance which prevents it from exerting its digestive action on
the cells within which it is made; damped, as gunpowder is
damped during transit. Or it may be that it is not a finished
ferment ; it may need a further addition to its molecule. During
activity, while the granules disappear, proteins accumulate at
the bases of the cells, giving to a tube of gland-cells the appear-
ance of a peripheral non-granular zone. This proteid sub-
stance must have come from the lymph, and the inference
seems inevitable that the cells have taken into their protoplasm
a supply of material which will serve for the manufacture of
additional granules. Hach gland-cell is therefore an indepen-
, out of which it Gateteoviies its own ibe products.
stores its products until they are wanted. Then by its own
activity it extrudes them into the lumen of the gland-tube. It
has, indeed, been shown that, when the nerve going to a salivary
gland is stimulated, the gland shrinks, notwithstanding the
great dilation of its bloodvessels. Under the influence of the
stimulation the granules in the gland-cells imbibe water, swell
up, and escape from the cells. The cells discharge their
accumulated stores, in the first instance, more rapidly than they
take up materials (even fluid) from the blood. For its know-
ledge (if the term may pass) of what is wanted the gland-cell is
_ dependent upon messages which reach it through the nervous
system. ‘T’hese messages take origin in the endings of the
sensory nerves of the mouth, pass up to the brain, and are
reflected down the nerves to the gland. So accurate is the
information conveyed to the glands, that when a horse transfers
the work of mastication from one side of its mouth to the other,
as it is in the habit of doing about every quarter of an hour, the
flow of saliva from the parotid gland on the masticating side is
increased; on the other side it is diminished. Two or three times
as much saliva is poured out on the one side as on the other.
Not only is the amount of saliva poured out in response to
stimulation proportional to the needs of mastication, but the
kind of saliva is adapted to the nature of the food. In a dog—
and this is an observation which can be made only on an
animal which lives on a mixed diet—it is possible to determine
the amount of the two kinds of saliva secreted and the relation
of flow to food. When meat is given to the animal, the
submaxillary gland yields its secretion; when it is fed on
biscuit, abundance of the watery aa saliva is poured
forth. A mouthful of sand also causes the parotid saliva to
flow, in order that the sand may be washed out of the mouth.
More remarkable than the response to direct stimulation is the
effect produced by the sight and smell of food. When meat
is shown to a dog, submaxillary saliva begins to flow; when
it is offered bread, parotid saliva is secreted. And the activity
of the glands is not merely a nervous reflex independent of the
animal’s mind. The moment the dog realizes that it is being
played with—that there is no intention of giving it the coveted
4 =
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when every physiological condition is demanding it. This is
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food—the flow of saliva ceases. An emotion may check secretion
the explanation of the Rice Ordeal. Dry rice provokes a flow
of saliva in the mouth of all save the guilty man. Response
to mental impressions is a matter of the greatest consequence ~
in the physiology of digestion. It holds good in the case of
the secretion of gastric juice equally with that of saliva. The
sight and smell of food sets the juice flowing into the stomach,
and the more desirable the food, the more attractive its appear-
ance, the more stimulating its smell, the more rapidly does the
secretion flow. Here we touch upon a theme which hardly
needs exhaustive treatment. It is not the stoutest people
who eat the most, although an impartial survey of one’s well-
- nourished friends will show them to be persons who “take kindly
' to their victuals.” A small quantity of food perfectly digested
_ is more nourishing than much food which the digestive organs
do not efficiently prepare for assimilation. Good digestion
waits on appetite ; and appetite, in civilized man, is something
more than a mere physical need of food. The hunger which leads
to the bolting of food without pleasurable anticipation, without
mastication, without any consideration of the quality of the
viands, is a harmful craving which ends in imperfect assimila-
tion. It is more profitable to toy with a hors duvre than
to engulf, unthinking, a plateful of beef. But we have said
enough to suggest reflections to those who take no thought
as to what they shall eat or what they shall drink ; and few
who take thought need to be convinced.
The Stomach.—The sight and smell of food, its presence in
the mouth, and the performance of mastication, which induces
a secretion of saliva, gives rise at the same time to a flow
of gastric juice. It is psychic stimulation and the act of
eating which cause gastric juice to ooze from the gland-tubes
of the stomach at the commencement of digestion, not
the stimulation of nerve-endings by food which has passed
down the cesophagus. As a consequence of gunshot wounds,
or as the result of operations performed for the purpose
of relieving patients whose cesophagus has become blocked,
numerous cases have been recorded in which a fistulous
opening into the stomach has made it possible to study the
interior of this organ. Such cases present an opportunity of
PAMEIEN AG NED COG
“TORONTO URTV.
| a) ae
ig the digestion of various foods introduced through the
ening, and of collecting gastric juice for purposes of analysis.
_ Asimilar condition has been established in animals by operative
means. The cesophagus having been cut, and the cut end
sutured to the margins of an aperture in the skin, food taken by
_ the mouth escaped by this opening instead of passing into
the stomach. A similar opening was made into the stomach
for the insertion of food, and for the purpose of studying the
effects of reflex stimulation of the gastric glands. As soon as
food was introduced into the mouth, gastric juice began to flow.
The advantage of this experimental method lies in the fact
that the juice secreted was a pure juice—not mixed with food,
as in all the earlier experiments in which, the stomach being
_ opened without diversion of the cesophagus, the presence of
food within it was the stimulus which led to secretion. No
juice flowed in the absence of stimulation; nor was the secre-
tion normal in composition when provoked by a mechanical
stimulus, such as the tickling of the gastric mucous membrane
by a feather.
My lord the stomach! He is not the only, nor is he the chief,
agent in digestion; but with him rests the decision as to
whether the food offered to the alimentary tract is suitable
in quality and quantity. He is offended if it be not offered
with all the circumstance and ceremony which becomes his
rank. As an intimation that he is about to receive food, he
accepts the news from the mouth that its nerve-endings are
subject to mechanical stimulation. But the chewing of india-
rubber would produce a like effect. The stomach, therefore,
confers with the organs of taste and smell. If their report is
favourable, he argues that the substance which the teeth are
crushing will justify an outflow of gastric juice. He responds
most generously when prolonged mastication assures him that
he may trust to receiving the food in a sufficiently subdivided
state. At our peril we neglect to propitiate my lord. Not
always debonair when treated with consideration, he is
morose or petulant when slighted. Never content with lip-
service, he exacts the labour of teeth and tongue and palate.
_ The tribute we offer may be of the best—savoury, wholesome,
well cooked, well chewed—but if it be not tendered with some
degree of love, if thoughts are concentrated on other things,
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ifn no attention is devoted’ to the wal if ibaeleee ikin
panies our offering, my lord the stomach « on his part affords
viands an indifferent reception. In consulting our own tas
we are to a large extent consulting the needs of the stom AC. ae
Ravenous and excessive feeding is not an exhibition of taste ;
it is a return to the instinct of the savage, who was never sure q
that he would get his full share, and was afraid to trust that i.
another meal would be obtainable when nature declared it due.
Some degree of epicureanism is favourable to digestion. The
flow of gastric juice in the stomach occurs reflexly in response
to the emotion of appetite, to stimulation of the nerves of
taste and smell, to the obscure sensations which accompany
the activity of the muscles of mastication.
The gastric juice secreted in a day amounts probably to about
8 or 9 pints. To this we must add, when considering the quantity
of fluid which passes through the stomach, the saliva, which
certainly reaches as much as 2 pints, and the beverages taken
with food.
. |. Gastric juice collected in the manner described above is a
f 6, clear, colourless, inodorous fluid. It is very acid, and so power-
\w fully peptic as to digest its own weight of coagulated white
/ | of egg. Its solid constituents amount_to 0-5 per cent. They
“\ | consist of the two ferments pepsin and rennin, with traces of
‘ proteins and mucin, and various inorganic salts. Its acidity
f is due to free hydrochloric acid to the amount of 0-2 per cent.
This acid is more or less in combination with the pepsin. In
pure gastric juice hydrochloric acid is the only acid present ;
| but when mixed with food the juice contains other acids also,
especially lactic.
When food first reaches the stomach, the alkaline saliva |
which accompanies it neutralizes the acidity of the gastric
| juice. For some time, probably about half an hour, the
conversion of starch into sugar is still carried on by the ptyalin
| of the saliva, owing chiefly to the difficulty which the gastric
| juice encounters in permeating the masses of masticated food.
The Bacillus acids lactict is always present in the stomach. It
converts some of the sugar into lactic acid; of this a small
quantity is further changed into but butyric and acetic acids, with
the formation of carbonic acid and h: hydrogen gas. ‘After a a while
the lactic acid is absorbed, and hydrochloric acid alone remains.
mm F Ly oF; ‘ oe (i - *, at We tale . a 4
le ples of 80 argh a eedecat
ssi. oe plant -cells to produce it without injury to the
, or for the stomach to contain it without self-digestion ?
By chemical and physical theories have been advanced in
B eliet that they rendered the process of its production less
4 if cult to understand. All such theories are, however, i 23
_ adequate to explain the secretion as a discontinuous process,
which occurs only as a response to demand. That the source
_ of the acid is the sodic chloride which the gland-cells take from, /
ydroe’ ates always aroused interest. How is it ‘
ty
the blood does not need assertion, but we cannot picture ‘the of
process by which this exceedingly stable compound is decom-
_ “posed otherwise than on the assumption that weaker acids,
or, rather, acid salts, are also absorbed by the cells, and that,
in accordance with the laws which govern the composition of
salts in solution, an exchange of acids occurs. If sodic chloride
and any acid salt—acid phosphate of sodium, for example—
are in solution in water, the salts do not retain their form as
we know them when isolated by crystallization. The mixture
contains “free”? hydrochloric as well as “free” phosphoric
acid. It may be assumed that within secreting cells a similar
exchange of acids takes place. By a process which we term
“vital,” the acids are kept apart, and the hydrochloric acid is
extruded by the cells. In the present state of knowledge this
vital action is mysterious ; but it is no more mysterious than
the isolation of pepsin, or any other metabolic event which
occurs within a cell.
The proteolytic ferment pepsin is active only in an acid
medium. Yet apart from its digestive function as an ally of
pepsin, hydrochloric acid by itself also exerts a valuable
disintegrating action on certain constituents of the food.
Possibly the most important results of the presence of free
hydrochloric acid in the great chamber into which food is first
received are due to its disinfective property. It destroys all the
putrefactive germs which accompany the food, and many germs
which, if introduced into the blood, would give rise to disease.
It also destroys the germs which multiply in the stomach
towards the end of each interval between two meals. When
withdrawn from the body, gastric juice will keep an indefinite
time, if evaporation of the acid be prevented.
8—2
resemblance to the salivary pers) "Probably. “’ i
blance is merely superficial. Minute examination —
points, apparently of great morphological importance, in wk
they differ. In the gland-tubes of the salivary glands, and, —
indeed, in all glands with the exception of the pancreas, us
secreting cells project into the lumen. The secreting cells
of the pancreas are invested internally by a layer of —
flattened scales .(intra-acinar cells). They lie, therefore,
between the basement membrane which invests them exter-
nally and this second layer of flattened cells which separates
them from the lumen of the tube. At a very early date in em-
bryonic life the gland-cells of the pancreas are filled with highly
refracting granules. As this occurs long before any digestive
action is called for, it may be taken as indicating that the
pancreas has functions which other glands—the salivary, for
example—do not possess. These granules do not, however,
appear in all parts of the tubes. Certain portions of the tubes
remain undeveloped—fail, that is to say, to acquire a secreting
function—even in adult life. Such patches of cells, not dis-
posed in gland-tubes, are known as islands of Langerhans.
When the pancreas is over-stimulated by artificial means,
leading to its extreme exhaustion, large portions of its
glandular substance return to this primitive condition. The
gland-cells not only discharge their stores of granules, but
they lose the greater part of their cell-protoplasm. It would
seem that, in their effort to meet the demand for ferments,.
they use up their own cell-substance in their manufacture.
Having exhausted their coal, they stoke the furnace with
the looms and furniture of the mill. It may be that other
glands would do the same if it were possible to stimulate them
as strongly as the pancreas can be stimulated. The result is
probably due to the extreme susceptibility of the pancreas
to the action of secretin, a substance made in the intestine.
Secretin can be isolated and injected into the blood. We
shall refer again to this chemical stimulation of the pancreas
when tracing the progress of food through the alimentary canal.
The secretion of the pancreas is a clear, colourless, alkaline
liquid of syrupy consistence. The quantity of juice secreted
is usu smal but the organic substances which it contains
ieee
117.
in a concen ia form. They constitute as much as 10 per
of the pancreatic juice. Proteins are present, if the
F juice be fresh. If it has stood for any length of time, they are
- found as peptones. The digestive ferments of pancreatic juice
4 are the most powerful which are seareted into the alimentary
canal.
~ Bile.—In its most important functions the liver has no rela-
tion to digestion. It is a storehouse of absorbed food. This
organ will therefore be treated in a separate chapter. The
bile which the liver secretes into the alimentary canal has no _
ene
chemical action on any of the constituents of food, with the
exception of a feeble tendency to digest starch. Yet it is in
some degree accessory to digestion. Poured into the second
portion of the duodenum through an orifice common to the liver
and the pancreas, it mingles with the semi-digested food, or
“ chyme,”’ which, about two hours after a meal, passes through
the pyloric valve. Gastric digestion has converted the greater
part of the proteid constituents of the food into peptones or
intermediate stages. ‘The proteoses or propeptones—a name is
needed for the intermediate products of proteid digestion which
does not commit us to any theory as to their chemical con-
stitution—are quickly peptonized by the pancreatic juice. But
portions of the proteins have escaped the action of gastric
juice, or have at most been affected by its acid only ; these
are precipitated by the bile-salts on the mucous membrane of
the small intestine, which is raised into projecting flanges for
_ the purpose of delaying the passage of the chyme, in order that
it may be thoroughly submitted to the digestive action of pan-
creatic juice. Bile-salts also favour the digestion of fat, and
its passage through the intestinal wall. The action of bile-
salts in spreading fats is well known to artists. Ox-gall is
smeared upon glass when it is desired to apply oil-paints to its
surface. When mixed with oil, it causes its emulsification,
or breaking up into microscopic globules. In the absence of
bile, but little fat passes into the lymph-vessels which convey
digested food from the intestine to the thoracic duct, and so
to the great veins of the neck. Its action is mechanical. It
favours the digestion of fats by rendering them easily amenable
to hydrolysis by pancreatic juice.
Bile as secreted by the liver is a clear, limpid fluid of low
a ee se we feed Ce ee oe j
BTS ee nse eee NO
BET eh Polar oa ee
411g “THE BODY AT WORK
specific gravity ; but during its stay in the ,
ponoentraed by absorption-of water, and mucin is added tots a
t
contains “ bile-salts ” of complex constitution. These salts _
favour the solution of certain by-products of cell-metabolism,
cholesterin and lecithin; substances which are formed in =)
many célls, both in animals and plants. Cholesterin occurs
most abundantly in nerve-tissue and in blood-corpuscles.
Lecithin also is a by-product of the metabolism of nerve-tissue.
Protoplasm appears to be incapable of oxidizing these sub-
stances, as it does other products of metabolism. Other sub-
stances of equally complex constitution are reduced to urea if
they contain nitrogen; to water and carbonic acid if nitrogen
be absent. Cholesterin and lecithin have to be eliminated
without further change. Some of the cholesterin is excreted
by the sebaceous glands of the skin. It is the chief constituent —
of “lanoline”’ prepared from sheep’s wool; an unguent which
owes its valuable properties to the resistance which cholesterin
offers to cell action, and therefore to the action of living fer-
ments. Bacteria ‘cannot turn it rancid. The sebaceous
glands have the power of directing metabolism into a channel
in which cholesterin is the chief product, but apparently all
cells make it in small quantity. The bile-salts carry choles-~
‘terin and lecithin into the alimentary canal, from which they
are not reabsorbed. Some of the bile-salts are lost to the body,
but the remainder re-enter the circulation, and recommence
their work as vehicles for these inoxidizable and insoluble
substances. In the gall-bladder cholesterin is apt to separate
out from the bile in the form of gall-stones ; but whether this is
due to an excess of cholesterin in the bile, or to an abnormal,
inflammatory condition of the lining membrane of the gall-
bladder, is still an open question.
Bile also contains bile-pigments. Their colour varies in
different animals, and changes according as the bile is exposed
to the air, or subject to the action of reducing agents. If
oxidized, the colour is green (biliverdin) ; if reduced, brownish-
yellow (bilirubin). Bile- -pigment is formed from hemoglobin,
the colouring matter of the blood, after the removal of its iron.
Worn-out red blood-corpuscles are destroyed in the spleen,
in the manner already described, but it is uncertain whether
the conversion of the hemoglobin thus set free into bilirubin
brane of the alimentary
ract bie pay as the middle of the rectum, is, as previously
d (p. 102), studded cadres a y secrete a
Its
1t-yellow fluid, alkaline in reaction, and opalescent.
line to Ne aa sueage the liver we
20st important property is due to a ferment which converts
‘ eamie-Sugar~into a mixture of dextrose and levulose, and
i oem maltose—the sugar produced by the action on starch
of saliva and pancreatic juice—into dextrose. It is in the
form of dextrose that sugar is carried about the body and
assimilated by the tissues.
‘ Intestinal juice also contains a ferment, erepsin, which shakes
to pieces the heavy molecules of peptones and partly formed
-_peptones. Under its influence they break up into compara-
tively simple bodies containing the radicle of ammonia.
Substances containing an NH, group—one H of NH, (ammonia)
having been given up, in order that the group may have a
** free arm ”’ with which to link on to the other component parts
of the molecule—are termed “‘ amides.” The amides which
are most characteristic of the action of erepsin are leucin, an
amidated fatty acid ; and tyrosin, an amidated aromatic acid.
The tendency of proteins to break up along these two lines—
the fatty acid line and the aromatic acid line—is of consider-
able interest. The one line is represented by acetic acid,
CH,,COOH ; the other contains the hexone radicle, C,H,. Ben-
zoic acid, C,H,COOH, is representative of the latter. It used
to be thought that proteins which were shaken into simple bodies
such as amides were lost to the economy. Their downward
career was a foregone conclusion. There could be no arresting
it before they brought up at the bottom—as urea, CO(NH,),.—
the diamide of carbonic acid. It was even supposed that this
disintegration of proteins was a provision for getting rid of
.the surplus animal food which we consume. Physiological
chemists now take quite a different view. They believe that
the epithelial wall of the intestine through which these sub-
stances are absorbed, or the liver, to which they are carried
by the portal blood-stream, has the power of recombining
these fragments into the complex protein edifice. It is even
"420 ‘THE BODY AT WORK _
thought that disintegration is a necessary preliminary to the —__
rearrangement of the sub-groups. A large variety of proteins
is ingested as food. Many of them, especially the vegetable
proteins, are quite foreign to the body. By the activity of
pancreatic juice and erepsin, they are broken into small and
relatively stable groups of atoms, which are again fitted
together into the particular forms of protein which are of use
to the economy.
The Story of a Meal.—The chemistry of digestion will be
understood most readily if the constituents of a meal are
traced from their entrance into the mouth to their absorption
through the wall of the alimentary canal, or abandonment as
indigestible. ;
We may describe as a typical meal one consisting of bread,
vegetables, cane-sugar, meat, milk, fat, and cheese. In the
mouth the various foods are crushed and mixed with the alka-
line secretions of the salivary glands. A certain amount of
the cooked starch contained in the bread is changed into
maltose. In the stomach the digestion of starch is continued
for a time, but a large part even of the cooked starch awaits
the action of pancreatic juice. A certain amount of cane-sugar
is converted into dextrose and levulose, which are rapidly
absorbed into the blood ; but this action is due to hydrochloric
acid, and probably affects a comparatively small part of the
cane-sugar swallowed. Fat is quite unaltered in the stomach.
All proteins are attacked by pepsin, but some yield to digestion
more readily than others, Gluten of bread, like all vegetable
proteins, is comparatively resistant ; but since it is presented
to the action of pepsin in small quantities and in a spongy
form—very suitable for digestion—it is probable that most of
it is peptonized in the stomach. Chemists experimenting
with gastric juice taken from the stomach, and reproducing
the conditions as to temperature, removal of products of action,
etc., as closely as it is possible to reproduce them in the labora-
tory, find that the various foods take different times to digest.
The proteins of meat are more quickly peptonized when raw
than after coagulation by heat. The same is true of white of
egg. Amongst different varieties of cooked flesh, beef is more
quickly peptonized than fish. The casein of milk is more
quickly peptonized than any other protein ; and it also is no
; tion “a a4 ale that digestibility i is diminished by cooking.
Similar data may be obtained for all foods. They are no doubt
useful indications of the course of action which we may expect
__ to occur within the stomach, but we can never be sure that
my lord will obey the ruling of the chemist. Practice with a
__ captive golf-ball is a useful preparation for the game ; but there
are conditions on the links which cannot be reproduced on the
lawn. In an artificial stomach the clean fibre of raw fish
_ digests more slowly than raw beef. Even when the beef is
roasted and the fish fried or boiled in the ordinary way, the
beef disappears through the dialyser (the bag of membrane
suspended in a vessel of warm water in which experimental
digestion is carried out) more quickly than the fish. Never-
theless, the living stomach is better disposed towards a mixed
meal containing a certain weight of fish than towards a meal
in which, the other constituents remaining the same, beef takes
the place of fish. Important conclusions may, no doubt, be
drawn from observations of the time occupied in the peptoniza-
tion of pure food—.e., fibrin, white of egg, clean meat, etc.—
under conditions simulating those which are present in the
stomach ; but they must be accepted with many reservations.
In the a eaark it is not pure substances, but mixtures, that the
gastric juice has to deal with. And here a most important
factor comes into play, to which further reference will be made
later on. The amount and quality of the secretion of the
gastric glands depends upon the nature of the food. Hence a
food, or a combination of foods, which digest readily in
the laboratory may take a long time to disappear from the
stomach, and vice versd. NDigestibility depends upon the
nature of the food. It depends also upon its physical state.
To take simple illustrations : Cheese contains coagulated casein,
one of the most easily digestible of proteins, but the casein is
intimately mixed with fat, upon which gastric juice can make no
impression. Even when finely divided, the particles of casein
are protected from the action of the juice by fat. In the same
-way the meat of pork is as digestible as mutton, but the fat
of pork is quickly melted and very liquid. In the process of
cooking the muscle-fibres become saturated with fat.
It is not the function of the stomach to complete digestion.
Its business is to initiate it. Food which reaches the stomach
tion heat a much larger proparian of intonmedians
ducts, proteoses or propeptones, than does digestion in t
duodenum. Such intermediate products are quickly dealt
with by pancreatic juice. Artificial tests of relative igen oe ;
bility do not, as a rule, take the amount of propeptones formed
in a given time into account. When considering the digestion -
of a typical meal, we must bear in mind that it is not the duty
of the stomach to pass as much sugar, peptone, and fat as
possible into the blood. In fact, very few of the products
of digestion are absorbed by th the bloodvessels of the the stomach,
The impermeability of its mucous membrane is shown by the
fact that hardly any of the water swallowed passes through
the stomach-wall. Practically all the water ingested leaves
the stomach through the pyloric valve. Various salts, some
sugar, and peptones are taken up by the vessels of the stomach ;
but the bulk of all the different kinds of food passes. into. the
duodenum in a semi-digested state. The function of the
stomach is to carry digestion through a preliminary stage. “The
process will be completed in the small intestine. It is to be
noted that, although water is not absorbed by the stomach-
wall, alcohol passes through it with great rapidity. The same is
true of the various crystalline nitrogenous bodies found in_
meat-extracts, and also of the essential principles of tea and
coffee, which chemically belong to the same class. All these
substances are degradation products of proteins produced by
oxidation, far advanced along the road to urea. In this selec-
tive absorption we see proof of the activity of the cells of the
mucous membrane. They take up the substances which it is
desirable to remove from the contents of the stomach. Some
may be wanted by the body for its immediate use; others
are better out of the way, because they are prejudicial to the
progress of digestion.
When contemplating the activity of the cells of the gastric
mucous membrane, we feel the need of an adjective which
shall express our recognition of the fact that they have a
power which we cannot confer upon our clumsy mechanical
imitation stomach. They can_discriminate. “Vital” is the
only term available, though much abused. Using it without
3 54
Lars ‘ > oe
ee he sane
123
- prejudi o, as lawyers say, we aphak of the “ vital activity ” of
the cells when we wish to imply that things happen i in a living
_ stomach for which we cannot make provision in a model. Of
_ ~ the many substances which make their appearance as digestion
__ proceeds, some are absorbed, others left in the mixture.
a The mucous membrane shows its powér~ of controlling
i digestion in yet another way. In the neighbourhood of the
‘ pylorus its structure is unlike that which it “presents elsewhere.
The gastric glands are short, and tend to branch. Their lining
cells are all of the same kind. Over the greater part of the inner
wall of the stomach the tubes are long. They do not branch.
The cells which line them are of two kinds: small cubical
cells (the term refers to their form as seen in section), similar
to those of the pyloric glands; large oval cells, placed with
their longest axes in the same direction as the axis of
the gland-tube. These oval cells do not project into the bore
or lumen of the tube, but are displaced from it by the cubical
cells. They rest on the investing, or basement, membrane.
All parts of the gastric mucous membrane secrete pepsin,
although the pyloric portion produces very little ; the area
which contains oval cells alone secretes hydrochloric acid.
If a short time after a meal an extract is made from some of
the mucous membrane near the pylorus, by pounding it with
salt-solution and sand to break up its cells, this extract, when
filtered and injected into the blood, stimulates the glands of the
cardiac end of the stomach. Under its influence they pour
out both pepsin and hydrochloric acid. The extract contains a
substance which acts as a chemical messenger. It is a repre-
sentative of a class of bodies which play a most important part
in co-ordinating the activities of the various organs. Hitherto
physiologists have concerned themselves with the visible or
*‘ external ”’ secretions of glands. They have shown how the
production of these secretions is controlled by the nervous
system. Recently they have discovered that another set of
influences has to be taken into consideration. Glands, and
possibly all other tissues, take from the blood the materials
out of which they make their characteristic secretions, or, if
they do not discharge secretions, the substances which they
require for the building of their own structures, and return
to the blood “ internal secretions ”’ which act as stimuli to other
— eee al
ie ee
124, | 'THE BODY AT WORK —
tissues with which they are linked in harmonious. co-operation, ——
The active principles of internal secretions have been termed
bend ’—from opudw, I announce. The glands of the
pyloric mucous membrane secrete a hormone which calls upon
the rest of the membrane to pour out gastric juice (cf. p. 89).
What induces the cells of the pyloric mucous membrane to
produce the gastric hormone ? Their activity in this respect
evidently depends upon the presence in the stomach of par-
tially digested proteid substances. ‘The cells judge, as it were,
when these substances come into contact with them, that there
is more work for the great bag of the stomach to do. They
call upon the part which is most active in secreting gastric
juice to pour it out quickly and get the business of digestion
over. Meat-extracts, which contain the products of protein
disintegration, have a similar influence in promoting the forma-
tion of the hormone. Hence, no doubt, the general custom,
found from experience to be beneficial, of commencing dinner
with soup; although it must be remembered that the rapid_
absorption of meat-extracts makes them peculiarly valuable
as restoratives. They afford very little energy, but what they |
have to give is quickly placed at the disposal of the economy.
Persons whose stomachs are unduly irritable are advised to
avoid soup. It leads to undesirable activity on the part of
the gastric glands, and especially of the acid-secreting cells.
Well-chewed bread also encourages the production of the
hormone.
Here it may be well to call attention to the evident division
of the stomach into two parts—the large bag, or cardiac portion,
which hangs down; and the smaller, funnel-shaped pyloric
end, which is almost vertical. The distinction between
these two parts is faintly visible in the resting stomach, but
even opening the abdomen tends to obliterate it. That
it is much more evident during active digestion has been
shown by adding subnitrate of bismuth to the food, and
throwing the shadow of the stomach on a screen with Rontgen
rays. When this is done, it is seen that the two parts work in
different ways. Food is churned round and round in the
cardiac portion, and pressed towards the pylorus. Its fluid
products, mixed with the abundant secretion of the gastric
mucous membrane, are wrung out of it by the pyloric funnel.
They are squeezed towards the pylorus, which opens at intervals
tolet them through. If lumps of solid matter reach it, the
pyloric valve closes tightly, until the undigested food has
fallen back into the dependent bag. Dyspeptics are sometimes
unpleasantly conscious of the contractions of the pyloric funnel.
=, ~ In fact, putting aside pain due to gastritis, all the discomfort
of dyspepsia is felt on the right side. Flatus accumulates
beneath the pyloric valve. The valve will not open to let it
pass. The pyloric portion of the stomach contracts strongly.
Notwithstanding the general trend of movement in the opposite
direction, the gases are squeezed back into the larger bag, and
escape through the cardiac orifice.
Tables have been prepared showing the length of time
which various articles of food take to digest. They are based
in part upon observations made upon the living stomach in
cases in which it has been possible to examine its contents
through a fistulous opening ; in part upon the results of arti-
ficial digestions carried out in the laboratory. It is hardly
too much to say that such observations are absolutely without
value as tests of the relative digestibility of the several articles
of diet consumed as parts of an ordinary meal. The fact that
the commencement of the flow of gastric juice depends upon
mental stimuli, and its continuance upon hormones, shows how
difficult it must be to reproduce the conditions which obtain
in a healthy living body. The most wholesome of foods taken
by itself may be longer in digesting, or may produce more
irritation, than many less desirable things taken in judicious
combination. Crushed chicken, hastily swallowed, sometimes
proves more difficult of digestion than meat so cooked and
served as to stimulate appetite and to demand mastication.
Returning to the story of a meal, vegetables pass almost
unaltered through the stomach. Some of the scanty proteins
which they contain are peptonized, but unless they are very
well masticated or cooked until they are soft, and therefore
easily pulped by the churning action of the stomach, the
gastric juice has to reach the proteins through* cell-walls.
None of the digestive juices are able to dissolve the cellulose
of vegetable cell-walls. Blocks of vegetable tissue pass down
the whole length of the alimentary canal in the form in which
they were left by the teeth. Hence the extreme indigestibility of
ill-chewed snuibilier or apple. The pyloric Salve af the stotiindl .
is forbidden to allow any lumps of food to pass until the very
last stage of gastric digestion. Pieces of ill-masticated vege-.
table tissue lie for a long time in the stomach, i |
of the gastric nerves, until at last the time comes for them to
be shot through the pylorus into the duodenum. Many salts
which vegetables contain, especially the earthy carbonates |
and phosphates, are dissolved by the acid of the gastric juice.
Meat consists of muscle-fibres supported by connective
tissue. In the stomach the gelatiniferous connective tissue
is dissolved, setting the fibres free. Further, the fibres being
surrounded by a membrane of the same nature—sarcolemma—
this is removed ; and although it may be hardly justifiable to
speak of “‘ Krause’s membranes ”’ (cf, Fig. 10) as gelatiniferous
septa, the fibres are certainly composed of segments—Bow-
man’s discs, sarcous elements—into which they break up under
the action of gastric juice. As a result, meat-fibre is reduced
to a finely divided granular condition. The capacity of gastric
juice for dissolving collagen (the substance of which connective
tissue is composed) may be regarded as its most characteristic,
as it is one of its most valuable, properties. Collagen, when
boiled or acted on~by acids, takes water into its molecule,
becoming gelatin. Under the influence of gastric juice gelatin
is rapidly hydrolysed into diffusible gelatin-peptone. Pan-
creatic juice is unable to act upon collagen, unless it has been
previously boiled, or swollen by the action of dilute acids.
Fat is composed of vesicles of oil supported by connective
tissue. Gastric juice, by dissolving the connective tissue and
the collagenous walls of the vesicles, sets the oil free. The oil,
even though it be as firm as suet when cold, is liquid, or almost
liquid, at the temperature of the body.
Thus, with the exception of raw vegetables, the hard fibre
of cooked vegetables, elastic tissue of meat, and a few other
indigestible substances, the meal is reduced in the stomach to
a cream-coloured, fatty, strongly acid “ chyme.” In this
condition it enters the duodenum, where it at once comes
into contact with an alkaline secretion. The passage of acid
chyme down this portion of the canal provokes the discharge
of gushes of bile and pancreatic juice. By precipitating
partialiy digested proteins and “ acid-albumin ”’ bile renders
I actecnee are. of panera converted into peptones, some of
ich are shaken down by the violent action of erepsin into
- simpler bodies, such as leucin and tyrosin, etc. The chyme
rat
ee alkaline, grey, and thin. All anidsancked akaeok 7a
ia unged into maltose, and this into dextrose. Cane-sugar
is converted into dextrose and levulose. These sugars
are absorbed into the blood. Milk-sugar, if not_ converted
into lactic a acid, remains as lactose (CisH:011), in which
condition it is absorbed without “i inversion.” Fats are split
by a ferment of the pancreatic. juice into fatty acid and
_ glycerin ; some of the fatty acid combines with alkali to form
Soak. but of this we shall have more to say later on.
he duct common to the liver and the pancreas opens into
the second part of the duodenum. The organs which produce
bile and pancreatic juice are comparatively remote from the
“place where their secretions come into contact with the food.
By what mechanism are they thrown into activity when the
assistance of their secretions is required ? As in the case of the
stomach, the agent is a hormone, a chemical messenger. The
hormone, termed “secretin,” is formed by the cells of the
Mucous membrane of the duodenum when acid comes in
contact with them. It is absorbed by the blood, which carries
it to the pancreas and the liver. When it reaches the pancreas,
it acts as a most powerful stimulant to the discharge of accumu-
lated ferments, and to the production of an additional supply.
It stimulates the liver to pour forth bile. At present we are
in ignorance as to the chemical nature of this hormone. It is
ee substance, nor is it aferment. If scrapings from
@ mucous membrane of the duodenum be crushed with sand
and hydrochloric acid, the mixture boiled, neutralized with
carbonate of soda, and filtered, the clear, colourless liquid
which results has a powerful effect upon the pancreas, when
injected, in even small quantities, into the blood. Apparently,
the cells of the duodenal mucous?membrane are constantly
producing and accumulating a substance which is converted
into secretin when acted on by acid. It is not necessary for
the acid to stimulate the living cells.*} If the mucous membrane
is ground up with sand and salt-solution, the filtrate is inactive:
re” Fee ee eS ee. oe ee ee eee
oi, ae oe on ae x =
128 THE BODY AT WORK
but an active extract is obtained by treating the crushed cells
with HCl. It changes some substance which they contain (pro-
visionally termed “ prosecretin ’’) into the efficient hormone.
In the lo ion of the small intestine any maltose
that: remains is converted into diffusible dextrose. A very
large amount of water has been poured into the canal in the
various digestive juices. This, together with water drunk,
is absorbed _in the large intestine. At the lower end of the -
alimentary canal tr remains but indigestible substances -
taken with food, chiefly cellulose, and the pigments and other —
bodies which, as already said, are eliminated in bile.
The absorption of water is checked by the ingestion of
extremely soluble salts, such as sulphate of magnesia, the
heavy molecule of which diffuses with difficulty. We attribute
the fact that sulphate of magnesia remains in the intestine,
and prevents water from diffusing out of it, to its slowness in
passing through a membrane, because this is what would
happen in dialysis ;* but we must remember that the living wall
* Notice the distinction between filtration and dialysis. If water containing
soluble and insoluble substances is placed in a porous jar, the water and the
soluble substances pass through the pores of the jar. The rate of flow varies
as the pressure. If water containing soluble substances is placed in a bladder,
and the bladder is suspended in a vessel of water, some of the substances
which it may contain—white of egg, for example—are non-diffusible ; others will
pass from the water inside the bladder to the water which surrounds it. But
every diffusible substance has its own osmotic value, Some pass through the
membrane rapidly, soon establishing a condition of equilibrium in the two
fluids; others take a long time. Further, if the water on one side of the
membrane contains a certain percentage weight of a salt, the molecules of
which are large—say sulphate of magnesia—and the water on the other side
the same percentage weight of a salt of smaller molecule—say chloride of
sodium—water containing the salt of smaller molecule will pass into the water
containing the salt of heavier molecule with a certain force. If, to start with,
the two solutions are at the same level, the level of the solution containing
the less diffusible salt, sulphate of magnesia, will at the commencement
of the experiment rise. It is therefore said to exert a greater osmotic
pressure than the more diffusible salt—chloride of sodium. Equilibrium will
not be established until the fluid on one side of the membrane contains the
same number of molecules per unit volume as the fluid on the other side. If
the molecules of magnesic sulphate are pictured as oranges, and the molecules
of sodic chloride as nuts, it will be understood that equilibrium is not
established until the oranges and nuts to the pint on one side equal in number
the oranges and nuts to the pint on the other. When these principles are
applied to the passage of water containing products of digestion through the
wall of the alimentary canal, it is evident that, if we understand all the condi-
tions, the process cannot be explained as merely an exhibition of osmosis. Take
the simplest illustration. When blood-serum is placed in the intestine it is
absorbed. If it were in a dialyser, there would be equilibrium between the
2 is not a te ee ‘The cells which line the
> take up substances far less easily diffusible than the
hate of magnesia which they refuse. Nevertheless, speak- « Fy
ng generally, iti is the less diffusible salts which act as aperients, a
a of Gas contents” of the alimentary canal is a buatetied by castor-oil. ./
_ The peristalsis of the intestines is stimulated by certain drugs, ~~.
_ such as jalap or the burnt products of tobacco. Another class 42
of drugs, of which aloes is an example, increases the secretion 4
of the intestines, small or large. Certain purgatives, such as ~
_ ¢alomel, podophyllin, etc., used to be regarded as cholagogues. / ,
___ It was supposed that they increased the flow of bile. Thisis ~
aneerror. Their action is complicated, but it affects chiefly
_ the peristalsis of the intestine. The poor misunderstood liver
still suffers from the libels of primitive medical science. It is
_ the most innocent of organs, in no way responsible for derange-
ments of digestion. It carries out its functions without haste / |
and without delay. With the possible exception of salicylate
of soda, no drug is known which can stimulate it to a more
rapid output of bile.
~ Absorption.—All the cells which line the alimentary canal
are capable of absorbing food, if it is presented to them in a
suitable form. In a suitable form means, speaking generally,
“in a diffusible condition, although it must not be supposed that
the epithelial cells are incapable, under certain circumstances,
of taking up non-diffusible substances, just as a unicellular
organism—an amoeba—can take in food. If soluble proteins,
such as white of egg or acid-albumin, are injected into the large
i
;
' serum inside the intestine and the lymph on the outside. There would be
no osmosis. Or, again, supposing water containing 2% of common salt is
placed in the intestine, we find that both salt and water pass through into
the lymph. In a dialyser water would pass from the lymph (which contains
salts equal to about 0°9 % of sodic chloride) through the membrane into the
stronger solution. A salt-solution needs to be very concentrated to cause
water to take the reverse course through the intestinal wall, and so to act as
a purgative. When we study absorption from the alimentary canal, we find
that its if it wants a salt or any other substance, sets the laws of osmosis
a If the salt is not-wanted; the ordinary phenomena Of osmosis
are exhibited. Sulphate of magnesia (Epsom salt) would be deleterious if
absorbed. The intestinal wall behaves towards it like a dead membrane.
The salt retains the water in which it is dissolved: possibly water passes
out of the lymph into the solution of the salt. The contents of the intestines
are in consequence unduly liquid. The salt acts as a purge.
9
ae ey
ot) 4 ve? *
ao THE BODY AT WORK |
intestine, a very considerable proportion of ‘he subones so.
injected is absorbed. It is possible, indeed, to supply in this = a
way the whole of the nitrogenous food needed by the system,
none entering by the mouth. If milk is injected, a certain
amount of the fat also is retained. It can be shown that
such absorption takes place when no digestion of the ‘food
occurs in the colon. The food is taken up by the epithelial
cells in the form in which it is in’ injected.
The organs specially devoted to a absorption are the villi,
which project into the contents of the small intestine. Haok |
is a conical process about 0-5 millimetre long. The _ villi __
are longest_in the upper half of the small intestine. Below
this level they decrease in number and size. A villus is
completely covered with epithelial cells of short, columnar
form. The free border of each cell is slightly hardened,
forming a disc or cap which appears striated in optical
section—an indication, as some think, that it is traversed by
pores. Others hold that the appearance of striation is due to
minute cilia-like projections which beset the free border of
each cell. In worms and other invertebrates the cells carry
motile projections of not inconsiderable size, which no doubt
free their surfaces from the unassimilable matter which tends
to accumulate upon them. Possibly they help to fix particles
which are suitable for absorption. In mammals the presence _
of cilia has not been demonstrated. The extreme minuteness
of the striz seems to point to their being merely indications
that the border is permeable to fluids, including droplets of fat.
The so-called basement membrane upon which the epithelial
cells rest must not be regarded as a membrane in the physical
sense. Rather is it a basket-work which supports the cells,
without in any degree limiting their power of disgorging into
the lymph-spaces of the villi the substances which they have
absorbed. Within the villus, connective tissue forms a sponge-
work, the spaces of which are filled with lymph, in which a con-
siderable number of leucocytes | roam, on the look-out, no doubt,
for any germs which may make their way between the epi-
thelial cells. In the centre of the villus is a lymphatic
radicle—z.e., a fusiform cul-de-sac—which is the dilated end
of a lymph-vessel. It, like all other lymph-vessels, is walled
by flattened endothelial scales. It communicates with the
131
Z y ieee The nenartinar poss ¢ vessels ae? in the MeNENtery
3 converge to the receptaculum chyli, the bulbous commence-
[ oa
ye... ’
“4 7
ment of the thoracic duct, which lies at the back of the abdomen
in front of the bodies of the vertebre. The thoracic duct runs
up the front of the vertebral column, through the thorax, and
then hooks over to pour the fluid which it conveys into the
great veins shortly before they join the heart. After a meal
containing fat the fluid in the lymphatic vessels of the mesen-
tery, the lacteals, has, as already stated (p. 43), the appearance
of milk. The fat absorbed by the epithelium covering a
villus is passed on into its lymph-space. From this into the
central lacteal receptacle, thence to the submucous and peri-
intestinal plexuses, the lacteal vessels of the mesentery, the
thoracic duct. Absorbed fat does not pass_ through the.
liver, but is carried into the heart ; thence through the lungs,
and back to the heart, which pumps it to all parts of the body.
In addition to the lacteal_radicle; the villus contains long
capillary bloodvessels, and the arteriole and venule in which
they commence and end. These traverse the lymph-spaces of
the connective tissue, which contains, not only the fat which
the epithelial cells have passed into it, but the other products of
digestion also. None of the fat traverses the walls of the
bloodvessels ; but the other products diffuse from the lymph,
through the walls of the vessels, into the blood. Many nerve-
fibres are found in the core of the villus on their way to epi-
thelial cells, or to one or two plain muscle-fibres which are
disposed in the direction of its long axis. For each villus is a
“as pump. By the contraction of the muscle- fibres iba
Sra noous vessels.
Two problems have to be considered : First, in what form
and by what mechanism are the several kinds of food ab-
sorbed? Secondly, what becomes of them after they have
been absorbed ?
Clearly, the epithelial cell is the absorbing mechanism. It
is not a membrane governed by the laws which regulate diffu-
sion of fluids through membranes, but a living cell. There
is hardly any limit to. its power of selecting { the food which it
a 9—2
Cael ee mc eee 8 eigen AP ve SO eee ~ le ee eee) en ory eS
m~ Pe tn ge ee Oe Se VR re oS ae
' ~ ;
; ee hee OTD bere its “ws
= ie ’ poste, ne yh ay Oat ae me i
yt ee a Saeki
132 THE BODY AT WORK
ingests. It could, and very possibly it does, ingest albumin and bs |
fats as such. Still, the elaborate provision which is made for __
converting albumin into diffusible peptone, and cane-sugar and
maltose into easily diffusible dextrose, suggests that substances
which will pass through membranes are more readily absorbed
than substances which will not. We are justified in looking
upon absorption as a physical problem up to a certain point.
But we must not dwell too much on the physical aspects of
the problem. If the absorption of food were merely a process
of diffusion, an enormous quantity of water would be required. _
to carry the diffusible products of digestion into the villi. The
passage of the foods is aided by the selective activity of the
epithelial cells. Peptonization greatly facilitates the work of —
the epithelial cells, but it is not a condition éssential to ab-
sorption, so far as soluble proteins are concerned. It i is, how-_
ever, essential that the proteins should be presented to the epi-
thelial cells in a soluble form. They could do nothing with the
solid fibres of meat, however much they might have been dis-
integrated by mastication and by the action of hydrochloric
acid. It is only after digestion by pepsin and by trypsin that
all the proteins of food are brought into solution. Digestion
is needed to reduce them to a condition in which the epithelial
cells can take them up.
Much thought has been devoted to the question of the form
in which fat is absorbed. Fat in the chemical sense—a pure
fat, that is to say—is a compound of a fatty acid and glycerin.
Suet, lard, butter, vegetable oils, etc., are mixtures of several
fats. All consist of glycerin united with fatty acids. The
acids are stearic acid, palmitic acid, oleic acid, and others
of less importance. Fats are insoluble in water; so also are
the fatty acids. A fatty acid combined with an alkali (in
place of glycerin) is a soap. Soaps are soluble in water. If
milk is examined under the microscope, it is found to contain
droplets of fat, varying in size, but all minute. The larger
droplets tend to rise to the surface as cream, but the smaller
droplets do not run together. If milk from which the cream
has been skimmed is sterilized, it retains its normal appearance
for an indefinite time. Its fat remains in droplets. In
technical language, milk is an emulsion. Theoretically oil
and water would make an emulsion, if the droplets of oil were
DIGESTION 133
rendered sufficiently minute. Sucha condition has been
almost obtained by agitating oil and water with powdered
glass. But the more viscous the medium through which oil
globules are distributed, the greater is the resistance to their
fusion. If oil which has become rancid—in which a certain
quantity of fatty acid has been liberated from the glycerin
with which, in a neutral fat, it is combined—is shaken with
water containing carbonate of soda, an emulsion is easily
formed. The carbonate of soda and the fatty acids form
soaps. about twenty minutes after a meal. After
] bef ore fresh food. antl it. cp may be that the last meal
| was too large or the interval too short. If the mucous membrane
\ ‘ol is in an unhealthy condition, its own secretions afford material
4 on which bacteria thrive. Nothing short of washing it out
with a stomach-pump will clean it up. The presence, at the
time of feeding, of food left over from the previous meal is
_ likely to perpetuate the unsatisfactory state of affairs. All
_ the glands of the alimentary tract exhibit a tendency to
periodicity. Their efficiency is greatest when activity follows.
_ aperiod of rest. If the stomach is not able to expel its contents,
it has not the opportunity of preparing for fresh duties. Fat_
undergoes a certain amount of rancid fermentation_ in the |
stomach: Proteins are not attacked by bacteria in the stomach
unless the condition of the organ is very unsatisfactory. The
odour of the products of their decomposition is then recog-
nizable in the breath.
Bacteric fermentations in the small intestine are unimpor- \
tant under normal coriditions, with the exception of the |
fermentation of cellulose. Cellulose has the same empirical |
formula as starch. It is completely insoluble, and is not
affected by any of the digestive juices. The greater part of the
cellulose consumed by herbivora is, however, broken up by
bacteria into acetic and butyric acids, carbonic acid, and
marsh-gas. In Man also a small quantity is similarly. de-
stroyed.
In the large intestine the bacteric fermentations are not
unlike those which occur in the stomach, with, in addition,
the destruction of proteins, or of products of proteid digestion.
The greater the quantity of undigested food which reaches
the large intestine, the greater is the development of bacteria.
When the stomach is dilated, the ascending colon, and-espe-
cially | its cecum, is usually dilated also. Bacteric fermenta-
tion in the large intestine, with resulting flatulence, is evidence
of imperfect digestion, due either to an excess of food or to
weakness of the alimentary organs, or, as is more commonly
PAR: oS! ye ae =
“ —" ,
138
_ the case, to the combination of these two factors. The relation 3 %
of fermentation to alimentation can be shown by counting
the microbes in a specimen of the contents of the large intes-
tine. In a particular case it fell from 65,000 per milligramme
upon a mixed diet to 2,000 per milligramme upon a diet of milk.
In the world at large bacteria perform many offices of the
utmost usefulness to other living things. They fix nitrogen in —
the soil, sweeten polluted rivers, reduce animal and. vegetable
matter to a condition in which it is available as plant-food. _
Their presence within the alimentary canal is inevitable ; but
it is somewhat doubtful whether, with the exception of the
fermentation of cellulose, they do the economy any service
with which it could not dispense. As parasites of the alimen-
tary canal, some kinds are less desirable than others. Recently
a method of limiting their variety has been introduced and
advocated with much enthusiasm, as favourable to the hygiene
of the digestive tract. In countries in which the cows are
driven, in summer, to mountain pastures, the peasants of the
plains live during their absence largely upon milk brought down
at intervals, and allowed to turn sour. Sour milk, in Bulgaria,
develops a bacterium of extraordinary vigour. It can live in
a medium containing as much as 10 per cent. of lactic acid, a
concentration fatal to other forms of Bacterium acidi lactict.
It is easily cultivated, and when ingested continues to multiply
in the alimentary canal. So peculiarly lusty is this bacterium
that it makes life impossible for other germs. As it dies out
after two or three months, it seems unlikely that a man who
swallows the Bulgarian milk-germ runs a risk of inviting a
repetition of the tragedy which followed the acclimatization
of the mongoose in Jamaica. Its supremacy has been attri-
buted to its capacity of developing a concentration of lactic
acid too high for the well-being of other bacteria ; but it is
improbable that it has the opportunity of doing this in the ali-
mentary canal of a person living on a mixed diet. The extinc-
tion of other bacteria (if they are extinguished) is more likely
to be due to an antagonism of a more subtle kind, at present
inexplicable, but not without parallel. The purifying influence
of the water of the Ganges has for ages been an article of faith.
Pilgrims from fever-stricken districts bathe in it, foul it, drink
it, with the corpses of their fellows floating down the stream.
~ ee Sie Ao oe a ee Fe IN ee Ve OO) . «,
4 cated pl a oe oe eS
> ee Sy * 5 ?- ’ . =
e fer ir 4 ¥?- > =
ao)
DIGESTION 139°
z Recently it has been shown that this belief is not without
foundation. The water of the Ganges at Benares contains
bacteria which are as tigers among lesser vermin. The germs
of cholera and typhoid fever disappear from cultures into which
these overbearing microbes are introduced. |
Conditions Requisite for Normal Digestion.—When M. Chev-
reul, Professor of Chemistry at the Jardins des Plantes of
Paris, attained his hundredth year, an interviewer very natur-
ally inquired of him, “ Have you always had a good diges-
tion 2?” To this the still vigorous Professor answered: “I
really cannot say, for I have never noticed.” So long as
it is well used, the stomach is an unobtrusive organ. It is
tyrannical when it deems itself the victim of inconsiderate
treatment. A study of its physiology serves to show that it
will work contentedly only upon certain clearly defined terms, of
which the following are perhaps the most important: The )
stomach exacts due warning that its services are wanted.
The nerves of smell and taste must announce the approach of
food and guarantee its quality. ‘“‘ What may I eat ?”’ asked
a large-framed, strenuous, eager, over-worked barrister of a
great physician. ‘‘ Eat, sir? You may eat whatever you like.
But be quite sure that you do like it.”’ Wise advice. The
human race would not have developed its strong preferences
for certain kinds of food if all foods were equally suitable to
satisfy its needs. Taste is not a matter of fashion. It is
the expression of the experience of mankind. Fanciful as
civilization has made us, and easily as appetite is perverted, if
we are sure that we really like, and want, a food, we may trust
that our liking will guide us as safely as it guides a buffalo
oradeer. ‘“‘ Eat what you like.” Hating with liking carries
with it the idea of obtaining the maximum of satisfaction from
the exercise of this necessary function. Most things which are
reckoned unwholesome are full in flavour or rich in consistency.
They satisfy the palate when spread out very thin. It is poor
economy to help oneself to caviare with a table-spoon. In
the second place, the stomach must be assured that the teeth
are doing their proper share of work. Among the many half-
truths which every year are exalted to the level of a revela-
tion or a rule of conduct is the doctrine of the “ chewers ”—
persons who take no meals, but industriously and almost
“at a sitting.” In from two to three hours the last of aha
food should have passed through the pylorus, allowing the
stomach to rest before it is called into activity again. As
proteins are practically the only foods which are digested in
the stomach, the work required of this organ depends upon
the quantity of proteins present amongst the constituents
of a meal. Meat is the food richest in proteins, although
bread, vegetables, milk, cheese also yield them. Some people
can digest three meat meals every day ; but others, probably
the majority, find that it is unwise to take any considerable
quantity of meat more than once in twenty-four hours. It is
only when the cells of the gastric glands have accumulated a
store of pepsinogen-granules that proteid digestion is vigorously
carried on. Fourthly, the food must be in a form in which
it does not irritate the stomach, provoking an outflow of acid
out of proportion to the pepsin which accompanies it. Ex-
perience alone can teach the foods which are to be avoided on
this account. But speaking generally, it may be said that the
stomach resents the presence of substances which cannot be
amalgamated into chyme. Its task is the reduction of the
mixture of foods which compose a meal to the consistence of
a smooth cream. Hot buttered toast or pie-crust are made
of wholesome constituents enough, but, fat being melted into
the starch, the fragments are impermeable to the gastric juice.
They act mechanically as irritants of the mucous membrane.
Again, it may be said that “ pure” foods are apt to provoke
acidity. Nothing could be more wholesome than eggs or
pounded meat or custard pudding; but taken by themselves
these articles of diet over-stimulate the mucous membrane.
They need to be diluted with starch-foods, or even with
cellulose.
_ And this calls attention to the dietetic value of vegetables.
Vegetables, which consist chiefly of innutritious cellulose,
_ distribute the digestible constituents of a meal and increase
_ its bulk, greatly favouring its progress through the alimentary
_ canal. Especially in herbivora is it important that the bulk
- and looseness of the food should be well maintained. Rabbits
thrive on sugar, starch, and albumin, mixed with such an
DIGESTION 141
absolutely indigestible substance as horn-shavings. If the
inert substance be omitted, they die of intestinal inflammation,
although fed on the same mixture of pure foods. Other rules
which govern digestion might be mentioned; and it is needless to
point out that, when the mechanism is deranged, steps adapted
to the particular malady must be taken to bring it back to a |
normal condition. There is, however, one precaution upon which,
ina certain number of cases, it is impossible to lay too much
stress. The digestion of proteins is seldom carried out satis-
factorily when much sugar, and especially much cane-sugar,
has been eaten at the same meal. Excessive lactic fermenta-
tion prevents the proper peptonization of meat. The chemistry
of digestion is not sufficiently well understood to enable the
physiologist to say what is amiss ; but probably by-products
of peptic digestion are produced. To many people this is of
little consequence ; but to those who exhibit a gouty tendency
it is, unfortunately, a most serious matter. Civilized races are
particularly subject to the uric acid diathesis. In the course of
nitrogenous metabolism uric acid is formed in place of fully
oxidized and easily soluble neutral urea. Although the
chemical sequence has not been discovered as yet, there is no
question but that imperfect gastric digestion means the forma-_
tion of uric acid, with all its lugubrious results : malaise, neck-
ache, emotional depression. Birds and reptiles form uric
acid as the end-product of nitrogenous metabolism, not urea.
So also do city-fathers, butchers, and others whose diet consists
too largely of meat. Many nervous, ill-nourished men and
women tend to do the same, however abstemious their meals.
It is useless to tell such persons to reduce the amount of
proteins in their diet. Their attempts at increasing the starch,
sugar, and fat at the expense of nitrogenous foods lead to
dyspepsia, which makes matters worse. They often find,
however, that if they are careful to restrict to the narrowest
limits the amount of carbohydrates (especially sugar) which
they take in conjunction with meat, fish, eggs, or other proteid
foods, the formation of uric acid ceases. Sugar, bread, fruit,
and other carbohydrates, may be taken in abundance, and
with great advantage, at breakfast and lunch, without proteid
food, if dinner consists of broth, fish, meat, cheese, vegetables,
with a minimum of bread.
The History of the Foods after ‘Absorptios. Alt for Pe
with the exception of inorganic salts and salts of various —
vegetable acids, fall into three classes: (1) Proteins—sub- —
stances of complex chemical constitution, containing nitro-
gen; (2) carbohydrates—so called because hydrogen and
oxygen, in the proportions in which they enter into the forma-
tion of water, are united with carbon ; (3) fats. Proteins of
various kinds are consumed as food. The peptones produced
from them by digestion also vary. Yet very little is known as
to the differences in physiological value which distinguish the
various kinds of protein when absorbed into the fluids of the
body (cf. p. 134). All carbohydrates after digestion and absorp-
tion appear as dextrose. The various fats preserve their in-
dividuality until they are taken up by the tissues. When fixed
in the tissues, they assume, except under somewhat abnormal
conditions, the composition characteristic of the fat of the
animal which has eatenthem. Ifa dog which has been severely
starved is fed upon mutton-fat, it puts on in the first instance
fat which resembles that of a sheep rather than the normal fat
of a dog. As soon, however, as it is well nourished (which
would never occur unless some protein and carbohydrate were
added to the mutton-fat), its fat assumes the usual form.
For practical purposes we are obliged to speak of the three
classes of food—proteid, carbohydrate, and fatty—as if there
were but one member in each class. And we have abundant
evidence that such a simple classification is fully justified. The
body has so large a power of altering chemically the nature of
the food which it absorbs that it makes little difference in the
further history of the food whether the protein supplied to it
be an albumin or a globulin; the fat, stearin, palmitin, or
olein ; the carbohydrate, starch or sugar.
In earlier days it was customary to regard the body as the
receiver of a. variety of foods which it could break down into
simpler substances by oxidation, but could not reconstruct.
Plants were regarded as the manufacturers of organic com-
pounds, animals as the destroyers of the complex substances
made by plants. The union of molecules, synthesis, was
looked upon as the function of the vegetable kingdom.
Animals built into their tissues the products elaborated by
plants ; some of these products they shook to pieces for the
a
a
“a
re 0 ieee pire energy sph others sslowly disintegrated —
S result. of tissue “‘ wear and tear.” Gradually it was
realized that many chemical changes occur ii ate bods oh
cannot be viewed as merely ‘exhibitions — of its analytical
- capacity. The tissues were recognized, as laboratories ii
_ which reactions occur which consist in something more than the
_ splitting of complex into simpler molecules. The instances
earliest understood were connected with the history of carbo-
_ hydrates and fats. In the disease diabetes an enormous
_ quantity of sugar is excreted, amounting in extreme cases to
between 1 and 2 pounds per diem. When carbohydrates are
present in the food, the amount of sugar excreted in diabetes
is greater than it is when they are withheld; on an almost
exclusively proteid diet the amount of sugar excreted far
exceeds the amount of carbohydrates in the food. Another
illustration of the power of making sugar possessed by the
animal economy is afforded by a dog fed upon lean meat, and
nothing else. Sugar is found in its blood, and a store of carbo-
hydrate (glycogen) in its liver. The formation of fat is an
instance of constructive metabolism. There is abundant
evidence that the quantity of fat produced may greatly exceed
the quantity contained in the food. Animals are fattened
for the market on a diet which contains less fat than that
which accumulates in their bodies. When nursing her young,
an animal may secrete in her milk much more fat than she
obtains as such in food. It was a great mistake to suppose
that the body is dependent upon its tradesmen for fat and
_ Sugar. It can make either of these substances out of a mixed
diet in which it is relatively deficient. It must, however, be
a mixed diet. An animal cannot live exclusively on fat or
exclusively on carbohydrate. It is impossible, therefore, for
us to determine whether, if given the one alone, it can turn it
into the other. Chemists were very unwilling to credit the body
with the power of performing even the simpler of these trans-
formations—the conversion of carbohydrate into fat. Proteins
are essential constituents of a fattening diet. Their immensely
complex molecule has always afforded a tempting field for
arithmetical ingenuity. It is easy to remove from it the
atoms needed for the composition of fat, and yet to leave such
groups of atoms as might reasonably be supposed to con-
va ss ee
stitute its ‘‘ nitrogenous moicty The Wide that thee :
metabolic capacity of the body is limited to analytical pro- =
cesses justified the supposition that, when more fat is laid on ;
than the food contains, the balance comes from proteid sub- _
stances, which split into nitrogenous and fatty moieties. It
has been shown, however, that an animal during fattening
may put on more fat than is contained as such in the food, or
obtainable from its diet, even though all the atoms of carbon
and hydrogen in its proteid food were devoted to its forma-
tion. The balance must come from carbohydrates. Perhaps
a still more striking illustration of constructive capacity is
the power of making glycerin. If a dog receive fatty acids in
its diet, it accumulates normal fats. The glycerin which, united
with fatty acids, constitutes the fat, was not contained in its
food. Starch and sugar are sources of fat. As yet there is no
evidence that fat can be converted into sugar.
The chemistry of ‘he nitrogen-containing compounds appears
to present more difficult problems. Plants build up proteins.
Is the animal’s relation to these substances limited to their
disintegration ? Do proteins inevitably descend from step to
step until they reach urea? There are reasons for think-
ing that, even when dealing with nitrogenous substances,
the metabolic power of the body is not exclusively ana-
lytical. The liver can make urea from ammonia-salts, such
as lactate, or even carbonate, of ammonia—substances more
stable, and therefore in the chemical sense simpler, than urea.
This is an indication, though a faint one, that the body has a
constructive capacity, a power of producing more complex
from simpler substances, even in the case of nitrogenous
compounds. Beef-tea, mutton broth, meat-extracts have long
been regarded as foods of value when the power of assimilation
is low. Chemists point out that the nitrogenous substances
which these decoctions contain are so near the bottom of the
ladder that the energy set free by their further oxidation to urea
is scarcely worth consideration. 'They admit that their ready
availability renders them useful as restoratives, but they deny
them the status of foods, on the assumption that their further
progress must be downward. As was stated when the conversion
of peptones into leucin and tyrosin was described, evidence is be-
ginning to accumulate which shows that within certain limits, at
De ce |
ipossible to define, the system can reconstruct its
‘from. amides and other simple products of their
- + ~ ~
4 a Tah x
5 i we se
=
*
LeoTradation.
_ The animal economy receives, and after due digestive pre-
_ paration absorbs, three classes of food—nitrogenous, fatty, and
_ carbohydrate. If either of the two latter kinds be deficient
_ in the diet, the body can to a certain extent produce it from
the other two. What is the special value of each kind of food ?
- What use is made of it? Before attempting to answer these
questions, we must endeavour to trace the further history of
the foods after they have traversed the wall of the alimentary
canal.
After leaving the stomach and intestines, the foods follow
two different routes. Proteins and carbohydrates are carried
by the portal vein to the liver. Fats are carried by the
thoracic duct to the general circulation. An excess of fat
is found in the blood in all parts of the body after a meal rich
in fat. The eventual destination and fate of fatty foods is
unknown. Under certain circumstances they are added to the
fatty deposits in connective tissue ; but if no additional fat is
being laid down, they go to other tissues, in which they are oxi-
dized into carbonic acid and water. When the amount absorbed
is excessive, a certain quantity of fat may be stored in the liver.
In the cells of this organ it is housed for a time, in order that
it may be distributed to the tissues after they have used
up the supplies which first reach them through the general
blood-stream.
Proteins are completely lost to sight after they are ab-
sorbed into the blood. They take part, of course, in the
formation of growing tissue, blood-corpuscles, skin, hair, nails.
It is also common to speak of them as making good the wear
and tear- of active tissues, although it is very doubtful whether
we can legitimately speak of the wear and tear of tissues.
The protoplasm which does the work of the body is not worn
out in the same way as the materials of which a machine is
made. There is no friction to rub it down. Proteins, like
other foods, are used up as sources of muscular energy and
heat. Eventually they are reduced to urea, carbonic acid,
and water. Chemists naturally seek for substances inter-
Mediate in constitution between proteins and urea. They
10
ee
oe » _ e
we THE BODY AT WORK
oO ee eee ae ee ee bib fv) Kee, isp Sidi
assume that the degradation of proteins will occur in regule rs
steps; complex, partially oxidized, nitrogenous compounds 5
being formed first—in the muscles, for example—to be further
oxidized in the glands. The existence in all organs of nitro- —
genous “ extractives,’ which can be separated out when the
organ is subjected to chemical analysis, seems to justify the
search for stages ; but hitherto this search has been singularly
unsuccessful. Urea is the final product. It is not found in
muscle, nor, indeed, in any tissue other than the liver, which,
as already said, has the power of making it, even from salts of
ammonia. It is therefore clear that if proteins are destroyed
in muscle and other tissues, and if all urea is made by the
liver, the antecedents of urea must be carried from the muscles
to this organ. The substance which is most characteristic of —
muscular metabolism is lactic acid. It is not impossible that
all the nitrogenous portion of the complex proteid molecule is
reduced to ammonia (NH), which may be regarded as the
simplest of all nitrogenous compounds, and that this, com-
bined with lactic acid (C,;H,O;) as lactate of ammonia
(NH,C,H,O,), is carried by the general circulation to the liver,
where it is converted into urea. A considerable amount of
lactate of ammonia may be injected into a vein without any
of it overflowing through the kidneys. It is all reduced to the
condition of urea, water, and carbonic acid. If the liver is so
diseased as to be functionless, or if by operative measures
it is thrown out of action, salts of ammonia are excreted by
the kidneys instead of urea. In birds and reptiles uric acid
takes the place of urea. Their livers yield uric acid on analysis.
If lactate of ammonia be injected into their blood, it is con-
verted into uric acid, so long as the liver is intact.
We know nothing of the forms assumed by the proteins
absorbed into the blood, of the organs in which they are stored,
or of the higher terms of the series of substances through
which they pass before they are finally excreted as urea,
water, and carbonic acid. No nitrogenous compounds are
found in lymph or blood which can be pointed out with confi-
dence as the products of tissue wear and tear. When consider-
ing the sources of muscular energy, we shall have something
more to sayregarding the part that proteins playin the economy.
If there is great difficulty in following fats and proteins
- DIGESTION 147
after their absorption, it is quite otherwise when we come
to deal with sugar. Carbohydrates are the great sources of
energy. Muscular work may be generated by the oxidation
of either of the three classes of foods, but undoubtedly the
carbohydrate glycogen is its most constant source. Pro-
_ vision is therefore made for the storing of glycogen in the
liver, and the distribution to the muscles of a regular supply.
After a meal the portal blood, on its way from the intestines
tothe liver, contains a higher percentage of sugar than the
blood in the hepatic vein or in any other vessel. If sections of
liver be examined after feeding, and compared with those
obtained after a period of starvation, it is found that the
cells of the well-fed liver contain glancing masses of a sub-
stance which takes a port-wine colour with iodine. This is
glycogen, or animal starch. It has the same empirical formula
as starch (Cj;H,,0;),. In the dry state it is a greyish powder,
which, unlike starch, forms an opalescent solution in cold
water. Like starch, it is non-diffusible. In the animal king-
dom it stands to sugar in the same relation as starch to sugar
in plants. If a sheep be killed while it is feeding in the
paddock, and its liver removed and weighed, it will be found
that it is from one-third to one-half heavier than the liver of
a sheep of the same weight obtained from a butcher ; for
butchers have the stupid practice of starving animals before
they killthem. It was long ago discovered that it is unneces-
sary to feed an animal for a day or two before it is killed,
and this option has been elevated into a prohibition. A
tradition has grown up that it is undesirable to give food for
some time before killing. Not only will the liver of a sheep
killed during active digestion be found to be heavier than
that of a starved sheep, but it will also prove more succulent ;
for it is loaded with sugar (into which glycogen is rapidly con-
verted after death), as well as with proteins and fats, which
are withdrawn from it when the animal fasts. It appears
that the liver cannot secure the whole of the sugar which is
absorbed after a full meal. Some of it passes into the general
circulation, and is stored in the muscles; but the liver
always maintains a considerable reserve. Even after prolonged
deprivation of food, it holds on to a certain quantity, especially
in carnivora. Smoeen 3 is found in the liver of a dog after a
: 10—2
“148 THE BODY AT WORK
ae The muscles lose during activity q
e glycogen which -they.c =
It has already been pointed out that the ea is not onkinel 4
dependent upon external agencies for the production of the —
sugar which it needs. When the supply is inadequate, it manu-_ :
factures glycogen for itself out of the other constituents of
the diet. It can, indeed, make it at the expense of its own
proteins. If a dog which has been caused to do muscular
work, without a sufficiency of carbohydrate food, until (as
judged from a control experiment) all glycogen has disappeared
from its liver, be placed under the influence of a narcotic drug,
which arrests the activity of its muscles, glycogen reappears.
Dietetics.— Even those who are most ignorant of the science
of physiology flatter themselves that they have one piece of
information : “‘ The whole of the body is renewed once in every
seven years.” I cannot trace the origin of this sapient apo-
thegm, which for generations has passed current. If seven
weeks or seventy years were the period allowed for the renewal
of the tissues, the statement would be equally near the truth.
Judging from the rate at which they are destroyed, it is unlikely
that blood-corpuscles live for more than five or six weeks.
Hairs are shed about two years after they first appear above the
surface. On attaining this age a hair drops off and a new one
takes its place. The superficial cells of the skin are shed in
great numbers every day, and their place taken by younger
cells which come up from the deeper layers. The cells of many
glands would seem to have a comparatively short term of life.
On the other hand, some tissue-elements are far more perma-
nent. By the time a child is a year old all its nerve-cells are in _
position. They last as long as the individual lives. When |
the statement with regard to the renewal of the tissues is
understood as meaning, not that the cells are destroyed and
replaced by new ones, but that within a period of seven years
all the molecules which enter into their protoplasm are ex-
truded from the body and replaced by molecules received as
food, the assertion verges on the transcendental. It is unlikely ©
that we shall ever obtain data against which it can be checked.
The essential part of every living cell is its spongework of
protoplasm. ‘‘Bioplasm ”’ is perhaps a better term to use when
we are speaking of protoplasm as a structure, since it does not
Vegex: any rejbitios with regard to its chemical constitution.
W: ithir the meshes of the bioplasm are nutrient materials,
s yet unused, and worked-up products in various stages. It
as always been taken for granted that when treating of nutri-
ti on, we have to consider the repair of the bioplasm, as well as
the provision of raw material which it can convert into the
specific products of the cell. Suppose that the cell belongs
_ to the class of supporting tissues ; let it be a cell of cartilage,
for example. The bioplasm manufactures a collagenous sub-
stance which remains in and around its meshwork. If it be
an epidermal cell, it forms horny substance. If a secreting
cell, it accumulates secernable products. If a muscle-cell, it
_ develops a large quantity of material, which by a change in
_ form produces movement. In this last case we suppose that
the energy set free as muscular force is due to oxidation.
More stable bodies take the place of a less stable substance.
After contraction the relatively complex contractile material
is renewed from the foods stored in the muscle-cell ; or if it
be not, in the ordinary sense of the word, destroyed, if it has
merely parted with certain oxidizable constituents, it obtains a
fresh supply of such constituents from the foods which the
muscle-cell contains. Even in the case of cartilage or epidermis,
we imagine that, since the matrix is “ alive,” it is always under-
going molecular change, and consequently always requiring
food. The fact that every tissue, however inert, dies when,
owing to the blocking of the bloodvessels which irrigate the part,
its supply of nutriment is cut off, justifies this belief that all
living tissue is undergoing change.
When we make up a balance-sheet of the body as a whole,
_ placing to the debit side the food which it receives, and to its
credit side the work done in external movement and in
the production of heat, we again find reason for believing
that every part of every cell is constantly undergoing
change.
_ The balance-sheet of the body can be drawn out in either
of two ways. We can estimate the quantities of nitrogen,
carbon, hydrogen, and oxygen supplied to it in the several
foods, and compare them with the amounts of each of these
four elements given off in urea, carbonic acid, and water,
making, of course, a note of the body’s balance in hand at
150 THE BODY AT WORK —
the beginning and at the end of the period of observation.
Or, we may estimate the amount of potential energy
contained in the food, and ascertain the use to which this
energy is put in doing external work, in maintaining the
temperature of the body, and in warming the breath and other
excreta.
If we are making up the balance-sheet of a fully-grown man,
we may take for granted that he is not making fresh tissue.
During the period throughout which he is under observation,
care is taken to avoid altering the conditions of his life in such
a manner as to lead him to develop additional muscle. If
he gains in weight while under observation, he is putting on
fat. If he loses in weight, he is sacrificing fat.
The whole of the nitrogen taken in leaves the body in urea,
unless, as we have said, growth of tissue is taking place. The
body has not the same temptation to store nitrogen as it
has to store carbon. Consequently, it is very sensitive to any
deficiency of nitrogen in the diet. If food does not contain as
much protein as is needed, the deficit is made up at the expense
of the tissues. It does not necessarily follow that under these
circumstances a man loses in weight. He may be putting on
fat, although losing in strength owing to waste of muscle.
For observations upon the income and expenditure of the body
to be of any value, a condition of “ nitrogenous equilibrium ”’
must be established. The nitrogen taken in must equal in
amount the nitrogen given out.
Very exact determinations of income and expenditure may
be made by placing an animal, or even a man, in a box through
which airis drawn. A record is made of the volume of air
drawn through the box. The percentages of water vapour and
carbonic acid which the air contains are estimated before it
enters and after it leaves. The solid food consumed and the
urea excreted are also measured.
If it is desired to measure the amount of heat given off, an
animal may be placed in a calorimeter.
Even when most passive, the subject under examination,
whether an animal or a man, is expending energy in keeping
the body warm, in movements of respiration, and in shifting
position. If it is desired to ascertain the relation of oxidation
to external work, it is easy to devise a form of resistance, such
; DIETETICS 151
as the turning of a wheel, or the lifting of a weight which can
be measured. |
| In testing diets, it suffices to make sure that nitrogenous
equilibrium is maintained, and then to estimate the gain or loss
in weight and the output of energy in external work.
The Relative Value of Foods.—Dried proteins contain
about 15 per cent. nitrogen, 54 per cent. carbon, 7 per cent.
hydrogen, 22 per cent. oxygen, a little sulphur, and frequently
some phosphorus. A large proportion of their carbon and
hydrogen is available for combustion. Fats contain 75 per
cent. of carbon, and a considerable quantity of hydrogen avail-
able for combustion ; carbohydrates, 40 per cent. of carbon,
with hydrogen and oxygen in the proportions in which they
occur in water. If 1 gramme of protein is oxidized to the con-
dition of urea, carbonic acid, and water, sufficient heat is liber-
ated to raise the temperature of 4,100 grammes of water
1 degree centigrade. Its calorific value is therefore ex-
pressed as 4,100 calories, the unit of measurement—a calorie—
being the amount of heat needed to raise 1 gramme of water
1°. The calorific value of 1 gramme of fat is 9,300 calories ;
of 1 gramme of starch, 4,100 calories. Thus, the energy poten-
tial in protein and in starch is the same; that in fat more
than twice as great as that in either of the other foods.
A Normal Diet.—Nitrogenous equilibrium and body-weight
can be maintained and work done on diets which vary widely
in percentage composition. This is a question which we shall
consider at greater length later on. In the meantime, for the
sake of illustration, it is necessary to formulate a diet which is
fairly representative of the selection of foods made by a man
of average weight—say 70 kilogrammes (145 pounds)—who
desires to do a moderate day’s work in comfort. It has been
found to amount to about 100 grammes of protein, 100 grammes
of fat, 240 grammes of carbohydrate, all measured dry and as
pure foods. If the several elements of such a diet be multiplied
by the figures which represent their calorific value, it will be
found that the man is supplied with 2,324,000 calories. The
illustration that we have chosen is the diet of a professional
man who is not engaged in hard physical work. The pure
foods would be found to the amounts stated in 17 ounces
lean meat, 4 ounces butter, and 17 ounces bread. The day’s
152 THE BODY AT WORK
diet would, of course, be much more varied than this, but itis
simpler to express it in these terms. | et
Such a diet would hardly answer the requirements of a man
doing hard muscular work. Experience shows that he would
expect to receive a more liberal supply of energy, and that to
obtain it he would increase slightly his allowance of proteins, and
very considerably increase the quantity of carbohydrates that
he consumed. The diet of European workmen is remarkably
constant in the relative amounts of its several constituents, no
matter what their nationality or the exact form of their work
may be: Proteins, about 135 grammes ; fats, 80 grammes ;
carbohydrates, 500 to 700 grammes—giving a supply of energy ©
equal to 3,500 to 4,000 kilo-calories.
Speaking generally, carbohydrates are the source of muscular
force, and fats of heat. In warm climates men work on carbo-
hydrates. The ’rickshaw men of Japan are said to eat only
rice on working days, and to reserve fish for days of leisure.
The Japanese, as is well known, consume extremely little fat.
The Esquimaux and other inhabitants of high latitudes eat
immense quantities of fat. Proteins constitute the luxurious
element of a diet. Not only are they more attractive to most
palates, and therefore preferred by persons whose dietary is
not severely regulated by price, but the body prefers them.
It works with greater alacrity when supplied with more protein
than, in a strictly physiological sense, it needs.
The supply of food must exceed the apparent demand. The
most efficient of motors cannot convert more than 15 per cent. of
the energy potential in its fuel into work. If aman endeavours
to obtain a better result than this from his muscular system,
if he tries to make his machine do more than 15 units of work
for every 100 units of energy with which he supplies it, he does
it at the expense of his own tissues. First he loses in weight,
owing to the consumption of fat ; then the excess of nitrogen
discharged over nitrogen consumed shows that he is burning
up the proteins of his own tissues. It is needless to add that
the weakness which results puts a stop to excessive work.
Muscles, as we shall find when we consider the relation of their
output of work to the energy supplied to them, can produce
a much better result than the best of engines; but we are
speaking of the body as a whole, which wastes energy in the _
a
DIETETICS 153
a - movements of respiration, masticating food, shifting position,
maintaining the body-temperature, etc.
Health may be maintained and work done on diets which
depart widely from the one which we have selected as a stan-
dard. Darwin found the Guanchos of South America living
exclusively on meat. Nansen and Johannsen, when seeking
the North Pole, lived for months on meat and blubber. Millions
of the inhabitants of India abstain from meat and meat-fat,
their diet consisting of rice, buttermilk, and a little fruit. In the
case of all persons with whom the price of food is an important
consideration, carbohydrates are preferred to proteins and fats.
Oatmeal is very much cheaper per unit of energy than meat.
A man may be a meat-eater or a vegetarian, although he is
probably unwise in overlooking the obvious teaching of his
teeth and digestive organs, which are those of an omnivorous
animal. His prehistoric human ancestors lived chiefly on the
harvest of their spears and tomahawks. If we insist upon
looking back still farther, we discern a cleavage of the race
into the arboreal fruit-eaters, which still retain pre-human
characters, and the more enterprising and energetic troglodyte
hunters from whom the human race was evolved.
A man may vary his diet within wide limits. In-
numerable considerations lead certain individuals to desire to
depart from the diet which we have termed “ normal ”—+7.e.,
typical of inhabitants of the temperate zone. One man rebels
against the expense of living ; he would fain reduce the quan-
tity and the cost of food. Another, having to traverse regions
in which food is scarce, wishes to ascertain the lightest, and
therefore the most portable, combination of its essential
elements. A third—and he belongs to a much larger class—
tormented with indigestion or harassed by gout, asks, ““ Why
must I consume things which give the stomach trouble, or
produce disagreeable and incapacitating after-effects ?”? Many
circumstances prompt to experiments in diet. Much latitude
is undoubtedly allowed. But there are limits within which alone
health can be maintained and work done. It is of great interest
to ascertain exactly how wide these limits are; and especially
important is it to find out the lower limit, the minimum
of food, and the minimum of each particular kind of food,
which will enable the human machine to work. The problems
—r tcl Shay ee A
154 THE BODY AT WORK
involved are somewhat complicated. If it were possible to
live on a single food, it would be as easy to ascertain the irre-
ducible minimum as it is to find out with how much coal or
with how much petrol an engine can be made to turn a wheel.
But:to support the body several different kinds of food are in-
dispensable. It is therefore necessary to determine, not only
the minimum quantity of the combined foods, but also the
minimum amount of each kind of food, and the effect upon the
total of variations in the relative amount of each of its several
factors. The problem is complicated, but certain limits are
impassably defined. In the first place, with regard to the
total amount, the work which the body does cannot under
any circumstances be reduced below a certain level. The
food consumed must provide a supply of energy equal, at the
least, to the performance of the minimum of work. The
body must receive each day food of due caloric value. Then
with regard to the amount of each several constituent. Many
considerations lead us to wish to increase one of them or to
diminish another. Some food is cheap, and other food is dear.
Economic reasons are in favour of the cheaper food. Even
ethical considerations are not without weight. We have,
perhaps, a prejudice against sacrificing life to supply the pot.
We have doubts as to whether our system can properly digest,
metabolize, and excrete meat. We need an unambiguous
answer to the question, To what extent can nitrogen-foods be
replaced by carbon-foods, and vice versa? A cell, as already
said, consists of a framework of bioplasm bathed in cell-juice
which contains nutrient substances and manufactured pro-
ducts. The bioplasm is alive; the proteins, carbohydrates,
and fats of the cell-juice are the materials with which it is
nourished, and upon which it works. Some physiologists
incline to the view that non-living substances must enter into
the bioplasm before they undergo metabolism. They consider
that the molecules of the non-living substance must at the
time when they undergo a chemical change be physically and
chemically a part of the living substance. Others take the
opposite view: that the living substance does not undergo
change, but brings about changes in the non-living substance
which is in contact with it, enclosed within its meshes. This
is a problem which is not likely to be solved, nor is its solution
DIETETICS 155
of great importance in relation to the question which we are
discussing. Whichever of the two views be justified, we have
to distinguish between the bioplasm of the cell—the machine—
and its raw materials and manufactured products. The
question to which we want an answer is the following : Must
the bioplasm undergo change ? There seems to be no reason
in the nature of things why it should. It is not, as we have
already pointed out, subject to wear and tear. A _ perfect
machine would in the absence of friction, which rubs down its
steel and brass, continue to turn out its products so long as it
was supplied with raw materials and the energy needed to
manufacture them. We could imagine the bioplasm as in-
destructible, receiving energy from a portion of the foods, and
expending this energy in the production of chemical change
in the remainder. We could imagine that when once the
tissues had attained their full growth they would require no
more protein for their own nutrition ; they would be occupied
in producing heat and motion from the non-nitrogenous foods.
But observation shows clearly that this is not the case. The
force which energizes the bioplasm, enabling it to evoke meta-
bolism in non-living substance, is obtained at the cost of its
own destruction. The bioplasm wastes unless constantly
supplied with proteid food. .
Under ordinary circumstances the amount of urea excreted
varies directly as the quantity of nitrogen contained in the
food. Since urea contains 45 per cent. of nitrogen, and
protein 15 per cent., every gramme of urea excreted represents
3 grammes of dry protein consumed ; or, in terms of nitrogen,
every gramme of nitrogen excreted represents 6-25 grammes
of protein consumed. [If all food is withheld, the excretion of
nitrogen falls, but it never reaches zero. Many observations
have been made on fasting men. On the second day of
fasting the nitrogen excreted falls to about 13 grammes,
representing 80 grammes of protein used up. It is generally
thought that by the second day all “ floating proteins ” are
exhausted, and that therefore nitrogenous metabolism is
reduced, as it were, to a business basis. So long as the supply
of food is abundant, the body has a luxurious habit of using
proteins in preference to non-nitrogenous food. But after a
day’s starvation there is no longer any fancy metabolism,
156 THE BODY AT WORK
no consumption of proteins as fuel when cheaper fats and sugar
- would answer equally well. In the case of Succi, who fasted
for thirty days, the nitrogen excreted fell to 6-7 grammes on the
tenth day, to 4:3 grammes on the twentieth, and to 3-2 grammes
on the last day. Clearly, we have to make a distinction, when
all food is cut off, between the oxidation of the protein which,
failing all other material, is withdrawn from the tissues for the
purpose of supplying the force absolutely necessary to maintain
respiration and such other movements as are inevitable, and
to keep up the temperature of the body—force which under
other circumstances might be supplied by non-nitrogenous
food—and the oxidation to which bioplasm is inevitably
subject, so long as it is alive. The oxidation of bioplasm
under ordinary circumstances of course supplies force ; but
it does not follow that this is sufficient to maintain the
respiratory movements and the contraction of the heart.
When a herbivorous animal is starved, it not infrequently
excretes more urea at the commencement of the starvation
period than it was excreting when well fed. Its activities
did not come to a standstill when carbohydrate food was cut
off. Fora time they were maintained at the expense of its own
tissues. On the other hand, the results obtained from the
observation of the man who went without food for thirty days
show that Nature is able to economize force by reducing the
metabolism of living substance below the normal. It might
be supposed that the irreducible metabolism could be ascer-
tained by giving a nitrogen-starved animal non-nitrogenous
food, but it is found that this scarcely affects the tissue-waste.
Becoming more active, the tissues, while saved from the neces-
sity of supplying fuel for the production of heat and motion,
suffer more waste. Again, it might be expected that if to an
animal which had been starved for a few days, until its urea
had fallen to the starvation limit, exactly sufficient protein
were given to supply this amount, the tissues would be saved.
It is found, on the contrary, that nearly twice as much urea
is excreted as before. If the quantity of protein be steadily
increased, equilibrium is at last established, but not until
the amount of nitrogen in the protein given is two and a half
times as great as the amount excreted during the starvation
period. Additional food at once gives rise to additional
_ DIETETICS ~ 157
waste. The tissues which during the period of scarcity had
reduced their oxidation to a minimum become more active at
the first hint of returning plenty.
This last experiment illustrates a general law. An increase
of proteid food within certain limits increases the metabolic
activity of the tissues—provokes them to extravagance. It
is possible, by adding protein to a mixed diet which sufficed
for the maintenance of body-weight and nitrogenous equili-
brium, to bring about a nitrogen deficit and to reduce the
body-weight. Or, if the body is gaining in weight, owing to
the accumulation of fat, the substitution of protein for carbo-
hydrate (weight for weight, since their caloric values are the
same) will lead to its reduction. It is difficult to avoid the use
of fanciful language in accounting for these results. The
animal economy is like an over-careful housekeeper, who, when
meat is scarce, doles out porridge also with a thrifty hand.
When meat is plentiful she is prodigal with every article of
diet. Protein is the most costly of foods. Any indication
that it is scarce leads to a shutting-down of activity. On the
other hand, no other food is so readily absorbed (unless the
digestive organs be protein-sick) ; none is so quickly incor-
porated in the bioplasm ; none is so easy to decompose. When
fed with protein the machinery hums. The insatiable appe-
tite for beef and eggs which overtakes a man of sedentary
habits after a long morning in a boat or on a bicycle does not
indicate that his muscular tissue is suffering from wear and
tear. It does not prove that he is setting free energy by
oxidizing proteid food. It shows that he is asking certain
tissues which are accustomed to a quiet life to exhibit pro-
digious energy. They will not shake off their customary sloth
unless he stimulates them with sumptuous fare. At the end
of a week he finds that proteins are not the best fuel for steady
work. If he consumes sufficient to supply all the energy
needed by his muscles, he is hampered by a quantity of nitro-
genous residues which have to be reduced to urea and elimi-
nated by the kidneys. He goes back approximately to his
old regimen, so far as proteins are concerned, and consumes
more carbohydrates for the supply of the force which his
increased muscular activity demands.
It is possible to live on meat alone, but the quantity required
158 - THE BODY AT WORK
is very great, involving the digestive organs, the liver, and
the kidneys in an excessive amount of work. On the other
hand, it is possible to reduce the consumption of proteins to
a minimum by substituting for them fats and carbohydrates.
But, again, after the proper balance is disturbed, the substitu-
tion ceases to be a simple problem in arithmetic. The carbon-
food has to be increased out of all proportion to the protein
which it replaces. If a dog which is being fed on a diet natural
toit—chiefly meat—is in a condition of nitrogenous equilibrium,
carbohydrate may be substituted for some of the meat. But
from the very beginning it is found that, if nitrogenous equili-
brium is not to be disturbed (if the dog is not to be induced to
consume its own tissues), a weight of carbohydrate must be
given considerably greater than the weight of the protein
withdrawn. The disproportion increases as the experiment
proceeds, until perhaps 12 to 15 grammes of carbohydrate
have to be substituted for every gramme of protein. The
proteid food has now come down to 1-5 gramme per kilo-
gramme of the animal’s weight. Owing to the increase of
carbohydrate, the caloric value of the total food, nitrogenous
and non-nitrogenous, is several times as great as the animal
requires. The surplus is oxidized without any equivalent in
work. At about this point the experiment is brought to an
end, owing to the failure of the digestive organs to deal with
so large a mass of food.
The value of gelatin as an article of diet is of interest in this
connection. Gelatin is not, strictly speaking, a protein, and
it cannot be built up into the tissues. It does not prevent,
nor even delay, starvation. Yet up to a certain point it can
be used as a substitute for proteid food. In the observation
just referred to, protein might be withdrawn at any stage,
without disturbing nitrogenous equilibrium, by substituting”
about 2 grammes of gelatin for every gramme of protein
withdrawn. It spares protein, although it does not take its
place. It is said that the minimum of protein necessary for
the maintenance of nitrogenous equilibrium may be reduced
to about one-half by the substitution of gelatin. This has
been interpreted as indicating that when we have reduced the
oxidation of nitrogenous substance to its smallest amount
the nitrogen comes from two sources in about equal propor-
>=
- tions—(a) the bioplasm ; (6) the food-proteins in contact with
it. It is inferred that gelatin, although it cannot be built up
into bioplasm, may take the place of proteins present in the
cell-juice. It appears to be impossible to starve the cell
until it consists of a bioplasm framework bathed in nitrogen-
free cell-juice. As the non-living proteins of cell-juice are
removed, they are, if no nitrogenous food be given, renewed
by the breaking down of bioplasm. When gelatin is absorbed,
it takes its place in the cell-juice, and the breaking down of
bioplasm is no longer necessary. When digestion is impaired,
or vitality lowered, decoctions of meat which contain extrac-
tives of low calorific value, useless, without synthesis (cf.
p. 144), for the purposes of tissue-repair, may to a certain
extent save tissue-waste. In the same way, gelatin, which is
very rapidly digested in the stomach, may cover the consump-
tion of proteins, although it cannot take their place.
To sum up: The requisite daily income of energy must
come from both nitrogenous and non-nitrogenous food. It
is impossible to reduce the nitrogenous factor below a certain
minimum. From this minimum upwards, until a certain level
is reached, every additional unit of nitrogenous food enables
the system to dispense with more than its equivalent of non-
nitrogenous food. When the proper balance of foods is
attained, there is no waste either of labour involved in
digestion, or of labour involved in metabolism and excretion.
The Liver.—The liver weighs from 3 to 34 pounds. It lies
beneath the diaphragm, more on the right side than on the left.
Its posterior border, which rests against the last three ribs
(separated from them by the diaphragm), is about 3 inches
thick. Its anterior border is thin, and keeps close along the
line of the ribs. If the organ is neither unduly enlarged nor
squeezed out of its place owing to the use of a tight corset,
it does not project below the ribs, save where it crosses the
space between the rib-cartilages below the end of the breast-
bone.
The liver is supplied with blood by the hepatic artery.
This vessel is small for so large an organ. Although
responsible for the nutrition of the liver, it does not bring it
the materials which are stored in its cells. A much larger
supply of blood is derived from the portal vein, which breaks
y oa
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into capillaries, or, to speak more accurately,
pseudo-capillaries, in the liver. The blood, rhether ¢
>From Spleen
Stomach &
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Biliary
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Fig. 7.—DIAGRAM OF A LOBULE OF THE LIVER DIVIDED VERTICALLY THROUGH ITS AXIS.
In its centre is a space, the intralobular vein, through which the blood falls into a branch of the
hepatic vein, on its way to the heart. An interlobular branch of the portal vein, which
brings the blood from the digestive organs, pours it by many smaller vessels over the
surface of the lobule. It filters into the lebule through innumerable pseudo-capillary
vessels, or spaces, between the radiating columns of liver-cells. Arterial blood is brought
to the lobule by a twig of the hepatic artery. Bile is drained away from it by an affluent
of the hepatic duct. In the lower part of the diagram seven liver-cells are shown, forming
a divided column, magnified about 300 diameters. The cells are loaded with glycogen, and
contain minute globules of fat. Red blood-corpuscles and two leucocytes are seen between
the columns of liver-cells. One of the leucocytes has ingested two blood-corpuscles.
ty
ee a a ee ee
‘THE LIVER ages |
; ae away by the hepatic veins. The plan of structure of
the liver is best understood when viewed with reference to
; Pths hepatic veins. These, if traced backwards, are found
to break up into fairly straight vessels, each of which has
a large number of lateral branches. Each of the lateral
branches is in the centre of a mass of cells, which are packed
round it in radiating columns. These masses, which have a_
diameter of from 1 to 2 millimetres,-are termed ‘“‘ lobules.”’
By mutual pressure the lobules are squeezed into a pentagonal
or hexagonal form. The vein in the centre of the lobule is the
intralobular vein. Turn now to the portal vein ; this is seen
to break up into branches which run between the lobules,
and are therefore termed “ interlobular veins.”” The branches
of the hepatic artery also run between the lobules, as do the
radicles of the bile-duct. Each lobule is a liver in miniature.
The blood of the portal vein, which has come from the spleen,
in which red blood-corpuscles are destroyed, and from the
stomach and intestines, from which it has absorbed the pro-
ducts of digestion, is poured over the surface of the lobule, to
be filtered through into its central intralobular vein. In its
passage from the interlobular veins (and branches of the hepatic
artery) to the intralobular vein the blood is confined to radiating
capillary channels ; but since these merely prevent the escape of
red blood-corpuscles without imposing any restrictions upon
the exudation of blood-plasma, the portal blood is to all intents
and purposes filtered through the columns of liver-cells. The
body-substance of the liver-cells is soft, destitute of envelope,
and capable, when free on the (warmed) stage of a microscope,
of changing in form, somewhat after the manner of a leucocyte.
Such cells have a great capacity for taking up the products
of digestion. Possibly they take up and store fats and pro-
teins, but undoubtedly it is their chief business to absorb
sugar which accumulates as glycogen in their substance.
The glycogen is handed out to the hepatic blood as required.
The pigment which results from the disintegration of red
blood-corpuscles in the spleen is secreted, along with the bile-
salts, into minute channels, or canaliculi, which groove the flat
surfaces of adjacent liver-cells. These canaliculi converge to
the bile-ducts. The liver is therefore at the same time the
storehouse of sugar which it takes up from tke blood when
11
162 THE BODY AT WORK 3
it is in excess, and passes out to the blood when it is deficient, a
and an excretory organ which eliminates the refuse of hemo- __
globin. The iron derived from hemoglobin it stores, and
returns to the blood. )
Another function of the liver has been referred to already.
It is the organ, and, as far as we know, the only organ, in ©
which urea is made in mammals, and uric acid in birds. If
the liver of a freshly killed animal be excised and a stream
of blood passed through it, the blood which leaves the organ
contains urea. If a salt of ammonia, even the carbonate,
be added to the blood, it is converted by the liver into urea.
When a bird’s liver is made the subject of the same experi-
ment, uric acid appears instead of urea. The liver can convert
many nitrogenous substances into urea, but it seems probable
that, normally, the salt with which it has chiefly to deal is
lactate of ammonia (cf. p. 146).
A few words must be added with regard to the functions of
the liver during prenatal life, obscure though these functions
are. The liver develops very early, and attains a relatively
enormous size. At the third month it weighs as much as the
whole of the rest of the body (cf. p. 34). Yet it cannot, one must
suppose, have to do much of the work which falls to its share
in postnatal life. Food is reaching the embryo in a constant
stream, and not as the result of intermittent meals. The
embryo has no need to store glycogen ; nor does its liver, on
analysis, yield much of this substance. In the embryo glyco-
gen is widely distributed throughout the tissues, not specially
accumulated in the liver. No digestion is occurring in the
alimentary canal. Bile is not needed to aid the hydrolysis
and absorption of fats. A small quantity of cholesterin and
less lecithin is being eliminated, but not much bile is needed to
facilitate this process.
A process which is proceeding at a great rate in the embryo,
in various situations, is the formation of red blood-corpuscles.
In this the liver takes part. But its duty in regard to blood-
formation is not sufficiently onerous to account for its size.
The formation of blood-corpuscles in the liver is observed with
difficulty in microscopic sections. It is therefore impossible to
speak with certainty as to the extent to which it is going on,
but it may be safely asserted that this function by itself cannot
FT. Zi
y account for the great size of the organ in embryonic
What other office it fills at this period is a question
vhich still awaits an answer.
There is no more curious chapter in medical history than the
_ story of the views held at various periods with regard to the
_ functions of the liver. From being a mere mass of “ paren-
chyma ”’ serving as packing for the abdominal viscera, it was
elevated to the rank of Grand Purifier of the “‘ humours ” of
the body. Next, its excessive activity became the cause of
that form of dyspepsia known as “ biliousness.”’ Still later
its want of activity was its chief vice. A “sluggish ”’ liver
was held responsible for mental perversity and moral dulness.
Calomel, podophyllin, and other drugs were used as whips to
stir it up ; and the increased secretions of the alimentary canal
were mistaken for bile. Poor patient organ! It is the still-
room of the body, in which the day’s supplies are stored, and
from which they are served out, without haste and without
delay. And it makes urea. What else it does we have yet
to find out ; and it is not impossible that when physiologists
have quite shaken themselves free from the explanations
based upon conjecture, which their predecessors have handed
down, they may discover that it has other duties which are
not obvious, but of great importance.
11—2 .
CHAPTER VII
RESPIRATION
Lire means change. We cannot imagine its continuance with-
out liberation of energy. Arrest of molecular activity is death.
There is no possibility of its revival. A watch that has stopped
may be started by shaking. On the cessation of molecular
activity an animate being becomes inanimate. Dead, it is
liable to further chemical changes. Bacteria invade it. They
shake down its complex unstable compounds into simple, stable,
so-called “‘inorganic groups”’; but the ordered combination with
oxygen, which constitutes living,can never recommence. Putre-
faction may be prevented by the exclusion of germs. The
inanimate mass of organic material may remain unchanged.
Its return to life would be a miracle. From time to time a
frog is found enclosed in old red sandstone, or some other rock
which for countless ages has lain beneath the surface. The
cleft through which the frog entered a few hours or days before
it was discovered is overlooked. It is supposed to have lived
‘in a state of suspended animation ”’ for millions of years.
The fact that no frogs are to be found among the fossils of the
old red sandstone is an objection too casuistical to be seriously
entertained. The physiologist’s demand to know what has
become of the mountains of solid carbonic acid, water, and
urea which the frog must have produced during its unimaginable
term of incarceration is regarded as the natural expression of
his prejudice—that life cannot continue without molecular
change. And he is bound to admit his inability to prove that
it cannot. Nevertheless, his experience that, whenever and
however he may, by experimental methods, arrest change, he
loses the power of causing it to recommence justifies him in
his conviction that life is change. Even a living seed is to his
mind an organism whose complex constituents are slowly—
164
/ SAMDENTE MED EOC
PORONTO | aunty.
"RESPIRATION.
however slowly—setting free energy by settling down the steps —
__ which lead to stability and ultimate, inanimate rest ; and the
only source of this energy is combination with oxygen. In the
case of a seed the oxygen need not come from without. Seeds
retain their power of germination after long occlusion in
3 nitrogen or other neutral gases. But all the time some change
____ ig occurring, some internal oxidation which resolves their less
stable into more stable compounds. Otherwise they would not
be alive. A physiologist is willing to believe that this may
continue for ten years, fifteen years—for any period that the
botanist tells him that he has, under verifiable conditions,
observed that it does occur; but when he is told that peas
taken from the hand of an Egyptian mummy, or seeds set free
by the spades of navvies after a far longer burial, have been
found to retain their vitality, his credulity is stretched beyond
breaking-point. He cannot imagine a change so slow as to
be spread over a geological period, still without exhaustion of all
changeable compounds.
The term “respiration”? has been extended until it is
synonymous with “ oxidation.”’ At one time it was supposed
that the combination of oxygen with oxidizable substances
occurred in the lungs. The lungs were the hearth of the body,
to which the blood brought fuel which burned in the air drawn
into them. When it was understood that the actual com-
bination of combustible material with oxygen occurs, not in
the lungs, but in the tissues, a somewhat illogical distinction
was made between “ external respiration ’’—the combination
of oxygen and blood in the lungs—and “‘ internal respiration ”’
—the combination of oxygen and tissue-substances. The
terms are not comparable. The taking up of oxygen by the
hemoglobin of blood is. a different process to the union of
oxygen, after the hemoglobin has parted with it, with the
carbon, hydrogen, and nitrogen of the tissue-substances.
The blood-stream carries both fuel and oxygen to the tissues,
but the fuel while in the blood is not in an oxidizable con-
dition. The foods are taken up by the tissues. They enter
into combination with their protoplasm. Oxygen also com-
_bines with tissue-substances. In proportion as the tissues are
active oxidized compounds are split off. They fall into the
lymph, whence they are absorbed by the blood. If they are
a) CS Ct Nis STS EOS ee : ae ans
Ailigg CVUCKOY poe, gone an Genme
nitrogenous compounds, they are carried to the liver, forme od
into urea, and passed to the kidneys for elimination. If car- —
bonic acid, it is carried to the lungs for exhalation. The water
formed by combination of hydrogen and oxygen may escape __
from the lungs, the kidneys, or the skin.
Two or three pounds of mixed foods are consumed every day. .
By the blood they are carried to the tissues, whence an equiva-
lent quantity of waste—that is to say, oxidized—material is F
removed. About 14 pounds of oxygen is required to burn the
day’s fuel.
The problems of respiration are twofold. In the first place
we have to consider the physics and chemistry of the combina-
tion of hemoglobin with oxygen, and of the elimination of
carbonic acid from the blood in the lungs ; secondly we have
to explain the transference of oxygen from hemoglobin to the
tissues, and the reception in the blood of carbonic acid pro-
duced by the tissues.
The apparatus by which air is brought into relation with
the blood consists of lungs and windpipe. At its upper end,
where it joins the portion of the alimentary tract common to
deglutition and respiration, the special respiratory tube is
protected by the larynx. The nasal chambers belong to the
respiratory tract; the gullet, or pharynx, is common to the
two functions.
The mucous membrane which lines the nose and windpipe
is kept moist in order that it may catch particles of dust
drawn in with the air. At the same time the nasal chambers
serve to warm the air, and to add moisture to it if it be too
dry; for the lining epithelium of the lungs would suffer if
dry air came in contact with it. The wall-surface of the
nasal chambers is increased by the projection of folded and
chambered “turbinate bones.”” The importance of warming
the air before it is admitted to the lungs is remarkably illus-
trated in the case of certain sea-birds. The nasal chambers of
the frigate-bird, and of some other birds which resemble it, are
exceptionally complicated. Since the animal is devoid of any
sense of smell, and the air which it breathes must be nearly satu-
rated with moisture, the only function which can be assigned
to these convoluted passages is that of warming inspired air.
The larynx will be more minutely described when it is con-
eae RESPIRATION — 167
__ sidered as the organ of voice. In connection with respiration,
- it must be regarded as primarily a valve which closes the
entrance to the windpipe during swallowing. It is overhung
by a leaf-like appendage—the epiglottis—formed of exceed-
ingly elastic tissue. It was thought until lately that the
epiglottis drops over the aperture of the larynx when food is
. passing down the gullet, and springs up again as soon as the
act of deglutition is over; but recent observations have
shown that during deglutition the epiglottis is pressed against
the back of the tongue, and that the closure of the larynx is
effected by its own sphincter muscles. The mucous membrane
of the larynx is extremely sensitive to stimulation by anything
which would be prejudicial to the tissue of the lungs. When
its sensory nerve—the superior laryngeal—is stimulated, the
larynx closes. It is the agent in carrying out many reflex
actions, in which not the larynx only, but also the muscles of
the chest and diaphragm, take part. For example, it imme-
diately stops inspiration if an irritating vapour is present in
the air. It stops respiration if any foreign body, such as a
crumb of bread or a drop of water, touches the mucous
membrane. When the trunk of the nerve is stimulated by an
electric current, respiration is inhibited. Further, under suit-
able stimulation the nerve brings about respiratory move-
ments in which inspiration is gentle and expiration sudden,
violent, convulsive. Rib-muscles and diaphragm combine to
produce a cough, which ejects the noxious body. Again, its.
stimulation in a different way probably helps to produce con-
striction of the smaller bronchi which regulate the amount of
air supplied to the air-cells of the lungs; although this con-
striction may be largely due to a reflex which starts in the air-
cells. The epithelium of the air-cells has an immensely rich
supply of sensory nerves. In some persons this protective
mechanism is very prone to overact its part. A little dust or
foul gas in the air leads to such marked contraction of the
bronchi that respiration becomes very difficult. Such an
exaggerated tendency to reflex action constitutes the neurosis,
asthma. In this malady the mechanism is unduly sensitive.
Very slight stimulation leads to a maximum discharge of
impulses to the muscular tissue of the bronchi.
The trachea has a length of about 4 inches.. It extends from
the neck beneath the = eal eves hus ge jor oe :
under side of the arch of the aorta, where it divides into there
right and left bronchi. The epithelium which lines the trachea —
and .bronchi is ciliated. The cilia propel the secretion which —
accumulates on its surface upwards towards the larynx. The a
wall of the windpipe is kept open by rings of cartilage which
are incomplete behind, where the trachea and cesophagus are
in contact. Rings and plates of cartilage also support the
bronchi. The bronchi divide and subdivide until their
diameter is reduced to about 0-2 millimetre. Each bronchiole
then breaks up into a bunch of very thin-walled, elongated
infundibula, club-shaped, and with a diameter about five times
that of the bronchiole with which they are connected. They
may be three or four times as long as they are broad. The
wall of an infundibulum is pitted like a piece of honeycomb
into shallow chambers—the air-cells or alveoli.
The walls of the air-chambers, or alveoli, are formed of a
membrane upon which is spread a network of capillary blood-
vessels. The air-chambers are so closely packed together that
a common wall separates one chamber from the next adjoining.
Minute bloodvessels pierce the partitions which separate the
chambers, appearing now on one side of the wall, now on the
other. The air-chambers are lined by thin epithelial scales or
tiles. The blood in the capillary vessels is separated from the
air in the air-chambers by the wall of the capillary; by
a lymph-space, probably rather potential than actual; and
by the epithelial tiles. This covering suffices to prevent
the escape both of red corpuscles and of plasma, yet offers
very little resistance to the passage of gases from the blood
into the air, and from the air into the blood.
Leucocytes make their way between the tiles, and creep
over their internal surfaces, searching for cell débris or foreign
matter. Anything that they find they carry to the clumps of
lymphoid tissue which occur in the outer wall of the bronchi.
In a town-dweller, leucocytes are found in these lymph-
thickets, charged with particles of soot. They show droplets
of fat and other evidences of degeneration. At other spots are
to be seen little collections of soot which have been left behind
after the dissolution of the leucocytes which brought them there.
io
EMR See ee Nicer, RE
i. fost ee) a 5
Ss RESPIRATION 169
It is not possible to make anything like an accurate
estimate of the number of alveoli in the lungs ; 725,000,000 is
a figure arrived at by measuring the average cubic capacity
of an alveolus, and comparing it with the total cubic capacity
of the lungs. Each alveolus supports some forty or fifty
capillary vessels. The superficial area of vascular membrane
exposed is placed at 90 square metres, or about 100 times the
area of the skin. Figures such as these convey very little
meaning, but they help one to realize the magnitude of the
provision made for the aeration of the blood.
Pneumonia is a condition in which the lining of the air-
chambers is inflamed, usually, possibly always, owing to the
entrance of bacteria. Lymph exudes through the walls of the
alveoli. Epithelial scales flake off. Pus-cells (dead leucocytes)
accumulate in the air-chambers. Respiration is curtailed, and
dyspnoea results. After a time, if the case progresses favour-
ably, “ resolution,” as it is technically termed, begins to occur.
The exuded substances are either expectorated or absorbed,
and the lung-tissue returns to a normal condition.
Here a few words may be devoted to respiratory sounds.
Spirare means to sigh. Breathing received the name by which
it is known in physiology from the sound which accompanies
the exit of air from the nostrils. Since the introduction of
auscultation as a means of ascertaining the condition of the
lungs, other sounds, not heard until the ear or a stethoscope is
placed against the chest, have acquired great importance.
These sounds, termed “‘ murmurs,” may be divided into two
classes. (a) When the ear is placed against the windpipe, or in
the middle of the back between the shoulder-blades, a murmur
is audible, due to the movement of air through the larynx.
If the larynx, the trachea, or the bronchi contain mucus, it is
a harsh, rough, bubbling, or crackling sound. It accompanies
both inspiration and expiration. (b) A softer, more delicate
murmur is heard when the ear is placed against the front or
the side of the chest. This is the vesicular or pulmonary
murmur. It is heard during inspiration, and is due to the
passage of air out of the smallest bronchi into the more spacious
infundibula in which they end. These two kinds of murmur
must be rigidly distinguished—the laryngeal murmur, heard
in situations in which no lung-tissue interveres between the
Ae he Ne ee Le ee Te et on fe, ae ie ne eS
Seg eben goat tett et Te he ; : Pee Pall ion < Da ee aa
170 | THE BODY AT WORK
ear and the great tracheal or bronchial tubes ; and the pul- a
monary murmur, heard over all regions where the bronchi
are buried in lung. Healthy lung is as bad a conductor of
sound as a sponge or a wad of cotton-wool. The laryngeal
murmur is inaudible in regions in which lung lies beneath the
chest-wall. It would be far beyond the scope of this book to
attempt to describe the very varied alterations in the chest-_
sounds which may be produced by disease. The student would
do well to familiarize himself with the nature of the sounds
which are heard in health, and the situations in which they are
heard, in order that he may be able, in abnormal conditions,
to recognize that something is wrong.
The chief departures from the normal may be grouped under
the following heads : (1) The pulmonary murmur may lose its
soft, smooth, sighing character owing to inflammation of the
alveoli and infundibula. It may be as loud in expiration as
in inspiration. Only a practised ear can estimate the signifi-
cance of these changes. (2) The laryngeal murmur may be
reinforced by “rales ’’—a convenient term for supplementary
sounds. The source of such rales may be a cold in the chest,
laryngitis, or bronchitis of various degrees. (3) The laryngeal
murmur may be heard in situations in which lung intervenes
between the ear and the larger bronchial tubes. This can be
due only to the lung being in an abnormal condition as a con-
ductor ef sound. Instead of being as spongy as well-made
Vienna bread, its air-spaces are filled with solid or fluid deposit.
It is as firm as dough. To such a condition it attains at the
height of pneumonia—a stage termed “ hepatization ”’ because
in section it looks like liver rather than lung.
Breathing is the enlargement and diminution of the chest,
which causes air to be drawn into and expressed from the
lungs. The windpipe being open, the air inside the lungs is,
of course, at the same pressure as the atmosphere. Expansion
of the chest results in the equal expansion of the lungs. Since
there is no air-space between the outer surface of the lungs
and the inner surface of the chest-wall, the lungs cannot
separate from the chest-wall when it expands. But the lungs
contain elastic tissue always slightly on the stretch. If the
chest be punctured, and air admitted between the chest-wall
and the lungs, the lungs collapse. The expiratory movement,
RESPIRATION 171
the contraction of the chest, is due to the elasticity of the
lungs. This tendency on the part of the lungs to contract is
_ sufficient in quiet respiration to restore the chest to its usual
size after inspiration, and thus to expel air. The lungs are
held open owing to the negative pressure in the space which
separates them from the chest-wall. This negative pressure
has a most important relation to their permeability by air.
Imagine the condition reversed. Picture a lung into which
air is forced by a muscular pump. After each stroke of the
pump the lung would collapse. Its finest tubes and their
dilated terminations could be maintained as open spaces,
between the strokes of the pump, only by giving a consider-
able thickness and firmness to their walls. Such a substantial
structure would be unfavourable to an interchange of gases
between the blood and the air. The reverse of this condition
is found in Nature. The lung is stretched from without. Its
tissue, delicate as crépe, cannot collapse even at the end of
the deepest expiration.
The ribs are united by intercostal muscles, disposed in two
sheets. The fibres of the external intercostals are directed
downwards and forwards, those of the internal intercostals
downwards and backwards. In tranquil respiration the chest
is enlarged by the external intercostal muscles, which raise the
ribs, and the diaphragmatic muscle, which renders peripheral
portions of the diaphragm flat. The role of the internal inter-
costal muscles is a subject still under discussion. For the
most part, physiologists regard them as accessory to expira-
tion, but some hold that they combine with the external inter-
costals in raising the ribs and twisting them outwards during
inspiration. The diaphragm is a partition which separates the
thoracic from the abdominal cavity. It is in the form of a
vault. The central portion of the dome is membranous, its
margins muscular. Its membranous centre is in contact with
the pericardium, which encloses the heart. The level of this
part is therefore fixed, except in forced inspiration, when it
descends slightly. It constitutes a fixed plane for the muscles
of the diaphragm, which are attached below to the vertebral
column and the ribs. When the muscles contract in inspira-
tion, the curvature of the marginal portions of the diaphragm
is diminished, and the chest-cavity consequently enlarged.
Speaae SE ‘the space here aha 3
diaphragm and the chest-wall closes up, and the lower | b
of the lung slips out of it.
There is a marked difference in the relaiive extent of one a
costal and diaphragmatic movements in men and women. In — Fa
women respiration is chiefly costal; in men it is chiefly
diaphragmatic. In men the abdomen moves forwards, as the ql 7
diaphragm descends in tranquil breathing ; in women the chest
' rises. Men who wish, for the purposes of athletics, or singing,
A B
iagay Right Ventricle -
Rib v(? x88 fo +}
Mid line of body
<
Fig. 8.—THE DIAPHRAGM AND ORGANS IN CONTAOT WITH IT—A, IN EXPIRATION ; B, AT THE
END OF A DEEP INSPIRATION. TRANSVERSE VERTICAL SECTIONS IN THE LINE OF THE
ARMPIT.
A, At the end of an ordinary expiration the lung does not extend below the upper border of
the eighth rib. From this level to the middle or lower border of the tenth rib the two
layers of the pleura covering respectively the inner wall of the chest and the upper surface
of the diaphragm are in contact. B, When the lung is distended with air it occupies the
whole of the pleural cavity.
or public speaking, to retain the power of making the most of
their chest-capacity are wise in not allowing themselves to fall
into the habit of lazy, abdominal breathing.
When additional pressure is required, when respiration is
forced, various external muscles attached to the spinal column,
the shoulder-blades, and the clavicles, as well as the muscles of
the abdomen, come into play.
The chest is lined and the lungs covered by a serous mem-
brane—the pleura. Normally there is only just sufficient
lymph in the space between the visceral layer of the pleura
eee ee ae ee ego A
Pree pre er '
= z - . ve
— - ORESPIRATION 1B
which invests the lungs and the parietal layer which lines the
chest-wall to prevent friction during respiration. When the
pleura is inflamed, one layer of the membrane rubs against the
other. In the early or “dry ” stage of pleurisy, the physician
recognizes this condition by the friction-sound which he hears
on placing his stethoscope against the chest. In a later stage
lymph (pleuritic fluid) is poured out. It accumulates in the
lower part of the chest, and is recognized by the absence of
the resonant note which, under normal conditions, is given out
by the chest when percussed.
The lungs are not compressed during expiration ; they are
not squeezed, as a pair of bellows or a sponge may be squeezed,
emptying it of its contents. At the end of tranquil expiration
the lungs still contain about 34 litres of air. At the top of
tranquil inspiration the volume of their contents does not
exceed 4 litres. It is evident, therefore, that air is not drawn
into and driven out from the air-chambers by the movements
of respiration. The tide of air does not extend far beyond the
ends of the bronchi. The gases in the air-chambers are ex-
changed with the fresh air drawn into the infundibula by
diffusion. The composition of the air which is in contact with
the bloodvessels is constant. It is about 4 per cent. poorer in
oxygen and 3 per cent. richer in carbonic acid than the outside
air.
Of the air drawn into the windpipe during an inspiration,
about one-third returns to the open with the following expira-
tion ; two-thirds remains in the lungs. If, therefore, the air
taken in at each tide equals one-seventh of the quantity
already in the lungs, and if of this one-seventh two-thirds
remains, each alveolus renews about one-tenth of its air. Its
contents are completely changed in ten respirations.
Fresh air is composed of 21 per cent. oxygen, 79 per cent.
nitrogen, and a trace (0-04 per cent.) of carbonic acid. Forced
by a syringe through lime-water, fresh air does not produce
any appreciable milkiness, whereas air breathed through a
tube into lime-water renders it turbid owing to the formation
of carbonate of lime. Carbonic acid (CO,) occupies the same
volume as its oxygen (O,) would occupy if free. The oxygen
which breathed air has lost slightly exceeds in amount the
carbonic acid which it has gained in exchange. The differ-
ence is due to the retention of some of the oxygen for the pur- * ;
pose of uniting with hydrogen to form water, and of forming
urea. The proportion between carbonic acid gained and
¢
CO, . ¢ . * 29
oxygen lost, 0,’ 8 termed the “respiratory quotient.” Its
2
value varies, of course, with diet. In a herbivorous animal,
whose food consists of carbohydrates, it departs but little
from unity; in a carnivore, which eats fat and nitrogen-
containing food, it is about 0-8.
The respiratory exchange is very much smaller in cold-
blooded animals than in animals which maintain the tempera-
ture of the body at a fixed level. In warm-blooded animals it
rises as the temperature falls, falls as it rises, the increased
oxidation warming the body, the diminished oxidation allow-
ing it to cool; whereas in cold-blooded animals it increases as
the temperature rises, owing to the greater activity induced by
warmth, and falls as the temperature falls.
The respiratory exchange is increased by muscular activity.
If the amounts of oxygen absorbed and carbonic acid given
out are measured while a man is at rest, and again while he
is doing hard physical work, it is found that during work the
respiratory exchange is twice as great as during rest. During
periods of starvation the respiratory exchange remains un-
altered, since heat has to be constantly produced if the tem-
perature of the body is to be kept from falling.
Since the purpose of respiration is to give to the blood the
opportunity of renewing its supply of oxygen, and of getting
rid of the carbonic acid with which it is charged, it might be
supposed that the respiratory exchange would be increased,
so far as the intake of oxygen is concerned, by breathing
oxygen gas instead of air; but it appears that under normal
conditions nothing is gained. When an animal is breathing air,
its blood takes up all the oxygen that it wants—all the oxygen,
that is to say, for which its tissues are asking. Offering it
pure oxygen in place of mixed oxygen and nitrogen does not
induce it totake up more. The hemoglobin is almost saturated
with oxygen when the blood leaves the lungs under ordinary
conditions. In certain diseases of the lungs, however, in which
the blood becomes unduly venous, the respiration of oxygen
may be beneficial ; but even in these cases the results are dis-
-- RESPIRATION 175
BS appointing, because the system is suffering much less from
= deficiency of oxygen than from accumulation of carbonic acid.
Substituting oxygen for air does not facilitate the escape of
carbonic acid. —
The nervous mechanism of respiration has been the subject
of much investigation and ‘of many experiments, without, it
must be confessed, the development of a quite complete or
satisfactory theory. Rospiration is a rhythmic process.
About seventeen times in a minute the intercostal and dia-
phragmatic muscles contract. Inspiration is immediately
followed by expiration, the falling movement being due,
as already explained, to the elasticity of the lungs, which
are stretched during inspiration. A slight pause intervenes
between the end of expiration and the commencement of the
next inspiratory movement. ‘Tranquil respiration is a succes-
sion of reflex inspiratory movements, the depth of which varies
according to the needs of the body—that is to say, according
to the condition of the blood. If the need for aeration of the
blood becomes urgent, the depth of inspiration is increased, and
expiration also becomes an active movement, certain muscles,
especially those of the abdomen, being called into play. In
this condition two sets of reflex actions alternate. A large
number of nerves are concerned even in tranquil respiration.
If a man in falling “ breaks his back ”’ at the junction of the
cervical and thoracic regions, costal respiration ceases. The
series of intercostal nerves which arises from the dorsal spinal
cord below the level at which it is injured are thrown out of
action. Diaphragmatic respiration still continues, because the
nerve of the diaphragm, the phrenic, arises from cervical roots.
The lungs are supplied by the vagus nerve. This nerve joins
the medulla oblongata as one of a group of three—glosso-
pharyngeal, vagus, and spinal accessory—which by a large
number of roots enter the groove between the olive and the
restiform body. ‘The vagus is the channel along which afferent
impulses from the lungs enter the medulla. Such impulses
call for respiratory movements. Cutting both vagi, however,
does not put an end to respiration. Inspiratory movements
continue, but they are much deeper and separated by much
longer pauses. Such a form of respiration is inefficient. The
blood is not properly aerated. The animal fells into a condi-
ts inc i Eee ieee ei LM
176 THE BODY AT WORK
tion of dyspnoea, which ends in death. When the central end
of the cut vagus is stimulated, the movements become more >
natural. Clearly, the respiratory reflex is not dependent
upon the vagus, since it continues after the nerve is cut,
although the impulses which pass up this nerve regulate its
rhythm. They govern the length of the inspiratory move-
ments, cut them short at the right moment, and secure their
succession at proper intervals.
The transfer of afferent impulses into efferent channels
occurs in the medulla oblongata. Long ago it was found that
if the brain above this level be removed, part by part, respira-
tion is not interfered with until the medulla oblongata is
injured. When a cut is made into the floor of the fourth
ventricle not far to one side of the middle line, the respiratory
movements on that side of the body cease. If the injury be
bilateral, even though very limited in extent, respiration stops.
This spot was therefore spoken of as the “ respiratory centre.”
Flourens, who first discovered it, believed that it was a
mere spot. He gave to it the fanciful name of neud vital.
It is the place at which the afferent nerves which call for
respiration are brought into connection with all the various
motor nerves which bring about the respiratory movements
of nostrils, larynx, chest, and diaphragm. Possibly the knife
in Flourens’ incision divides the tract of fibres which distri-
butes afferent impulses, but whether the junction be a defined
tract or no, injury to this region of the medulla throws the
nervous mechanism of respiration out of gear. At this par-
ticular spot lies the “centre ”’ for respiration—the one part
of the nervous system which must be intact if the movements
of respiration are to be carried out. There is no reason for
thinking that respiratory impulses are generated at this spot.
It is a centre in the same sense in which Crewe is a centre for
distributing the goods of Lancashire and other parts of England
to North Wales. The use of the term “ nerve-centre ”’ has been
very much abused. Centres were supposed to be collections
of cells, each group of which had some prerogative of initiation.
Reasoning from the analogy of human institutions, it was
thought necessary that the nervous system should be organized
into departments severally responsible for the administration
of the activities of certain sets of muscles : one centre controlled
177
The centres were dependent one on another; each regulated
lower centres, and was governed by those above it, in this
bureaucratic scheme. We know nothing of any function of
nerve-cells other than that of transmitting impulses. All that
we know about nerve-cells is that they place afferent and
efferent routes in communication, and interpose resistance
into nerve-circuits. Every nerve-cell of the grey matter of the
brain and spinal cord gives off processes which ramify. The
ultimate twigs into which a branch divides are in connection
with other sets of twigs derived from the end-branchings of
nerve-fibres or processes of other nerve-cells. A nerve-fibre
is but the axis-cylinder process of a nerve-cell. Impulses en-
counter resistance in passing along the neuro-fibrillee (cf. Fig. 22)
contained in the twig-connections of the ramifying processes of
nerve-cells. There is no reason for supposing that anything like
the same resistance is offered to the passage of impulses along
the fibrillee where they lie within the stout branches of the cell-
processes or within the body of the cell. It is easy to make a
pictorial representation of such a mechanism. Imagine a
model of the stem of a tree made by binding together a large
number of wires ; its branches as containing small groups of
wires ; the ultimate twigs as separate wires. Carry wires from
the roots of one tree to the branches of another. Trees so
constructed might be taken as representing nerve-cells. We
have not as yet succeeded in demonstrating the isolated neuro-
fibrille as they pass over from the end-twigs of a nerve-fibre to
the end-twigs of a nerve-cell branch, but we have abundant
reason for believing that they do so pass, and that the resistance
to the passage of a nerve-impulse is interposed in this neutral
or junctional zone. This resistance has to be overcome. It
is overcome by the summation of impulses. All nerve-
impulses are vibratory. The first vibrations fail may to get
through ; but if the vibrations continue, they exert a cumula-
tive effect. After a time they overcome the resistance ; sen-
sory impulses flow through the centre into motor channels.
In this way we endeavour to explain the rhythmic discharge
through the respiratory and other centres. It has not been
found possible to determine the source of all the afferent
impulses which reach the centre. Respiration continues after
12
- eele lee Qe Ss Pcie Se eT ee LE ro ge SE ee eet me Oe te ee 7-1 .
7 I ea +) in 7 pny ‘ ae v bs
am . UP aimee e = bd hb ee 8 na % ‘ t + Le. i
mae
< ~ . a
.
at .
178
all accessible nerves have been cut, including even the posterior Fe
roots of the cervical nerves. Probably it is a mistake to look
for definite afferent channels in the medulla and the rest of the
brain. All parts of the body need aerated blood. From all
parts, including nerve-tissue itself, arises the demand for
respiration. Possibly nerve-centres have the power, as it were,
of storing impulses, and discharging them after the stream of
fresh arrivals has ceased to flow. They may acquire a habit.
The resistance in the centre is profoundly affected by the
condition of the blood. As the blood becomes more venous,
impulses pass across the nerve connections with ever-increasing
force. Kept in the first instance to definite channels, they
spread as the centre becomes more excitable farther and
farther afield, reaching one group of muscles after another,
and pressing them into the service of respiration. When, in
dyspnoea, every muscle which can in any way help the move-
ments of the chest is doing its best, others which are useless
for this purpose receive the reflected impulses and join in,
producing general convulsions. The increased activity of the
respiratory centre which is produced by slight venosity of the
blood is shown in the rapid and deep inspirations which are
caused by violent exercise. Perhaps it is justifiable to go a
step farther, and to assert that there is something in blood
which has been rendered venous by muscular activity which
is specially exciting to the respiratory centre. If the blood
from a limb be prevented from returning to the general cir-
culation, by compressing or tying its great veins, and if the
muscles of the limb be strongly stimulated by an electric
current, their activity, so long as the passage through the
veins is blocked, has no influence upon respiration. But, on
relaxation of the pressure on the veins, respiration may
become twice as deep and twice as frequent as it was before
the muscles were stimulated, although the limb is now in a
condition of perfect rest.
What is the special action of the vagus nerve ? Its superior
larnygeal branch checks inspiration and induces expiration, as
already said. The impulses which pass up its main trunk
bring about ordered movements. They are not dependent for
their generation upon the condition of the blood in the lungs.
When the chest is filled with nitrogen, inspiration and expira-
ee RESPIRATION — a
tion alternate in the usual way, although the blood is growing
_ steadily more venous. The failure of inspiration to bring about
aeration of the blood does not lead to a prolongation of the
inspiratory effort. Inspiration is cut off and expiration estab-
lished in regular sequence. In performing “ artificial respira-
_ tion”? (cf. p. 184) for the purpose of saving life, in cases in which
respiration has ceased owing to the lungs being filled with
water, or for other reasons, the chest is enlarged by raising the
arms above the head, and diminished by pressing the elbows
against the sides. Enlargement promotes a tendency to ex-
piration, compression a tendency to a natural inspiratory effort.
Evidently there is a connection between the movements of the
chest and the stimulation of the respiratory centre. If respira-
tion is being carried on artificially, by forcing air from a bellows
into the trachea, the nostrils dilate as the chest is distended,
and contract as it is emptied, so long as the vagus nerve is
intact, just as they do in normal respiration. This shows that,
when the chest is emptied, a message is sent through to the
nucleus of origin of the nerve which supplies the dilator muscles
of the nostril. When the lungs are full, a message calls upon
the nostrils to contract. The only factor which is common to
pressing in and pulling out the ribs, and filling and exhausting
the lungs with a bellows, is the alteration in the form of the
lungs which is produced by the two methods. It is impossible
to resist the conclusion that the stretching of the tissue of the
lungs stimulates the nerve-endings of the vagus. The impulses
thus induced automatically stop inspiration, and lead to an
expiratory effort.
There are many indications that the nervous mechanism of
respiration is a double one, certain stimuli inducing expiration,
with inhibition of inspiration, others inhibiting expiration and
inducing inspiration. There are, however, many difficulties in
the way of formulating a satisfactory theory of the relation of
these antagonistic actions. We may frequently observe indi-
cations of such an antagonism between the two phases of the
respiratory mechanism. Cold water dashed on the back of the
head (when the head is being shampooed) induces a long in-
spiration with inhibition of expiration. A blow in the pit of
the stomach “ knocks all the wind out of a man.” Expiration
is prolonged until the lungs are unusually empty, and yet the
12—2
180 “THE BODY AT WORK
victim of the blow feels as if he would never again be able to —
draw breath. :
Modified Respiratory Movements.—The object of cought
is to expel foreign matter from the windpipe or larynx; of —
sneezing, to clear the nose. The former action consists of a
long deep inspiration; the closure of the glottis; a forcible
expiration. The blast of air encountering a closed glottis
acquires considerable pressure. When the resistance of the
glottis is overcome, the blast rushes through, carrying with it
mucus or bread-crumb, or whatever the substance may be
which irritated the endings of the superior laryngeal nerve.
In sneezing, the back of the tongue is thrust against the palate,
closing the aperture of the fauces. Inspiration is prolonged.
A strong expiration follows. The blast rushes through the
nasal cavities. This reflex is usually provoked by a tickling
of the endings of the fifth nerve in the nasal mucous mem-
brane. It is also caused in many persons, through the optic
nerve, by a bright light ; an apparently purposeless reflex about
which we shall have something more to say in a subsequent
chapter. Laughing and crying are modified respiratory move-
ments as useless, so far as any immediate purpose is accom-
plished, as sneezing in response to a bright light. As means
of expressing emotions they have been cultivated by the
human race. Possibly a case for crying might be made out
on physiological grounds. Under certain circumstances it
relieves a feeling of distress which, while it lasts, is detrimental
to the proper functions of the body. Laughing undoubtedly
is beneficial. The rapid movements of the chest quicken the
circulation. The shaking of the midriff favours the discharge
of digestive secretions, accelerates the movements of the alimen-
tary canal, and generally is beneficial to digestion. But
“laugh and grow fat ” is not necessarily the order of cause and
effect. An efficient digestion and a good capacity for assimila-
tion lead to a sense of bien-étre which predisposes to a merry
view of life.
Yawning is a deep inspiration with open mouth and larynx.
It commences usually at the end of a normal inspiration, a
slight pause being followed by further inspiration, deep and
prolonged. Its commencement seems to be due to impulses
generated by the relaxation of the tone of the muscle which
181
a —
Papen, pe the j nee to fall. A sehen Sibhsauties of the
- muscles which open the mouth immediately follows. Muscles
_ of the neck and head also come into play. Not improbably
the yawn ends in a general stretch. If the origin of this reflex
is obscure, its usefulness is marked. The circulation is
_ quickened, the blood is changed, nervous system and muscles
again become alert.
“‘ Apnea ”’ is the condition of arrested respiration. If aman
about to dive into the water breathe deeply and rapidly half a
dozen times, he abolishes for a while the desire to breathe.
One is naturally inclined to explain this as due to a surplus of
oxygen taken into the blood, but a moment’s reflection shows
that this cannot be the cause. In the first place, as we have
already pointed out, the blood which leaves the lungs in tran-
quil respiration is very nearly saturated with oxygen. It can
take up but little more. Again, the deep inspirations do not
change the air in the air-chambers; time is required for
the renewal by diffusion of their gaseous contents. It is
improbable that the constitution of the air in the alveoli is
sensibly altered by a few deep breaths. Probably the explana-
tion is to be found in the effect upon the nerve-centre of dis-
tention of the chest. Stretching the nerve-endings of the
vagus in the lungs inhibits inspiration. If the stimulation be
excessive, inspiration is inhibited for a considerable time.
That this is the right theory of apnea is proved by repeatedly
inflating the lungs of an anesthetized animal with a pair of
bellows. The same arrest of inspiration is induced whether
the lungs are inflated with air or with a neutral gas, such as
nitrogen, so long as the vagus nerve is intact. If this be cut,
inflation with a neutral gas no longer produces apnea.
** Dyspnea ”’ is the term applied to the complex conditions
and movements which result from deficient aeration of the
blood, or, rather, from the distribution of insufficiently aerated
blood to the centres in the medulla oblongata. The blood of
the rest of the body may be in a satisfactory condition, but if,
owing to ligature of the carotid and vertebral arteries or other
causes, the blood supplied to the brain be inadequate to its
proper nutrition, the phenomena of dyspnoea are as marked
as they are when air is prevented from entering the lungs.
Rite a hes.
ass
182 THE BODY AT WORK _
That the excitability of the nerve-centres in the brain is greatly i
increased when this organ is supplied with venous blood, and _
that their tendency to transmit impulses which call for respira-
tion is consequently exaggerated, is remarkably shown by the _ Ee:
following experiment: Two rabbits—A. and B.—are placed
under the influence of chloroform. Their carotid arteries are
cut, and a crossed circulation established by connecting the
proximal ends of A.’s arteries with the distal ends of B.’s, and
vice versa. The head of each rabbit is now supplied with
blood from the heart of the other, the rest of its body by blood
from its own heart. A.’s chest is now opened, so that its lungs
collapse and cease to take part in respiration. The animal
continues to make the movements of respiration in a tranquil
manner, whereas B. is thrown into violent dyspnea. The
animal whose brain is receiving aerated blood remains normal,
notwithstanding the fact that its lungs and the rest of its body
are poisoned with venous blood. The animal whose brain is
supplied with venous blood becomes dyspneeic, although its
lungs and body are receiving pure arterial blood.
There is a regular sequence in the phenomena of dyspnoea
leading up to the final stage termed “ asphyxia.” If the trachea
be suddenly blocked, so that no air can pass, the respiratory
movements at once become deeper and more rapid. This con-
dition is termed “‘hyperpnea.” In a comparatively few
seconds the system appears, as it were, to find out that inspira-
tion is not needed. Expiratory efforts begin to preponderate.
They increase in violence. All accessory muscles are brought
into play. The cry for air is heard even by muscles which
cannot help. Muscles of the limbs contract, although their con-
traction has no effect upon the capacity of the chest. Every
expiratory effort is accompanied by convulsions of a flexor
type. At the end of two minutes there is usually a sudden
change. Attempts at expiration cease. Slow, deep, infre-
quent inspirations take their place, accompanied by convulsions
of extensor muscles. Pupils are widely dilated, mouth open,
head thrown back. The subject is absolutely insensitive to
every kind of stimulus. The pulse shows a high arterial ten-
sion. ‘The beating of the heart is slow and strong. In about
four minutes from the time at which the windpipe was blocked
respiratory movements cease. The arterial tension falls. The
183
heart’s action grows rapidly weaker, although for two or three
minutes longer it may still continue to flicker. Recovery is
possible until it finally gives up. After death the right side of
the heart is found gorged with blood, the left side empty,
showing that the heart had been unable to force the blood
through the capillaries of the lungs.
Under all ordinary conditions the sequence of phenomena
of asphyxia is the same—a stage of exaggerated breathing
(hyperpneea), a stage marked by the co-operation of muscles
which are not called into action in tranquil breathing (dyspnea),
followed by the condition of asphyxia properly so termed.
An animal whose supply of fresh air is cut off passes through
these three stages, whether it be enclosed in a small space or
in a very large one. It must, however, be noted that in
asphyxia several factors combine in varying degrees. Carbonic
acid is in excess in the blood, oxygen deficient. The nervous
mechanism which regulates respiratory movements is thrown
out of gear. Motor and inhibitory impulses are in conflict.
It is important, if these complex phenomena are to be analysed,
that one factor only should be altered at any given time. For
example, carbonic acid may be allowed to increase in the air
while a constant oxygen-tension is maintained. Under these
circumstances the dyspneeic contractions are much less marked.
No convulsions follow. The paralysing action of carbonic acid
predominates. Anesthesia passes into complete unconscious-
ness. Death is tranquil. And this, speaking generally, is
what happens in disease of the lungs. Asphyxia comes on
slowly. The supply of oxygen is undiminished, but carbonic
~ acid accumulates in the blood, acting as a narcotic poison which
lowers the excitability of the nervous system, suspends con-
~ sciousness, and slowly brings the vital activities to a standstill.
In cases of drowning, when the lungs are filled with water,
the resistance to the passage of blood through their capillary
vessels is greater than it is when they are still filled with air.
The heart is sooner beaten in its effort to drive the blood
through them. Usually it stops in about four minutes. Yet
it is difficult to say for how long after a person has been im-
mersed in water it may be still possible to resuscitate him.
Reports vary, owing in large measure to uncertainty as to the
exact time at which the immersed person sank and his lungs
See
Xe, a
filled with water. It is a wise precept that t artificial respiration “S
should be tried in every case, without waiting a single instant __
to ascertain whether the heart still beats. The first thing to —
do is to empty the chest of water. Then place the subject on
his back. Kneel on the ground behind his head. Grasp an
arm just below the elbow, in each hand. Draw the arms up
above the patient’s head, so that the pectoral and other muscles
drag on the ribs, enlarging the chest ; then lower them, and
press them into the sides. This must be done with the natural
rhythm of respiration, and not more frequently than twenty
times in a minute. It is well if an assistant draws the tongue
forward, to give free admission to air. Presumably the
slight exchange of air brought about by mechanical expansion
and compression of the chest favours the passage of blood
through the capillaries of the lungs; but the real object of
artificial respiration is to stretch the endings of the vagus nerve,
and in this way to originate impulses which will call the
respiratory centre into action. Perhaps it may not be super-
fluous to point out that the failure of the pulse must not be
taken as an indication that the heart has ceased to beat.
Owing to the obstruction to the circulation through the lungs,
the left side of the heart is almost empty. Very little blood is
pumped into the aorta. None reaches the wrist.
Exchange of Gases in the Lungs.—In the lungs each red
corpuscle takes from the air a charge of oxygen which it carries
to the tissues. In the tissues the plasma of the blood receives
carbonic acid, which escapes from it when it reaches the lungs.
Water dissolves oxygen and carbonic acid. Towards animals
and plants which live in it, water plays the same role as the
atmosphere towards dwellers on land. The quantity of a gas
which will dissolve in water is proportional to the pressure to
which it is subjected. If water were the circulating fluid, some
oxygen would enter it in the lungs ; some carbonic acid would
be taken up in the tissues and liberated in the lungs. But it
is clear that the small quantity of fluid which the vascular
system will hold would be incapable of serving as an efficient
medium of exchange between the tissues and the lungs.
When a given quantity of venous blood is agitated with air,
five times as much oxygen is taken up as the blood could
carry if the gas were simply dissolved. Both oxygen and
ee ga Pe 5 Ms rem Ad, ane A
r Cc acid. are held by the blood in chemical combina-
a Z
ULU he
"The condition in which oxygen is carried was discovered in
1864 (cf. p. 68). From all time it had been noticed that the
blood which flows from a vein is darker and of a more purple
tint than the blood which spurts out of a cut artery. Shortly
before the date mentioned above, the spectroscope had begun
to be used to distinguish more accurately than the eye can do
the groups of rays which a coloured solution transmits. The
colour of a ray of light depends upon its wave-length. The
light of the sun, when its rays are sorted by a prism, accord-
ing to their wave-lengths, shows all colours from the long
waves of red to the short rays of violet, with certain gaps. At
intervals where rays are missing, the spectrum exhibits dark
bands—Fraunhofer’s lines. The colour of a solution is measured
by placing’a flat-sided vessel containing it in the course of a
beam of the sun’s light, on its way to a prism. When the rays
are spread. out, it is observed that certain groups have been
absorbed by the coloured fluid. The colour of the solution is
due to the rays which it transmits. It had been pointed out in
1862 that blood diluted with water absorbs parts of each end
of the spectrum, and also two groups of rays lying between the
fixed bands of Fraunhofer which spectroscopists had labelled D
and E. Stokes observed that this is true only of arterial blood.
Venous blood absorbs a broad band in this part of the spectrum
in place of the two narrow bands. He showed that, “like
indigo, it is capable of existing in two states of oxidation, dis-
tinguishable by a difference of colour and a fundamental dif-
ference in the action on the spectrum. It may be made to
pass from the more to the less oxidized condition by the action
of suitable reducing agents, and recovers its oxygen by absorp-
tion from the air.”’ The reducing agents of which Stokes made
use were alkaline solutions of ferrous sulphate or of stannous
chloride containing some citric or tartaric acid. These sub-
salts of iron and tin very rapidly absorb oxygen from the air
or from any chemical substance which parts with it readily.
With thes esolutions Stokes replaced the tissues. He abstracted
the oxygen from the oxyhemoglobin; then, shaking the
solution of reduced hemoglobin with air, he reprodnces the
action which occurs in the lungs.
ne Ae Se eee Merle a Se er
ee a ol sey eS Fy ee ae A ey = ;
- : ; - : . . od : ? “
° r i = *
ieee THE BODY AY WORK
: If the hand be held between a spectroscope and the sonal 4
of light, in such a position that the beam passes through the
thin tissue of two fingers where they are in contact, the spec- _
trum of oxyhemoglobin is obtained. If now the circulation
through the fingers be impeded by putting strong indiarubber
bands round them, the blood becomes venous, and the two
narrow bands of oxyhemoglobin give cia to the broad band
of reduced hemoglobin. q
Although very soluble, hemoglobin may be obtained in |
crystals, the form of which varies in different animals. When
obtained from human blood, the crystals are rhombic prisms ;
from the guinea-pig, tetrahedra ; from the squirrel, hexagonal
plates. Yet it is unlikely that the hemoglobin of one
animal differs chemically from that of another in any proper
sense of the term. Probably the form of the crystals depends
upon the amount of water of crystallization. The apparent
polymorphism of hemoglobin may be associated with the great
size of its molecules (cf. p. 66).
Even when in the crystalline form, hemoglobin can take up
oxygen ; but the difficulties which attend its purification and
crystallization render somewhat uncertain the amount of
oxygen which a gramme of crystallized hemoglobin can absorb.
In solutior, 1 gramme can take up 1-34 cubic centimetres.
The whole of the hemoglobin of the body would, therefore, if
it were all in the oxidized condition, hold about 4 grammes of
oxygen.
It is not with oxygen alone that hemoglobin can combine.
It can absorb the same volume of carbonic oxide or of nitric
oxide gas. Both of these gases it holds more firmly than
oxygen. Neither carbonic oxide-hemoglobin nor nitric oxide-
hemoglobin is of any use to the tissues. If the blood becomes
charged with the fumes of carbonic oxide (CO) given off by a
coke-fire, this gas proves extremely poisonous. The blood does
not lose it in its circuit through the body, nor is it exchanged
for oxygen in the lungs.
The instability of the compound of hemoglobin and oxygen
is Shown under the air-pump. The pressure of air in the open
equals 760 millimetres of mercury. When the pressure falls to
about 250 millimetres, the oxygen is rapidly given off. This is
a matter of considerable interest in its bearing upon the ques-
: Fee ion | 187
- tion of the height to which it is possible for a human being to
ascend. An animal placed in a chamber from which the air is
pumped dies when the pressure falls to 250 millimetres of
mercury. It has been ascertained that a man under the same
circumstances can bear with impunity a reduction to 300 milli-
metres. How much lower must the pressure fall before it
proves fatal 2? Of three aeronauts who ascended in the balloon
Zenith to a height of 8,600 metres (26,500 feet), two died. The
third, Tissandier, became unconscious, but recovered during
the descent. The pressure of the atmosphere at such a height
is 260 millimetres. The greatest mountain heights yet at-
tained are 23,100 feet (Aconcagua, in the Southern Andes),
reached by Fitzgerald, and 23,400 feet (Trisul, in the Garhwal
Himalayas), reached by Dr. Longstaff and his companions.
The pressure at this height was 320 millimetres. From these
facts it is clear that mountaineers have just about reached the
limit ; but since they have not as yet mounted to a height at
which the barometric pressure is less than 300 millimetres, it
is possible that slightly higher mountains are still waiting to
be conquered. At 23,000 feet the oxygen contained in arterial
blood does not exceed 10 volumes per cent. (cf. p. 190). It is
therefore about half the normal amount. Hence the breath-
lessness and sense of feebleness experienced by climbers. The
least exertion leads to the consumption of all the circulating
oxygen. But since the effects of want of oxygen are felt at
altitudes much lower than those to which reference has been
made, it is clear that the question cannot be regarded as simply
ore of physics. The nervous system suffers when an attempt
is made to do work with a deficient oxygen-supply. Violent
headache and nausea attack most persons long before a level is
reached at which the combination of hemoglobin with oxygen
ceases to be possible. The occurrence of this “‘ mountain sick-
ness’? reminds us that we must not take for granted that the
nervous system will continue to do its work right up to the -
altitude at which oxy-hemoglobin is dissociated. Still, the
figures show that, apart from these nervous symptoms, which
disappear after a time, no serious disturbance occurs even though
the atmospheric pressure be but little higher than the absolute
minimum at which hemoglobin combines with oxygen.
The capacity of the blood for rapidly absorbing oxygen in
aN THE BODY AT WORK _ |
the lungs and readily parting with it to the goatee is nate ;
and completely explained by the property which hemoglobin ©
possesses of forming an unstable compound with this gas.
It is quite otherwise with regard to the liberation of carbonic
acid. The problems presented by the solution of this gas in
blood and its elimination in the lungs are difficult to solve.
Less than one-tenth of the volume of carbonic acid which
can be extracted from blood by the air-pump is simply in solu-
tion. The remainder is in loose chemical combination, the
chief agents in holding it being the alkaline carbonates which
the plasma contains. With an excess of carbonic acid they
form acid carbonates, which give up carbonic acid and again
become normal carbonates in the lungs. About one-third of
the carbonic acid is, however, held by the blood-corpuscles—
partly in virtue of their alkaline carbonates and phosphates,
partly in combination with their globulin. The affinity of these
several vehicles for carbonic acid is sufficient to enable them to
take it from the lymph, and to hold it while the blood is in the
veins. When they reach the capillaries of the lungs, they part
with their burden of carbonic acid to the air. It is in connec-
tion with this renunciation that certain difficulties remain to be
explained. The carbonic acid is given up with greater readi-
ness than our knowledge of the chemistry of the compounds
into which it enters in the blood would lead us to expect.
Why does oxygen enter blood as it circulates through the
lungs, and carbonic acid leave it? We have referred to the
immense surface which the lungs expose to air. If a soap-
bubble be filled with a mixture of oxygen, nitrogen, and
carbonic acid, and if the oxygen be in smaller proportion, and
the carbonic acid be in greater proportion, than in the air of the
room, oxygen will enter the bubble, and carbonic acid will leave
it, by diffusion. If, instead of filling a bubble with gas, we fill a
bladder with water charged with carbonic acid, but destitute
of dissolved oxygen, a similar exchange with the gases of the
air will take place. It is merely a question of “‘ gaseous
tension.” The tension of the gases in the lungs is measured by
passing a small tube down the trachea, and along one of the
two chief bronchi until it becomes blocked in a bronchus just
large enough to admit it. Respiration is carried on under
normal conditions in the remainder of the lung ; but in the lobe
P
_ + RESPIRATION — 180
which the catheter blocks diffusion from stationary air to tidal
is no longer allowed. At the same time, since the circulation is
not interfered with, the gases in the blood of the occluded lobe
of the lung are not in markedly different proportions from those
in the air-chambers of other parts. If at the end of a sufficient
interval the air of the occluded lobe is drawn off and its gases
measured, their tensions can be compared with the tensions of
gases in specimens of arterial and of venous blood. If from
10 c.c. of fluid 1 c.c. of gas can be removed by the air-pump, the
volume of gas dissolved is 10 per cent. of the volume of the fluid
which dissolved it. Commonly this is written “ 10 volumes per
cent.” To ascertain experimentally the tension of a particular
gas in a particular fluid when dissolved to the amount of
10 volumes per cent. at the ordinary pressure of the atmosphere
and at the temperature of the body, it would be necessary to
place it in an open vessel in air containing a sufficient admixture
of the gas to prevent its escape from the fluid. Suppose that it
were found that, when the fluid containing the dissolved gas
was placed in air mixed with the same gas to the extent of
one-tenth of its volume, the fluid neither gave up gas nor
absorbed more gas, the tension of the gas would be equal
to one-tenth of an atmosphere. Since the pressure of the
atmosphere equals 760 millimetres of mercury, the tension
of the dissolved gas would be 76 millimetres. If more gas
were added to the air, more would dissolve in the fluid ; if
some of the gas were removed from the air, gas would escape
from the fluid. Gas passes from the medium in which its
tension is high to the medium in which its tension is low.
The tension of carbonic acid in tissues, particularly in muscles
and glands, is higher than in lymph ; in lymph higher than in
blood ; in blood higher than in air. Hence it passes by these
several stages from the tissues in which it is formed to the air
in the lungs. Much ingenuity has been devoted to perfecting
methods for the determination of the tension of carbonic acid
in lymph and in venous blood. Frequently results have been
obtained which seemed opposed to the doctrine that carbonic
acid progresses from one medium to another in accordance with
the law of pressures ; but such perplexing results were probably
due either to imperfections in method or to the establishment
of abnormal physiological conditions during the course of the
fant in the tissue-spaces where the exchange occurs. “The “=
experimenter in such a case was in error in supposing that the a
spetimen of lymph which he examined contained as much
carbonic acid as did the lymph in the tissue-spaces from which
the blood which he compared with it received its supply of this
as.
; We have already given the figures for the composition of the
air in the air-chambers of the lungs. The figures commonly
accepted as correct for the percentages of the several gases in
the blood are, at 0° C. and 760 millimetres of mercury
pressure :
Oxygen. fag nea Nitrogen, -
In 100 vol. of arterial blood .. =. 20 39 1-2
In 100 vol. of venous blood .. 8-12 46 1-2
This table shows the gain in oxygen and the loss in car-
bonic acid which results from the passage of blood through
the capillaries of the lungs. The aerated blood returned to the
heart by the pulmonary veins contains 8 to 12 volumes per cent.
more oxygen, and about 7 volumes per cent. less carbonic acid,
than the blood which the pulmonary artery carries to the lungs.
As to the physics of this exchange, the air in the recesses of
the lungs contains about 16-36 per cent. of oxygen, and an
amount of carbonic acid variously estimated at from 2-57 per
cent. to 3-84 per cent. Of the 760 millimetres of mercury which |
the atmosphere holds up in a barometric tube, the oxygen
760 x 16-36 oe.
in the alveoli of the lungs supports nee 124-33 milli-
metres ; the carbonic acid, at the lower figure quoted (2-57 per
cent.), 19-5 millimetres.
The tension of gases in arterial blood is ascertained by open-
ing an artery into a closed vessel which contains nitrogen mixed
with oxygen and carbonic acid at about the tensions which it is
computed that they have in the blood. If the amounts of these
- gases are exactly right, no exchange occurs between the blood
and the mixture of gases. The mean of many observations
~ made in thie way aie various physiologists i is, for oxygen in the
blood 72-2 millimetres mercury pressure, for carbonic acid
5 ai 20-5 millimetres mercury pressure. At a glance it is seen that,
since the tension of oxygen in the blood never exceeds 72 milli-
_ metres, whereas its tension in pulmonary air never falls beneath
124 millimetres, there is no difficulty in accounting for its
passage from air to blood. The position is somewhat otherwise
__ with regard to carbonic acid. Aeration continues in the lungs
until the tension of this gas in the blood returning to the heart
does not exceed 20-5 millimetres ; whereas the tension in pul-
monary air, even accepting the lowest figure obtained by ex-
perimental means, is as high as 19-5 millimetres. This leaves a
very small margin of pressure to account for the escape—and it
is undoubtedly a rapid escape—of carbonic acid from blood as
it circulates through the lungs. As was said regarding the
fixation of carbonic acid in the blood, it is somewhat doubtful
whether the problem has been completely solved.
The carbonic acid exhaled contains all the carbon of the
digestible food, with the exception of a comparatively small
quantity given off in urea. It amounts to about 900 grammes
per diem.
How are we to determine the quantity of air which an
individual requires ? We can but make the general statement
that it must be sufficient to dilute the carbonic acid exhaled to
an extent which precludes poisoning. It is impossible to fix a
limit. Breathing becomes embarrassed, and frontal headache
and other symptoms make themselves felt when 10 per cent. of
pure carbonic acid is mixed with air. Even in so large a pro-
_ portion as this, carbonic acid is not fatal to life. Yet an atmo-
sphere in which there is present a hundredth part of this amount
of carbonic acid, produced by respiration, is extremely injurious
to health under the ordinary conditions in which people live.
It may be asserted, therefore, that under ordinary conditions
0-1 per cent. is the extreme limit for wholesome living. But
again we are obliged to add that air contaminated to this extent
is not under all circumstances injurious to health. The explorers
on the recent Antarctic Expedition were obliged at times to
sleep three men in one sleeping-bag, with the aperture of the
bag tightly closed. The atmosphere must have been heavily
laden with carbonic acid. Dr. Wilson assures us that it was
OP eater Manne age
——— 7 sah ate
ee UE gd tne
a ca att rae.
impossible to keep a pipe 6 AGES inside the ie Not that any By
man so placed would desire, one would imagine, to add the |
combustion-products of tobacco to those given off from the
lungs! The survival of the explorers proves that it is im-
possible to fix a limit of safety even for the carbonic acid in air
vitiated by respiration. It is, however, a matter of common
observation that air which is moist and warm, owing to respira-
tion, and tainted with the odours of humanity, is extremely
prejudicial to those who live in it. Such an atmosphere is a
favourable medium for the conveyance of germs, whether of the
common cold or of a more virulent type. At one time it was
supposed that the volatile emanations which can be condensed,
along with water, by hanging a vessel of ice to the ceiling of a
crowded room, were actively poisonous ; but this statement has
not been confirmed by recent research. It is unnecessary to
call any such evidence in support of the thesis that human
beings thrive better in fresh air than in foul. The admirable
results achieved by the “ fresh-air cure ” show that there is no
degree of vitiation which can be pronounced innocuous. Never-
theless, public opinion demands that sanitarians should give
some figure as a guide. Commonly they fix the maximum of
carbonic acid compatible with health at 0-06 per cent., the
quantity of carbonic acid being taken as the measure of all
impurities present. An adult exhales about 0-6 cubic foot of
CO, per hour. Fresh air already contains about 0-04 per cent.
If, therefore, the percentage is not to rise higher than 0-06 per
cent., each adult must be supplied with 3,000 cubic feet of air
per hour. With good ventilation air may be changed four
times an hour, and therefore 800 cubic feet is regarded as
sufficient space for each occupant of aroom. The figure may
pass. It is a reasonable basis from which to calculate the
packing capacity of a dormitory. So long as a man has
800 cubic feet of air to himself, he may safely feel that he has
room to stretch his lungs. Dwelling on this figure may make
him feel uncomfortable when he finds himself in a railway
carriage, seated five on a side, with the windows closed. In the
theatre or in church he may doubt whether he has all the fresh
air to which his humanity entitles him. But, as a philosopher
rather than as a physiologist, he reflects that, whether on the
Antarctic icecap in a sleeping-bag or standing on a summit in
—a0,
193
% the Alps, he takes all that he can get, for fresh air is one of the
_ few good things of which one can never have enough.
Tissue Respiration.—A frog will live for seventeen hours in an
atmosphere of nitrogen. Under these circumstances it is clearly
impossible for it to take up oxygen, yet for several hours it gives
off as much carbonic acid as it would do if it were living in air.
Such an observation as this proves that oxidation does not occur
in the lungs, but deeper in the body. At one time the blood
was regarded as the seat of oxidation ; the products formed by
the splitting up of proteins in the tissues were supposed to be
passed into the blood, where they came in contact with the
oxygen carried by hemoglobin. A certain amount of oxidation
does take place in the blood, as in all other tissues, for blood is a
living tissue and needs to respire. But the oxidation which
occurs in the blood is small in amount as compared with that in
the organs which the vessels traverse. Muscle and other tissues
detached from the body and free from blood give off carbonic
acid. It is possible to wash the blood out of the vessels of
a frog and to replace it with a solution of salt. In an atmo-
sphere of oxygen such a “ saline frog’”’ lives for a day or two,
taking in the same quantity of oxygen and giving off the same
quantity of carbonic acid as a normal frog. The oxygen is
chiefly absorbed through the skin, the carbonic acid discharged
from the lung. This experiment shows that blood is not
essential for oxidation. Oxidations do not occur in the salt
solution with which blood is replaced. Taking all the evidence
together, it seems to be safe to conclude that the tissues absorb
the oxygen which the oxyhemoglobin brings into their neigh-
bourhood, and that they have some capacity of storing it. A
piece of detached muscle which gives off carbonic acid in an
atmosphere of nitrogen would appear to be holding a store of
oxygen, much as hemoglobin holds it. The proof is not quite
so definite as might be desired ; but we are probably justified in
holding the belief that the main part of the respiratory exchange
occurs in the tissues. Lymph dissolves oxygen which it obtains
from the blood. The tissues take it from lymph. Tissues set
free carbonic acid which lymph dissolves. Its tension being
higher than in blood, carbonic acid diffuses from lymph, through
the walls of the capillary vessels, into blood, from which it
passes into the air in the lungs.
13
CHAPTER VIII
EXCRETION
Many things enter into the alimentary canal. If an analysis
were made of a day’s food and drink, from the cup of tea on
waking to the cocoa or other potion which is regarded as a
necessary preliminary to settling for the night, it would be
found that a great variety of substances were included in the
food or taken as adjuvants to food. All these things, differing
widely in chemical constitution, must leave the body. Some
are not digested. They do not, properly speaking, enter into
the diet. Such are the cellulose of vegetables, especially skins,
husks, woody fibres ; elastic fibres of meat ; horny substances,
etc. The quantity varies greatly, according to the nature of
the diet. About 2 ounces (weighed dry) is the average. With
this indigestible refuse is included undigested food, if the diet
be excessive, and a variety of substances secreted by the liver,
such as cholesterin and bile-pigment, some residues of the
secretions of the alimentary canal, and products of bacteric
fermentations. All food which is digested and absorbed is
oxidized. It leaves the body by the lungs, the kidneys, or the
skin. Foods, as already stated, are classified as proteins,
carbohydrates, and fats. The chief excreta are carbonic acid,
water, and urea. Carbonic acid makes its exit from the lungs ;
water from the lungs, the kidneys, and the skin ; urea from the
kidneys. The three great groups of foods and the three great
groups of excreta overshadow in amount all the other sub-
stances which pass through the system. A balance-sheet in
which proteins, carbohydrates, and fats appear on one side,
carbonic acid, water, and urea on the other, is substantially
correct. The energy which is set free by burning in a calori-
meter the items entered on the debit side, after deducting that
yielded by burning the urea (carbonic acid and water are in-
194 *
“4 ri
"EXCRETION: 195
& - capable of - further oxidation), gives a ae s income. Other
constituents of the diet are so small in quantity as to be negli-
gible in making up the body’s accounts. The chemical changes
which they undergo add practically nothing to its capacity for
work. Yet some of them are essential to the maintenance of
health. Of such are common salt (sodic chloride), alkaline and
earthy carbonates, sulphur, phosphorus, etc. These things,
together with some products of action of the bacteria in the-~-
alimentary canal, the final stage of hemoglobin, imperfectly
oxidized nitrogenous substances, and other soluble substances
which enter with, or are formed from the food, are removed
by the kidneys. We speak of the elimination of waste products,
as excretion. Not that there is any physiological distinction
between excretion and secretion. Both terms refer to the
selection or production and the discharge of materials by cells.
If the product discharged has a useful function to perform—if
it be a digestive ferment, for example—it is said to be secreted.
If it is of no further use to the economy, we say that it is ex-
creted—got rid of. In some cases either term is equally appro-
priate. The sebum prepared by the sebaceous glands is useful
as a lubricant of the skin. It is thrown off. We may speak of
the glands as either secreting or as excreting this fatty sub-
stance.
The Kidney.—From worms upwards, all animals possess
organs for the removal of waste products in solution. This
statement might, indeed, be widened so as to include animals
even lower than worms. All animals which have a ccelomic
cavity—a space between the alimentary canal and the body-
-wall—have organs for the removal of soluble waste. The seg-
mental organs of worms are obviously the same organs as the
kidneys of mammals ; the latter are distinguished from their
prototypes by greater concentration of structure and specializa-
tion of function. The kidney is the oldest of organs, if its
antiquity be estimated as the length of time during which it has
had a form practically identical with that which it now pre-
sents. The lungs are of late appearance in the animal scale.
Alimentary canal, heart, brain, have passed through many
transformations. The kidney assumed its permanent form
very far back in the history of the animal kingdom. The
most primitive animal which has a digestive cavity, and
13—2
196 | THE BODY AT WORK
vessels in which the products of digestion circulate, needs an —
organ which provides for the overflow from the body-fluids of
all substances which are injurious or effete.
The kidney is an aggregation of long urinary tubules. The
head of each tubule is dilated into a globular capsule, into
which a tuft of bloodvessels depends. This is the sink into
which the waste-water of the blood drips. The long urinary
tubules are lined with cells well qualified by form and con-
stitution to search the blood in the capillaries which border
them, for substances which, not being easily diffusible,
have to be forcibly dragged from it and added to the water
trickling down the pipe which connects the rain-water head
with the sewer. The hydrostatic conditions of this apparatus—
the provision for greater or less flow of blood through the tufts
(glomeruli) which hang in the capsules, and for longer or shorter
exposure of the blood to the purifying activity of the epithelium
of the renal tubules—will be described after a very brief account
has been given of the structure of the organ.
The outer border of the kidney is convex, its inner border
concave. The concavity is termed the “hilus.”” The central
depression of the hilus is embraced by the expanded end of
the ureter—the tube which carries the secretion of the kidney
to the bladder. The renal artery and the renal nerves enter,
and the renal vein leaves, the kidney at the hilus.
If a kidney be split longitudinally, it will be noticed that its
outer part, the cortex, is darker in colour than its inner part,
the medulla (Fig. 9). The glomeruli already referred to occur
in the cortex. The medulla is occupied by radiating tubules,
collected into groups. Those of each group converge towards
a common duct. From twelve to eighteen ducts open into the
expanded end of the ureter, each at the apex of a pyramid. If
the section of the kidney be examined with a lens, it will be
seen that narrow rays from the medulla extend into the
cortex. The cortex is therefore made up of interdigitating
pyramids of dark substance, consisting of glomeruli and the
contorted tubules, about to be described, and of lighter sub-
stance, consisting of straight tubules continuous with those of
the medulla.
The urinary tubules are the separate pieces of apparatus
of which the kidney consists. The problems connected with
nS og ) CRETION pry * 197
subule are therefore the problems of the kidney as a
Fia. 9.—THE UPPER END OF THE LEFT KIDNEY, VERTICALLY DIVIDED, AND MAGNIFIED.
It is invested by a capsule with which, at the hilus, the dilated end of the ureter blends. A
portion of a papilla (the end of a pyramid) is shown projecting into one of the calices into
which the ureter dilates. The peripheral portion of the kidney containing glomeruli and
contorted tubes is termed its cortex, the central portion medulla. At A is shown a single
urinary tubule. Commencing at the third glomerulus, it winds in the cortex, descends
into the medulla, turns in a loop of Henle, again winds in the cortex, and ends in a collecting
tube, which joins a duct. The arrangement of the bloodvessels is shown at B. A straight
artery and a straight vein lie side by side. The artery gives branches to the glomeruli.
The venules from the glomeruli again divide into capillaries, which supply the contorted
tubes and loops of Henle. The ducts are supplied by long arterial capillaries. C shows
the structure (magnified) of a glomerular tuft of capillary vessels, invested by a capsule
which closes into a contorted tube, ct ; dH, a descending limb; aH, an ascending limb
of a loop of Henle; d, a duct.
tion of any one of them applies to all. Each begins as a
capsule containing a glomerulus. The wall of the bulb—which
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198 ‘THE BODY AT WORK
is merely a thin basement membrane covered by epithelial
scales—is involuted by the tuft of bloodvessels. The vessels
do not penetrate its capsule. Between the tessellated epi-
thelium which covers the tuft and the similar epithelium which
lines the capsule there is a space communicating by a narrow
aperture with the next portion of the tubule—termed its
“contorted ”’ part, because it is twisted about like a tangled
thread in the cortex of the kidney. The contorted tubule is
of relatively large calibre. The cells which line it are irregular
in form and indistinct in outline. The basal half of each
cell, between its nucleus and the basement membrane, is
vertically striated, or ‘‘ rodded,”’ as it is usually termed. Such
an arrangement of the protoplasm of a cell is commonly
associated with a habit of absorbing fluid. It would seem to
indicate in this case that the cells take water and various sub-
stances dissolved in water from the direction of the basement
membrane. After a time the contorted portion of the tubule,
although still sinuous, becomes more nearly straight—the
“spiral portion ’’—and assumes a radial direction. In the
zone between the cortex and the medulla, the spiral portion
tapers into an exceedingly slender tubule which, after running
some distance in the direction of the hilus, turns back again
towards the cortex, making a loop, known as the “loop of
Henle.” The ascending limb of this loop is of larger calibre
than the descending limb. The descending limb is lined by
flattened epithelium, each cell so thin that (in microscopic
sections as ordinarily prepared) its nucleus bulges into the
lumen of the tube. The cells of the ascending limb are more
nearly cubical in form. On reaching the cortex, the tubule
again becomes contorted. The second contorted portion
narrows into a “collecting portion,” which joins a ductule.
The ductules unite together, until at last a single duct is formed
which opens at the apex of a pyramid. The cells of the duc-
tules are cubical or columnar. Their cell-substance is clear,
whereas that of the cells lining other parts of the tubule is
cloudy in appearance.
Such a tubule, viewed as a hydrostatic mechanism, presents
three portions, evidently fitted for different functions : (1) The
glomerulus is an apparatus which allows of the rapid exuda-
tion of water from blood. (2) The contorted portions of the
4 tubule present the appearance of a secreting siechiniam. The
large soft, cloudy cells which line them are eminently fitted to
take from the blood, or rather from the lymph which fills the
tissue-spaces which intervene between the walls of the capillary
bloodvessels and tubules, the various substances which they
excrete. (3) The loop of Henle is a remarkable piece of
apparatus, the purpose of which has been a subject of much
controversy. Looking at it from the point of view of hydro-
statics, it seems safe to conclude, from its extremely narrow
bore, that it raises the pressure of the fluid in the glomerulus and
first contorted portion; but it may have other functions also.
A consideration of the arrangement of the bloodvessels of
_ the kidney bears out the conclusion that the secreting apparatus
is divisible into at least two separate portions, possibly into
three. The glomeruli are supplied by short and relatively
wide arterioles. Each arteriole breaks up, as soon as it enters
the capsule, into a bunch of capillary vessels, which, in the
same abrupt manner, reunite to form a venule. On leaving
the capsule, this little vein behaves in a fashion for which the
only parallel is to be found in the portal system of the liver.
Instead of uniting with a larger vein, it again breaks up into
capillary vessels, which supply the contorted tubules and
loops of Henle. The medulla of the kidney is supplied by long
arterial capillaries of the usual type. The short arterioles of
the glomeruli are controlled by nerves which, constricting
them, or allowing them to dilate—possibly by actively causing
them to dilate—rapidly diminish or increase the amount of
blood passing through their tufts of capillary vessels. Here,
_ therefore, is a mechanism by which the glomeruli can be sud-
denly flushed with blood—a condition favourable to exudation
into the urinary tubules. The interposition of a second set
of capillaries prevents this sudden flushing from unduly disturb-
ing the pressure in the vascular system as a whole. In the
renal-portal capillaries of the kidney the blood-pressure is
fairly constant and, presumably, low. The use of the term
*‘renal-portal ”’ is justifiable, not only on the ground that
the vessels of the kidney behave like those of the portal system
of the liver, but also owing to the very significant fact that in
fishes and amphibia the kidney actually has a double blood-
supply. In such an animal as the frog the glomeruli are
supplied with arterial, the tubules ih venous, ‘Tplood: “the
glomeruli receive branches from the renal artery, the tubules © zi
from a portal system derived from veins of the abdomen and
hind-legs.
Sir William Bowman, who in 1842 gave the first detailed "3
"a
description of the microscopic structure of the kidney, con-
cluded that, whereas “ the tubes and their plexus of capillaries
are probably the parts concerned in the secretion of that
portion of the urine to which its characteristic properties are
due (the urea, lithic acid, etc.), the Malpighian bodies [7.e., the
glomeruli] may be an apparatus destined to separate ros the
blood the watery portion.”
All physiologists are in accord in regarding the glomeruli as
the principal seat of exudation. There is great diversity of
view as to the function of the tubules. In 1844 Ludwig
advanced the opinion that all the constituents of the urine
pass through the glomeruli in a large excess of water, and
that in the course of the tubules this excess of water is re-
absorbed. This theory was based, among other considerations,
upon the extreme thinness of the epithelium which covers the
glomerular tufts ; he judged that water would filter through
it very readily. A large amount of experimental work has
been directed to the solution of these two problems—viz.,
(1) Do urea and other similar substances pass through the
glomeruli ? (2) Is water returned from the tubules to the
venous system ? Our views as to the functions of the kidney
as a whole will not be greatly influenced by the answers that
may eventually be given to these questions ; yet their discussion
is of very great interest, owing to the nature of the evidence
which may be marshalled on either side.
There is, perhaps, no other organ in the body the problems
with regard to which seem to be so nearly plain questions of
hydrostatics. It is easy to make a model of a urinary tubule
and its blood-supply. If such a model were shown to a sanitary
engineer, and he were asked to explain the working of the
drainage system of the body, and especially to answer the two
questions which we have propounded, he would say that there
could be no doubt as to the part of it through which most water
enters the tube, the glomerulus. He could give no opinion
as to whether urea, uric acid, and other substances of a like
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EXCRETION 201
nature, accompany the water until he had tried the experiment
of separating blood from water containing the inorganic salts
of urine by a permeable membrane—the blood being at such a
pressure as the physiologist told him he might expect it to
have in renal arterioles, the water at such a pressure as he might
expect it to have at the upper end of a urinary tubule. He
would find that urea, and still more uric acid, is very reluctant
to pass through the membrane. Again, when asked whether
water, in which urea and other things were dissolved, would
leave the tubule—say from the loop of Henle—to pass back
into the blood, he would repeat his experiment with a mem-
brane. This time he would allow the urine and the blood to
be at the same pressure (or, possibly, would assign a higher
pressure to the former), and he would dilute the urine to make
the conditions agree with those which Ludwig supposed to
exist ; but his experiment would prove to him that, unless the
urine were very dilute indeed, water would still tend to pass
into it from the blood, and not vice versa. And here it may be
remarked that the results of these experiments might have
been predicted by calculation. When Ludwig advanced his
theory, osmosis was a mysterious phenomenon. Its laws
have since been accurately ascertained. Given the molecular
weights of bodies in solution and their degree of concentra-
tion, the direction in which they will pass through a mem-
brane can be predicted. The force with which water will
tend to pass from one solution to another can be calculated.
Urine as secreted contains far more urea, sodic chloride, and
other salts than blood. It has a much higher degree of con-
- centration. The concentration of blood is 0:55 ; that of urine,
1-85. Water passes from a less concentrated to a more con-
centrated solution, not vice versa. As a solution of a problem
in hydrostatics Ludwig’s hypothesis is untenable.
Osmosis.—Cells of all kinds, both vegetable and animal, are
limited, or surrounded by a layer of cell-substance which is
firmer than, and probably different in constitution from, the
substance in the interior of the cell. This outer layer is a
living membrane. The nutrition and growth of the cell are
dependent upon the capacity of its limiting membrane for
regulating the ingress and egress of water and of substances
dissolved in water. ‘The phenomena of osmosis—that is to
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202 = THE BODY AT WORK
say, of the passage of water and of solutions through mem- hs
branes—are of such high importance in relation to the life of
the tissues that it may be permissible to make a further digres-
sion for the purpose of describing them (cf. pp. 40, 128). A very
simple apparatus will suffice to exhibit a phenomenon which
will give an idea of the meaning of osmosis. If the top of a
glass funnel, covered with a piece of bladder, so fastened to
its edge as to make it water-tight, be fixed in an inverted
position in a glass vessel, the glass vessel filled with water, and
the funnel filled to the same level with a solution of sugar, it will
soon be evident that water is passing through the membrane
into the funnel. The level of the sugar-solution will rise in the
tube of the funnel. If, instead of water outside the funnel and
sugar-solution inside it, a strong solution of sugar be placed in
the funnel and a weaker solution outside it, water will leave the
weaker for the stronger solution, and sugar the stronger solu-
tion for the weaker. If some of the solution in the funnel
be removed from time to time so that the pressure in it is
kept down to the same level as that outside it, water will
continue to enter through the membrane and sugar to leave
the contents of the funnel until the concentration of sugar is
the same on the two sides. The fluids will then be of identical
composition, and therefore isosmotic. In the further con-
sideration of the phenomena of osmosis, a distinction must be
made between permeable and hemipermeable membranes.
Suppose in the first instance that a permeable membrane is
used. Let it be so placed as to separate two watery solutions
of different constitution, yet of the same osmotic pressure.
By their being of the same osmotic pressure is meant that
they are of the same molecular concentration. The liquid A
contains certain salts in solution ; but the liquid B may con-
tain the same salts in quite different proportions. It so happens,
however, that the salts are so balanced that the total tension
of the salts in A is equal to the total tension of the salts in B.
At first there may be some change in level in the two liquids,
owing to differences in rates of diffusion through the mem-
brane of the various salts which they contain ; but after a time
the levels of the two liquids will be the same. To outward
appearance, nothing will have happened. Nevertheless, if the
experiment has been continued for a sufficient length of time,
; wil ek found that ‘seo Woihes have occurred in shi con-
| F ‘stitution of the two liquids. At the commencement,
g aoe their total tensions were equal, the proportions in
which the various salts were distributed in A, and therefore
their partial tensions, were very different to their proportions
and partial tensions in B. At the end of the experiment
each of the several salts is equally divided between A and
B, supposing the volume of A to equal that of B. This
experiment shows that the molecules of substances in solu-
tion are free to move. They behave like gases. Gases diffuse
through a membrane until their partial tensions are the same
in the two spaces which the membrane separates. The ether
in which physicists picture gases as dissolved offers no re-
sistance to the migration of their molecules ; neither does the
solvent—water, for example—prevent the movement of salts
which are distributed through it.
One other illustration of the phenomena of osmosis will
suffice to give an idea of the laws by which they are governed.
In the case just cited the membrane was permeable to all the
salts in solution. When the phenomena of osmosis were
first investigated, a distinction was drawn between substances
which will pass through membranes—crystalloids—and sub-
stances which cannot pass—colloids. We have already had
occasion to note that, whereas albumin is a colloid which does
not diffuse, its hydrate, peptone, is a crystalloid which does.
The term “ crystalloid’ indicates that substances which can
be crystallized are diffusible. Substances which are diffusible
are therefore allied to those which crystallize. The nature
of the membrane used to test diffusibility was not at first
taken into account. Now a distinction is drawn between
membranes which are permeable to all diffusible substances,
and membranes which are permeable to the solvent, but im-
permeable to the substances which it dissolves. The latter
are termed “ hemipermeable.” Imagine now that water is
separated from a solution of sugar by a membrane which stops
sugar, but is permeable to water. Water will pass through
the membrane into the solution of sugar. The level of the
solution will rise. Pressure will be needed, and a very con-
siderable pressure, to prevent its rising—to prevent endos-
mosis, that is to say. The force needed to resist osmosis is
204 THE BODY AT WORK
directly proportional to the degree of concentration of the
solution. If the solution contain 1 per cent. of sugar, a
pressure of 500 millimetres of mercury is needed ; if it contain
2 per cent., a pressure of 1,000 millimetres; if 6 per cent.,
of 3,000 millimetres.
In the next experiment separate two solutions, A and B, by a
hemipermeable membrane. Let A contain one salt only—X ;
let B contain several salts—X, Y, Z. Water will pass from A
to B, or vice versa, unless the osmotic pressure of the salts which
the solutions contain is the same, The osmotic pressure will be
found to be the same if the total number of molecules dissolved
in A equals the total number of molecules dissolved in B. If
in A there be N molecules of X (per unit volume), and if in B
there be nX, n’Y, n”Z, the osmotic pressure will be the same
providedn+n’+n"=N. This, it will be seen, is a very different
matter from equality of percentage composition. Some mole-
cules are light; others are heavy. The percentage weight of
X+Y+Z in B may be very different from the percentage
weight of X in A. To estimate the osmotic pressure of a
mixed solution, it is not sufficient to add together the per-
centages of the various salts which it contains. ‘‘ Concentra-
tion,”’ in the sense in which it was used in regard to blood and
urine, refers to the number of molecules of dissolved substances
in a given volume, not to their weight.
It would be undesirable to attempt in this place to enter
upon the theory of osmosis. Enough has been said to suggest
to the reader that he should, when endeavouring to apply its
laws to the explanation of physiological phenomena, bear the
following facts in mind: Some membranes are permeable to
water and to the crystalloids which it dissolves; others,
although permeable to water, are impermeable to substances
in solution. Some substances are diffusible through per-
meable membranes ; others are not. Osmosis of water occurs
from the solution of lower to the solution of higher concentra-
tion. Diffusion of crystalloids is their escape, owing to their
own molecular movements, from a situation in which they
are denser to a situation in which they are less dense. It must
be added, however, that various circumstances prevent the re-
duction of the laws of osmosis to simple terms—the tendency of
salts to dissociate when in solution, their bases and acids acting
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EXCRETION 205
as independent ‘‘ions,” is an example of the complications
which produce apparent departures from these laws. It must
further be added, and with emphasis, that, important though
it be that anyone who attempts to explain the interchanges
which occur between the various fluids of the body should be
conversant with the laws of osmosis, it is impracticable, and in
some cases misleading, to rigidly apply them. Living mem-
branes and dead membranes do not necessarily control diffusion
in the same manner. Still. less do the laws which govern
diffusion through dead membranes hold good, without qualifica-
tion, to living cells.
To return to the sanitary engineer whose opinion we asked
regarding the mode of working of the drainage system of the
kidney. Probably he would deny that the problems came
within his province. “‘ They are not physical, but vital,’ he
would say. “I know nothing about the vital action of the
cells which line the tubule.” Objection may be taken to the
_ form of expression, albeit he was fully justified in declining to
discuss the question any further. He does not know enough
about the internal structure of a cell to be able to predict the
phenomena of osmosis which will occur within it. No one can
say what capacity living cells may have of taking substances
from the blood, returning some of them, and excreting others.
This unknown capacity leads to results which, when they do
not appear to be in accordance with the laws of physics,
are commonly termed “ vital.” The term is a stumbling-
block which has tripped up generations of physiologists. The
expressions ‘‘ vital action’ and “ physical phenomena ”’ have
been used as if they were antithetical, whereas all vital
actions are physical phenomena. “Vital’’ in this sense connotes
‘‘as yet unknown.” Yet, in truth, there is abundant excuse
for the use of a term which covers ignorance, so long as its con-
notation is not extended until it assumes a positive, anti-
physical sense. “Physical”? and “vital” are expressions
which point a contrast constantly present to a physiologist’s
mind. He knows perfectly well that the passage of water and
- salts through a membrane, and their passage into and out of a
living cell, are equally phenomena of osmosis. But the former
process he can test and measure in his laboratory ; the latter he
can but observe in much obscurity in the living body. He
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206 THE BODY AT WORK —
cannot make a model of a living cell. In the case of the
salivary gland, as we have already seen, living cells take water
from lymph, and discharge it as saliva in apparent opposition
to osmotic force. They reverse the direction of the flow which
would occur were lymph and saliva separated by a membrane.
But a cell is not a membrane. It is an extremely complicated
structure with an elaborate architecture of its own. As well
might we compare the distribution of water by a County
Council water-cart and its passage through a brewery. Accord-
ing to all the laws of hydrostatics, the water which flows into a
brewery should leave it through its drains. Its exit in barrels
on drays is antiphysical. When the physiologist can explore
the living cell, he will discover that the imbibition and extrusion
of water, the selection, retention, and discharge of salts, are
phenomena as strictly physical as their passage through a
dialyser in his laboratory. In the meantime he can but con-
template the cell with a certain degree of awe. His best
devised model of a urinary tubule may lead him into error, for
the simple reason that he cannot line it with living cells. A
living cell has a power which upsets all calculations, falsifies all
experimental findings. Its protoplasm can isolate and place
out of action any of the substances which enter it. If observa-
tions eventually prove to us that water passes from the urinary
tubules into the blood, “‘ in the face of osmotic force,”’ we shall
be constrained to explain this antiphysical phenomenon as due
to the action of living cells. The cells, we shall say, take up
fluid from the urinary tubules, fix its urea and other salts in
their protoplasm, discharge its water into the venous blood,
return the urea and other salts to the urine. Given this
property of protoplasm, such a process is strictly in accordance
with physical laws.
Enough has been said regarding the theory, or want of theory,
of the action of the kidney. Turning now to matters of obser-
vation, it can easily be shown that the epithelium of the
tubules has the power of excreting into the urine highly complex
materials which diffuse with difficulty. If a substance soluble
in blood, but insoluble in urine, an alkaline salt of indigo, for —
example, be injected into the vascular system, it is rapidly
excreted by the kidney. The indigo is precipitated even before
mre
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EXCRETION 207
r it comes in contact with the acid urine. If the animal be killed
a short time after the administration of the indigo, the con-
_ torted portions of its tubules and the ascending limbs of the
loops of Henle are strongly coloured blue. An ammoniacal
solution of carmine may be used for a similar experiment ; but
the results are not nearly so sharply limited to the large-celled
portions of the tubules. Even the glomerulus is coloured red,
a fact which has been interpreted as showing that, although
the greater part of the carmine is excreted into the tubules,
some of it accompanies the water which exudes from the blood
through the glomerular tufts.
The practical identity in structure of the kidney in birds and
reptiles and mammals would seem to have an important bearing
on this controversy. The urinary excretion of birds consists
almost exclusively of uric acid. As seen under the microscope,
it is a semisolid white deposit, made up of crystals, supposing
no special precautions have been taken to obtain it fresh. The
water, pigment, and salts which are essential elements of the
excretion of mammals are practically absent. Yet the kidney
of a bird presents the same arrangement of glomeruli and
tubules as the kidney of a mammal, although the glomeruli are
relatively smaller. Uric acid diffuses with great difficulty.
If it is, so to speak, washed through the glomeruli, and the
water which dissolved it reabsorbed by the tubules, an enor-
mous quantity of water must pass through the kidney in order
that it may carry the uric acid in its stream. If uric acid be
excreted by the epithelium of the tubules, it is difficult to
account for the presence of glomeruli, since no water leaves the
_kidney. Crystals of uric acid are to be seen in a section of the
kidney, not only in the cells of the tubules, but also in the
glomeruli; but it may well be that in both situations crystalliza-
tion has been induced during the preparation of the section. It
jars an histologist’s conception of the constitution of a secreting
cell to contemplate the formation within its network of proto-
plasm, and the extrusion from it, of sharp-angled crystals. As
a matter of fact, it is not in its crystalline form that uric acid is
excreted by birds, but as quadri-urates—i.e., salts containing
only one-fourth of their ‘normal’ complement of base ;
crystalline spheres or amorphous deposit, not angular crystals.
These quadri-urates decompose very quickly, setting free
A tS ee ee eee ee
208 : THE BODY AT WORK —
crystals of uric acid. It must be confessed that,in whatever
way one attempts to account for the excretion of uric acid by
birds, the similarity of structure of their kidneys and those of
mammals is difficult to reconcile with the wide difference in
consistency and in chemical composition of the excrement. —
Reflecting upon all the evidence bearing upon the mechanism
of the mammalian kidney, the majority of physiologists come —
to the following conclusions: The greatest outflow of water
occurs in the glomeruli. The water is accompanied by salts,
including a small quantity of urea. The contorted and spiral
portions of the tubule and the ascending limbs of Henle’s loops
add to the urine the remainder of the urea, together with
various bodies still less readily diffusible.
It may be that the chief function of the loops of Henle is to
oppose resistance to the passage of fluids, thus heading up the
secretion, and favouring the osmosis of water into it from the
blood of the glomerular capillaries. It is possible that the
calibre of the slender descending limbs is influenced by external
pressure, their partial occlusion being increased, and the
pressure in them raised, when the organ is very active and
its intermediate zone turgid with blood.
Various drugs influence the secretion of the kidney. In
some cases their action seems to be mainly hydrostatic. They
change the rate of flow by altering blood-pressure. Digitalis in-
creases the force of the heart. The heart beating more strongly,
blood-pressure rises. Higher blood-pressure is accompanied by
a more copious secretion. This action of digitalis is far more
marked when the heart is out of order than when it is healthy.
In heart-disease the blood-pressure is unduly low, and the tissues
become water-logged in consequence. When the _blood-
pressure is restored and a brisker capillary circulation estab-
lished, water and waste-products, which have accumulated in
lymph, pass, as they ought to do, into the veins. Carried into
the general circulation, they overflow from the kidney.
It is a little difficult to realize the abundance of the body-
fluids. From one-quarter to one-third of the whole body-
weight is due to lymph, using this term in its most general sense.
The waste-products of tissues collect in the lymph. The
blood circulating through capillary vessels which traverse
lymph-spaces takes up water and waste-products. Its just
- EXCRETION 209
a peuapoatbion is maintained by the eliminating activity of the
_ kidneys.
Even in the diuretic action of digitalis we see indications of
something more than an alteration of the hydrostatics of the
blood-supply of the kidney. The brisker circulation carries
waste-products to the liver ; the liver transforms nitrogenous
refuse into urea; urea stimulates the renal epithelium. It
would be a mistake to lay too much stress upon the direct effect
of the drug upon the blood-pressure in the kidney. Other
illustrations throw the mere hydrostatics of the problem into
the background. Adrenalin (extract of suprarenal capsule)
causes a severe contraction of the small arteries, which raises the
general blood-pressure considerably ; but the increased blood-
pressure is not accompanied by diuresis, because the glomerular
arterioles share to a full extent, perhaps to a disproportionate
extent, in the general constriction. In migraine and certain
_other disorders it frequently happens that the blood-pressure in
the aorta is unduly high, yet very little fluid enters the renal
tubules. If a “saline diuretic,” potassic nitrate, sodic acetate,
or some other drug of the same kind, be administered, a copious
flow is established, the blood-pressure is relieved, the distressing
symptoms disappear. Then, again, certain diuretics, such as
‘ sweet spirits of nitre,” tea, gin, etc., may bring about a flow
out of all proportion to the alteration they produce in the
hydrostatics of the circulation. The diuretic action of these
various drugs is clearly due to increase in permeability of the
renal epithelium. And, of all stimulants to secretion, urea, the
natural stimulant, is the most effective. If a kidney be
removed from the body, a cannula inserted into its artery, and
defibrinated blood caused to circulate under pressure through
the organ, water may or may not drip from the ureter. On
addition of urea to the blood, a copious excretion is set up. In
explaining the mode of working of the kidney, as, indeed, in
explaining that of every other organ of the body, the mechanical
aspects of the problem must be kept in the background. When
we are contemplating the plan of construction of the kidney,
the hydrostatics of the circulation attract attention; but
alterations in hydrostatic conditions are not the initiating cause
of a greater or less flow of urine. The chemical condition of the
blood circulating through the kidney is the initiating cause,
14
ar THE BODY AT WORK ——
When the presence in it of urea demands a more copious — =
the hydrostatic conditions are adjusted to this need. In the —
case just cited of the isolated kidney, it might be urged that the _
flow caused by urea is a mechanical effect. The cells of the
contorted portions of the urinary tubules remove urea from the
blood. They secrete it into the tubules. The solution of urea,
being headed up towards the glomeruli, owing to the resistance
offered to its passage down the tubules by the narrow,
descending limbs of Henle’s loops, surrounds the capillary
tuft. Urea rapidly attracts water from the blood. &, “are - :
: “i =-4e eS AS 2S. |
\. > * a ~ aL i‘. rw J. > P
: PS as — ?
214 THE BODY AT WORK |
we gave as the predominant cause of gout acid fermentations _
in the stomach. It does not, by any means, follow, however,
that we were right in correlating imperfect digestion with an
excessive formation of uric acid. It may well be that the gouty
symptoms to which hampered peptic digestion gives rise are
due in larger measure to a disturbance of the composition of
the body-fluids which renders them unfit to carry uric acid to ~
the kidneys in such a form, or in such relation to the fluid in
which it is dissolved, as will insure its escape into the urinary
tubules. The interference with the efficient working of the
system caused by accumulation in it of uric acid gives a par-
ticular interest to all that is known regarding the nature and
origin of this substance.
Uric acid has the formula C;H,N,0;. It is a more com-
plicated and a more stable body than urea. The deposits of
guano in Peru contain uric acid (the excrement of birds) which
has remained practically unchanged for years—for centuries,
perhaps. Its chemical nature is not completely understood.
It can be readily made to yield urea ; and it can be formed by
conjugating urea with a nucleus derived from lactic acid
(cf. p. 13). Its formula is therefore commonly represented as
that of a diureide—a substance containing two urea radicles :
(HN—-CO
CO , Cz art
CO
[HN —— C——NH
But notwithstanding this inclusion in its molecule of two
radicles of urea, it is safe, when one thinks of the contrast
between urea and uric acid, to lay stress, in the case of the
former, on the binding of nitrogen to hydrogen ; in the case of
the latter, on the binding of nitrogen to carbon.
Uric acid is soluble with difficulty ; it crystallizes in rhombs.
It forms salts, normal and acid. Those which appear in the
urine are always acid salts. As a treatment for “stone,”
lithia water has long had a reputation which it probably
deserves, the acid urate of lithium being the most soluble salt
_ EXORETION = ° ~_—_—s2B
s as uric acid which the kidney can secrete. When uric acid is
in excess in urine, brown crystals of uric acid are deposited as
“ oravel”’ soon after it is passed. Even when not in excess,
uric acid crystals appear after a sufficient time. In other cases
uric acid, when in excess, is thrown down in the form of a cloud
of acid urates of sodium and other bases, which renders the
urine turbid. These urates are redissolved when the water is
warmed.
The more fortunate of human beings need never concern
themselves with the chemical history of uric acid. It is always
present in their body-fluids. It is excreted by the kidney. Its
formation is of no greater interest than that of creatinin and
other nitrogenous compounds which escape the almost universal
reduction to urea. Persons who have a uric acid diathesis are
in a very different plight. KEvery scrap of evidence bearing
upon its origin is of supreme importance. Unfortunately, the
evidence collected as yet is scanty, and its application for
remedial purposes impracticable.
The only disease in which uric acid is invariably in excess is
leucocythemia. This is a condition or habit marked by the
presence in the blood of a very great number of white blood-cor-
puscles and a paucity of red ones. The connection between this
disease and the production of uric acid is made plain by certain
experiments in diet. If flesh which contains relatively a
large proportion of cell-nuclei is eaten, the uric acid excreted
is markedly increased. Sweetbread, especially “‘ neck sweet-
bread ’’—1.e., thymus gland—is a mass of comparatively small
cells with large nuclei. If thymus gland be substituted for
all other meaty foods, the quantity of uric acid appearing in the
urine is doubled. A large increase in the quantity of ordinary
meat or fish consumed also increases uric acid, because all
meat-fibres contain nuclei. If egg-albumin be taken instead
of meat, uric acid is not increased. A sudden excess of mus-
cular work leads to an increase in uric acid, owing presumably
to the unusual activity of the tissues. This used to be very
noticeable in the case of young men during the first few days
of “training”’ under the old system; but it may have been
due to the generous consumption of chops and steaks, rather
than to the increase in physical work, and consequent destruc-
_ tion of tissue. Nuclei contain nucleo-proteins, which split into
=o, Se, Re
a De ee oe)
proteins and nuclein. Chemically, it is reasonable to attribute
to nuclein the parentage of uric acid ; a plausible line of descent _
can be traced. The association of leucocythemia with the
production of uric acid is probably due to the destruction of
leucocytes which are present in abnormal numbers (cf. p. 53). a
Such is the evidence at present in the hands of physiologists. __
Naturally, physicians have endeavoured to turn it to account. |
Patients have been recommended to avoid animal foods which
contain nucleo-proteins—to take, instead of meat and fish, eggs,
milk, cheese, vegetable-albumins. Certain physicians contend
that such a diet is followed by the happiest results ; others,
equally competent, and perhaps less biassed by ‘“ medical
theory ’—the most dangerous of handicaps for anyone who
practises an art which must ever remain empirical—are satisfied
that equally good results are obtained by excluding from the »
diet eggs, milk, and cheese. Physiological discoveries suggest
treatment. Modern medicine is in the fullest sense applied
physiology. But treatment based upon theory must be con-
trolled by unprejudiced observation. It is possible that the
gouty diathesis may be held in check in certain cases by the
exclusion from the diet of certain kinds of nitrogenous food.
The experience of generations has taught us that the injudicious
use of such articles of diet as fruit, pastry, sugar, which do not
contain nitrogen, is the main factor in inducing an attack of
gout ; that imperfect digestion, sluggish circulation, insufficient
activity on the part of the kidneys, lead to the accumulation
in tissue-spaces of the fons et origo malorum. Even sweet-
bread, which with the precision of a chemical experiment
increases the production of uric acid by a healthy person, is
not necessarily found unwholesome by those who are inclined
to gout. It is amongst the most digestible of all meat foods,
and easy digestion covers a multitude of metabolic sins.
CHAPTER IX
THE CIRCULATION
THE blood circulates in a closed system of tubes, continuous
from the heart back to the heart. The walls of these vessels
separate the blood from the tissues. Nowhere, except in the
spleen, does it come into contact with any cells other than the
lining cells of the vessels in which it flows, and the exception
made by the spleen is more apparent than real. The spleen
(p. 79) is a kind of sponge invested with a firm capsule. Small
arteries discharge their blood into its spaces ; small veins collect
it. But the organ is essentially a part of the vascular system.
Its spaces take the place of the capillary vessels which connect
arteries with veins in other situations.
The blood makes a double circuit. From the right heart it
passes through the vessels of the lungs. Returning to the left
heart, it is driven through the body. Although the heart con-
sists of two separate pumps, it makes but asingle organ. Its
division into right auricle and ventricle and left auricle and
ventricle is but slightly indicated on the surface. In most
invertebrate animals the two pumps are distinct. In some the
- lung-heart and the body-heart are on opposite aspects of the
body. But one must not, when thinking of the morphology of
the vertebrate heart, picture it as formed by the juxtaposition
of two, originally separate, pumps. Truly, in its very earliest
stage of growth, it is represented by two tubes which lie, in the
embryo, far apart. But these, before we can speak of the
existence of a heart, fuse into a single tube, with four con-
_ tractile bulbs in series. As the heart develops, the dilatation at
its hinder or venous end and the dilatation at its anterior or
arterial end disappear. A partition is formed which divides the
two middle bulbs into right and left auricle and right and left
ventricle respectively. Immediately after birth the lungs are,
217
vessels of the pew s lungs.
auricular septum is perforate.
R.Carotid A. L. Carotid A.
Artery to right arm I l 4 Artery to left arm
7o Lungs
To Lungs
\\ oe : Hii} ‘ae oe Hi}}})
From Lungs SS From Lungs
From Liver '<
To Stomach
To Spleen
Fig. 10.—THE HEART CUT IN THE PLANE OF ITS LONG AXIS, AND THE VESSELS WHICH OPEN
INTO AND OUT OF IT.
Chord tendines attach the margins of the auriculo-ventricular valves to musculi papillares
which project from the inner aspect of each ventricle.
foramen. When the lungs are expanded by the forcible
enlargement of the chest-cavity which contains them, their
bloodvessels are distended by the same extensile force. Blood
is sucked into them from the right side of the heart. » i _ ~~.
7 a “ _ a,
Pay 1) : oy i
ey }
eM aL
y Cy", a eee ’
q 5, ty ’ ’
AVS o Ky
nm +
above the head, and slowly lowering them again, has a remark-
able effect in quickening the circulation—increasing the blood-
Supply of the brain. Changes of posture, by relieving pressure
on subcutaneous veins, removes an impediment to the flow of
blood. 3
The second of the factors to which we have referred as
adjuvant to the heart’s action is the negative pressure of
inspiration. In explaining the effect of this force upon the
circulation, the relation of the lungs to the thorax must be
taken into account. The box in which the lungs are enclosed
is too big for them ; nevertheless, being extensible and elastic,
they always fill it. They follow its movements when in
inspiration the muscles between the ribs enlarge it, and when
in expiration it diminishes again. No air or fluid, save the
moisture which lubricates the surface of the pleura, reducing
friction, occupies the (potential) space between the lungs
and the chest. But the moment the chest is punctured the
lungs collapse. Air is sucked into the pleural cavity. The
lungs fill the chest only so long as there is neither air nor fluid
between it andthem. Lung-tissue is extremely delicate. Hach
air-cell is a cup of thin membrane holding together a basket-
work of capillary vessels. Solong as the chest-wall is stationary
the negative pressure in the pleural cavity has no effect upon
these slender tubes. But when the chest expands, the capil-
laries are between two minus pressures, the pull of the chest-
wall and the resistance offered to the entrance of air into the
lungs by the passages through which it has to pass. The
calibre of the lung-capillaries is increased, just as it would be
increased were they hanging in an air-pump while the piston
was drawn out. More blood passes to the left heart through
the wider capillaries. Ejected into the aorta, it raises the
pressure in the arterial system. A record of the pressure in
an artery shows a rhythmic rise for each heart-beat. It shows
also a rise with inspiration and a fall with expiration. These
larger undulations correspond with the movements of the
chest, although they are necessarily somewhat late on respira-
tion, for the first effect of the dilatation of the capillaries is to
cause them to hold more blood and to deliver less. The first
effect of expiration, on the other hand, is to urge on the blood
which the dilated vessels contain. In any case a single beat is
Yih, ate ieee ert ET GR Pele a, tell ee hy ae a Ce % ~ SK wm oss re =. Me ail i.
Se elit en eerie Bete ony Ly SL Eh” San) eee cients eae nee
Soe ee ae 5 : > ro Ce
=> E = eas coe :
. .
it ee
222 ‘THE BODY AT WORK Shs
needed to throw into the aorta the blood which has ‘boa
received by the right auricle.
The expansion of the chest influences the flow of blood in
yet another way. The heart and the great vessels which join
and leave it are themselves enclosed within the chest, subject
to the negative pressure produced within that cavity by the
elasticity of the lungs. The lungs pull upon the pericardium,
the membranous covering of the heart. When this pull is
increased owing to the forcible expansion of the chest, blood is
sucked into the great veins, just as air is sucked into the wind-
pipe. The thick-walled aorta, containing blood at high pressure,
does not feel the effect of slight variations in the pressure round
it. The soft-walled veins are expanded during inspiration to a
not inconsiderable degree. What relief a deep yawn gives by
hastening a languid circulation! Leaning over an account-book
late in the afternoon, every condition is unfavourable to the
flow of blood. It accumulates in the legs and in the abdomen.
The head is thrown back and the mouth opened wide, while the
chest expands in a long deep inspiration. Down on the liver,
stomach, and intestines presses the flattened diaphragm,
squeezing their blood towards the heart. The negative pressure
within the chest sucks this up, and draws down the blood con-
tained in the great veins of the neck. The capillaries of the
lungs are widened, allowing blood to pass more quickly from
the right side to the left side of the heart. ‘The heart responds
to the call upon it, throwing all that it receives into the aorta.
Only a great effort of the will had kept the pale brain at work ;
in the attic it suffers more than organs on the lower storeys
from insufficient pressure. For a short time after the yawn
it finds itself nourished with an adequate supply of blood.
The negative pressure in the thorax is considerable at all
times. If a manometer—a U-shaped tube with mercury in its
loop—be connected with a cannula passed through the wall of
the chest, the difference of level of the mercury in the two limbs
of the U is a measure of the force with which the lungs are
endeavouring to shrink away from the chest-wall. Even at the
end of expiration the mercury in the limb next the chest stands
about 6 millimetres higher than the mercury in the outer
limb. During a deep inspiration the pressure in the chest
falls 30 millimetres below the atmospheric pressure. Hence a
eee Meee fg
mae PD gs ane oe
; THE CIRCULATION — 993
a E problem is eae of which no completely satisfactory
solution has yet been given. How comes it that lymph is not
sucked into the pleural cavity ? In health there is no more
pleural fluid than just suffices to keep the membrane moist.
The endothelial cells which cover the surface of the pleura resist
further exudation. Valves in the lymphatic vessels prevent
backward flow. Yet in disease, when the pleura is inflamed,
lymph pours out quickly, often to be reabsorbed with equal
rapidity when the pleurisy subsides. This flow uphill, from a
lower to a higher pressure, can be explained only as a pheno-
menon due to the “secretory” capacity of endothelium. As
an answer to the hydrostatic problem this is hardly satisfactory.
The circulation of the blood is the result of the difference
between the pressure in the vessels through which it leaves the
heart, and that in the vessels through which it is returned.
The pressure in the aorta amounts to about 200 millimetres
of mercury. In the vene cave it is nil, or, ovals to the
aspiration of the thorax, less than nil.
The Heart.—Inspection of the liver, the spleen, or ve kidney
helps but little to the comprehension of the mechanism of these
organs. It is quite otherwise in the case of the heart. Its
mechanics being comparatively simple, physiology is concerned
with measurements, with the conditions under which it can
and cannot work, and with the action upon it of the nervous
system and of drugs. The heart of any mammal will suffice
for anatomical study. A sheep’s heart is about the same size
as that of a man, and exactly similar, save in minute par-
ticulars, which do not appreciably affect its mode of working.
The heart is a hollow muscle, composed of minute contractile
cells. Each cell is a cylinder, about twice as long as it is broad,
with an oval nucleus in its centre. There is no impropriety in
speaking of the heart as a single muscle. Muscles which we can
move at will, “ voluntary muscles,” consist of fibres, each from
1 inch to 2 inches long, and of about the thickness of a piece of
thread (Fig. 16). Every fibre is surrounded by a membranous
sheath, its sarcolemma, which completely isolates it from the
others. Each has its separate nerve-supply. A voluntary muscle-
fibre is a cell-complex. The single embryonic cell which grew
into the fibre underwent nuclear division until hundreds of |
nuclei were formed, but its cell-substance was not divided into
territories appertaining to the several finelet) “tn Wee
on the other hand, nuclear division has been followed by cell- ee,
division ; but minute protoplasmic bridges are left between the __
cells. The whole of the heart-substance is thus in structural _
continuity. The cells are not invested with sarcolemma. As ~
the result of this arrangement, an impulse started in one part of
the heart spreads over the whole, with certain limitations as to
the directions in which it is able to travel, whereas in voluntary
muscle a separate impulse must be delivered to each fibre.
The wave of contraction commences in the great veins, the
ven cave and pulmonary veins, near their junction with the
heart, spreads from cell to cell throughout the auricles, and
onwards down the ventricles to the apex of the heart. The ~
substance of the heart has not, however, a homogeneous appear-
ance. Its cells are collected into fascicles, which lie in various
planes and cross the axis of the heart at various angles. Ina
boiled sheep’s heart it is easy to separate one fascicle from
another, and to distinguish the sheets into which the fascicles
are collected. The four valves of the heart lie in almost the same
plane. They are supported by a fibrous plate divided into four
rings (Fig. 11). Most of the fascicles are attached to this plate,
though some which encircle the auricles are independent of it.
With one or with both ends attached to the plate, fascicles loop
over the auricles. They run down the ventricles with a twist
from right to left. Those on the surface turn in at the apex of
the heart, and run up the inner surface of the ventricles. Some
of them go to form the free columns which are found on the inner
surface of the ventricles, pointing towards the valves—musculi
papillares. The fibrous plate which supports the valves cuts
off almost all of the muscle which makes the walls of the
auricles from that which constitutes the ventricular walls ; but
a thin sheet is continued from the inner surface of the auricles
down the interventricular septum. Toa considerable extent
the walls of the two auricles and of the two ventricles are
respectively continuous, insuring synchronous contraction.
The arrangement of the fascicles accounts for the changes in
form which the heart undergoes when it contracts. Systole
commences in the cardiac ends of the venz cavee and pulmonary
veins. They empty the last of their blood into the auricles,
and close to prevent regurgitation, their mouths not being
1,
wi
Fr
papillares. As soon as ventricular systole has commenced, the
wo, ®
en the auricles quickly shrink in all dimensions, and
F
as soon as their contraction is at its height the ventricles
_ contract, while the auricles relax. The ventricular wave runs
_ from base to apex too rapidly to be followed with the eye,
%
and ends, owing to the involution of the fascicles, in the musculi
auricles relax. After emptying their contents into the aorta
and pulmonary artery, the ventricles relax, their contraction
giving way first at the apex, and being longest held at the base.
Then follows a pause (diastole), during which both auricles and
ventricles are flaccid. If the pericardium is open, the heart is
Fig. 11.—A SECTION APPROXIMATELY AT RIGHT ANGLES TO THE LONG AXIS OF THE HEART,
EXPOSING THE FOUR VALVES WHICH LIE VERY NEARLY IN THE SAME PLANE.
The semilunar valve which guards the aperture of the pulmonary artery is the nearest to the
breast-bone.
seen to become round instead of oval in transverse outline
during systole. Itshortens. Its apex twists a little to the right,
and projects forward. But if it is within its pericardium the
shortening is not accompanied with any displacement of the
apex. Instead of the apex mounting, the base descends. The
front of the right ventricle, at some little distance from the
apex, presses the chest-wall forwards in the fifth intercostal
space, about an inch to the inner side of a line falling vertically
through the nipple. This pressing forwards is felt as the
“impulse of the heart.”
The contraction of the heart is not a see-saw of auricles and
ventricles. During diastole blood is falling from the veins
15
Po eee
SS de gs eS See at ee a en
226 THE BODY AT WORK
through the auricles into the ventricles. Ina sense, the auricles
are not necessary parts of the double pump. They collect
blood while the ventricle is contracting, thus preventing it
from heading up in the veins. They save time. Their con-
traction completes the filling of the ventricle, so that the
instant the ventricular contraction begins blood enters the
aorta and pulmonary artery.
The Valves.—If ever expressions of admiration were appro-
priate in a treatise on the animal body, such preface might be
permitted to a description of the cardiac valves. Which means
no more than this: Men make pumps. Therefore they are in
a position to appreciate the mechanism of the heart. We
cannot admire what we do not understand. If we made secret-
ing organs or self-contracting springs, glands and muscles
would evoke our commendation. We should recognize that —
Nature’s apparatus is even better adapted to its work than
any that men can make. This is the admission which is forced
from us when we study the heart.
The apertures connecting auricles and ventricles are ex-
tremely wide, allowing the contents of the former to be emptied
into the latter almost instantaneously. If we attempted to
make a pump fulfilling this condition, we should find that it
failed in several respects. In the first place, the rush of fluid
from the one chamber into the other would press the flaps of
the valves back against the wall of the second chamber. They
would cling to the wall, and would not float up quickly into
place when the second chamber was squeezed. Let us call the
two chambers A and V for brevity’s sake. When V contracted,
some of the fluid would be thrown back into A, because, the
resistance in that direction being lower than the resistance
offered by the column of fluid above the pump (the resistance
in the aorta is very high), the contents of V would rush past the
margins of the A-V valve. This would happen even though its
flaps were not pressed back against the wall. Further, at
the height of contraction the membranous valve would bulge
backwards into A, making a cup towards V which V could
not empty. In the heart these difficulties have been overcome.
The tricuspid valve, which separates the right auricle from
the right ventricle, has three flaps. The mitral valve, on the
left side of the heart, has but two. The flaps are composed of
ey 1 RW ee ae ee
ee me -OROMNT 1 ney UNIV:
227
tough membrane, but are comparatively thin. The following
_ direction for deciding at an autopsy whether or not they were
healthy at the time of death was given many years ago by a sur-
geon of repute : “‘ You ought to be able to see the dirt under your
thumbnail when you place it beneath one of the flaps.”’ Surgery
has improved in cleanliness as well as in other ways ; indeed,
the possibility of advance has been due to the recognition of
the need for transcendental cleanliness. But this is a digres-
sion. The margins of the flaps are crenulated.. Threads—
chorde tendinee—are attached to them like the stay-ropes of
a tent. At their other end these tendons are attached to the
musculi papillares already mentioned. The bunch of tendons
from each papillary muscle spreads, to be inserted into the
contiguous margins of two flaps. We have mentioned some of
the difficulties which have been overcome in the construction
of the pump. (1) The flaps do not flatten back against the wall
of the ventricle during systole of the auricle. It must be re-
membered that during diastole of both chambers blood is
flowing through the auricle into the ventricle. The latter being
partly filled before systole of the auricle commences, the flaps
are floated up. This is greatly favoured by the form of the
inner wall of the ventricle. It is not flat, but raised in pillars
—columnez carnee. The spaces between these pillars cause
backwash currents, which lift the flaps and help to bring them
into apposition as soon as systole of the ventricle commences.
(2) No blood which has entered the ventricle is thrown back
into the auricle. The valve “balloons” over the blood in
the ventricle before the contraction of the auricle has ceased.
The thin margins of its flaps come together with great rapidity.
The tendinous cords holding their edges on the ventricular
side, they meet, not edge to edge, but folded flap to folded
flap. (3) The valve does not bulge into the auricle. On the
contrary, at the height of systole it is pulled into the ventricle
by the contracting musculi papillares. As the ring to which
the valve is attached is diminished in size, by the contraction
of the base of the heart, which continues, it will be remembered,
until after the apex has begun to relax, the edges of the flaps
are folded farther and still farther over by the pull of the
musculi papillares, and the blood is squeezed out from between
the wall of the ventricle and the indrawn valve.
15—2
Be Gee ee nee
Vii 7 OT OWOD mio -
228 THE BODY AT WORK
Veen 7 eo
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The “ semilunar valves,’ which close the apertures into the
aorta and pulmonary artery, have each three flaps. The aortic _
semilunar valve, which has the higher pressure to bear, shows
its characteristic features in a rather more marked degree
thar the other. Each of its three flaps is a half-cup. At the
centre of the margin of the half-cup is a small fibrous nodule.
The edge of the cup on either side of this is very thin. Fine
elastic fibres radiate from the nodule to all parts of the flap.
The wall of the aorta shows three bays, or “ sinuses,” one
behind each flap. Hence, when the valve is forced by the rise
of pressure in the ventricle, the flap is not flattened back
against the wall of the aorta. There is always a certain
amount of backwash in the pocket behind it. The instant the
pressure in the ventricle begins to fall, the three flaps come
together with a click, so smart as to be plainly audible over
most of the front of the chest. The click is the “‘ second sound ”
of the heart. The auriculo-ventricular valves also make a
sound when they close; but this “ first sound of the heart ”’
has a different character. It is prolonged, soft, low-pitched.
It is customary to represent the sounds by the syllables “‘ libb
dtip—lubb dtp,” the pause during diastole being of about
the same length as the sounds when the heart is beating with
its normal rhythm. The duration of systole is little affected by
variations in the rate of beat. It is diastole that is shortened
or prolonged. ‘The second sound is due entirely to the closure
of the semilunar valves. It is heard most clearly when the
stethoscope is placed over the region where the aorta comes
nearest to the wall of the chest—at the second rib cartilage
on the right side of the breast-bone. The first sound is loudest
near the apex of the heart. It is generally agreed that it is
not wholly due to the closure of the auriculo-ventricular
valves, but possesses a second constituent. Some persons
assert that they can with the ear distinguish the clearer
valvular sound at the commencement from the general rumble
which overtakes it. The main part of the sound, if it have
two constituents, or the whole sound, if there be no dis-
tinguishable valvular constituent—observers differ—is just
the noise of a distant cab (bruit du cab) or the waves on a far-
off beach ; it is the sound which the ear picks up from any
irregular mixture of tones which it cannot analyse. It is
.. = ig the gis Ca part in adders the first sound cannot be
doubted, whether by their first closure or by their subsequent —
vibration. We should be inclined to attribute to them the
whole performance, were it not that the first sound, or at
any rate a sound, is heard during the beating of a bloodless
heart. If an animal be killed and the heart removed from its
thorax with the utmost despatch, it will beat for about a minute
while lying in the palm of one’s hand. When a stethoscope is
applied to the ventricle, a “ first sound”’ is heard. This was
described as a muscular sound, owing to a misconception. It
is similar to the sound which is heard when a stethoscope rests
upon a contracting biceps. Until recently the voluntary con-
traction of a muscle was believed to be vibratory—a tetanus.
The sound corresponds to a rate of about thirty-six vibrations
to the second. There being reasons for thinking that muscle
contracting naturally does not vibrate as fast as this, the
- sound was interpreted as the first overtone of the muscle-
note. Muscle was said to vibrate eighteen times a second.
The similarity of the first sound of the heart and the ordinary
muscle-sound led physiologists to infer that the contraction of
the heart also was a tetanus. But this was a mistake. Neither
voluntary muscular action nor the contraction of the heart
is an interrupted contraction in this sense. In the case of the
musculature of the heart especially, contraction is a steady
shrinking, followed by a steady relaxation. The sound pro-
duced by the bloodless heart is due to the various displace-
ments which occur when it contracts. Its interior is very
irregular, with its columns, papillary muscles, tendinous cords,
valves. The displacement of these various structures is re-
sponsible for the noise.
The sounds of the heart afford to the physician a means of
ascertaining with the utmost nicety the condition of the valves.
If the sounds are altered from the normal in the least degree,
the valves are not healthy. Alteration of the structure
of a valve is in ordinary parlance heart-disease. It is usually
indicated by an addition to the normal sound. Such addition
is termed a “ murmur ”’; in French, wn bruit de souffle. Hither
term is somewhat misleading to the tyro. We remember a
fellow-student to whom our chief had in vain expounded the
‘Ugtenins (2 eet Pe et
eee
230 —
nature of a murmur. “Surely, Mr. S., you can hear the
murmur in this case.” We others could hear it as we stood
around the. bed. After listening for a minute, S. replied:
“T think I could hear it, sir, if the heart wasn’t making such
a thundering noise.” The thundering noise was the murmur.
It is the business of the physician to recognize that there is
a departure from the normal, to analyse its character, to deter-
mine the time at which it is heard in relation to the cardiac
cycle, and to locate the place on the chest where it is heard
most loudly. He is then in a position to state which of the
valves is affected and what is the nature of its lesion. Is it
a lesion obstructing an orifice, or is it causing regurgitation
of blood? Or is one of the valves, as is commonly the case in
heart-disease, imperfect in both respects ?
A murmur, in the strictest sense, is a sound added to a heart-
sound. It is due in all cases to vibration of a fluid column
(“ fluid vein’ is the term in physics). When fluid passing
‘under pressure along a tube of a certain calibre enters a tube
of smaller calibre, no vibration occurs. When it passes from
a tube of smaller calibre into a larger tube or space, it is thrown
into vibration. Under normal conditions no vibration occurs
in the heart. The auriculo-ventricular orifices are so large that
auricle and ventricle form a single cavity when the valve is
open. The ventricles drive the blood into tubes of smaller
dimensions than themselves. These are not the conditions which
set up vibration in a fluid column. But if one of the orifices is
constricted, owing to thickening or partial adhesion of its
valve, the fluid column vibrates on entering the space beyond
it. The sound is propagated forwards, beyond the constric-
tion, not behind it, and transmitted to the wall of the ventricle,
aorta, or pulmonary artery, as the case may be. When either
of the auriculo-ventricular orifices is constricted, the vibration
of the fluid column can be felt as well as heard. The finger
placed against the chest-wall at the spot where the impulse of
the heart occurs is sensible of a thrill. The vibration may occur
whilst blood is flowing through an auricle into a ventricle,
before the auricle contracts. In time, it is presystolic. The
murmur produced byregurgitation into an auricleis synchronous
with systole. The murmur due to regurgitation into a ventricle
past an incompetent semilunar valve is postsystolic.
231
We have said that the heart is so formed that no vibrating
fluid vein is produced when it is functioning normally. Mur-
murs are due to alterations in the valves which are visible
after death. This statement needs modification. Not in-
frequently functional murmurs are heard, which disappear
again after a time—in a few weeks, or even days, perhaps. The
explanation of murmurs of this class is very difficult. They
are heard most frequently in anzmic persons, and appear in
these cases to be due to the heart having shrunk, owing to the
blood in circulation being deficient in quantity, until the cavities
of the ventricles have a smaller diameter than that of the .
great arteries into which they expel their contents.
Such is the explanation of the physical cause of murmurs
given by Chauveau and Marey, the physiologists who have paid
most attention to this subject. But it must be remembered
that the valves which, when diseased, are the sources of the
murmurs are membranous structures. It may be that fluid
veins would be produced by them if they were rigid ledges
which jutted into the blood-stream ; but, being membranous,
they are capable of vibration. Certain physicists are of opinion
that a murmur is caused, not by the vibration of a fluid vein,
as such, but by the vibration of the membranous structure
which impedes the passage of the fluid. The physics of the
problem is of little consequence to the physician. The murmur
is produced at the spot where a diseased valve is situated, and
is propagated forwards. It enables him to ascertain with
~ accuracy what is amiss with the heart.
Bloodvessels.—The greater circulation occurs through a
closed system of vessels which unite the left ventricle with the
right auricle. The aorta gives off lateral branches. Its
branches branch. Subdivision continues until the vessels are
just wide enough to allow blood-corpuscles to pass in single
file, or but little wider. When a bough of a tree divides, the
united cross-sections of its twigs, their soft bark being stripped —
off, may be a little larger than the cross-section of the bough ;
but the disparity is usually small. The united cross-sections of
the smaller arteries is considerably greater than that of the
trunks which give origin to them. By the time the capillaries
are reached,their total bed—their united cross-section—is about
640 times as great as that of the aorta. This estimate is based
~
upon the diminution in the rate at which blood flows throu; h
the vessels. The velocity with which a stream flows through %
a channel varies as the cross-section of the channel. In a
capillary vessel the blood flows at the rate of from 0-5 milli-
metre to 1 millimetre per second. In the aorta the velocity
is about 320 millimetres per second. In the re-formation of
the venous system a converse process of reduction occurs, but
not with anything like the same rapidity. The united calibre
of the two vene cave, in which the reduction is complete,
is about twice that of the aorta. From this it follows that
the veins hold much more blood than the arteries ; and since
veins are more easily distended, the amount that they can
hold varies within wide limits. They constitute to some
extent a reservoir for blood.
The capillary vessels are the tubes of the circulatory system
in which blood comes into use. On the average they are
about 0-5 millimetre long. Through them the blood flows
slowly. Through their walls alone is there any exchange worth
-mentioning between the blood within the vascular system and
the lymph by which it is surrounded. Interest therefore centres
in these vessels. Their walls are formed of endothelial tiles.
In the centre of each thin transparent tile is a boss, where
its lens-shaped nucleus is situate. The outline of the tile is
sinuous. Its margin dovetails with the margins of those
adjacent to it. Oxygen and carbonic acid, nutrient sub-
stances and waste-products, pass rapidly through the endo-
thelial cells. Leucocytes have the power of pushing the cells
aside, in order that they may make their way out of the blood
into the lymph which fills the tissue-spaces. With. the ex-
ception of the lens and cornea of the eye, cartilage, and the
various epidermal structures, all tissues are traversed by
capillary vessels. It is not difficult to calculate the number
of such vessels in the body exclusive of the liver and the lungs.
The diameter of the aorta is 28 millimetres, that of a capillary
about 0-008 millimetre. The cross-section of all the capillaries
added together is 640 times that of the aorta, as already stated.
Many schemata have been devised to illustrate the vascular
system ; but all are misleading, inasmuch as they fail to give
any idea of the extent to which the subdivision of its vessels
is carried. If the water-pipes supplying a town branched
~-e
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oe
~« Bee elise er :
233
until the original conduit was represented by five to six thou-
sand million little pipes, the friction which the pumping-station
would have to overcome would be very great. But little force
would remain in the water when it reached the smallest pipe.
Still greater is the resistance to the flow of blood, which is
slightly viscous, and contains solid corpuscles, which increase
friction. Two thousand miles of capillary tubing in the body
of a man, without reckoning the vessels of his liver and lungs !
Water is supplied to houses in rigid tubes. Arteries are
elastic, and their elasticity is self-regulating. The cause of
this will be apparent if a section of an artery is examined. It
~ Lining |
Z Fpitkelium
Perforated
Elastic Membrane
a Musele
Ss, Fibres
Zee Flastic Fibres
FE cut across
Fig. 12.—-A PORTION OF THE WALL OF A SMALL ARTERY CUT TRANSVERSELY AND HIGHLY
MAGNIFIED.
Its inner coat consists of a lining sheet of epithelial scales supported by connective tissue and
a strong elastic membrane. This membrane is perforated with holes which place the
lymph-spaces on its two sides in continuity. The middle coat is composed of plain muscle
fibres and patches of elastic membrane ; the outer coat of elastic fibres, mostly longitudinal,
and connective tissue.
contains much elastic tissue. It also contains plain muscle-
fibres. The smaller the artery, the greater is the amount of
muscle relatively to the other constituents of its wall. The
wall of a vein contains very little muscle, and not much elastic
tissue. The muscle of all arterial walls is in a chronic state of
tone. To some extent the degree of tone is varied automatically.
Pressure within an artery acts as a stimulus to the muscle-
fibres of its wall. Any increase leads the fibres to contract
more strongly. Any diminution induces them to relax. The
arteries resist distension; they do not narrow to any great
extent when pressure falls. But more important than this
automatic mechanism for maintaining a uniform pressure in
234 | THE BODY AT WORK
the capillaries in general are the changes of pressure in par- — ; a
ticular localities, brought about by the mediation of vaso-
constrictor and vaso-dilator nerves. In almost all organs and
parts of the body the automatic tone of arteries is enhanced
by impulses which flow continuously down vaso-constrictor
nerves. These impulses start from, or, to speak more accu-
rately, pass through, the vaso-motor centre in the medulla
oblongata. From every part of the body impulses ascend to
this centre, urging it to keep up the blood-pressure by universal
constriction. Yet no separate organ would be interested in
sending such a message if it were not open to it to ask at the
same time that the constriction of its own vessels might be
relaxed. Hence it may be said that every individual in the
community is crying out for universal economy, with more
generous treatment of himself. The response made by the
State to the latter part of his demand is in proportion to the
vehemence with which it is presented.
If the spinal cord of an animal be cut across near the medulla
oblongata, respiration being maintained by pumping air into
and out of the lungs, the heart continues to beat with un-
diminished force, but the pressure in the large arteries falls to
one-third of its normal height. Constricting impulses no longer
pass down the spinal cord from the vaso-motor centre. This
experiment also illustrates the truth of the statement that
models of the vascular system—arrangements of pumps and
indiarubber tubes—are more likely to mislead than to inform.
In an artificial schema the relaxation of the constriction of the
small tubes on the proximal side of the capillary vessels would
reduce friction. Fluid would reach the capillaries in larger
quantity, and pass through them more quickly. The pressure
in the tubes which represented veins would consequently ap-
proach more nearly to that on the arterial side. But when the
spinal cord is divided the pressure falls in the veins, as well as
in the arteries. This is due to another factor, and one of very
great importance in the regulation of the circulation. The
blood from the digestive organs is collected by the “ portal
system ”’ of veins. These do not join the inferior vena cava ;
they go to the liver, where they again break up into capillaries.
It is not until after this second distribution through minute
vessels that the blood is re-collected by the hepatic veins and
J 1 o- ay oe ee se Be i SES a. a a
wih See ee SS ae ie or
>
=
ey )
= URW CIRCULATION 235
_ forwarded to the heart. As in the case of the arteries, the
portal system of vessels is controlled by the nervous system.
When the spinal cord is divided they also dilate. The whole
vascular system becoming more capacious, blood-pressure falls
in veins as well as in arteries.
When the digestive organs are active, other parts of the body
are kept short of blood. It chanced to the writer, in his student
days, to spend the early summer in Paris, with a big healthy
Yorkshireman as companion. We dined together each night
at one of the restaurants of the Palais Royal @ prix fixe. After
dinner, with British regularity, my friend called for the Times.
Then followed a short period of placid reading, interrupted by
the remark : “ How cold it is!’ Half an hour later, giving
himself a shake : “‘ Suppose we go and dine somewhere else 2”
His well-ordered digestive organs had made short work of the
two-franc dinner. They had been ably supported by the vaso-
motor system of nerves which provided them with the bulk
of the blood, while limbs and skin ran short.
Vaso-constrictor nerves leave the spinal cord by the roots
(called “rami communicantes”’) of sympathetic ganglia.
Beyond the ganglia they apply themselves to the large arteries
whose course they follow. The constrictor nerves for the face
and neck leave the spinal cord within the chest by the roots of
the first four thoracic nerves. They do not at once apply them-
selves to the great artery of the head. Until the upper part
of the neck is reached, they traverse the ganglionated sympa-
thetic cord, which lies behind the carotid artery and internal
jugular vein. If in a rabbit this cord be cut, the vessels of its
ear dilate, as evidenced by the rosy blush which is observed
when a light is held behind it. If the upper part of the sympa-
thetic cord be stimulated, the ear grows pale. The redness of
the ear remains for many days after section of the nerve; but
gradually the engorgement diminishes, and the vessels acquire
the power of automatically regulating the flow.
The classical experiment with the rabbit’s ear suffices to
show the relation of bloodvessels and nerves which holds good
for all areas of the skin. The condition of the skin is the chief
factor in regulating the temperature of the body. In a cold
atmosphere its vessels are severely constricted to limit loss of
heat. When one passes into a warm room the constriction is
236
relaxed. The skin is flushed ; heat is thrown off by radiation.
The sweat-glands secrete water, which is evaporated by the
heat of the skin. Constriction and ‘remission of constriction are
the processes which diminish or increase loss of heat.
This mechanism is different in the case of glands and some
other structures which, when active, require an abundant supply
of blood. Such organs are provided with vaso-dilator in
addition to vaso-constrictor nerves. The most conspicuous
example of this is to be seen in the case of the submaxillary
gland. The nerve to this gland runs for some distance as an
isolated thread—the chorda tympani. Stimulation of the
chorda tympani has the double effect of dilating the arteries of ~
the gland and of causing it to secrete. But the administration
of atropin prevents secretion. Vaso-dilation is then the only
visible effect. Stimulation may increase sixfold the outflow of
blood from the veins of the gland. It rushes through with such
rapidity that it retains its bright arterial hue. The gland also
receives a twig from the sympathetic cord in the neck, which, as
already stated, controls the vessels of the face. By stimulating
the one nerve or the other the physiologist can at will increase or
diminish the amount of blood flowing through the submaxillary
gland. Stimulating any sensory nerve causes in a reflex manner
an increased outflow of constrictor impulses from the centre in
the medulla oblongata to all parts of the body, with the excep-
tion of the part to which the sensory nerve appertains. Its
own constituency receives an increased supply of blood. It is
not difficult to appreciate the importance of this double action.
A part is injured. The restrictions placed upon its supply of
blood are suspended. Lest its increased consumption should
lead to a general fall in pressure, all other parts have their
supply curtailed. The effect is even more pronounced than
this. The whole blood-pressure is raised above its ordinary
level. The flow of blood to the injured part is therefore greater
than it would be were relaxation of its arteries the only change.
The most important of all constrictor nerves are the
splanchnics which control the supply to the stomach and
intestines. When these nerves are cut, the digestive organs
become engorged to such an extent that a pronounced fall of
the general blood-pressure is the result. Their stimulation
renders the digestive organs anemic. We have already shown
re et ry ok .
: tion of vaso-constriction occurs in a reflex
The reflex relaxation of the splanchnic area is a
matter of great importance, because it can be brought about
by stimulation of one of the sensory nerves of the heart. The
higher the blood-pressure, the harder the heart would work if
left to itself. It is an impetuous organ, always trying to
quicken its pace and to increase the force of its beat. Ex-
cessive zeal would get it into trouble if severe precautions were
not taken to hold it in check. True, it is encouraged by certain
“‘ accelerator nerves ’’—sympathetic filaments which leave the
spinal cord by the anterior roots of the second and third
thoracic nerves ; but the influence which the accelerators exert
under normal conditions is not, it would seem, very pronounced.
The nerves which restrain the heart are much more in evidence
than those which urge iton. The arrangements for diminishing
the work of the heart are of two kinds. In the first place,
branches derived from the vagus act as a continuous check.
From a certain spot in the medulla oblongata, the cardio-inhibi-
tory centre, impulses are always descending to slow the heart.
They are of reflex origin, but a high blood-pressure in the centre
increases the facility with which they are transmitted. Some
of these stimuli originate in the heart itself, ascending and
descending the vagus nerve. The remainder come from various
sources. A severe injury to any part of the body slows the
heart. Injury to the intestines, such as occurs in peritonitis,
is particularly effective in increasing vagus inhibition. Slowing
of the heart lowers blood-pressure. When both vagi are cut,
the heart begins to gallop whatever may be the pressure against
_ which it has to work.
A sensory nerve of the heart, termed the “ depressor,”’ is the
chief agent in lowering ‘blood-pressure. Its course is not the
same in all animals, but it runs more or less in conjunction with
the vagus. Usually it joins its superior laryngeal branch. Im-
pulses which ascend this nerve inhibit the constriction of the
splanchnic vessels. They open a floodgate which brings down
the general pressure. The severe pain and extreme distress of
angina pectoris are the cry of the heart when blood-pressure is
too high—when it feels unable to work against it. This was
recognized by physiologists long before a remedy was known.
A systematic search was instituted for a drug which could be
té |
Probably even such figures as these would be thrown intel the
shade if we could estimate the minimum amount of human
effuvium which will enable a dog to follow his master’s trail.
Explanations have been sought in alterations in the vibrations —
of molecules of air caused by the presence amongst them of —
relatively heavy molecules of volatile substances ; but the diffi- —
culty of accounting for the generation of nerve-impulses in the —
sensory cells remains as great as ever. The hairs borne by ~
olfactory cells are so short that it is impossible that they should
project beyond the film of moisture on the surface of the mem-
brane. This seems to preclude an answering vibration. Yet
an increase in the thickness of this layer and in its density,
due to the presence in it of mucus secreted during a catarrh,
renders the sense-cells incapable of responding to odorous
particles.
Smell in an animal is not a test of the quality of the air it
is breathing, but a source of information as to the direction
in which it may seek its prey ; or, although far more rarely, as
to the direction from which the advance of a foe is to be feared.
Hunting animals depend for the most part on the nose. Hunted.
animals rely chiefly on the eye.
If we attempt to analyse our smell-sensations, we find that
we can pick out a number of varieties which appear so unlike
as to have nothing in common: Putrid meat, burning india-
rubber, sulphuretted hydrogen, ammonia, roses, onions, lemon
verbena, methylated spirit. Everyone can make for himself a
list of typical odours which seem to have specific qualities—
odours so distinct that he never confuses one with another.
He can also class together scents about which he is often un-
certain. The type-odours he can distinguish when present in
a mixture ; whereas odours which are less distinct reinforce or
modify one another. It has been found, by careful experiment,
that certain type-odours even tend to neutralize each other.
Musk and bitter almonds, for example, if present in small
quantities and properly proportioned, produce a very dim
sensation, whether supplied as a mixture to both nostrils, or
the one assertive odour to one nostril and the other to the other.
This last observation is of great importance. It proves that
their mutual destruction does not occur on the olfactory mem-
brane. It is not due to physical interference. The sensation
‘SMELL AND TASTE 367
of musk is delivered to one side of the brain, the sensation of
___ bitter almonds to the other ; but when attention is directed to
these two sensations there is found a quality in the one which
is irreconcilable with the quality of the other.
In certain persons and under certain pathological conditions,
sensitiveness to particular odours, or groups of odours, is
absent, while for the rest the sense is normal. Methylated
spirit, prussic acid and mignonette, constitute a group which
not infrequently drops out. Instances have also been reported
of persons unable to smell vanilla (to which some are hyper-
sensitive), and of others insensitive to violets, although
normally sensitive to the scents of other flowers. The notes
sounded in consciousness extend over a long gamut ; but there
are reasons for thinking that the number of keys on the clavier
which odoriferous substances strike is limited. Eleven is the
number provisionally adopted. The effect in consciousness
varies according as one key or another is struck, or several at
the same time with varying degrees of force.
- Many attempts have been made to associate the sensation-
qualities of the various odours with the chemical or physical
properties of their odorants, with but little success as yet. To
excite the sense of smell, a gas must be at least a little heavier
than air. No volatile body, it is stated, is so heavy as to be
odourless ; on the contrary, speaking generally, heavy molecules
are more stimulating than light. The quality of a smell-sensa-
tion would therefore appear to depend upon the period of vibra-
tion of the molecules of the substance which evokes it; but, as
already stated, a consideration of the apparatus which responds
to stimulation by odoriferous particles does not help us to an
understanding of the way in which the particles act upon it.
Taste is far more limited in its range of sensations than smell.
The back of the tongue is sensitive to bitters, the tip to sweets
and salts, the sides to acids. Mixtures of these qualities are
distinctly analysable by the sense of taste. Our sensations of
taste donotfuse. Slight differences in the way in which the
organs on the different parts of the tongue react to stimulation
enable us to recognize that a sapid substance is a mixture.
When, with a great flourish of trumpets, saccharin was intro-
duced as a safe sweetener for gouty people, an attempt was
made to provide them with saccharin-sweetened jam. The
suppression, followed by nauseating sweetness. The se
a
a
organs which subserve the sense of taste are clusters of fusiform. ,
epithelial cells, collected in “ taste-bulbs” (Fig. 26). Each a
gustatory cell bears a minute bristle, which projects through —
PSO Ori
[|
| | | hS CGR
rrr
Fic. 26.—HIGHLY MAGNIFIED SECTION THROUGH THE WALL OF A CIRCUMVALLATE PAPILLA OF
THE TONGUE, SHOWING TWO TASTE-BULBS.
i) <
These sense-organs are groups of elongated epithelial cells, set vertically to the surface. Their
cells are of two kinds—the one fusiform, slender, bearing each a bristle-like process which
projects through a minute pore left between the superficial cells of the general epithelium ;
the other thicker and wedge-shaped. Nerve-fibres are connected with the fusiform cells.
the pore left by the cells of the surrounding epithelium which
constitute a globular case for the bulb. As in the nose, eye,
and ear, a second thicker variety of epithelial cell is also present.
The nerve-fibres of the taste-bulbs are not, as in the olfactory
membrane, processes of their cells, but branches of the fifth
nerve which ramify amongst them. On the back of the tongue
taste-bulbs are much more numerous than elsewhere. They are
~~
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~
SMELL AND TASTE | 369
not as sensitive as the cells of the olfactory membrane ; never-
theless, they enable us to detect 1 part of quinine in 2,000,000
parts of water. |
- Sensations of taste and smell endure for a long time after
stimulation, because the odorous or sapid substance remains
in contact with the sense-organs. This accounts for the con-
fusion into which a man is thrown if he sip alternately port
and sherry. After a short time he cannot tell the one from
the other. The organs are quickly fatigued, using the term
loosely. How intolerable patchouli would be to the ladies
who use it were it otherwise! If for some time one sniffs the
odour of mignonette, it ceases to be recognizable ; whereas,
turning to a rose, the olfactory membrane is found to be as
sensitive as usual. When the sense is fatigued for a particular
smell, it is dull for others of the same group, thus affording an
opportunity of classifying smell-sensations according to their
qualities ; but the method is difficult to apply. Taste-organs
are greatly affected by temperature. Quinine is not tasted
just after drinking ice-cold water. Alcohol, ether, or chloro-
form paralyses the organs much in the same way. Castor-oil
slips down the throat unnoticed if the mouth, just before
swallowing it, has been rinsed with brandy or with a strong solu-
tion of tincture of chloroform.
Englishmen make but little use of their sense of smell. It
might teach them much regarding the various emanations
from putrid matter which are produced by bacterial action ;
but, dreading drains, they decline to cultivate proficiency in
the exercise of this sense. The nose is valued for the warning
it gives of “nasty smells,”’ but is not allowed to analyse them.
Burnt milk, soap-boilings, rancid oils, are taboo, because they
are associated with bungling in the kitchen. With moderated
ardour, we allow our sense of smell to distinguish foods and
beverages, but we are not a race of epicures. The perfumes
of flowers are classed as “ nice smells.”’ The idea of greediness
is not associated with their enjoyment ; besides, they remind
us of gardens, sunshine, pretty forms and colours. When
bottled, musk, orange-blossom, violets, lavender, are valued
not so much for their own sweetness, as for their singular
efficiency in obscuring nasty smells. Few persons practise
the recognition and distinction of even pleasant odours. Very
24
370 THE BODY AT WORK
few, on first coming across a scented herb or shrub, pay suffi-
cient attention to its perfume to impress it on their memories.
They note the shape of its leaves and the colour of its flowers,
but they are unable to identify it by its odour when they meet
with it again. It is not much to be wondered at, therefore,
that this slighted sense tends to leave us after middle life. It
has been asserted—and probably the statement is justified—
that rarely is the olfactory bulb of a man over forty free from
signs of atrophy. We have no statistics concerning the brains
of Japanese, who regard the sense of smell as one of the chief
avenues of pleasure ; but it may be that in this respect their
brains present a contrast to our own. Yet the deadening of
the sense is scarcely noticed, since its results are of little con-
sequence as compared with those which follow loss of sight or
loss of hearing. Many a man, as he grows older, declares that
the cook of his club has lost his cunning, or frankly asserts
that he “‘ no longer cares for kickshaws. Cold beef, beer, and
pickles, are good enough for him.” He little suspects that his
palate has lost its power of distinguishing the flavours of dainty
meats and wines. Others continue to be exacting, because
their imaginations still endow food with the qualities which
they remember, just as people eat preserved asparagus or
tinned peas because they look—however little they taste—like
the gifts of Spring.
Taste accompanies the reception of food in the mouth. We
have no knowledge of the situation of our own olfactory mem-
branes, and therefore we suppose that a flavour, whether it
be due to stimulation of taste-bulbs or olfactory membrane, is
in the mouth. The odour of a flower we mentally project
to a distance, because we associate the sight of a flower
with its perfume. A dog, able to judge the freshness or stale-
ness of a scent, must project its sensations of smell in the
same way in which we project our sensations of sight. It
forms an estimate, of a sort, of the time that it will take in
reaching the source of the scent. Its excitement increases as
the trail grows fresher.
Taste and smell are heavily laden with affective tone.
When disagreeable, the feeling which they evoke is near akin
to pain. It may gather head until, like hunger, it causes
the discharge of motor neurones; but under its influence
‘SMELL AND TASTE 371
food is ejected, instead of preparation being made for its
reception.
Taste and smell are senses which afford us no information
with regard to time or space. They give rise to massive sensa-
tions. Such sensations, devoid of detail, produce a frame of
mind rather than thought. The smell of tobacco does not
distract attention. On the contrary, the steady flow of im-
pulses to which it gives rise helps to inhibit, to subdue, the
yapping of more exigent sensations. And since sensations of
smell have no features of their own, they form a background
to sensations of other kinds, entering with them into memory.
No two scenes are exactly alike. One cannot recall another.
But the scent of syringa is always the same. Wherever
smelled, it opens the pathways in the brain in which were first
associated a June evening and syringa, with a scene and a
situation upon which memory loves to dwell.
24—2
CHAPTER XIII
VISION
THE eye is enclosed in a globe of fibrous tissue, of which the
front part, or cornea, being transparent, admits light. The
epithelial layer which covers the cornea, conjunctiva, is also
transparent. No bloodvessels enter these colourless tissues,
unless as the result of inflammation due to infection or to
exposure to sunshine or dust. For nutrition they are depen-
dent upon the plasma which, exuding from, and returning to,
the vessels which surround them, circulates in their tissue-
spaces. In advancing years, when the circulation is less brisk,
a ring of opaque tissue, arcus senilis, encroaches on the cornea.
In the interior of the globe, just behind the cornea, is a pro-
jecting shelf, formed of a ring of tissue supported by buttresses,
ciliary processes. It is continued inwards as the iris, a mus-
cular curtain. The “ hyaloid membrane ” lines the back por-
tion of the globe. Continued on the inner side of the ciliary
processes, it splits into several layers, which pass, one in front
of the lens, others to its edge, to which they are attached, and
still another, very thin, behind it. Since it holds the lens in
place, the anterior portion of the hyaloid membrane is known
as its ‘‘ suspensory ligament.’ Thus the eyeball is divided into
three chambers. The anterior is filled with watery lymph,
aqueous humour. In it, resting on the anterior surface of the
suspensory ligament of the lens, is the iris. The middle
chamber contains the lens. The posterior chamber is filled
with a liquid jelly, vitreous humour.
By the contraction of the circular fibres of the iris, the
aperture of the pupil is diminished, limiting the light which
enters the globe. This adjustment occurs when the illumina-
tion is bright. It is also brought into action for the purpose
372
VISION _ | 373
of cutting out divergent rays, which would not be clearly
focussed when objects near at hand are looked at. The
posterior surface of the iris and the inner surfaces of the ciliary
processes are covered with dense black pigment. It is this
pigment, showing through the uncoloured connective tissue
and plain muscle-fibres of which the iris is composed, that
gives their colour to grey and blue eyes. In many eyes the
iris contains a brown pigment in its substance.
Central Artery--°*" |
of Retina
Fic. 27.—HORIZONTAL SECTION THROUGH THE RIGHT EYE.
The slight depression in the retina in the axis of the globe is the fovea centralis, or yellow spot
the optic nerve pierces the ball to its inner or nasal side. The lens, with its suspensory
ligament, separates the aqueous from the vitreous humour. On the front of the lens rests
the iris, covered on its posterior surface wtih black pigment. On either side of the lens is
seen a ciliary process, with the circular fibres of the ciliary muscle cut transversely, and its
radiating fibres disposed as a fan.
The back portion of the globe of the eye is covered with a
curtain, the retina, formed by the spreading out of the fibres
of the optic nerve in front of various layers of nerve-cells and
the sensory cells of the organ of vision, rods and cones. The
retina lies between the hyaloid membrane, which encloses the
vitreous humour, and a layer of pigment which “ backs ”’ it,
as a photographer backs a plate when he proposes to use it
towards a source of light—to take a photograph of a window
from within a room. The serrated margin of the retina is
somewhat anterior to the equator of the eyeball. The pig-
ment which backs the retina is contained in a sheet of cells
, i oe hi, a Oe ee Se tee a ed 4 a.
of Se Soe ae ee ey ‘ =
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374 | THE BODY AT WORK |
which belongs to the pouch of brain that extended outwards 4
towards the eye-pit (p. 334). Properly speaking, therefore, it —
_ is a layer of the retina.
Three sets of tissues take part in the development of the
eyeball. (1) The epithelium covering the surface of the head
is depressed as a pit, which gradually closes into a hollow
sphere. This sphere, when its cavity is filled up, owing to the
great elongation of the cells of its posterior half, becomes the
lens. It breaks away from the rest of the epithelium of the
surface, which clears to transparency as that part of the con-
junctiva termed the “ corneal epithelium.” (2) The retina, as
already stated, is a hollow outgrowth from the interbrain.
i)
SS Pee.
@&
(oA
Qp ey
QAADAS
S
A B
Fig. 28.—DIAGRAMS SHOWING THE MODE OF FORMATION OF THE CRYSTALLINE LENS.
A, A pit in the epithelium on the surface of the head has closed into a hollow sphere. 8B, The
cells of the posterior wall of this sphere are growing forward, as the fibres of the lens
pag traverse its whole thickness, with the exception of the cubical epithelium on its
As this pouch approaches the lens, its anterior half is pushed
back into the posterior half, forming a cup with a double wall.
The anterior, or inner, sheet of the bowl of the cup develops
into the nervous layers of the retina, the posterior sheet into
its pigmented epithelium. (3) Connective tissues are trans-
formed into the other constituents of the globe—cornea, iris,
vitreous humour, etc. The globe is complete, except at a
spot on the nasal side of its posterior pole where the optic nerve
pierces it.
The bloodvessels of the retina, entering with the optic nerve,
ramify on its anterior surface. Under ordinary circumstances
we ignore the shadows which they cast, as we ignore the blind-
spot which coincides with the disc of insensitive tissue presented
ous te ae of the pea’ nerve, and many other imperfec-
tions ;. but it was shown by Purkinje many years ago that
by a very simple manceuvre they may be forced upon our
notice.
By making use of Purkinje’s figures, it can be proved that
the level in the retina at which undulations of light give rise
to the impulses which evoke visual sensations coincides with
the back of its anterior sheet—.e., with the layer of rods and
cones. A person stares fixedly at a white sheet in a dimly
lighted room while an assistant, by the help of a lens, focuses
Fig. 29.—PURKINJE’S SHADOWS.
A beam of light traversing the eyeball in the direction A throws a shadow of the vessel v,
lying on the front of the retina, upon the sensitive layer at its back. When the light is
moved from A to B the shadow moves from atob. The mind, supposing the shadow to be
a dark mark on the nearest wall or screen, infers that this mark moves from A’ to B’.
a strong light on the front of his eyeball, to the outer side of
the cornea. The rays, traversing the white of the eye, throw
shadows of the retinal vessels on the layers behind them ; but
this not being the way in which light normally enters the eye-
bali, the person experimented upon supposes that he sees the
shadows in front of him. He mentally projects them on to
the white sheet. The pattern of his retinal vessels appears on
the sheet in grey streaks. When the spot of light is moved, the
shadow-pattern shifts, and in the same direction; since,
376 THE BODY AT WORK
as the retinal image is reversed, a movement from right to
left is interpreted by consciousness as a movement from left
to right. Given the angle through which the light is moved,
and the apparent displacement of the shadows, it is a simple
matter to calculate the distance behind the bloodvessels of the
sensitive layer of the eye. So definite are Purkinje’s figures
that the shadows of individual blood-corpuscles can be fol-
lowed, and the rate at which they are moving in the capillaries
of the retina calculated.
The retina is the organ of vision. Cornea, iris, lens, vitreous .
humour, are parts of the camera in which this sensitive screen
is exposed ; and of the retina, the sensitive layer is the layer
of rods and cones. Interest therefore centres in these struc-
tures. They are disposed with the utmost regularity on the
posterior surface of a thin, reticulated membrane—the outer
limiting membrane. But rods and cones are only the outer
halves of sensory cells, the inner portions of which, reduced to
a minimum in thickness, except where they contain their
nuclei, lie in the outer nuclear layer. Rods are the larger
elements. Each consists of an outer segment, or limb, of
relatively firm substance transversely striated, and liable to
break into discs ; and an inner limb of much softer substance,
again divisible into two parts, the outer longitudinally striated,
the inner granular. Cones are almost identical in structure
with rods, save that their outer limbs are much smaller, their
inner limbs rather fuller. In frogs and various other animals,
but not in Man, each cone contains at the junction of its two
limbs a highly refracting globule of oil, often brightly coloured,
red, yellow, or green.
The layers in front of the rods and cones contain nervous
elements accessory to them. In the “inner nuclear layer ”
are the ganglion-cells of the retina, homologous with the cells
of the ganglia on the posterior roots of spinal nerves; but, in
the retina, bipolar and extremely minute. On either side of
the rather thick layer occupied by the nuclei of these ganglion-
cells (and of cells of other types which, for the sake of clearness,
we omit) is a felt-work of nerve-filaments in which their two
extremities arborize. The most internal, or anterior, layer
consists of a single sheet of rather large collecting cells and of
their axons, which stream towards the optic nerve. Each
—
377
Utne
Zep
NP NEN
\
(fenew
Fig. 30.—THE RETINA IN VERTICAL SEOCTION—A, AFTER EXPOSURE TO BRIGHT LIGHT;
B, AFTER RESTING IN THE DARK.
The arrow shows the direction in which light traverses the retina. C, Retinal epithelium, with
its pigmented fringe. 1, Layer of rods and cones, separated by the external limiting
membrane from 2, the layer of the nuclei of the rods and cones. 3, The ganglion cells
of the retina, which are homologous with the cells of the afferent root of a spinal nerve.
Their peripheral axons ramify beneath the sensory epithelium (rods and cones and their
nucleus-bearing segments), their central axons in 4, the inner molecular layer. D, Collect-
ing cells on the front of the retina; aaa, their axons which conduct impulses to the brain ;
b, an efferent fibre from the brain.
378 THE BODY AT WORK
cone has its proper ganglion-cell, collecting cell, and efferent
fibre. Rods are served in groups by ganglion-cells and col-
lecting cells. From this it may be inferred that a cone is a
sensory unit, an inference confirmed, as we shall show presently,
by direct evidence. The connections of the rods show that
they also are sensory elements, although it may be doubted
whether they are sensory units. The optic nerve contains a very
large number of fibres—about a million—all small, but some
distinctly larger than the rest. The largest very probably belong
to the collecting cells of rods. But the retina certainly does
not contain a million collecting cells. A considerable residue
of fibres is therefore unaccounted for. It is supposed that .
they are afferent to the retina, but we have no knowledge
regarding the nature of the impulses which descend from the
brain.
The retinal pigment is not merely a backing for the sensitive
screen. It undoubtedly plays an important part in vision.
That it is not essential is evident from the fact that albinos,
whose eyes appear pink owing to the absence of pigment, and
the consequent showing through of the blood in the exceedingly
vascular membrane which lies behind the retina, can see;
although their visual sense cannot be described as normal.
They are exceptionally sensitive to an excess of light. We
shall return to this subject after describing the differences in
manner of functioning which distinguish rods from cones,
differences so marked as to justify us in speaking of two kinds
of vision.
During twilight warm tones gradually fade out of the
landscape ; cold blues and greys predominate. A time arrives
when scarlet poppies look black, although yellow and blue
flowers and green leaves can still be dimly distinguished. In
full daylight colours are seen at their brightest in the high
lights ; where the light is dim they tend to appear in different
shades of grey. At night, if the sky is star-lit, all colours give
place to a slightly bluish grey in the high lights, black in the
shade. But a not very uncommon abnormality is night-
blindness—inability to see at all when the light is not bright
enough for the recognition of colours. In persons so affected
the rods do not function ; for it is with the rods that we see in
weak light. They record differences in intensity between the
.
VISLON “379
lower limit of their sensitiveness and the higher degree of
brightness, at which they are superseded by cones ; but they
afford no information regarding colour. Their monochrome is
interpreted by the mind as a bluish grey, apparently because,
since they are insensitive to red rays, the sensations of which
they are the source are associated with the blue end of the
spectrum. When the cones are stimulated very slightly, the
reinforcing grey of the rods enables us to distinguish all other
colours, save red, which appears black. In bright light
the rods are in a permanent state of exhaustion ; they do not
contribute to vision. Rods respond to stimulation more
slowly than cones. This fact enables us, by a very pretty
experiment, to distinguish the two kinds of vision. A disc
of green paper about the size of a threepenny-bit is pasted
on a red surface. Held at arm’s length in a room lighted
by a single candle, the disc looks dull green when the gaze
is directed at it; but if the gaze be directed 2 or 3 inches
to one side of it, it appears brighter than before, but less
distinct and almost grey. The explanation of this is to be
found in the fact that at the posterior pole of the eye there
is a shallow cup—fovea centralis—which carries cones only,
without rods. This small depression is the area of direct vision,
the only spot at which we see things quite distinctly. At the
fovea the nuclei and nerve-cells of the retina are withdrawn
from in front of the cones to the margin of the cup, in order
that they may not interfere with the passage of light. The
pit and the ring round it contain some yellow pigment. Hence
it is usually termed the “ yellow spot.” When we are looking
straight at the green disc, it is focussed on the yellow spot. It
then excites a sensation of greenness; but since this is not
reinforced by any rod-sensations, the green is dull. When it
is focussed outside the yellow spot, it stimulates rods and the
sparse cones which lie amongst them ; and the rods being more
sensitive than cones to light of low intensity, the disc looks
brighter. If, while the observer is still gazing fixedly at a spot
to the side of the disc, the red paper be waved rapidly, but
gently, to right and left, a brightish grey cover seems at each
movement to slip off the dark green disc, and to regain its
position a moment later, with a jump. The grey rod-sensation,
developing more slowly than the green cone-sensation, is, as
380 THE BODY AT WORK
it were, left behind. The two are separated at the moment
when the paper starts to right or to left.
Astronomers have long recognized that one of the smaller
stars which catches the attention when they are not looking
directly at it may be invisible when the gaze is directed to the
spot where it ought to be. It was visible when focussed on
rods, but it is not visible when focussed on cones. In most
birds the retina shows cones alone. To anyone who for the
first time enters a dovecote at night the experience is very
curious. A candle is for him a sufficiently strong illuminant,
but it does not give light enough to enable the pigeons to see.
Although evidently alarmed by the noise made by the intruder,
they allow themselves to be taken down from their perches —
without making any attempt to escape. If, startled by
the touch of a hand, they take to flight, they fly against
the wall. Pigeons are night-blind. The retina of an owl
bears chiefly rods, the outer limbs of which are exceptionally
long.
The outer limbs of the rods are coloured reddish purple.
This colour is quickly bleached by light. If a frog which has
been kept for a short time in the dark be decapitated, its
head fixed for ten minutes in a situation in which a window
is in front of it, then carried to a photographic dark-room,
where an eye is taken out by red light, opened, and the retina
removed, a print of the window will be seen upon it. Such an
optogram may be fixed by dipping the retina in alum.
The retina is easily detached from its pigment-layer. If it
has been bleached by exposure to light, it regains its “ visual
purple”’ when again placed in contact with its pigment.
Evidently the visual purple is renewed from the pigment which
lies behind (and around) the rods.
From the cells of the pigment-layer a fringe of streaming
processes depends amongst the outer limbs of the rods and
cones (Fig. 30). Ina dull light the processes hang but a short
way down ; in a bright iight they react almost to the outer limit-
ing membrane. They supply pigment to the rods, but their
relation to cones is not understood. It is clear, however, that
the cones, although they are not coloured, are dependent upon
the pigment-fringe, since they always remain in contact with
it. Their inner limbs elongate in the dark, lifting them to the
VISION 381
pigment, and shorten in bright light. These movements may |
merely indicate that the cones require a backing of pigment,
but it would seem more probable that, like the rods, they
absorb a substance which is sensitive to light, although we
cannot recognize it by its colour.
The responsiveness of the rods to light is due to visual
purple. As every lady is aware, colours, especially mauves
and lilacs, are bleached by light. The chemical change affected
by light in the colour of the outer limbs of the rods is the
stimulant which originates impulses in the nerve-fibres con-
nected with them, and it is generally believed that cones—the
more highly specialized sensory cells—are stimulated in the
same way. Visual purple is particularly abundant in all
animals that range at night, with the exception of the bat.
But its absence in the bat does not militate against the theory
that it is the cause of night-vision, for it has been shown that
a blind bat flies with almost as much freedom, and avoids
obstacles—even threads stretched across the room—with as
much skill as one that can see. It is guided by the bristles
of its cheek. So, too, is the cat, which has the reputation of
being able to see in the dark. Undoubtedly a cat’s eye is an
exceptionally efficient organ in dim light, just as it is excep-
tionally sensitive to sunshine—it is provided with an iris which
contracts the pupil almost to a pinhole—but the cat trusts
to the bristles of its cheek for information regarding the things
which block its path.
Most of the peculiarities which distinguish the reactions of
the eye from those of other sense-organs can be explained by
its mode of stimulation—the initiation of a nerve-current by
a chemical change. No stimulus, if sufficiently strong, can
be too brief. The retina reacts to an electric spark in the same
way as a photographic plate ; but, unlike the plate, the retina
is restored to its previous condition of sensitiveness in about
one-tenth of asecond. A visual sensation lasts about one-tenth
of a second. This prolongation of the sensation is, however, a
mental, not a retinal, effect. The mind continues to see an
object which has been illuminated by a flash until the retina
is again in a condition to send brainwards a second impulse.
Were our sensations coincident in duration with the stimulation
of our sense-organs, we should live in a flickering cinemato-
389 THE BODY AT WORK _
graph. When one is watching a moving point of light—the
glowing end of a match, for example—the prolongation of
sensation has its disadvantages; the moving point is inter-
preted as a streak of light. If the illumination be very
brilliant, the object seen may give rise to a prolonged after-
image. A glance at the sun leaves in the mind for seconds,
or even for minutes, the image of a glowing disc. Sensations
due to stimulation of the yellow spot last longer than those
which originate in the peripheral retina. If, in a train, one is
being carried at a certain pace, past a fence composed of up-
right palings, one sees the separate slats until the eyes are
directed towards them, when they fuse into a continuous screen.
The phenomena of negative or complementary images are of
retinal origin. The bright image of the sun, if the stimulus
has not been too violent, gives place to a black disc. If one
closes the eyes after staring at a window, a black surface crossed
by bright lines is seen in place of a white surface with dark
frames to the panes. If, after staring at a red surface, one looks
at the ceiling, a green patch is seen ; after yellow, blue. Every
colour has its complement, which may be determined in this
way. There is much uncertainty as to the exact terms in which
this phenomenon is to be accounted for, but little doubt as to
its being due to the peculiar mode of reaction of the retina to
light. Chemical substances which have been used up have
to be restored, and during the period in which they are coming
back to what may be termed a neutral condition the retina
delivers to the brain impulses of the opposite sign.
Contrasts which are experienced simultaneously are more
difficult to understand than those which appear successively.
In Fig. 31 the half of the grey cross which is surrounded by
black appears brighter than the half which lies on white paper.
A grey cross on a red background looks green; on a green
background, red ; on yellow, blue ; on blue, yellow. If green is
on red, it looks greener than if it is on white or black. These
simultaneous contrasts are seen best when the strength of the
colours is reduced by covering them with tissue-paper. It is
as if activity of any one part of the retina is accompanied by
activity of the opposite sign in the remainder. But it is unsafe,
in explaining our various sensations, to lay too much stress
on the mode of stimulation. The mind judges sensations in
VISION 383
the light of previous experience. In anatomical language, the
effect of sensations upon the personality depends upon the
paths which impulses follow in the brain, and the associations
which have been established by previous impulses which have
followed the same paths. The retina enables us to distinguish
tone and colour. By the variations in tone, the juxtapositions
of light and shade, we recognize form. All streams of impulses
which do not present tone-variations—do not, that is to say,
reproduce the details of a scene—are interpreted in terms of
colour. Every child discovers that the tedium of the intervals
during which it is proper that his eyes should be closed may
be relieved by pressing his knuckles against the lids. Although
Fic. 31.—SIMULTANEOUS CONTRAST.
The shading of the two V’s is exactly similar ; but the figure in half-tone on black appears
brighter than the figure in half-tone on a white ground.
the world is shut out, a phosphene offers itself for his considera-
tion—a yellow or white disc of irregular form with a red
margin, changing into lilac bordered with green, and then into
yellowish-green with a blue edge. Such, if my recollection
can be trusted, were the pictures which I used to see as a boy ;
but no adjustment of pressure calls them forth with anything
like the same vividness now.
All the senses show a tendency to rebound after activity,
exhibiting contrast-phenomena ; but the contrasts of vision are
more marked and varied than those of the other senses, as
everyone who is curious in the observation of his own sensations
is aware. Negative after-images are generally referred to the
retina ; but various other kinds of after-image and contrast-
phenomena must be attributed to the judgments passed by the
\
384
mind upon the sensations which it receives ; and not to physical
changes in sense-organs. Positive after-images are well-marked
appearances, although less common, perhaps, than the phe-
nomena of reversal of sensation of which we have just written.
On waking in the morning, one looks at the window ; shifting
the gaze to the ceiling, an after-image of the window appears,
just as one saw it, with bright panes and dark frame. The
‘dark adapted eye,” being exceptionally sensitive, yields the
same persistent positive after-image as the eye in its usual con-
dition yields, after being directed towards the sun at mid-day.
Movement-after-images can be explained only by referring
them to misdirection of judgment. If the gaze is fixed on a
rock close beside a waterfall, then shifted to a bank covered
with grass or bushes, the part of the bank which occupies the
lateral part of the field of vision appears to rush upwards,
reversing the movement of the water. When the gaze has
been fixed upon falling water—a narrow stream sparkling in
sunlight—a central strip of the field moves upwards, the
margins remaining stationary. If one stares at the spot on the
surface of a basin of water on which drops are falling from a
tap, and then looks at the floor, it is seen to contract towards
the spot looked at, reversing the movement of the ripples in
the basin. These observations reveal a fact of great import-
ance in the physiology of vision. It is, probably, impossible
truly to fix the gaze. The muscles of the eyeball keep the
retinal field in constant movement—larger movements with
minute oscillations superposed. When, as in watching a
waterfall, movement has for a time taken a definite direction,
its cessation is judged to mean reversal.
The anatomical unit of sensation is a cone. The fovea
centralis, the only part of the retina capable of receiving sensa-
tions sufficiently discrete for reading, contains cones alone.
If the gaze be directed but a very few millimetres on to the
white margin of the page, letters lose their form. In the fovea
the centre of one cone is 3-6 » distant from the centre of the
next. Two stars are visible as separate stars if they subtend
an angle of at least 60 seconds with the eye. Their images on
the retina are then 4 » apart. Parallel white lines ruled on
black paper, held at such a distance as causes them to subtend
angles of 60 seconds with the eye, appear not straight but
‘VISION © 385
wavy, showing that their images are taken up, not by a con-
tinuous substance, but by the mosaic of cones. So far the
explanation of the visual unit is strictly anatomical; but it
must be added that trained observers can recognize the
separateness of objects which subtend angles of much less
than 60 seconds—not more than 5 or 6 seconds. This can be
accounted for only on the hypothesis that images far closer
together than the width of a cone produce a specific effect in
passing across the anatomical unit.
In 1807 Thomas Young, the physicist, formulated a theory
to account for colour-vision. He supposed that the retina
contains three kinds of apparatus—a, 6, and c—each especially
responsive to a particular kind of light, all three slightly
stimulated by rays of all colours. (Young imagined three
kinds of nerve, but modern supporters of his theory suppose
three different substances chemically changed by light.) A
prism spreads out the rays which are combined in white light
into a band in the order of their wave-lengths—those which
have the longest wave-length (0-8 ») and the slowest rate of
vibration (381 billions to the second) at one end, those which
have the shortest wave-length (0-4 ») and the most rapid
vibration (764 billions to the second) at the other: between
these two extremes every intermediate grade of length and
rapidity. These are a mere fraction—a small group—of the
waves which the ether transmits, but they are all that we
can see. The long, slow vibrations give rise to sensations
which we describe as red; the short, rapid vibrations we
describe as violet. Our names for the tints which intervene
are singularly old-fashioned and unsatisfactory, but all persons
agree that they recognize in the spectrum a certain number of
definite colours. Some normal-sighted persons say twelve,
others eighteen. It is largely a question of terminology.
Many considerations show that it is quite unnecessary to
imagine that the retina is affected in a different kind of
way by every kind of light, or by each ,of several groups
of waves. If the red of the spectrum is mixed with yellow,
we receive an impression of orange, which is identical with the
impression produced by waves of the mean length of red and
yellow ; orange and green give yellow; yellow and blue, green.
Any two complementary colours yield white. By taking three
25
duly mixed, not white light a but light 0 of any obbee ti,
although not of spectral purity, since it is mixed with whites
Young considered that all the conditions of colour-vision would —
be satisfied, all our various sensations provided for, if the
retina contain three kinds of apparatus which light, according ~
to its quality, affects in varying degrees ; and with this theory
of three kinds of apparatus—a, 6, and c—the theory of three
elementary or fundamental colour-sensations is indissolubly
linked. The colour x produces its intensest effect when a is
stimulated, with the least possible stimulation of b and c; y is
the reaction of 6, z of c. Recent studies of the curves of
intensity give us the tints of 2, y, and z as carmine-red, apple-
green, and ultramarine blue.
The blending of sensations is illustrated with the well-known
colour-top. But perhaps the most striking proof that three
elementary colour-sensations are adequate to produce our
visual world is afforded by photographs taken with the three-
colour method. Three plates are exposed—(a) behind a red
screen, (6) behind a greenish-yellow screen, (c) behind a blue
screen. ‘They are fixed in such a way that the portions acted
upon by light are rendered insoluble, whereas the rest of the
film can be dissolved away ; a is then stained red, 0 greenish
yellow, c blue. The three are superposed, and the result
appears to the eye as an exact reproduction of the subject
of the photograph in all its hues. It shows every shade of
orange and green and violet. It is as bright—that is to say,
as full of white light—as the original.
Various objections may, however, be brought against
Young’s theory. Of these, the most weighty are: (1) The retina
does not contain three kinds of apparatus, as Young sup-
posed; nor can we find three kinds of photochemical substances,
as required by the theory in its modern form. If we could
find them, a fresh difficulty would arise; for we have no
reasons for supposing that one and the same nerve-ending can
receive stimuli of three different kinds. (2) The theory offers
~no explanation of negative after-images—the complementary
colours experienced when the eye is closed after staring at a
brightly coloured object. (3) It does not adequately account
for the various deficiencies of colour-blindness.
VISION 387
It is well recognized that there are various degrees of colour-
blindness, and that the colour-vision of persons considered
normal presents different grades of refinement. Nevertheless,
- the abnormalities of colour-blind persons are so marked that
cases fall into definite classes. Those whose cones do not
function—which means that their yellow spots are either
undeveloped or diseased—see all things grey. They are totally
colour-blind. Excluding these, the colour-blind may be
grouped in one or other of two divisions—(a) those who confuse
red and green, (b) those who confuse yellow and blue. One
person out of every thirty-five is red-green blind. The pro-
portion is even higher if males only are considered, showing
how very unfortunate is our choice of warning signals. A man
who is red-green blind cannot tell the port from the starboard.
light. Blue-yellow blindness is, on the other hand, extremely
rare. According to Young’s theory, colour-blindness is due
to the absence of one of the three sets of visual apparatus.
But cases do not altogether conform to this hypothesis. We
knew an amateur water-colourist, since deceased, who derived
intense pleasure from the beauties of Nature, and showed no
mean skill in reproducing them with his brush, notwithstanding
the fact that he was red-green blind. Each night his sister
arranged his paint-box for him, and only rarely did he use
vermilion to fill in a foreground of lush green grass. But this
mistake, when he made it, did not destroy his own satisfaction
in the picture. It was clear that red had a value for him,
although he confused it with green. It is impossible for a normal
person to see through the eye of one who is colour-blind, and
there is no other means of comparing his sensations with our
own. The mistakes which the colour-blind make in sorting
coloured objects and in naming mixtures of light selected from
various parts of the spectrum show the range of their de-
ficiency, but give us no information regarding the qualities
of the sensations which they retain.
The test of colour-sensitiveness usually employed is the
grading of a large number of wools of different tint. The order
in which the colours should be arranged is not a matter of
opinion. They must be placed in the order in which they
occur in the spectrum—.e., arranged according to their wave-
lengths. In the cases of colour-blindness which are most fre-
25—2
388
quently met with the defect may be described as due to an
absence of the sense of redness, or as an absence of the sense
of greenness. The two conditions can be distinguished. But
since the eye is not dark for red (although in certain cases
vision is very weak for the red end of the spectrum) or dark
for green, the abnormality cannot be adequately accounted
for on structural grounds. It is not explicable on the hypo-
thesis that one of three sets of responsive sense-organs (or
nerve-fibres) or photochemical substances is absent from the
eye. Again, it is generally agreed that the sensations of white,
yellow, and blue of the red-green colour-blind are similar to
those of normal persons. This is not in harmony with the ~
theory of the omission from their eyes of one of three pieces of
colour-apparatus.
Professor Hering, of Leipsic, adopting the generally accepted
view that light effects chemical changes in substances contained
in the retina, to which changes stimulation of nerve-endings is
due, formulated a theory of colour-vision which many physio-
logists prefer to Young’s. He imagines that the retina con-
tains three kinds of pigment, each of which is, as he believes
all living substance to be, in a constant state of change. It
is at the same time being built up and destroyed. Using the
terms which connote the opposite directions of metabolism,
the pigment is simultaneously undergoing anabolism and
katabolism ; the two processes, when the retina is at rest,
maintaining equilibrium. When light acts upon either of the
substances, it hastens, according to its quality, either the one
process or the other ; and the chemical change, whether it be
constructive or destructive, stimulates the endings of optic
nerves. Hering assumes, therefore, that there are six elemen-
tary qualities of visual sensation—red, green, yellow, blue,
white, black. Red, yellow, white are due to anabolism of the
visual substances ; green, blue, black are due to their kata-
bolism. The installation of yellow amongst the unanalysable
colours is a relief to many minds. It is almost impossible to
think of yellow as a compounded colour. White also, we feel,
is not a compounded colour, despite our knowledge that a prism
scatters from it all the hues of the rainbow. Black, many per-
sons assert, gives them a definite sensation, and not merely a
sense of rest. (Parenthetically, it may be observed that the
389
feeling that a colour is pure or mixed is not to be trusted. It
_ may be based upon the chromatic aberration of the eye, or it
may be reminiscent of the paint-box. We know that we cannot
make yellow by mixing red and green pigments, hence we feel
that it is pure. Of green we are not by any means sure ;
gamboge and Prussian blue come into our minds.) Except
when the light which falls upon the retina is giving rise to one of
the four pure colour-sensations, all three substances are simul-
taneously affected, although one may be undergoing katabolism
while the other two are being built up, or vice versa. Hering
accounts for simultaneous contrast by assuming that the
activity of any one part of the retina induces an opposite kind
of change in the remainder, and especially in the vicinity of
the primarily active part. When a certain patch is developing
a sensation of red, the rest of the retina develops a sensation
of green.
The great merit of the theory is, however, to be found in its
offering an explanation of complementary after-images. The
green patch seen with closed eyes after one has stared at a
red object is due to the rebound of metabolism. In returning
to a condition of chemical equilibrium the retinal substance
acts as a stimulant which evokes the antagonistic colour. But
it is a theory which makes very large assumptions. It assumes,
for example, the possibility of the existence of a substance
which is built up by light from one end of the spectrum, and
decomposed by light from its centre. Not that Hering regards
the existence of three retinal substances as essential to his
theory. He is prepared to transfer to the brain the seat of the
substances, or the substance, which, by their, or its, anabolism
and katabolism, produces antagonistic colour-perceptions ; but
in this he is abandoning physiology for metaphysics. We have
no warrant for imagining that there exists in the brain any
substance which, by undergoing physical changes of various
kinds, produces various psychical effects. The problem to be
solved is physiological. Rays of light of different wave-
lengths excite the retina to discharge impulses which are
variously distributed in the brain. The effects which they
produce in consciousness depend upon their distribution. The
impulses to which the longest rays give rise evoke sensa-
tions of red, those due to the shortest, sensations of violet.
390 “THE BODY AT WORK —
And what is true of the retina as a whole is true, apparently, of —
each individual cone. In what way does light act upon a
cone? It is one of the most fascinating problems in physi-
ology. Round it our thoughts revolve whenever we are trying
to form conceptions of the nature of stimulation, sensation,
and perception. Each of the two theories which we have ex-
pounded above helps to group together certain of the more
striking phenomena of colour-vision, but neither gives a satis-
fying explanation of their causation.
The sensitiveness of the retina is in a remarkable degree
adjusted to the intensity of the light. When a dark room is
entered, the pupil dilates ; but one’s power of distinguishing
objects continues to increase after the pupil has reached its
maximum size. At the end of ten minutes the eye may be
twenty-five times as sensitive as it was when the room was
entered. This adaptation to darkness is due in large degree to
the substitution of rods for cones as the organs on which vision
chiefly depends. But it cannot be wholly due to this, since
it occurs when one is working with a red light. Probably the
red used in a “ dark room ”’ is not sufficiently near the end of
the spectrum to be completely without influence upon visual
purple, but it is a colour to which rods are comparatively
insensitive. Other evidence also points to an adaptation of
cones as well as of rods.
Accommodation of the eye for distance is brought about by a
mechanism which allows the lens to change in shape. It
becomes more convex when a near object is looked at than it
was when adjusted for an unlimited distance, which is its con-
dition when the eye is at rest. Adjustment for near objects
involves muscular action, and is accompanied by a sense of
effort, however slight. Whilst the eye is at rest the lens is
mechanically compressed against the anterior layer of its sus-
pensory ligament. Accommodation for near vision is effected
by the ciliary muscle, which is placed in the shelf of tissue which
projects into the interior of the eyeball. This muscle is made
up of a ring of circular fibrés, and to the outer side of this,
of fibres which radiate backwards and outwards. The longi-
tudinal, or radiating, fibres obtain their purchase by attach-
ment to the firm wall of the globe just beyond the cornea.
They spread into the front of the loose chorioid membrane
io, ss 1 iP “ et ‘
- >
A
¥ Vv
B
Vv
y as
y eee
re
L \
(eat i Ge eas
NY ro ‘4 eS
Fig. 33.
A The normal eyeball, in which, when the ciliary muscle is relaxed, parallel rays are brought
to a focus on the retina.
B, A hypermetropic eyeball. Its depth being less than normal,
parallel rays are not brought to a focus on the retina when the eye is adjusted for distant
vision without the aid of a convex glass. C, A myopic eyeball. Its depth being more
than normal, a concave lens is needed to diminish the convergence of parallel rays.
or may accompany insufficient length or too great length of
the optic axis.
It is due to unequal curvature of the cornea.
Usually the curvature is sharper in the vertical than in the
horizontal meridian (cf. p. 269) ; as a consequence, points in a
vertical line are focussed in front of points in a horizontal line.
:
é
fal)
eat Pei,
be en 2° ee
— — /
VISION | | 393
Cylindrical glasses, not lenses, are required to correct this
defect. And here it may be well to call attention to the fact
that rays of light are more sharply refracted by the surface of
_ the cornea than they are by the crystalline lens. The lens has
a high index of refraction (1-45), but it does not lie in air (the
index of refraction of which is 1), but between two humours
which have about the same index as water—namely, 1-336. The
bending by the combined action of the cornea and the lens of
rays of light which come from a source so distant that they may
be considered as parallel brings them to a focus on the retina,
when the lens is at its flattest. When the lens is at its roundest,
rays which diverge from a point only 5 inches in front of the
eye are focussed on the retina. The lens is therefore essential
for accommodation, but, after its removal for cataract, vision,
even for near objects, is rendered possible by the use of convex
glasses.
A star or a distant gas-lamp is seen as a point of light with
rays. Usually this figure, which has given origin to the
expression “ star-shaped,” shows three greater rays alternating
with three lesser rays. Such an image is not produced by a
point of light near to the eye, since it is due to the puckering
of the lens when flattened against its ligament. It brings
into evidence the three axes on the front of the lens and the
three axes which alternate with them on the back, with regard
to which the lens-fibres are disposed.
As an adaptation of living tissues to optical purposes the
eye is above admiration, yet it presents many defects, which
an optician corrects in the instruments which he manufactures.
A remarkable fact in the physiology of- vision is our uncon-
sciousness of the imperfections of its organ. An unusual
experiment is needed to bring them to our notice. If we look
through a common glass lens uncorrected for unequal refrac-
tion of rays of different wave-lengths, we recognize that a
bright object is shown with a colour-fringe, yet we take no
cognizance of the colour-fringes which surround the images of
all bright objects focussed upon our retine. If we think about
the matter, we recognize a feeling that blue in a window of
stained glass appears farther away than red; but this might
well be due to association. Blue glass is chiefly used for the
sky. If we look at a bright object through purple glass, we
394 THE BODY AT WORK
see it either red with a blue fringe or blue with a red fringe, —
according as the eye is focussed for red or for blue. The
purple glass having absorbed all intermediate rays, we become
aware that we cannot focus the two extreme ends of the
spectrum at the same place. Since a greater effort of accom-
modation is needed to focus red, we judge that the bright
object is nearer to us when it appears red than when it appears
blue.
Spherical aberration is another fault of the lens. The rays
which enter its margin are brought to a focus sooner than those
which pass through its centre. This is due to the fact that
its surfaces are regularly curved, whereas a glass lens is cor-
rected by grinding it flatter towards the margin. This defect
is partly corrected by the cornea, which has an ellipsoidal
surface, and partly by the greater density of the centre of the
lens. Yet it is still necessary for the eye to be “stopped
down ”’ by the iris when a near object is looked at, although
less light is entering the eye than when it is directed to the
horizon—a condition which would lead a photographer to open
his iris-diaphragm.
Of all the imperfections of the eye which the mind ignores,
the most remarkable is the gap in the field of vision, due to the
gap in the sensitive layers of the retina, which occurs where
the optic nerve enters it—the blind spot. Hold this page of
the book 10 inches from the face, keeping the lines of print
horizontal. Close the left eye and look at X with the right
eye. The black disc disappears, because its image is focussed
of
on the blind spot. Since the picture on the retina is reversed,
it is clear that the optic nerve enters the globe to its inner side,
and slightly above its horizontal meridian. But, unless we
employ an unusual test, we are quite unconscious of the fact
that a definite hole is punched in the picture. The mind fills
it in, and the way in which it does so is extremely suggestive.
VISION 395
It lies about it—in a downright ingenuous fashion if it is con-
fident of credence, in a more subtle way if a simple falsehood is
_ likely to be challenged. In place of the black disc make nine
conspicuous crosses:
ay ee
aX
>. ae, Sea 4
Hold the paper in such a position that X falls upon the blind
spot. It ought to disappear, but the mind assures you that
there is a cross at that spot. The mind completes the field.
In place of the crosses use noughts and crosses, thus :
O X O
», ee. Cn». <
O X O
Now let X fall on the blind spot, and allow the eye to go just
a little out of focus. The four marginal crosses draw inwards :
O O
xX
xX
O O
The mind contracts the field. Still denying the gap, but not
having sufficient data from which to invent an object, the
fraudulent nature of which would not be found out the instant
that the gaze is shifted, the mind lies regarding the position on
the paper occupied by surrounding objects.
Is it quite fair to the mind to say that it lies about the
blind spot? The mind judges sensations in the light of
experience. An association of previous sensations teaches me
that the wall of the room is not pierced by a round hole a foot
in diameter opening into outer darkness. Many sensations
396 THE BODY AT WORK _
have discovered to me the fact that the designs on a wall-
paper succeed one another with unbroken regularity. Fixing
my gaze on one of them, I cannot by any effort of attention
efface the pattern which happens to be focussed on the blind
spot. I know that I shall see it the instant that I move the
eye. If I let my eye roam until the face of my wife falls on
the blind spot, its image disappears. I know its lineaments
far better than I know the pattern on the wall-paper, but I
cannot fill it into the picture. Her hands are visible, and the
work which is resting in her lap, but in a mysterious way the
background draws together where the face should be. My
mind refuses to pass a false judgment; but it also refuses to
see that there is a gap.
This exceedingly instructive observation teaches the rela-
tivity of sensations. It shows that a sensation has no objec-
tive value until judgment has been passed upon it by the
mind. The meaning of this we express in figurative language,
none other being available. We speak of a new sensation as
being compared with sensations previously received—taken -
into the picture-gallery of the mind, and placed in its due
position amongst the infinitely numerous records which are
stored there. If we try to make a nearer approach to cor-
relating physical with psychical activity, we say that sensa-
tion has no value save that which it acquires from its temporal
relation in the sequence of sensations to which attention is
directed, and that this value depends upon the relation which
similar sensations have possessed in former sequences. There
is no gap in binocular vision. An object focussed on the
inner (nasal) side of the right eye, where the blind spot is
situate, is focussed on the outer (temporal) side of the left eye.
The left eye sees the object to which the right eye is blind.
Since we have almost invariably used two eyes in the past,
experience teaches that there is no gap in the field of vision.
Hence the new group of sensations which alleges that there is
a gap must be corrected. The field must be filled up in the
way which experience shows to be most likely. The retina is
a sheet of rods and cones, each of which has a nervous con-
nection with the brain proper to itself. The retinal field
is associated with the brain-field. But this does not imply
that we may think of the mind as having a spatial distribution
ie en o
‘ Pes oe <= oe 4 Per oe
VISION : 397
in the brain. Pressure on button A or button B in the retina
causes bell A’ or bell B’ to ring in the brain, but it does not
follow that perception A” or perception B” will be heard in
the mind. It will be heard if this is the association established
by custom, since mind is the product of experience. But the
new sensation is creating precedent as well as being judged
by it.
Point A in the right retina is associated by experience with
point a in the left, and point B with 6. These are termed
corresponding points, because they are similarly stimulated in
binocular vision. ‘The mind, therefore, judges that it receives
the same information from each pair of corresponding points.
The position of corresponding points will be understood if the
right retina is imagined as put inside the left, precautions being
taken to make the yellow spots coincide, and to avoid twisting
the retinal cups in taking them out of the eyeballs. Great
care is taken to maintain the points in correspondence during
the various movements of the two eyeballs. In addition to
the four recti muscles which move the eyeball upwards, down-
wards, to right and left, two oblique muscles give it the re-
quisite amount of rotation. We have learned to give the
same value to the impulses from two corresponding points.
But under changed conditions the correspondence changes.
When a squint develops in childhood, it follows one of two
courses ; either the obliquity of one of the eyeballs increases
until it looks towards the nose, and its images cease to inter-
fere with the images in the dominant eye—they are ignored by
the mind—or a fresh correspondence is established between
points in the oblique eye and points in the eye which looks
straight forward. If we are severely critical, we find, from a
study of the form of the eyeball, that it is impossible that the
same rods and cones should occupy corresponding points in
different positions of focus and with different degrees of con-
vergence of the eyeballs. To permit of this the retinal cups
would need to change in shape. But again mechanical cor-
respondence is of little consequence. In the light of experience
the mind judges that points correspond. When we are gazing
at a flat surface, the mind judges that corresponding points are
giving it similar information. It does not see a flower on a
wall-paper twice as bright or twice as red with two eyes as
with one. If the eyes are normal the ae or
solid objects, the image on one retina is not the same 2 the ,
image on the other. One eye sees farther round the object on —
the one side, the other on the other; and it is just this dis- —
parity in the pictures, aided by the feeling that the eyes are
converging, that gives the impression of solidity. Correspon-
dence of points, on the other hand, is not necessarily sufficient
by itself to convince the mind that the pictures presented by
the two eyes are identical. When a flat triangle such as this
is regarded with the two eyes, its black lines fall on correspond-
ing points ; but the figure is associated in the mind with other
sensations—sensations of movement and touch. Notwith-
standing the identity of the retinal images, the mind tries to
see them as disparate. The figure troubles the eyes. At one
moment the meeting-point of the three central lines projects
forwards, at the next it recedes. That similarity of retinal
images counts for something is shown by closing one eye.
The uncertainty of shape of the figure is rendered more trouble-
some. It changes still more rapidly from convex to concave.
When the point seems to be in front of the page, the accommo-
dation of the eyes is adjusted for nearness ; when behind the
page, for greater distance. But the illusion that the object
occupies three dimensions is not dependent upon the sense of
contraction of the ciliary muscle. When the paper is moved
towards the eye, its centre recedes ; it is left behind until the
ciliary muscle has had time to contract. When it is moved
away from the eye, it projects until the ciliary muscle has had
time to relax. Accommodation follows judgment, not judg-
399
_ the veracity of its newsagents. Disparateness of images, con-
vergence of the eyeballs, shifting of accommodation for the
various levels of an object in space, should be indisputable
evidence of solidity or of hollowness. Conversely, the absence
of either factor should be conclusive proof of flatness. But
the mind does not trust to isolated sensations ; it looks for —
associations of sensations. When the finger hints, “I could
touch that sharp point,” it is useless for the eye to aver that
there is no point to be touched.
If two exactly similar photographs are placed in a stereo-
scope, the fact that the eyes are not converged gives to the
common picture an appearance of depth, notwithstanding the
fact that corresponding points on the two retine are stimu-
lated. If the two photographs have been taken, as they
should be taken for this purpose, with a double camera, the
disparity of the retinal images immensely enhances the impres-
sion of solidity.
It is impossible to exaggerate the dependence of sensation
on judgment. At birth a child commences the long process
of education which enables it to associate the sensations
derived from its retinal images with the movements which
place it in contact with things. It discovers that, when it is
necessary to make the eyes converge, the object is near at
hand. It also associates the voluntary action of contracting
its ciliary muscle with nearness. Unconverged and unaccom-
modated eyes come to mean distance. So, too, do indistinct-
ness due to absorption by the atmosphere, blueness due to the
same cause, a small image on the retina. But there are
obvious limits to its power of ascertaining the distance of an
object, and therefore, conversely, of its power of estimating
size. We have no idea of the size of the retinal image of the
sun. Very few people would be prepared to believe that the
angle which the sun subtends with the eye barely exceeds half
a degree. (The first finger, viewed in profile, at arm’s length,
covers one degree of arc.) A disc of paper of the right size,
placed at the right distance, looks far too small to represent the
sun. The most brilliant of orbs bulks larger than this in our
minds. Everyone who for the first time looks at the sun
through well-smoked glass, or, better, through a flat-sided
400 _ THE BODY AT WORK
vessel filled with ink and water, is astonished that it looks so —
small. Nor are we prepared to accept the evidence of a
camera that the sun at the zenith does not produce a smaller
image on the retina than the sun when rising above the horizon.
Yet if a photographic plate is exposed to the rising sun, and
again, without changing its focus, to the sun at the zenith, the
two images are practically equal. There is a slight difference
due to the greater refraction of rays passing tangentially through
the atmosphere, but it is so slight as to bear no relation to the
difference between our two judgments of size. When the sun is
rising behind trees and houses, we compare it with objects which
we know to be large and distant ; yet it looks almost as large
when rising out of the sea. One of the causes of the illusion is
our conviction that the sky is flattened ; and this, again, is due
partly to its paler tint—its less substantial blueness—near the
horizon, and partly to our impression that it is spread out
over a flat earth. When the sun is in what we deem to be
the more distant part of the vault of heaven, we judge it to be
farther from us, and therefore larger than when it is above us.
Yet the last word has not been said in explanation of a pheno-
menon which has been studied by mankind since the dawn of
science. Helmholtz attributed the apparent greater distance,
and consequent greater size, of the sun and moon when near
the horizon to the indistinctness of their discs. When its
image is so reflected from the zenith as to cause the moon to
appear to rest upon the horizon, it does not, he said, increase
in size. In answer to Helmholtz’s explanation, it may be
objected that, when at midnight he brought the full moon
down from the zenith, he did not bring with her the conditions
of light and colour by which she is customarily surrounded
when floating on the horizon. If, when watching the moon
which has just risen, vast in diameter, out of the sea, one
interposes between it and the eye a sheet of paper in which a
small hole has been made, and looks at the moon with one eye
through the hole, it instantly shrinks to the size which it
appears to have at the zenith. It is not even necessary to
blot out the whole of its trail of light on the sea. At the same
time, it appears to retreat to a great distance. This shows
how complicated are the associations upon which judgments
of size and distance are based, and to how small an extent
are de termi | LAER
obs rvation is aes surprising if made one or two nights
full 1 moon, when twilight is already dim at moon-rise.
Our estimate of the distance away from us of an object on
t is based upon the time and effort which experience
“tells us we should need to spend in reaching it. The untried
_ appears shorter than the tried. Anyone who compares his
feeling of the number of yards he would have to climb up a pole
reaching to the zenith with his feeling of the number of steps
he would need to take to reach the horizon will recognize that
the horizon appears to him to be the farther away.
ee vat
Fig. 36.—A SYMMETRICAL ARCH, DIVIDED BY A VERTICAL LINE, A, WHICH PASSES THROUGH
Ivs APEX.
In representing a solid object an artist conveys the idea that
light is falling obliquely upon it. One side of the object,
therefore, is more strongly illuminated than the other. By
depth and gradation of shade he indicates.the extent to which
the thing projects forwards, if solid, or falls back, if hollow,
He makes the margin of a ball hazy, in the expectation that
the spectator will look at the spot nearest to him—an artifice
which he may easily press too far, since the eyes wander rest-
lessly over a flat surface. In representing distance he is
dependent upon giving to the various objects in his picture
sizes equivalent to the sizes of their images on the retina,
making them brighter or paler and more or less distinct. Yet
he cannot hope to simulate the convincing evidence of distance
which is afforded by our sense of the degree of convergence of
26
402 THE BODY AT WORK
our eyes. Hence, as Francis Bacon pointed out, a picture —
appears more real when one eye is closed than when both are ©
open. Its middle distance at once falls back. .
> Innumerable are the illustrations which may be given of
errors of sensory judgment, but none are more striking than
the various figures which may be drawn with converging or
diverging lines. The mind under-estimates acute and over-
estimates obtuse angles. It is impossible to convince oneself
that in Fig. 36 the line A bisects asymmetrical arch. Equally
difficult is it to believe that in Fig. 37 the line with diverging
terminal segments and the line with converging terminal seg-
ments are of exactly equal length. In the Ruskin Museum
ier
ee
Fie. 37.—Two HORIZONTAL LINES OF EQUAL LENGTH—THE ONE WITH DIVERGING, THE
OTHER WITH CONVERGING, TERMINAL LINES.
at Sheffield there is a sketch by the master of the facade of a
church which shows a vertical tower to one side of a triangular
pediment, or, rather, this is what the sketch was meant to
show, and does show, when measured on an architect’s table.
In effect the tower appears to be leaning towards the pediment.
_ Errors of judgment of this type have been attributed to the
curvature of the lines of a rectilinear image on the retina, the
mind judging the distance between two points by the length
of the chord, and not the length of the are which joins them.
This is very simply illustrated by the example of the apparently
greater length of a filled space than of a vacant one.
A B C
A B looks longer than B C. If A BC be represented as a
curved. line, the arc A B will, of course, be longer than the
chord B C. But it is not safe to suppose that the mind com-
pares the length of an arc with the length of a chord. Judg-
ment is based upon experience, and probably the illusion is
due to more subtle causes than the curvature of the retina.
Dyce ane i ee a eT et
ware PERT Pe Sieh: fal fons 6 mae
fe aS re . ee = me
hc ee ee 2 Ane et
a peat
ee VISION 403
_ The mind does not look at the retina. If it did, it would find
_ the reversal of the picture the least of the inaccuracies which
it had to correct. It would find it very difficult, for example,
_ to superpose in its stereoscope the photographs of a vertical
tower taken simultaneously by the right eye and the left. The
curved images on the retina of the vertical lines which define
the angles of the tower, as seen with one eye, could not be
made to correspond with the images focussed by the other eye.
The Greeks felt this when they settled the form of a column.
The canon of the swelling entasis and increasing taper above
it did not destroy the appearance of uniform thickness which
the shaft presented. It gave to the eye just the slight help
which it needs to enable it to picture the shaft as of the same
thickness from base to capital.
26—2
CHAPTER XIV
HEARING
Tue ear, like the eye, records amplitude of vibration ; loudness.
It also records rapidity of vibration, musical pitch, which corre-
sponds with colour. But it seems to have a more difficult
task than the eye, since it has to analyse, or at any rate has to
transmit information regarding the form of compound vibra-
tions. ‘The meanings of these distinctions may be illustrated by
reference to a tracing on the cylinder of a phonograph. A
needle attached to the posterior surface of the thin metal plate
against which one speaks scratches the surface of a rotating
cylinder of hardened wax. Examined with a lens, the record is
seen to be an irregularly changing line. The depth of the marks
is a measure of loudness. Their varying number in a given
time indicates the changing pitch of the voice which produced
them. Their form is a record of the quality of its tone. The
work of the ear, so far as it consists in the estimation of the
amplitude and rapidity of pulsations of sound, is easy to
describe, but the acoustics of form are complicated.
Light is transmitted as vibrations of ether. They are
transverse to the direction in which the light is travelling.
Sound cannot travel through a vacuum, since it is dependent
upon displacements of material particles. The particles move
forwards and backwards in the direction in which sound is
progressing. Sound is a sequence of pulsations, alternate con-
densations and rarefactions of the media which conduct it.
Their particles are first pressed together, and then rebound to
positions farther apart. A sequence of to-and-fro movements,
each smoothly continuous throughout the whole duration of a
pulsation, would produce a pure musical tone. Tuning-forks
carefully bowed settle down after a few seconds into unbroken
oscillations, which convey to the air the to-and-fro movements
of pure tones. Such tones vary in nothing but loudness and
pitch. If their pulsations are slow, we speak of the pitch as
“low”; if they are rapid, we say that their pitch is high.
404 ;
HEARING ; 405
- But if the sound produced by tuning-forks (and low-toned
_ stopped organ-pipes) be omitted from the list, no pure tones
- reach our ears. The notes of flutes, fiddles, trumpets, pianos,
have each a certain “ quality’ characteristic of the instru-
ment. Even in a violin the G string has not the same timbre
as the D string. Owing to the elasticity of the substances
which originate and of the substances which transmit sound.
its pulsations are not simple to-and-fro movements, uninter-
rupted from beginning to end. Each pulsation is partially
broken at intervals ; and the quality of the sound depends upon
the number and relative acccntuation of these partial inter-
ruptions. Sound travels through air at the rate of 1,100 feet
per second. This figure, divided by the number of vibrations
per second of a tone, gives the wave-length in air of a tone of
that particular pitch. For example, the middle C has a
vibratory rate of 256. Its wave-length is, therefore, somewhat
over 4 feet. The lowest tone of an organ has a wave-length
of 37 feet ; its highest of 3} inches. These figures give no infor-
mation, however, regarding the movement of the particles
which pass on the sound. When air is transmitting a note—
say the middle C—its separate molecules do not move through a
distance of 4 feet. Each molecule moves but a short distance,
varying with the loudness of the tone; but the “wave” of
crowding runs straight forward from the piano-string to the
ear, the molecules at the end of each stage of 4 feet taking on
a backward movement, so that the crowding, so far as the mole-
cules of that particular section are concerned, returns to its
starting-point. Between the piano-string and the ear there is
a crowding and forward movement at 0, 4, 8,12... feet; a
spreading and backward movement at 2, 6, 10, 14... feet.
Most illustrations which are intended to aid the mind in
forming a definite picture of the transmission of sound are
liable to be misinterpreted, because they translate rectilinear
movements into waves. They represent the movements of the
string, and not the movements of the molecules of air between
the string and the ear; but with the aid of the imagination
one may picture the positions of the particles in this path.
The pulse, we will suppose, has just reached the limit of 12 feet.
Half-way from its 8-foot halting place the molecules are again
crowded, althotigh not so densely. One-third of the distance
from the same point there again appears a tendency to
es the two ee WAVES, divided into fou
these intervals are other points at which the :
closed together, the distances from a nodal point ¢ deper
upon the number of waves involved, and, speaking general
growing less marked as the number increases. Such are t
very complex pulsatile movements which reach the ear,
Every musical sound produced by a piano, a violin, or other! :
instrument, is compounded of a fundamental or prime tone, —
and overtones, partial tones, or harmonics. The following —
table shows the more important partial tones which accompany ©
the prime tone when the middle ( on a pianoforte is struck :
Note sl areca Interval. Ratio, | Sete
gui 2,048 7th
’ 8
\ Super-second 7
B''b 1,792 6th
Sabdninordhied a
G!! 1,536 5th
Minor third =
Ell 1,280 Ath
5
Major third 7
Cll 1,024 ) 8rd
> Fourth =
G' 768 < Qnd
38
r Fifth Qo 2
C! 512 Ast A
°
. Octave a
C 256 ,
- HEARING | | 407
The Stall of a atin note depends upon the number
and relative loudness of its overtones. When several notes
are sounded simultaneously, they blend into a chord or har-
mony, provided the intervals which separate them are equal
to the intervals which separate the simpler overtones. Each
of the notes yields overtones. The tones blend into a concord.
Their partials are in unison. The variations in air-pressure
of the compound tone are strictly periodic. If the ratios of
the frequencies of its constituent notes are simple the product
is a rich, full sound, such as a common chord.
At least one other character of the pulsations of sound must
be taken into consideration if we wish to picture the nature
of the force to which the ear responds. Tones which reach it
from several instruments simultaneously are not necessarily in
unison, or even in harmony. The overtones of a single note
sounded on a piano or violin—the statement does not hold
good for bells, nor is it strictly true of flutes or horns—must
necessarily bear a simple proportional relation to their prime
tone. They divide the grand pulsation into fractions ‘ with-
out a remainder.” But the vibrations of two tuning-forks
which are slightly out of unison interfere one with the other at
regular intervals. They produce “ beats.” Everyone is familiar
with the curious effect which is produced upon the eye when
one row of railings is seen through another, or one expanse of
wire-netting behind another. Sets of lines which occupy
nearly the same positions in the line of sight combine to make
a large pattern, which overlies the smaller pattern of the rails
or netting. The same thing happens with sounds which coin-
cide at considerable intervals, although in the case of sounds
interference is as marked as reinforcement. If whilst a tuning-
fork yielding 101 vibrations per second is singing another of 100
vibrations is brought into play, the vibrations of the second
fork are superposed on those of the first. At a certain moment
the forward movement of molecules of air induced by the first
fork is reinforced by a forward push from the second. But
half a second after this coincidence of phase an opposite result is
produced—504 vibrations of No. 1 have passed, but only 50 of
No. 2. No. 2 is going backwards (inwards), whilst No. 1 is
moving forwards (outwards). The same molecules are im-
pelled backwards by No. 2 and forwards by No. 1. The result
408 THE BODY AT WORK
is a pause. The compound sound produced by the two forks
reaches the ear in throbs. If the forks were vibrating at the
rates of 101 and 99, there would be two pauses and two beats in
every second ; if at the rate of 202 and 198, four. The number
of beats per second equals the difference in frequency of
vibration of the tones. A pianoforte tuner does his work best
if he has a musical ear, yet he may discharge his duties with
competence without one. Having struck a note, he sounds
its octave, holding both keys down, and listens for the beat.
If the first note gave no beat with his tuning-fork, the second
is in tune when it likewise gives no beat with the first. We
have met a tuner who did his work in this way ; but it must be
admitted that his tempering of the intervals of the octave
with which he commenced, and consequently of the other
octaves above and below it, left something to be desired. The
result might have been satisfactory had he been provided with
twelve tuning-forks.
The question as to whether beats, when sufficiently rapid,
blend into a tone has been much discussed, without a decision.
Probably they do not. The complementary question as to
the cause of dissonance is also not completely closed. Two
notes harmonize, as we have seen, when the ratio of their
frequencies is a simple fraction. Musicians are not quite
agreed as to the level of numerical complexity at which a
compound tone first produces a feeling of discomfort. A good
deal depends upon its position in the scale and the instruments
which are combining to produce it. A minor third (&) is on
the safe side. This is the first chord in our list of intervals in
which a beat can be detected. Slow beats, however, do not
distress us. It is the rapid beats of conflicting overtones
which give a harsh, rough character to a compound note. The
level at which a line is drawn between harmony and dissonance
seems to depend to a considerable extent upon musical educa-
tion, using the term in its widest sense. In primitive music—
Hungarian, Scotch, Welsh—intricate minor chords predomi-
nate. The minute subdivision of the octave in Indian music
is quite incomprehensible to a European ear. Musical cultiva-
tion tends to eliminate complex fractions. It is, however, to
be noted that the history of Western music also shows the
influence of an opposite tendency. Later generations have
409
_ admitted as harmonies combinations which earlier generations
could not tolerate.
Pitch, quality, harmony, and dissonance are distinguished
by the human ear. These are the attributes of musical or
periodic sounds. In a separate class must be included noises
of all kinds, termed in acoustics “ aperiodic,’”’ because the
vibrations which cause them are not rhythmic. The teeth of
a policeman’s rattle may click a hundred times a second, but
it does not make music. Even with a rapidity of interruption
greater than this (at least 500 times per second) a succession
of noises fails to blend into a smooth, continuous sound. The
ear recognizes the loudness, duration, and even to a very high
frequency the repetition of unmusical sounds.
The ear as a sense-organ can be followed down the zoological
scale to jelly-fish. In its primitive form it is a chamber lined
with epithelial cells bearing hairs, containing an otolith, or
ear-stone. Otoliths are rounded calcareous masses which
play an important part in the ears of all animals up to fishes.
Even in man they are found in the more subdivided form of
otoconia. Contact of the otoliths with the sensory hairs
originates impulses in the nerves with which primitive ears
are abundantly provided. Advisedly we use the word “ ear ”’
in place of “auditory organ.” In all animals this organ
affords information of a double nature—movement of the
external medium in which the animal lives, and movements
of the animal in the medium. When the animal moves, its
sensory hairs are displaced with regard to the otolith ; when
the water in which it is swimming pulsates, its otoliths are
shaken against the sensory hairs. Displacements of the
animal and agitations of the water produce similar effects. The
ear in this stage is an organ of touch. It might well be ques-
tioned whether an animal fitted with a piece of sensory appar-
atus of this kind is endowed with a sense which we may
properly, after reflecting upon our own sensations, term
“hearing.” It is, however, stated that certain transparent
crustaceans, in which the functioning of the ear-organs may
be watched through a lens, show in these organs hairs of
varying length which vibrate to tones of different frequency.
This observation apart, it might be doubted whether fishes
hear, if we mean by the word “ hearing ” the recognition and
410 THE BODY AT WORK
discrimination of tones of high frequency—musical tones.
Their ears serve equally to inform them of the changes in
position of their heads and of the tremblings of the sea. The
shocks transmitted through the sea are near akin to the
slower vibrations of sound, if the fishermen of the Mediter-
ranean are justified in their practice of beating a wooden
clapper which rests upon the seat of the boat as they row
backwards and forwards in front of a curved net. They
believe that the fish are frightened by the noise ; but it matters
little whether we describe the fish as hearing a noise, or as
feeling the percussions of the clapper conducted through the
water. To the more rapid vibrations of the clapper, the fish
are probably insensitive. The cochlea, which we have every
reason for regarding as the organ by which sound is analysed,
is not possessed by fishes. It makes its first appearance in
reptiles. Birds, it is evident, are able to distinguish musical
tones. Their cochlee are very short, and are destitute of
“rods of Corti.” For a moment this appears surprising,
but it must be remembered that the range of tones which any
bird discriminates is very short, however nicely it may value
the notes within its range. In mammals the ear is clearly
divided into three parts, to which the three functions which
have grown out of the specialization of the sense of touch are
allocated. (1) The semicircular canals are concerned with
the sense of orientation. (2) The utricle and saccule rever-
berate to noise—the rumbling of trains, the boom of guns,
the beats of dissonant musical tones. We do not know how
to classify the agitations of the atmosphere which surrounds
us and of the earth on which we stand, nor can we point with
any certainty to the groups of stimuli which for us have
taken the place of the grinding of stones on the beach and
slapping of rocks by waves. (3) The organ of Corti in the
cochlea discriminates and analyses musical sounds. To these
three sense-organs, which are situate in the inner ear, certain
structures are accessory.
The concha, which enables a horse or a cat to collect sound
and to localize its source, is in ourselves merely an ornament
to the side of the head.
_ The external meatus is a curved tube, about an inch long.
Frequently a tuft of hairs guards its entrance. The wax
| by its wall serves to attach particles of dust, and to
deter insects from entering the tube. The air at the end of it
is at a uniform temperature. It is closed by the membrana
tympani, or drum. This membrane receives the vibrations of
ee
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esis
= ets
‘
.
GIGRIANY
prsece
cay 08
Fig. 38.—THE EXTERNAL, MIDDLE, AND INTERNAL EAR OF THE LEFT SIDE.
From right to left, the figure shows the concha and,lobule of the ear in profile; the external
meatus (abbreviated) ; the drum, divided vertically, its posterior half visible ; the hammer-
bone, with the tip of its long arm attached to the drum, an arrow indicating the point of
attachment and line of action of’ the tensor tympani muscle; the anvil attached by a
ligament to the bony wall of the middle ear; the stirrup, with its foot-plate almost filling
the oval window ; the labyrinth, with the three semicircular canals above, and the scala
vestibuli below. ‘The curled black line shows the situation of the scala media, or ductus
cochlese (which contains the organ of Corti). Pulsations of sound which move the mem-
brana tympani are transmitted by the three bones to the oval window. They shake the
perilymph, producing waves which travel along the scala vestibuli to the apex of the
cochlea, whence they return by the scala tympani to the round window (if they do not
take a shorter course through the ductus cochlese). The Eustachian tube opens out of the
lower part of the middle ear.
sound ; and, in order that it may collect them with absolute
impartiality, it is in every respect the opposite in shape and
structure to the top of a drum. The stretched parchment
which covers a drum is flat. Its tension is uniform in all its
412 THE BODY AT WORK
parts. Movements have the greatest amplitude at the centre.
Every precaution is taken to insure its emitting, with as little
confusion as may be, the particular note to which it is tuned.
The drum of the ear is shaped like the mouth of a trumpet,
depressed to a point, but convex from this point outwards.
Its elastic fibres, which are partly radial, partly circular, are
at many different tensions. Its deepest part, to which the
long arm of the hammer-bone is attached, is not its centre.
The “middle ear” is an irregular cavity communicating
with the pharynx by the Eustachian tube. It is filled with
air at the same pressure as the atmosphere. Except during
the act of swallowing, when it is at first shut tightly and then —
opened, the pharyngeal end of the Eustachian tube is gently
closed. When one is dropped in a lift rapidly down the shaft
of a mine, the difference in pressure between the external air
and. the air in the middle ear stretches the drum to such an
extent that deafness to low tones is produced. Conversation
becomes inaudible. The deafness is remedied by swallowing
saliva, and thus opening the end of the Eustachian tube. The
commonest cause of permanent deafness is inflammation followed
by thickening of the mucous membrane of the lower end of the
Kustachian tube, with its consequent closure, due to frequent
sore-throats. The air in the middle ear is slowly absorbed.
It needs to be constantly renewed through the Eustachian
tube.
On the inner wall of the middle ear are two small apertures
—the oval window and the round window. Both are closed
with membrane. Into the oval window is fitted the sole-plate
of the stirrup-bone. Three bones—hammer, anvil, and stirrup
—combine in transferring the movements of the membrana
tympani to the oval window. They constitute a jointed lever,
which swings about an axis passing through the ligament of
the anvil (Fig. 38), the excursions of the long arm of the
hammer being reduced in amplitude by one-third at the
stirrup-plate. As the oval window has only one-twentieth of
the area of the drum, the movements of the latter are trans-
mitted with concentrated force. Two points in the mechanism
of these bones may be specially noticed : (1) The head of the
hammer is free to rotate in the cavity of the anvil, checked by a
cog. Every inward movement of the drum is faithfully trans-
HEARING 413
mitted to the oval window ; but when the drum moves out-
_ wards, the hammer does not necessarily carry the anvil with
it. (2) A muscle—tensor tympani—is inserted near the elbow
of the long arm of the hammer. When high notes are listened
to its contraction tightens the drum, rendering it more respon-
sive to rapid vibrations. It has a tonic action, but it does not
make any special contraction for low notes.
Behind the two windows, within the solid bone, is the inner
ear, which our ancestors very aptly termed a “labyrinth.” It
is filled with fluid—perilymph—which is shaken by every
movement of the stirrup-plate. Since water is incompressible,
no waves could be raised in the perilymph were there no
second aperture. Every vibration conveyed by the stirrup-
plate after passing through the labyrinth ends as a vibration
of the membrane which closes the round window.
Nowhere does perilymph come in contact with auditory cells.
All the endings of the nerve of hearing are contained within a
membranous labyrinth which lies within the bony cavities.
The way in which the waves of the perilymph are dispersed
over the surface of this closed sac can be inferred from the
diagram (Fig. 38). They sweep round the utricle and saccule,
are lost in the narrow spaces which surround the semicircular
canals, run up the scala vestibuli of the cochlea. The course
of the waves which traverse the cochlea is of especial interest
in connection with the physiology of hearing.
The cochlea—snail-shell—is a spiral tunnel of three turns, in
hard bone, about an inch in length. A shelf of bone—
lamina spiralis—projects into the tunnel on its convex side.
From the free margin of this spiral lamina two membranes
extend to the outer wall of the tunnel—one firm, containing
straight, stiff, and probably elastic fibres which radiate out-
wards (the basilar membrane) ; the other an extremely delicate
film of connective tissue. The tunnel is thus divided into
three compartments, known as the scala vestibuli, scala media,
scala tympani. The scala media belongs to the membranous
labyrinth. Waves transmitted through perilymph pass, as
we have already explained, up the scala vestibuli. At the
apex of the cochlea the two scale are in communication ; but
the aperture is small, and it is unlikely that waves reach the
lower passage from the upper through this opening. ‘They pass
414 THE BODY AT WORK
through the thin membrane which roofs the scala media, shake
its endolymph, and reach the lower passage through the basilar
membrane. It is noteworthy that, since the round window
at the lower end of the scala tympani is, with the exception of
the oval window, the only opening of the bony labyrinth, all
waves transmitted through the oval window must travel part
of the way or all the way up and down the cochlea.
The organ of Corti is spread out on the basilar membrane.
It is an epithelial structure of extreme regularity and uni-
— ——=t fg Ae
SS P= Oe
— \ fj
com
para SS Vines
Va
Fig. 39.—A SECTION THROUGH THE AXIS OF THE COLUMN OF THE COOHLEA.
The spiral sheet of nerve-fibres which supplies the organ of Corti is cut in eight places. If
the bundle to the lowest coil of the shell (on the left side of the diagram) is followed, it will
be seen to bear ganglion cells where it enters the bony spiral lamina. This lamina divides
the tube into two canals—scala vestibuli above, scala tympani below. From the edge of
the lamina the membrane of Corti stretches to the outer wall. Above the organ of Corti
is the membrana tectoria, and above this a very thin membrane which cuts off the ductus
cochlee from the scala vestibuli.
formity. Near to the edge by which the basilar membrane is
attached to the spiral lamina rests a double row of rods of
Corti, stiff pillars which lean one towards the other, over the
tunnel of Corti, the convex head of the outer rod fitting into
a concavity in the head of the inner one; in some places one
outer rod fits against two inner rods, as the latter are rather
the more numerous. On the inner side of the inner rod is
seen, in transverse sections a single plump cell filled with cloudy
protoplasm, and bearing on its free surface a tuft of very short
| HEARING 415
hairs. On the outer side of the outer rod are three or four
hair-cells, each with a cloudy outer segment containing the
nucleus, a granular middle segment, and a stiffish stalk, which
attaches it to the basilar membrane. Between the hair-cells
are supporting cells, thicker below, tapering above, containing
‘in their substance a firm fibre. Still farther to the outer side
are epithelial cells, of no special interest. The purpose of the
rods of Corti and the supporting cells is to give attachment
and support to a reticulated membrane of exquisite delicacy,
through the oblong apertures of which the hairs of the hair-
cells project into the endolymph. The spiral lamina is
))
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4) lo |O| Oo eV
cor A Yr~PakhrDV%-sel-rtoe OLY ~~ > ~
eh, 22 Qe # lo 1O"7_2 BAS eN a
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= NS
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Fig. 40.—ORGAN OF CORTI.
The spiral lamina, on the left of the drawing, gives attachment to the membrane of Corti,
which stretches to the opposite wall. Below the membrane is a bloodvessel which runs
its whole length beneath the tunnel of Corti. The tunnel is formed by pillars—the inner
on the left, the outer on the right—which meet above it. On the left of the inner pillar
is a hair-cell ; to the left of this a nerve-cell with two nuclei. To the right of the outer
pillar is a space ; to the right of this four hair-cells alternating with four supporting cells,
which hold up the reticulated membrane through apertures in which the tufts of hairs
project. Three nerve-fibres are seen in the spiral lamina ; they cross the tunnel to ramify
between the rows of outer hair-cells. The lamina tectoria rests upon the tufts of hairs.
traversed by a vast number of fibres of the auditory nerve,
which, losing their medullary sheaths, pass across the tunnel
of Corti as naked axons, to end amongst the hair-cells. Above
the organ of Corti, attached by its edge to the spiral lamina, is
a thick, gelatinous, fibrillated structure—membrana tectoria—
which rests as a coverlet on the surface of the organ. It has
been supposed that it serves to damp the vibrations of the
hairs after they have been set in motion by the waves passing
across the scala media ; but it not impossibly plays a more
active part in hearing than this.
The organ of Corti is, beyond doubt, the apparatus which
analyses sounds; but the problem of the way in which it
Me tu) eee
416 THE BODY AT WORK
responds to tones of different pitch, or analyses compound
tones, is not as yet even approximately solved. To escape the
acoustic difficulties which have to be faced by anyone who
endeavours to expound the theory of the cochlea as a piece of
analytical apparatus, various suggestions as to the possibility
of an action en masse have been advanced. For example, the
basilar membrane has been compared to a telephone-plate
which takes up vibrations and transmits them through the
auditory nerve to the brain. But if the organ of Corti be the
transmitter, there is no ear in the brain to analyse the vibra-
tions given out by a receiving telephone-plate ; and without a
receiving plate and a listening ear a telephone is purposeless.
According to this hypothesis, the basilar membrane vibrates as
a whole, moving the hair-cells in various “patterns ”’; the pres-
sure of the hairs against the tectorial membrane causing irrita-
tion of the cells which bear them, and hence producing stimula-
tion of various groups of nerves. Other pattern theories are
somewhat similar. But it is obvious that all hypotheses of the
vibration of the whole of the basilar membrane, or of large
parts of it, simultaneously, leave to the mind the responsibility
of reading the pattern which the impulses generated in the
organ of Corti make in the brain. It is conceivable that every
fraction of a semitone which a musician can discriminate, and
every combination of tones which he can analyse, is trans-
mitted to the brain by a large number of co-operating nerve-
impulses ; but such a theory involves a complexity of mental
associations difficult to contemplate.
According to the general principles enunciated in this book,
analysis of stimuli is the function of sense-organs. It cannot
in all cases be compared with the analysis effected in a physical
laboratory ; nor is this necessary ; but it must be carried so far
that nerve-impulses which have no specific qualities apart from
their source shall give rise to effects in consciousness which
have no basis other than the topographical distribution of the
said impulses in the brain. There may be sensory impulses
of different orders; there may be in the brain psycho-
physical substances which react to impulses of various orders
in various ways; but until we have some hint of the
existence of specific impulses and specific psycho-physical
substances, we are not justified in postulating their existence
a 4
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:
HEARING 417
7 | simply in order that we may escape from physiological em-
_ barrassments.
The organ of Corti has in the highest degree the appearance
of a piece of apparatus for the analysis of sound. If the
basilar membrane, with the cells which rest upon it, be cut
out and laid flat, the suggestion of some kind of instrument is
very strong. It is a long narrow ribbon, narrowest at the
bottom of the spiral, increasing to about twice the width at
the apex. It is crossed by radiating fibres, presumably
elastic. The cells which rest upon it carry vibrating hairs,
and are supplied with nerves. The rods of Corti hold up the
reticulated membrane, which keeps the hair-cells in place. It
is not to be wondered at that when its structure was first
discovered it was thought that the problem of the analysis
of musical tones was solved. If two pianos in perfect tune are
in the same room, when one is played the corresponding
wires of the other twang. Anyone who sings into a piano,
whilst the loud pedal raises the dampers, feels an increased
fulness in his voice. This is the familiar phenomenon of
resonance. Why should not the fibres of the basilar membrane
resonate to the tones conveyed to the ear—the shorter ones at
the base of the cochlea to high tones, the longer ones at the apex
to low tones ? This is the order in which we should expect the
pulsations of sound which ascend the scala vestibuli to be
taken up—the more rapid near its commencement, the less
rapid farther up it. But an explanation of the physics of the
selection of vibrations of different frequencies by different sets
of the elements which make up the organ of Corti, if such selec-
tion occurs, is still to seek. In the first place, the fibres of the
basilar membrane are so exceedingly short. What could a fibre
less than 0-5 millimetre in length make of the vibrations of
a 36-foot organ-pipe? Even if this objection be waived, as
certain eminent physicists hold that it may be, there is not a
sufficient difference in length between the longest and the
shortest fibres to account for the great range of tones which
we are able to discriminate ; nor is there any evidence that
some fibres are more tightly stretched than others.
A further consideration which tempts physiologists to look
upon the organ of Corti (including the basilar membrane) as
a series of resonators is the somewhat remarkable agreement
27
418 THE BODY AT WORK
between the number of separate pieces of apparatus of which —
it appears to be composed and the number of different musical —
sounds which, if it were a series of resonators, it might be called
upon to discriminate.
The squeak given by a bat at each turn in its flight has a
pitch of about 11,000 vibrations to the second—the sixth E
above the middle C (Tyndall). In a group of persons listening
for the squeak there are usually some who cannot hear it.
Above this the range of hearing is very variable. The sudden-
ness of transition from perfect hearing to total want of per-
ception makes experiments with small pipes or with a siren
somewhat amusing, when a number of persons are tested at the
same time. One complains that the note is intolerably loud and
shrill, whilst others assert that there is perfect silence. Thirty-
three thousand vibrations is usually regarded as the upper
limit for the human ear, but certain physiologists place it at
40,000, or even higher. The upper limit is of little consequence,
since there is very little power of discriminating rapidities
above the highest note used in music—the piccolo stop of the
organ, with a pitch of 4,096. It is possible that a sound with
a lower frequence than 27 (the contra-bassoon) may be heard
as a tone—16 according to certain writers; but again our
power of discriminating very low notes is small. Over a
certain range a skilled musician can tell that a note is out of
tune when it is one sixty-fourth of a semitone higher or lower
than it ought to be. If we assume that by allowing equal
sensitiveness for a range of seven octaves, the excess of the
allowance over the actual sensitiveness towards either end of
this stretch would compensate for the comparatively few
distinctions which the ear can make either below or above it—
64x 12x7=5,376. A much higher estimate, based upon
observations which seem to show that the ear can distinguish
sounds less than one sixty-fourth of a semitone apart, places
the total number at 11,000.
On the assumption that one piece of apparatus is tuned
to resonate for every distinguishable sound, between 5,000
and 11,000 pieces of apparatus would be required. Taking
one of Corti’s arches as the centre-piece of the resonator,
although the rods are certainly not vibratile structures, we
find the number to be 3,848 (the number of the outer rods) ;
HEARING 419
if either rod with a hair-cell, or hair-cells, is the analytical
- element, 9,438. Counting gives 3,487 inner, 11,700 outer,
hair-cells. The fibres of the basilar membrane are estimated
at 24,000 ; the fibres of the cochlear nerve at 14,000. It will be
understood that the counting of structures as minute as these
yields results which cannot be more than approximately
accurate. Helmholtz, assuming that each are of Corti indi-
cates an analytical element, accounted for the apparent
deficiency in their number by assuming that a tone of which the
pitch fell between two arches set both in sympathetic vibra-
tion, the arch which was nearest in pitch to the tone vibrating
the more strongly. In this way he anticipated an objection
which has often been brought against his theory of a long
series of resonators.
In opposition to Helmholtz’s theory it is pointed out that
when a violinist runs his finger up a bowed string, the pitch
rises with perfect smoothness ; it does not bump along from
resonator to resonator. LEspecially in the case of very high
tones given out by a siren, it is urged that at the rare intervals
at which a resonator in the ear is tuned for the tone which the
siren is emitting it should sound much louder than when the tone
falls midway between two resonators. But the whole question
of the nature of the response of the analytical elements is too
obscure at present for the discussion of points so nice as this.
Many who think that Helmholtz’s theory of resonators is
based upon principles of physics and of physiology which must
be regarded as the starting-points of any explanation of the
analysis of sounds by the ear and the mind, hold that it goes
too far in searching for a separate resonator for every dis-
tinguishable tone. The cochlea, as we have already said, does
not offer anything like so extensive a choice as this, if regard
be had to the tension or length of its elements, and not to their
numbers. Those who accept it as an axiom that the cochlea
contains a series of responding instruments—but a series far
more limited in range than the gamut of our sound-percep-
tions—seek to discover in musical tones qualities which unite
them in groups. Just as in the case of colour-sensations they
recognize four (or six) elementary qualities which excite four
(or six) pieces of responding apparatus, so also in the case of
hearing they seek for a limited number of tone-qualities and
27—2
“
420 THE BODY AT WORK
a correspondingly limited number of elementary sensations.
The ideal of those who take this view is an octave of qualities
and of elementary sensations sounded in the middle of the
scale when x nerve-endings are stimulated, as the octave above
when 2x nerves respond, the octave below with st Such a
conception seems to guide thought round insurmountable
barriers. There is, however, a risk of making too much of the
periodic intervals, because they take so important a place in
music. At one side of the gap which sound bridges between
the individual and his environment is an elastic body shaking
at any possible rate within the range of hearing. At the other
side of the gap is the ear. If, having arranged several thou-
sands of stones along the side of the road in order of size, I
were to state, picking up No. 512, “‘ This is the fundamental
of which No. 1,024 is the octave,’ answer would be made to
me: “It may be that the larger could be broken into halves,
each as heavy as the smaller stone ; but I recognize no differ- —
ence between the stones in shape, colour, or hardness.” A
vibrating string divides into equal segments, each of which
vibrates within the vibrations of the whole string, sounding the
octave. We recognize a similarity in quality between tones
and their octaves because we are accustomed to hear the
octave, the most prominent of overtones, in all musical sounds.
Hence, from association, it has become more difficult to dis-
tinguish a note from its octave than it is to distinguish it from
its fifth ; but it does not follow that the effect of 1,024 vibrations
upon the sensory cells more nearly resembles the effect of 512
than does that of 768. But at this point we are compelled to
construct some hypothesis as to the way in which the vibra-
tions affect the sensory cells. The protoplasm of the cells
is not directly sensitive to them. We can account for the
generation of impulses in the nerve connected with a par-
ticular cell, or group of cells, only on the supposition that a
resonating mechanism which responds to vibrations of a certain
frequency shakes the cell. Even then it seems necessary to
suppose that there is an accessory mechanism which disturbs
the cell protoplasm sufficiently to render the shake effective,
probably the hairs rubbing against the tectorial membrane.
Anatomical study gives us no confidence in the theory of the
HEARING — 421
existence of several thousands of resonators tuned to as many
notes of different pitch. It remains for the physicists to say
whether or not we may picture one of these minute resonators
as responding to a given note in 10 separate octaves, another in
9... another in only 1. The physicists, on their part, may
very properly ask the anatomists to point out the resonators,
and even to reproduce them in models of dimensions which
allow of experimental investigation.
It is generally agreed that the sensation of a chord is com-
pounded of the sensations to which each of its constituent
tones gives rise, and that our power of analysing the com-
pound is a question of attention. A musician can direct his
attention to either sensation at will. It is not equally certain
that a person who has no knowledge of music can do the same.
Familiarity with musical instruments gives us so exact a know-
ledge of the way in which compound tones are produced that
it becomes a difficult matter to decide whether, when we say
that we can pick out the E or the G of the common chord, it
means that we can hear it as distinct from C and C’, or whether
it means that, knowing the constitution of the chord, we think
about the E or the G when we hear the compound tone, to
the exclusion of its other constituents. Then, again, the
several strings which we try to strike simultaneously do not
actually “toe the line.” Their vibrations are not in the same
phase, even though the strings be in absolute tune. Dis-
crepancy of phase may favour the singling out of the several
constituents of the chord. There we touch upon a problem
which we passed over in silence when attempting to give an
idea of the nature of the pulsations which reach the ear. We
then (p. 405) described the partial pulsations which are super-
imposed upon the main pulsation as if they necessarily started
simultaneously with it. We assumed that the phase difference
of the partials was zero. But it is clear that differences of
phase of its constituent tones may produce an almost infinite
number of variations in the form of a compound “ wave ”’ of
sound. Is the ear variously affected by different forms of
wave ? Does difference of phase result in difference of sensa-
tion ? In broad terms, the answer to this question must be
in the negative ; although it can be shown that in certain cases
a change in phase of the several constituents of a compound
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429 ‘THE BODY AT WORK
tone, without any alteration in their number or their loudness, —
makes a change in its acoustic quality. Any attempt to corre- —
late physical changes—the movements of air in the outer ear
—with the effects which they may be supposed to have upon
the organ of Corti must take into account this wide range of —
variation of wave-form. We have called attention to the diffi-
culties which it introduces ; but have no hope of indicating the
way in which they may be overcome.
; Nothing connected with the physiology of the sense of hear-
ing is more remarkable than its capacity for education. The
cochlea of one human being is as extensive and as elaborate in
structure as that of another, yet some men can make an
infinitely more refined use of it as an analytical apparatus than
can others. A native of the Torres Straits cannot distinguish
as two separate notes sounds which are less than a semitone
apart. Sir Michael Costa could distinguish sounds into the
sixty-fourth parts of semitones. The cochlea of a cat is not
less elaborate than that of a man, yet Man’s mental life is
based upon the analysis of auditory sensations. His supreme
advance in the animal scale has depended upon the invention
of language, by means of which he communicates and receives
information, thus rendering experience eternal, notwithstand-
ing the transience of the individuals who acquire and transmit
it. An animal is born, finds out, dies. A man starts with the
wisdom of the race beneath his feet.
Hearing has a nebulous origin in sensations of movement or
displacement. The connection between the two special senses
—the sense of orientation and the sense of hearing, properly
so-called—remains always intimate. David danced before the
Ark of the Lord. All people, savage and civilized, associate
music with movement. High in the animal scale appears the
sense-organ which enables its possessor to discriminate musical
tones. By its use Man has developed with great rapidity—as
secular time is reckoned—an intelligence which removes him
from all other animals a planet’s space. The sounding of his
organ of Corti by pure tones and combinations of pure tones
gives him extreme pleasure, although it in no way ministers
to his intelligence. Yet there is in the enjoyment of music a
quality of pleasure which makes it near akin to the satisfac-
tion which we experience in exercising the intellect. ;
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CHAPTER XV
SKIN-SENSATIONS
THE senses, according to a time-honoured classification, are
five in number—smell, sight, taste, hearing, and common
sensation, or touch ; but such a classification of our sensations
and of the organs which originate them is too crude for modern
needs. Already we have shown that, whereas the nose and
the tongue afford the same kind of information, the ear affords
information of two, perhaps of three, different kinds. Within
the realm of common sensation we pick out three special senses
served by specialized sense-organs—touch, cold and heat—and,
possibly, a fourth, served by non-specialized nerves, to which
alone the epithet ‘“‘ common ”’ properly applies.
The skin is supplied with nerves—naked fibrils—in the
richest abundance. They are most easily demonstrated in the
layer which covers the cornea, thanks to its transparency ;
in this, as shown in Fig. 41, having branched on the front
of the fibrous tissue of which the cornea is composed, the
nerves pass towards the surface, forming connections with
every one of its cells, or, at any rate, with every cell of the
more superficial of the three or four layers of which the
epithelium is made up. Ramified nerve-twigs of this type
do not, under ordinary conditions, convey any sensations to
consciousness. So long as the skin-cells with which they are
connected are healthy, the nerve-twigs establish for them con-
nections with the central nervous system by which their nutri-
tion is regulated ; but they carry no impulses to which we can
direct attention. The movement of blinking is accompanied
by no sensation until the edges of the eyelids come in contact.
A pencil pressed against. the lid evokes touch-sensations from
the skin, but none from the cornea which underlies it. When
423
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424 THE BODY AT WORK
a tiny beetle injures the surface of the cornea by scratching —
the epithelial cells with its horny wings and legs, the ruptured
nerve-filaments convey to consciousness impulses, or, aS We —
prefer to express it, an influence which is felt as pain. But
even the pain caused by injury to the cornea is trifling as com-
pared with that which originates in the under-sides of the lids,
' where not only is the epithelium supplied with branching
nerve-twigs, but specialized organs of touch are present to
localize the seat of injury. Everywhere the epithelium
Fig. 41.—VERTICAL SECTION OF THE EPITHELIUM WHICH COVERS THE SURFACE OF THE CORNEA,
AND OF A SMALL PORTION OF THE CORNEAL SUBSTANCE, HIGHLY MAGNIFIED.
The black lines are naked nerve-fibres (stained with chloride of gold), which are distributed
amongst the cells of the more superficial strata of the epithelium in very great abundance.
The corneal substance is composed of sheets of transparent fibres with intervening cells.
As the fibres of the several sheets cross one another at various angles, they are cut, some
transversely, others in the direction of their length.
covering the surface of the body is so abundantly supplied
that a successful staining of nerve-filaments induces one to
think that every epithelial cell has its nervous affiliation.
These are the nerves of common sensation, if we retain the
term ; but sensation so common, so obscure, so little differen-
tiated that we know no more about it than we know about the
air which envelops our hands and faces on a warm, windless
day. Yet the air, when it moves, gives rise to a dim, broad,
generalized sensation, which may be focussed into definiteness
by a sensitive nerve.
An observer who has devoted himself for many years to the
investigation of skin-sensations, and especially of the “ re-
ferred pains ’’ which are due to diseases of the viscera, recently
caused the large cutaneous nerve which supplies the thumb
a 2 ‘ ~
_SKIN-SENSATIONS 425
side of the forearm and hand to be cut in his own arm, in order
that he might study carefully the revival of sensations. He
found that he never lost his ability to recognize displace-
ments of the tissues beneath the skin. Pacinian bodies and
other end-organs of deep-lying nerves recorded pressure and
tension caused by pushing or rubbing with a blunt instrument.
Seven weeks after the injury he began to recognize stimuli that
do harm—hot things, cold things, pricking with a pin—
although his power of localizing the spot injured was extremely
vague. In seven weeks, that is to say, the protopathic nerves,
which do not follow the same definite lines as the nerves of the
special senses, but form open networks with many alternative
paths, had re-established their skin connections. Only gradu-
ally and very slowly did critical sensations return—the ability
to distinguish degrees of warmth, to recognize as separate two
points of a pair of compasses, to feel a touch with cotton-
wool.
According to a theory set forth in this book (p. 312), pain
is not a set of sensations, but a condition of the central nervous
system which renders it unduly excitable, or excitable in a
particular manner, to impulses which have the same local
origin as the nerve-current which sets up the condition of
pain. When a nerve of the skin has been cut, the epithelial
ramifications are renewed before any specialized tactile or
other sense-organs have regained their nervous connections.
When the area which has regained its surface ramifications, but
has not regained its sense-organs, is injured, no localization of
pain results. Indeed, the obscure sensations which are then
experienced if the skin be injured can hardly be described as
painful. The ramified nerves pour their agitation into the
grey matter of the spinal cord ; but it is not the agitation per se
which causes pain. It is the passage of impulses through the
agitated area that gives to them, when they reach conscious-
ness, not only a topographical meaning, but also a distressful
feeling. Until the specialized organs of the skin have been
restored to working order, there are no impulses to pass through
the agitated grey matter, and therefore no feelings of pain.
According to this view there are two systems of afferent nerves,
the protopathic and the specialized or critical. The former
is very widely and very abundantly distributed to the surface
426 THE BODY AT WORK
of the body, the lungs, the alimentary canal, and other viscera. —
It has no end-organs, no defined tracts in the central nervous —
system, no definite connections with the cortex of the great —
brain. The currents which it conducts, if they originate in the —
visceral part of this system, have no direct effect in conscious-
ness ; but if they originate on the surface of the body, orinthe —
alimentary canal at the lower end of the csophagus, or in
certain other situations, they co-operate with stimuli of heat,
cold, or traction. The critical system works in a more definite
way. Its impulses originate in sense-organs. Starting with a
certain potential, they are transmitted by the discharge of a
succession of linked neurones. When they reach the cortex
their potential is sufficiently high to evoke consciousness.
Their distribution in the cortex is as definite as their origin.
Specialized sense-organs are necessary for the origin of all
sensations. Within the epithelium are certain cells which look
as if they were specialized for sensory purposes. ‘The deeper
sheet, or derma, of the skin is abundantly provided with struc-
tures in which nerves end in the most elaborate and compli-
cated ways (Fig. 42). They are found especially in the
papille of connective tissue, which, set in rows, form the ridges
that one can see at the finger-tips and in various other situa-
tions. All of these organs are made up of groups of epithelial
cells which, displaced from the epidermis, have sunk into the
derma, with the nerves connected with them. In their further
development the nervous part of the apparatus is complicated
by branching, the branches being thickened and usually
flattened into ribbons, which lie on the external surfaces of
the cells or between them. A more or less marked capsule
is provided for the organ by condensation of connective
tissue.
Anyone can convince himself that the skin is not uniformly
sensitive. He may test it first for the minimal stimulus which
excites a sensation of touch. With a hair of the head—it must
not be a very fine one—cut across with scissors, and held
between finger and thumb at the right distance from the cut
end, the skin of the palm of the hand is prodded. Every here
and there a spot is found which is insensitive to so slight a
pressure. These spots are neither large nor very close together.
If the hairless skin of the arm between the elbow and the
427
armpit be investigated in the same way, much larger blank
areas are met with—oval patches more than } inch in diameter.
When a hairy surface is tested, it is found that contact with a
hair can always be felt ; and when the hairs are shaved, the
touch-spots are found to extend around or from the points at
which hairs pierce the epidermis. Touchless areas lie between
them. Hair-follicles receive tufts of nerve-filaments, and it
appears that they are the chief organs of touch. ‘‘ Touch-
corpuscles,” which are found in great numbers in the papillz
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Fig. 42.—SENSE-ORGANS SUSCEPTIBLE TO PRESSURE.
All are formed on essentially the same plan; a fibrous capsule invests a group of epithelial
cells amongst which a nerve ramifies. The simplest form is known as a Grandry’s cor-
puscle—a nerve ending in one or two plates between two or three epithelial cells. These
organs are found in great numbers in the bills of aquatic birds. If a duckis watched
whilst it is gobbling mud at the margin of a pond, it will be seen to have a remarkable
capacity for discriminating between the shells of small snails, which it can crush, and
stones, which it needs to drop from its bill. Its bill is also provided with small Pacinian
corpuscles (Fig. 43). Touch-corpuscles, more elaborate in form than the one figured, are
found in the papille of the skin of the fingers and elsewhere. They appear to be modified
hair-follicles. End-bulbs occur in the conjunctiva and elsewhere, and especially in the
peritoneum. Together with Pacinian corpuscles, they are accountable for sensations con-
nected with the distension of the stomach and intestines.
of the skin of the fingers and elsewhere, may probably be re-
garded as, genetically, hair-follicles which have not developed
hairs.
If sensitiveness to pain is investigated by tapping very gently
with a needle—or, better, by using a stiff horsehair fixed in a
cleft stick, from which it projects about + inch—it will be
found that every here and there are spots which are exceedingly
sensitive, whilst adjoining them are areas which are moderately
sensitive, and between these areas small spots or stretches of
-:
skin which do not give the anata oundlide even
the horsehair be pushed untilitdoublesup. =
Testing now for sensitiveness to cold with a cold plate
point, “‘ cold-spots ” can be mapped on the skin. If the 1
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These organs are especially numerous in the neighbourhood of tendons and ligaments. They
are also present beneath the skin of the hands and feet. Their capsules are formed of a
great number of concentric lamelle of connective tissue, enclosing lymph-spaces. Within
the capsule is a core of finely granular substance, which also shows a tendency to a lamellar
disposition. The structure of these relatively large sense-organs is highly suggestive of
sensitiveness to pressure, traction, or rubbing.
is warmed to about 50° C., “ heat-spots”’ are found. The
different kinds of spot are very irregularly distributed. They
may coincide, or overlap, or leave blank spaces. Their relative
abundance varies. In some regions touch-spots, in others cold-
SKIN-SENSATIONS 429
: spots, in others heat-spots, are more closely grouped. The
_ tongue and the hand, and especially the tips of the fingers, are
most sensitive to touch; but whereas the tongue is also ex-
ceedingly sensitive to warmth, the hands are relatively insensi-
tive. Yet, speaking generally, parts especially sensitive to
touch are little sensitive to temperature, and vice versa. Sensi-
tiveness to cold is much more widespread than sensitiveness to
heat. It is concentrated in the skin covering the abdominal
viscera. A cold douche directed between the shoulders is
doubtfully felt as cold. There is no doubt whatever about it
when it strikes the skin over the stomach.
From these observations it appears that the skin contains
three sets of organs sensitive respectively to touch, cold, and
heat. Certain investigators hold that it also contains specific
organs, or nerve-endings, sensitive to painful stimulants ; but
in this case there is the obvious difficulty of distinguishing
between pain and touch. At no spot can pure pain be evoked
free from any consciousness of touch.
To a certain extent the combinations of epithelial cells and
nerve-endings in the skin fulfil the negative requirement of
sense-organs ; each kind, whilst specially sensitive to its own
specific stimulant, is insensitive to stimulants of other kinds.
But mutual exclusion is not absolute in the case of cold
and warmth. If a warmed metal point be applied to a
cold spot, it produces a sensation of cold. Our feelings of
warmth and cold are to a large degree comparative. Luke-
warm water feels cold to hands just taken out of hot water ;
moderately cold water appears luke-warm to hands that have
been in contact with ice. The sensory apparatus for cold and
heat soon adapts itself, or, in physiological language, it is soon
fatigued. If after a prolonged bath at the body temperature
a foot be plunged into very hot water and withdrawn quickly,
the feeling which first ensues is one of cold. It is indistinguish-
able from the feeling provoked by dipping the foot into cold
water. The sensation of cold subsequently gives place to one
of painful warmth. This does not indicate that the heat-spots
have been waked out of their lethargy by excessive stimula-
tion. On the contrary, it is the cold-spots which, when they
were first stimulated by the very hot water, answered “ Cold,”
that now cry out “ Hot ”’; for both cold-spots and heat-spots,
} take deta past, one Bg alr warm ad the othe
applied simultaneously to two closely adjacent spo 3
the resulting sensation is “hot.” When the cold -
withdrawn, or replaced by a second warm point, the ser ns
sinks to “‘ warm.’ 7
CHAPTER XVI
VOICE AND SPEECH
A out carried horizontally backwards across the cartilage
which projects forwards as Adam’s apple, a quarter of an inch
below its notch, would show that it is V-shaped, the point of
the Vin front. Each limb of the V isa broad plate. In the
mid-line is a gap, the rima glottidis, through which the wind-
pipe communicates with the pharynx (Fig. 45). It is overhung
by the stiff leaf-shaped epiglottis, the edge of which can be felt
with the finger behind the tongue. (yAwrris, the mouthpiece of
a reed-pipe, is the term commonly used, for short, for the rima
glottidis.) When air is being drawn into the lungs, the glottis
is widely open. In speaking or singing it is almost closed. It
is tightly shut whilst food is passing down the gullet.
The glottis is bounded, as to its anterior two-thirds, by two
membranous folds, the vocal cords. In its posterior third it has
a triangular cartilage, the arytenoid, on either side. A distinc-
tion is sometimes drawn between the anterior part, bounded
by the vocal cords, and the whole glottis, the former being
termed “ rima vocalis ”’; but it is scarcely justified, for, although
it is true that the anterior part is essentially the organ of voice,
and its margins alone vibrate when high notes are sung, the
anterior ends of the arytenoid cartilages also vibrate during
the production of low notes. (The substance of: these pro-
cesses is not, properly speaking, cartilage ; it resembles the
epiglottis in containing a great abundance of elastic fibres.)
And here we must warn the reader not to picture to himself a
vocal “cord ’”’ as a kind of fiddle-string. It bears no resem-
blance to a cord, as we ordinarily understand the word ; it
is but a fold of mucous membrane, such as one might pinch
up between finger and thumb from the inner side of the cheek.
431
distended, and vast numbers of exceedingly slender | oh 3
oe
fibres which traverse it.
The first cartilage below the thyroid—it may be felt Be bic
the finger—is termed “cricoid”’ (xpixos, a ring), from its.
resemblance to a signet-ring. Narrow in front, its large signet
= Here se seiner Epiglottis
(Gy (3\-- Ayoid Bone
----/}\-- Ventricle _
Li. False Vocal Cord,
-}-- Thyoarytenoid Muscle
s --t--— Voeal Cord
---- Thyroid Cartilage
, ~------- Cricoid Cartilage
0
O}------- Wind-pipe
()):
(\
Fig. 44.—THE ANTERIOR HALF OF THE LARYNX SEEN FROM BEHIND.
The drawing shows the folds of mucous membrane, the vocal cords, which stretch from the
tips of the arytenoid cartilages to the recess behind the median portion of the thyroid
cartilage. To the outer side of each vocal cord is seen the thyro-arytenoid muscle (cut
across), consisting of a broad outer portion, chiefly concerned in closing the glottis during
the act of swallowing, and a smaller internal portion, which regulates the length and the
thickness of the segment of the cord allowed to vibrate.
projects upwards, within the V of the thyroid, behind, and on
the top of the signet rest the two arytenoids. Each arytenoid
is a triangular pyramid, its anterior, external, and upper angles
prolonged into processes. It is united with the cricoid by a
swivel joint, which allows its anterior process to swing inwards
or outwards under the influence of two antagonistic muscles
attached to its outer angle—the lateral and posterior crico-
arytenoids. Another muscle attached only to the arytenoids
draws them together. Still another muscle—or two muscles,
— =f
, 3 7
f
VOICE AND SPEECH 433
for it is in two separate bands—unites the anterior process of
the arytenoid with the back surface of the thyroid just on the
outer side of the attachment into that cartilage of the vocal
cord. The internal thyro-arytenoid muscle is a comparatively
narrow band; the external thyro-arytenoid muscle is thick
and broad.* By the simultaneous contraction of the encircling
wes
B C
D E
Fig. 45.—THE APERTURE OF THE GLOTTIS SEEN FROM ABOVE.
The leaf-like structure in front of it is the epiglottis ; the two triangular structures at the back,
the arytenoid cartilages ; the white bands on either side, the vocal cords. A, The glottis is
widely open during inspiration. Arrows show the lines of action of the muscles which
rotate, and approximate, the cartilages. Attached to their outer angles, and pulling these
_ angles forwards, the lateral crico-arytenoid muscles ; pulling them backwards and inwards,
the posterior crico-arytenoid muscles. Drawing the cartilages together, the arytenoid
muscles. 3B, The glottis during speaking in a deep chest-voice, or when a low note of the
lower register is being sung. C, During the production of a high note of the lower register.
D, During the production of a note of the head-register. E, During the act of swallowing ;
the arytenoid cartilages are drawn towards the epiglottis ; the aperture is folded into a T ;
the pharynx (the tube behind the glottis) is distended.
muscles the larynx is closely squeezed together, the anterior
portion of the slit forming a T, with the transverse limb in
front. This occurs only in swallowing. Under the co-operat-
ing contractions of the several muscles, the glottis assumes a
variety of shapes. The external crico-arytenoids rotate the
anterior angles of the arytenoid cartilages inwards (Fig. 45, A).
* A bullock’s larynx is an admirable object of study. In almost all points
of form and structure it is practically identical with the human larynx, and
its large size makes it easy to dissect.
28
434 ‘ THE BODY AT WORK
If at the same time the arytenoid muscle draws the cartilages
together, the glottis is reduced to a slit (Fig. 45, C). The
posterior crico-arytenoid muscles rotate the cartilages out-
wards. If the arytenoid muscle is at the same time relaxed,
the glottis gapes to its fullest extent (Fig. 45, A). The freer
the opening, the less is the resistance to the blast of air,
the gentler the vibrations of the cords, the lower the voice.
The closer the slit, the greater is the resistance which the air
in the windpipe has to overcome in passing through it, and
consequently the more ample the vibrations into which it
throws the vocal cords.
The vocal cords are the tongues of a reed-pipe, which, com-
mencing in the chest at the point where the great bronchi join
to form the windpipe, comprises the larynx, and, above the
larynx, the complicated chambers of the throat, mouth, and
nasal cavities, including the spaces within the bones of the
head which open out of them. The pitch of the voice depends
upon (1) the length of the vocal cords, and (2) their tension.
The first factor is fixed for every individual. The voice is base, ©
baritone, tenor, in a man ; contralto, mezzo-soprano, soprano,
in a woman—in proportion as the cords are long, of medium
length, or short. A man’s vocal cords measure, on the average,
15 millimetres, a woman’s 11 millimetres. When a boy is from
twelve to fifteen years of age his vocal cords double in length,
and the “ breaking ” of the voice occurs as he gives up trying
to get high notes out of his longer cords, and allows them to
produce manly tones of an octave lower.
The lower posterior angles of the thyroid cartilages articulate
with the cricoid. If the four cartilages are freed from all soft
tissues without disturbing the thyro-cricoid, or crico-arytenoid
joints, and if, while the thyroid is held in one hand, a finger of
the other is placed on the front of the cricoid, it will be found
that as this is depressed the arytenoid cartilages which rest
upon its signet are tilted upwards and forwards within the -
thyroid ; as it is raised, they are tilted away from it. In life
this movement is effected by a muscle—the crico-thyroid -
(Fig. 46)—attached to the front of the cricoid cartilage and
to the under border of the lateral plate of the thyroid. This
is the muscle of supreme importance in the production of the
voice. The thyroid cartilage is slung in a fixed position by
*»
ee Pe © Pee Te oe a ae
ie ety o Ot | ir pea hee ey if
ae ee 2 al ie “
VOICE AND SPEECH 435
the hyoid bone (to be felt in the neck above it). The crico-
thyroid muscle, being unable to depress the thyroid, raises the
front of the cricoid cartilage, tilts back the arytenoids, tightens
the vocal cords. As the voice ascends the scale, the tension
of the cords is progressively increased, and their vibrations
rendered proportionately more rapid. The range of the human
voice is about three and a half octaves; of individual voices
about two octaves ; if the shrill cry of a baby, which may reach
the third G above the middle C, or even higher (E’’” or F’’”’),
Fig. 46.—THE LARYNX FROM THE RIGHT SIDE.
From above downwards: the hyoid bone, thyro-hyoid membrane, thyroid cartilage, cricoid
cartilage, trachea. The upper and posterior angle of the wing of the thyroid cartilage is
suspended from the hyoid bone ; its lower and posterior angle articulated with the cricoid
cartilage. On the summit of the cricoid cartilage it articulates the arytenoid. Dotted
lines indicate the position of the vocal cord. The crico-thyroid muscle, which raises the
front of the cricoid, tilting the arytenoid cartilage backwards and tightening the vocal cord,
extends, fan-like, from the front of the cricoid to the lower border of the wing of the
thyroid. 7
be excluded. Exceptional voices have a range far greater
than two octaves. Falsetto voice is produced by throwing
half of the vocal cord out of vibration (the way in which this
is accomplished is not clear), and at the same time raising the
back of the tongue to the wall of the throat in such a manner as
to cut off all the lower part of the upper resonating chamber,
leaving it only the mouth and the cavities of the nose.
So far the mechanism of voice is easily understood. As the
scale is ascended, the vocal cords are progressively tightened
by the contraction of the crico-thyroid muscles. But an analysis
: 28—2
436 THE BODY AT WORK
of the feelings experienced during singing (and of the quality
of the sounds produced) shows that by themselves these
muscles are not able to make changes in the tension of the
cords sufficient to account for the full range of the voice. Or,
put in another way, the tension of the vocal cords is not
altered to the extent which would be necessary if upon it alone
depended a range of from two to three octaves. It is obvious
that by some means the length or thickness, or both, of the
portions of the cords vibrating is changed as the scale is
ascended. If commencement be made on a low note, a point
is reached, after a certain number of notes have been sung, at
which a sudden change occurs. There is an alteration in the
quality of sound, the more marked, the less well trained the
singer. The singer experiences a feeling of relief. If a finger
be placed on his crico-thyroid muscle, a relaxation of its anterior
fibres can be detected. As he proceeds up the scale, these
fibres again tighten. At a certain point there is again a change
in the quality of voice, and in the feelings which accompany
its production. The two points at which change occurs are
said to divide the voice into three “ registers ”—the lower, or
chest-register, the middle, and the upper, or head-register. A
great effort is needed to hold either register above its natural
range.
The physiology of the registers is a subject far too thorny
for handling in this book. The larynx can be watched with
the laryngoscope during the production of notes of different
pitch, but observers are not in accord regarding the appearances
which it presents, or their interpretation. The possibilities
of changing the reed which vibrates, the vocal cord, otherwise
than by increasing the direct pull upon it exerted by the crico-
thyroid muscle, appear to be as follows : (1) During the pro-
duction of the lowest notes the elastic portion of the arytenoid
cartilage may be included with the cord. It may be thrown out
of vibration by its rotation inwards (under the action of the
lateral crico-arytenoid muscle) until it is pressed against its
fellow. (2) Certain portions of the cord may be damped by
partial contractions of the internal thyro-arytenoid muscle.
It has been frequently stated, although the statement is not
accepted by all anatomists, that some of the fibres which
take origin from the arytenoid cartilage end in the cord,
VOICE AND SPEECH 437
instead of passing right through to the thyroid. It is supposed
that by their contraction they throw the posterior portion of
the cord—even, it is asserted, as much as its posterior two-
thirds in the higher head-notes—out of vibration. (3) It
appears that the width (thickness) of the cord vibrating is also
regulated by the contraction of the thyro-arytenoid muscle.
Those who regard the diminution in the thickness and width of
the vibrating fold of mucous membrane and underlying elastic
tissue as the chief factor in the adaptation of the larynx for
the middle register lay great stress upon the sense of relief
from muscular effort which accompanies the transition. Less
force is needed to tighten the thinner cord. They also call
attention to the loss in volume of the voice when the lower
register is left, and to its greater softness. The lower is spoken
of as the thick register, the middle as thin, and the upper (on
the hypothesis that part only of the cord eee as the small
register.
Singing reveals the possibilities of the Ltt as a musical
instrument. In speech the larynx plays a part, but the form
of the syllabic sounds and the relative prominence of over-
tones in the vowels is of more importance than pitch. Flexi-
bility of voice is dependent upon ability to increase or diminish
at will the size of the resonating chambers of the throat.
mouth, and nose, or the freedom of access to them. Con-
versation is carried on in the lower or chest-register. When
a practised speaker mounts a platform, he spends the first few
minutes in ascertaining the pitch of the hall—that is to say,
the pitch of his voice to which the room resonates most freely.
Having found the proper tone, he endeavours to maintain a
uniform tension of his vocal cords, and therefore a uniform
pitch. He relieves the monotony of speech by suitable varia-
tions of its overtones. Nothing is more uncomfortable to
listen to than an oration delivered in cadences. The speaking
voice should be full, round, and musical, and free from affecta-
tion—as guiltless of the intoning or preaching quality as it is
of harshness or of vulgar flatness. A flexible voice is capable
of producing, as occasion calls for them, tones of any and every
quality. With the throat and mouth set for the syllable
“haw,” it is impossible to do justice to such words as “king ”
and “queen.” The voice-tones of a superior person are’as
438 THE BODY AT WORK
distasteful to the hearer as those of a vulgarian. Unpleasant
also is a nasal twang, illogically so called, since it is due, not
to the opening of the resonating chambers of the nose, but
to the restriction of the entry of air into them. In this it
is somewhat similar to the effect produced by a severe cold.
Resonance in the nasal chambers produces a clear, ringing
voice.
A little consideration of the varying qualities of different
voices suffices to show how largely they depend on resonance.
When vowel-sounds are analysed, it is found that the dis-
tinctive character of each of them is dependent upon the
overtones which it contains. For every vowel the overtones
are fixed, or very nearly so, no matter what may be the pitch
of the note to which the vowel is sounded.
It is much to be regretted that the alphabet was settled
before the physiology of speech was understood. Were it
based upon reasonable principles, children would be spared
the bewilderment which overtakes them when they endeavour
to establish in their minds some kind of relation between the
names of consonants and their effects upon the blast of air
as it passes through throat and mouth, and between tongue
and palate, teeth and lips. The vowels, had physiologists
defined them, would have been real pure vowel-tones—60, 0,
ah, €—sounds which can be sustained for an indefinite time,
and allowed to die away without deterioration in their quality.
A (é as pronounced in France) is doubtfully pure—it has a
tendency to tail off in @é; 4 is frankly a diphthong, az (ah-2).
Try to hold a long final note on the syllable “nigh”! An
international standard of vowel-sounds would have been fixed,
by giving the vibrating periods of the tuning-forks for which
in each several case the resonating chambers are shaped, and
defining the relative accentuation of each overtone. Greatest
boon of all, the irruption of the Essex dialect would have been
dammed. It would not have been allowed to inundate London,
or to submerge Australia, debasing our English tongue. In
Cockney speech vowels degenerate down the line of greatest
indolence. Aw becomes or, or ar; a becomes 2. It requires
a greater effort to pronounce a full a than a flat a, a definite
flat a than 7. And worse than a Cockney’s unwillingness to
take the trouble necessary for the production of dignified
VOICE AND SPEECH 439
vowel-tones is his reluctance to make the effort required for
the holding of any tone. In his mouth virile, self-reliant
vowels are replaced by emasculated diphthongs, which collapse
as they present themselves to the ear. It costs trouble to fix
the mouth-chamber before a vowel is sounded and to hold
it steady until it is finished. Ah slides down through
ai to 2; 7% slips into a. “Cow” becomes kyow; “ you,”
ye-u-ow ; “cart,” kyart. And just as the effort needed for
the filling of the vowels is shirked, so also is grudged the
expenditure of an accessory blast for their aspiration.
When a vowel is whispered, although the vocal cords do not
vibrate, the blast passing through the resonating chambers
produces the overtones characteristic of the vowel. Anyone
who feels his own larynx while he sings, to the same note, the
various vowels between 66 and @é—he may please himself as
to the number of ai, eu, and % vowels he interposes between
these two extremes—will recognize that it is pulled farther
and farther upwards by the muscles which surround it. The
cavity of the mouth is at the same time made shorter and
broader for each succeeding vowel. Singing the several vowels
before a piano, and at the same time striking various keys, it
is felt in the mouth that the resonance of that chamber is
reinforced by certain selected notes. Certain tuning-forks,
when sounded in front of the mouth shaped for a vowel, ring
out more loudly, because the mouth cavity resonates to their
prime tones. The overtones of the vowels can be analysed in
this way. Conversely, by sounding simultaneously an appro-
priate selection of tuning-forks, each with the right degree of
force, the overtones of a vowel can be synthesised. Thus if
whilst one tuning-fork is sounding B,b (Bb above middle C),
two others be added giving B,b (loud) and F, (soft), the com-
posite sound resembles the vowel o. If to these same three
forks, with F, sounding more strongly, B,b and a loud D, be
added, the sound changes to ah.
The organ of voice is a combination of a reed-pipe with
resonating chambers, the shape of which can be changed at
will. The quality characteristic of a vowel is given to it by
adding to the note produced in the larynx sounds due to the
resonance of the throat and mouth. On the assumption (not
allowed by all authorities) that, since the resonating chambers
~
440 THE BODY AT WORK
are not sound-producers, they can only add to the Jarynx- —
tone, as “formants ’’ of a vowel, its own harmonics—sounds
which they have picked out of it—it follows that, if, when the
prime is changed, the resonators were not adapted to the
new note, they would be dumb. If this attitude in regard to
the question be justified, there must be a certain amount of
variation in the quality of a vowel as the scale is ascended.
But a vowel is not a musical tone ; it is a conventional sound.
Its whole value depends upon its retaining, as nearly as may
be, the same quality, whatever be the pitch of its prime tone.
By adjusting the form of the throat and mouth, we can not
only prevent one vowel from passing into another, but we can
keep it so nearly true to itself as to convince the ear that its
quality is unchanged: 06 remains 60, and ah ah, although
the form of the sound as produced on Cy is different to its
form when sung to C.
Apart from the general distinction that low notes are taken
more easily with vowels requiring a large mouth-cavity, and
high notes with those providing a small one, there are certain
very distinct relations between vowel-sounds and musical tones
which need to be borne in mind in setting words to music.
A singer changes a word when he feels that its vowel-tone
does not allow him to give to the note to which it is set the
fullest expression of which he is capable. |
An account of the physiology of the production of con-
sonants is to be found in most text-books of grammar.
INDEX
AxssorpTIoN from alimentary canal, 129
Accelerator nerves of heart, 237
Accommodation of the eye for distance,
391
for light, 390
Acromegaly, 93
Addison’s disease, 91
Adrenalin, action on the kidney, 209
formed in suprarenal capsule, 92
Air, quantity inspired, 173
quantity needed by individual, 191
Air-cells of lungs, 168
Albumin made by plants, 12
Alcohol, effect on nerve conduction, 301
Alimentary canal, morphology of, 98
nerves of, 104
Altitude, highest, attained by climbers,
187
Alveoli of lungs, their number, 169
Amides produced from proteins, 119
Ameeba, irritability of its protoplasm,
nitrite, effect on vascular
system, 237
Anemia, treatment with iron, 67
Anesthetics, influence on protoplasm,
11
Analysis by animals, 12
Angina pectoris, 237
Angler fish, its nerve-cells, 31
Animal machine and its driver, 354, 358
Animals, hunting versus hunted, 366
not reflex machines, 358
relative insensibility to the knife,
361
Antitoxins, formation by protoplasm,
Aorta, diameter of, 232
Aphasia, 352
Apnea, condition of arrested respira-
tion, 181
Appendicitis, increased frequency of,
Appetite, a safe guide, 114
Arteries, blood-pressure in, 234, 239
structure of wall of, 233
Artificial respiration, 183
Asphyxia, 182
Association zones in the cortex of the
great brain, 348
Asthma, due to reflex contraction of
small bronchi, 167
Astigmatism, correction by glasses, 393
due to modern print, 269
Attention, effect of, in heightening pain,
361
Bacteria, diminution of number in
intestine on milk diet, 138
of alimentary canal, 135
of Bulgarian sour milk, 138
of the River Ganges, 141
in an infant’s intestine, 136
their réle in nature, 20
Balance-sheet of body, how drawn up,
149
Balloon, highest altitude attained in, 187
Basket-cells in nervous system, 324, 340
Bat’s squeak, number of vibrations, 418
Bats, flight not dependent on vision,
381
Beats in music, explanation of, 407
Beetle, muscle of, 261
Belladonna, physiological action, 109
Bile, composition, 117
function in regard to absorption
of fat, 133
relation to digestion, 117
Bile-pigment, origin from hemoglobin,
69, 82, 118
Bioplasm, the essential substance of a
living cell, 148
Birds, sense of hearing of, 410
Blind spot, how filled in, 395
Blisters, 41
Blood, amount ejected by heart, 219
circulation-time, 219
composition of, 59
gases of, amount, 190
tension, 61
lodged in abdominal veins, 234, 236
Blood-eorpuscles, cellular nature, 28
life-story, 62
number, 61
origin, 63, 64
structure, 60
Blood-platelets, 74
Blood-poisoning, 57
Blushing, 243
Bowman’s description of kidney, 200
discs in muscle, 259
Brain. Cf. Cerebellum,
cerebrum
blood-supply of, 352
Bread, digestion of, 120
Breathing, mechanism of, 171
Bruises, explanation of play of colours, 69
Bulgarian milk-germ, 138
Cortex of
44]
PPAR Y OS sre Se Nol a ee en ee eT TP ey oe ee
442
Capillary vessels, circulation of blood
migration of leucocytes from,
~ + 232
structure of their walls, 38
Carbohydrate foods, chemical composi-
tion, 147
Carbonic acid, carried by blood, 60
liberation in lungs, 61, 189
Carbonic oxide, compound with hzemo-
globin, 187
Carnivora, absorption of fat from
alimentary canal of, 133
Cartilage, growth, 28
Catalysis, 17
Cell theory, 26
Cells, constituent parts, 26, 28
size, 30
specialization of function in, 35
Cells of Purkinje in the cerebellum, 303,
340
Cellulose, digestion of, 137
Cerebellum, cases of deficiency of, 341 -
connections with cerebro - spinal
axis, 340
development of granules of, 299, 303
lobes, 338
minute anatomy, 339
phylogeny, 338
relation to tone of muscles, 342
Cerebral hemisphere, an outgrowth to-
wards olfactory pit, 334
in animals with various sen-
sory endowments, 349
Cerebro-spinal fluid, 50
Chemical activity of protoplasm, 12
messengers, 89, 123
processes in plants, 15
Chemiotaxis of leucocytes, 56, 364
Children, brain in, 346
development of astigmatism in
eyes of, 269
Chill, catching a, 242
Chloroform. C/. Anzsthetics
Cholesterin, 118
Chromatolysis in nerve-cells, 320
Chrome-silver method of colouring
nerve-tissue, 293
Chyme, food converted into, 126
Circulation of the blood, 218
Circulation-time, 219
Cirrhosis of liver, 42
Coagulation of blood, 69
Cochlea, anatomy, 413
Cockney dialect, the degradation of
vowel-sounds, 439
Coke fire, poisonous fumes from, 186
Cold-spots in skin, 429
Collaterals of nerves, 297
Colon, length and disposition of, 101
Colour-blindness, 385
THE BODY AT WORK
Colour vision, 385
Colours, reason for apparent fading in
twilight, 378
Conductivity of protoplasm, 248
Consciousness, does not come within
physiological investigation, 360
its part in animal life, 359
Control experiments, their value, 72
Convolutions of brain, 345
Cooking, effect upon digestibility of
meat, 120
Corneal epithelium, sensitiveness of,
424
Corpus striatum of brain, 344
Cortex of cerebrum, discovery of ex-
citability of, 344
-
fissures and convolutions, 345
functional areas, 352
myelination of its fibres, 345
sensory and association areas,
346
structure of, 347
variations in different animals,
349
Corti, organ of, its structure, 414
theories of function of, 416
Coughing, mechanism of, 180
Crayfish, tone of claw-muscle of, 273
Cretinism, 85, 90
Cricket, chirp of, 261
Crypts of Lieberkiihn, 103
Curdling of milk, 75
Dancing, association of sound with
movement, 422
Day’s work, food required for, 151
Deafness due to sore throat, 412
Degeneration of nerves after section,
326
Depressor nerve of the heart, 237
Diabetes, excretion of more carbo-
hydrate than contained in food, 143
Dialysis, explanation of the process, 40,
128
Diaphragm, function in respiration, 171
Diastases, destructive ferments, 18
Diet, limits of possible variations in,
153
of labouring classes, 152
Digestibility of bread, meat, fish, etc.,
120, 125
Digestion, mechanism of, 96
vascular changes during, 235
waits on appetite, 114
Digitalis, action on heart and kidney,
209
Diphtheria, antitoxin of, 20
Diuretics, 209
Dog’s sense of smell, 370
Dreams, theory of, 362
Dropsy, 42
oikga resuscitation from, 183
» Pp
Dru ysiology of, 95
Ductless glands, 94 =
Dyspnea, difficult respiration, 181
Ear, anatomy, 411
bones of, 412
differentiation into separate sense-
organs, 410
in fishes, 410
phylogeny, 409
Eel’s blood injected into mammal, 20
Effector, an organ which exhibits
change in response to stimulation, 253
Egg-albumin destroyed by blood, 19
Electric organs, 288
phenomena of muscles, 279
Emotions, their relation to vaso-motor
changes, 242
Energy, expended by body, 151
source of the body’s, 152
of stimulus compared with energy
of muscular response, 254
Engines, body compared with, 152, 256
Epiglottis during swallowing, 433
Equilibrium, maintenance of, in walk-
ing, 342
Erepsin, ferment of intestinal juice, 119
Errors of sensory judgment, 402
Excretion, 195
Eye, accommodation for distance, 391
adaptation for darkness, 390
blind spot, 394
optical defects of, 393
phylogeny, 334
refractive media,
image by, 391
Eyeball, abnormalities in shape of,
392
anatomy, 373
development, 374
muscles of, indefatigable, 269
formation of
Fat, absorption of, 131, 132
accumulation of, relation to foods
consumed, 144
chemistry, 132
digestion, 133
laid down in connective tissues, 145
stored in liver, 145
Fatigue, causes of, 45, 268
Fermentation, 16
Ferments, chemical nature, 18
. classification, 16, 18
physiological importance, 18
Fibrin of blood, its antecedents, 75
Fireflies, source of their light, 291
Fish, sense of smell of, 365
supposed to be frightened by noise,
410
Flatulence, cause of, 114, 125, 136
443
Foods, classification, 142
history of, after absorption. 142
relative value, 147, 151, 153, 157
residue after digestion and absorp-
tion, 194
Foramen ovale of heart, sometimes per-
forate, 218
Frigate-bird, turbinate bones of, 166
Frog, supposed to be found entombed
in rock, 164
Functional interdependence of organs, 94
Functions transferred to other organs,
87
Gall-stones, cause of formation of, 118
Galvani’s observation of contraction
of a frog’s muscles, 277
Ganges, purifying water of, 138
Ganglia of sympathetic chain, function,
325
Ganglion cells of retina, 376
spinal, 299, 333
Gaseous tension, meaning of expression,
188
Gases of blood, their exchange in the
lungs, 184
Gastric glands, structure, 123
juice, amount secreted, 114
composition, 114
digestive action, 115
Gelatin as article of diet, 158
Giant cells, 65
Glands, vaso-motor nerves of, 109, 241
Glycogen, formula, 147
as muscle food, 148
stored in liver, 147
Goitre, cause of, 84
Granules, appearance of, in glands, 110
of cerebellum, development of, 299,
303
Grey matter, formation of paths in, 356
Growth, a function of protoplasm, 24
a reaction to work, 47
Hematin, 68
Heematoidin, 68
Hemochromogen, 68
Hemoglobin, crystalline form, 66, 186
formula, 66
as oxygen carrier, 66, 186
spectrum, 68, 185
Hemophilia, non-coagulability of blood,
76
Hallucinations, 362
Headache, a pain in the scalp, 106, 319
the brain’s warning of fatigue, 269
from strain of eye-muscles, 268
Hearing, analysis of compound vibra-
tions, 405
capacity dependent upon educa-
tion, 422
fe ee Se
44 + HE BODY ATWORK
Hearing, Helmholtz’s theory of analysis
of sounds, 419
range of sensations, 418
sense of, 404
upper limit, 418
Heart, anatomy, 217
automatism of, 238
development, 218
murmurs, 229
muscular tissue, minute structure,
261
nerves regulating beat, 237, 239
sounds of, 228
valves, their mechanism, 226
work done by, 219, 223
Heat, production of, by muscles, 254, 256
Heat-spots in skin, 429
Helmholtz’s theory of organ of Corti, 419
Hering’s theory of colour-vision, 388
Hormones, meaning of term, 89, 124
of pancreas and liver, 127
of stomach, 123
Humours in ancient medical theory, 79
Hunter, experiment of grafting cock’s
spur in its comb, 47
Hydrochloric acid, part taken in diges-
tion, 114
Hydrophobia, protective inoculation, 78
Hyperpnoea, excessive respiratory
efforts, 182
Hypoblast, a layer of the embryo, 97
Illusions of movement, 335, 384
of size and distance, 400
Immunity, acquisition of, 20
Impulse of the heart, 225
rate of passage in muscle, 280
in nerve, 278, 280
theory of nerve conduction, 282
Inhibition, explanation of term, 311
of reflex actions, 311
‘Insects, efficiency of their muscles, 261]
Instinct, due to brain-pattern, 359
Intelligence of animals, 359
Internal secretions, 83
Intestinal juice, digestive action, 119
Intestine, large, sacculation of its walls,
101
small, folds and glands of mucous
membrane, 102
Intestines, movements of, 103
nerves of, 105
size and situation, 100
Iodine, importance of, to economy, 89
Todothyrin, goitre due to deficiency of, 90
‘Iris, its function in regulating admis-
sion of light to eye, 394
Iron in food, 67
in hemoglobin, 67
use of, in treatment of anemia, 67
Irritability, a function of protoplasm, 10
Japanese, cultivation of sense of smell —
y, 370 ‘
Judgment of angles, 402
of distance and size, 401
of meaning of sensations, 396, 399
Kidney, ancestral history, 195 vn
elimination of indigo by, 207 —
of birds and reptiles, 200, 207
hydrostatic mechanism, 189
minute anatomy, 196
Kinesthetic sensations, absence from
dreams, 363
part played by, in voluntary
actions, 354
representation in cortex o
rain, 350, 352
Knee-jerk, 274
Labyrinth of ear, 413
Lactate of ammonia, relation to urea,
13
Lacteals, lymphatic vessels of ali-
mentary canal, 43, 131
Lactic acid produced in muscle, 46, 146
Larynx, closure during swallowing, 433
structure of, 430
Latent period of muscle after nervous
impulse reaches it, 278
Laughter, respiratory mechanism of,
180
Lecithin produced by metabolism of
nerve-tissue, 118
Leech, ganglion cells of, 298
Leucocytes as protective agents, 52
death of, 54, 57
migration of, 49
number in lymph and in blood,
49, 61
origin of, 33, 51
source of fibrin-ferment, 74
Leucocythemia, excess of leucocytes
in the blood, 215
Levers to which muscles are attached,
286
Light, emission of, by animals, 291
Lime, influence upon coagulation of
blood, 75
curdling of milk, 75
Lithates, or urates, constituents of
calculi, 213
Liver, destruction of red _blood-cor-
puscles in, 83
form and structure of, 160
former theories of its functions,
129, 163
manufactures urea and uric acid,
146, 162
of well-fed sheep, 147
origin of, in vertebrate phylogeny,
34
Rate ee INTER
Liver stores food, especiall lycogen,
46, 145, 147, 161 iain
Locomotor ataxy, 341
— view of mechanism of kidney,
Luminous glands, 291
Lung, exchange of gases in, 173, 184, 190
nerve-supply, 178
structure, 168
Lymph, amount of, in body, 37
composition, 49
relation to blood, 51
Lymph-spaces, 37, 43, 49
Lymphatic glands, structure of, 54
Lymphatic vessels, 43
Malapterurus, electric organs, 288
immense neurones of, 295
Manometer for measuring blood-pres-
sure, description of, 238
Man’s ancestry, 153
Massage of abdominal viscera, 101
of muscles, 48
Meal, the story of a, 120
Meat, diet consisting solely of, 157
digestion of, 121
extracts of, as articles of diet, 159
Megacaryocytes, 65
Memory, physiological explanation, 356
Metabolism, chemical change in living
tissue, 12, 273
Methzmoglobin, 69
Microscope, its discovery, 26
Migration of birds, 359
Milk, call for secretion of, by a hormone,
chemical and physical constitu-
tion, 132
digestion of, 127
Milk diet, reduction of bacteria in ali-
mentary canal on, 138
Mind, physiology of, 354
Mosquitoes, production of sound by, 261
Motile cells, 32
Mountain sickness, 187
Mountains, highest climbed, 187
Mucous membrane, use of term, 97
Murmurs, in chest, in diseases of lungs,
169
of heart, 229
Muscle, change in appearance under
microscope during contraction,
263
chemistry of contraction, 266
coniraction a phenomenon of os-
mosis, 258
electric phenomena of, 278
means of promoting growth of, 271
measurement of its power, 285
nature of impulse which leads to
contraction of, 282
445
~ Muscle of heart, its minute structure,
224
of insects, its efficiency, 261
plain, its minute structure, 258
plasma, its coagulation, 266
rhythm of voluntary contraction,
279 :
theory of its structure as a mechan-
ism liberating energy, 234, 255
tone of, 272
tracings taken of contracting, 278
voluntary, its minute structure, 259
wastes when its nerve is severed,
274
work done by, proportional to
load, 286
Muscles, arrangement in regard to the
bones which they move, 286
co-operation in lifting a weight, 287
Muscular energy, source of, 235
Muscularis mucose of alimentary canal,
103
Musculi papillares of heart, 227
Music, chords admissible in, 408
Indian, division of octave, 408
primitive, prevalence of minor
chords, 408
Musical tones and overtones, 406
Myelination of nerves, order of, 345
Myxcedema, dependent on disease of
thyroid gland, 85
Myxomycetes, fusion of cell bodies of, 27
Nasal chambers, air warmed in, 166
Negroes, their long heels, 285
Nerve, conduction in, theory of, 282
degeneration, 326
electrical phenomena, 279
indefatigable, 282
regeneration, 326
structure, 296
Nerves, depressor, 237
experiment of crossing, 327
fifth, 316
of heart, 239
of intestines, 426
protopathic and critical systems
of, 425
secretory, of the salivary glands, 109
splanchnic, 236
superior laryngeal, 178
vagus, 104
vaso-motor, 239
Nerve-cells last throughout life, 148
limitations of their functions, 321
store of energy in, 320
transfer of impulses from cell to
cell, 177, 300
their relation to muscle-fibres, 274
' varying size of, 295, 322
Nerve-centres, 176
cer improper use of expression,
81
Nerve-impulses, distribution in grey
-matter, 305
reinforcement of, 320
resistance to, at synapses, 306
Nerve-nets, pericellular, 301, 319
Nervous system, neuronic and extra-
neuronic conduction, 310
phylogeny of, 332
Neuro-fibrille, 298
Neurone, origin of term, 293
transmission of current by, 328
various types of, 296, 323
N ight- blindness, 378
Nissl’s bodies, source of nervous energy,
320
Nitric oxide, combination with hzemo-
globin, 186
Nitrogenous equilibrium, 150
food, stimulating effect of, 157
waste, 210
Neud vital of Flourens, 176
Normal diet, 151
Normal salt solution, 82
Nucleo-proteins, source of uric acid, 215
Odours, classification of, 366
(Edema, or dropsy, 42
Olfactory membrane, structure, 366
Optic nerve, number of fibres, 378
Organ of Corti, structure, 415
theory of function, 417
Organs that have lost their prime func-
tions, 87
Orientation, sense of, 335
Osmosis, 40, 128, 201
cause of muscular contraction, 235
Osteoblasts, bone-forming cells, 32
Osteoclasts, bone-eating cells, 65
Oxygen, amount required per diem, 166
carried by red blood-corpuscles, 66
Pain, influence of, upon action, 359
referred from viscera to surface of
body, 316
relation to sensation, 313, 425
theory of, 312, 425
Pancreas, structure, 116
Pancreatic juice, constitution, 116
fat-splitting ferment of, 133
Papille of the tongue, various forms of,
Parathyroids, 86
Pepsin, digestive action, 115
Peptone prevents coagulation of blood,
717
Pericellular nerve-nets, 301
Perspiration, cools the surface of the
body, 236
repressed during fever, 257
Poyer’ s SERS: of lymph follicles in is
ee 53 : ony, 6.
agocytes, germ-eating leuc
yi fan of red blood - cor-
puscles by, 82
Phosphenes, developed by pressure on
eyeball, 383
Phosphorescence, cause of, 291
Phrenology; 343
Pictures, suggestion of solidity in, 401 —
Pineal body, phylogeny, 334
Pituitary body,
Plants, Edler ffi’ by ether, 12, 24
their metabolism, 15
their respiration, 24
Pleura, lining membrane of chest, 172
Pleurisy, pain of, 313
Pleuritic fluid, absorption of, 223
Pneumonia, changes in lung during, 169
Portal system of bloodvessels, 80
regulator of vascular tone, 236
Power of muscles, 285
Precipitins formed in blood, 19
Proteins, absorption by alimentary
canal, 145
chemical constitution, 6
dietetic value, 157
fate after absorption, 212
Protopathic nerves, 425
Protoplasm, arrangement in cells, 30
constitution, 7
Huxley’s ae 6
Pulse, cause of, 2
records of, 248
variations, 247
Purgatives, theory of action, 128
Purkinje cells of cerebellum, 303, 340
shadows of retinal vessels, 375
Pus, origin of, from leucocytes, 57
Pyramids of cortex of great brain, 346
Rabbit’s ear, vaso-motor changes in, 235
Receptor, an organ specially sensitive
to stimulation, 253
Referred pains from viscera, 316
Reflex action, inhibition of, 311
of scratching, 330
vinegar experiment with frog,
307
Regeneration of nerves, 326
Renal-portal circulation, 199
Renewal of tissues, 148
Rennin, ferment of milk, 16
Resistance in nervous system, laws of,
177, 307
Respiration, artificial, 179
effect on circulation, 221
a function of protoplasm, 23, 164
movements of, 171
nervous mechanism, 175, 179
in tissues, 165, 193
. Respiratory centre in medulla oblon-
gata, 17 > 178, 182
Respiratory quotient, 174
Retina, structure, 374
Retinal pigment, relation to vision, 381
Rice ordeal, arrest of secretion of
saliva, 112
Rigor mortis, 266
Rods and cones, respective functions in
vision, 378
Rowing, value of, as exercise, 287
Saccharin, taste of, 367
Saline frog, respiration in, 193
Saliva, chemical constitution, 107
function of, 96, 107
Salivary glands, mechanism of secre-
tion, 108
nerves of, 109, 236
Salts, absorption of, in alimentary
canal, 128
Scientific method, definition of, 71
Scratch reflex, in dog, 330
Sea-sickness, 106
Secretin, hormone of pancreas and liver,
127
Secretion, accumulation of granules in
cells, and their discharge, 110
a response to stimulation, 111
not a process of filtration, 110
Semicircular canals, their functions, 410
their positions in space, 335
Sensations, their apparent fusion, 356
many which escape attention, 318,
355
neutralization of one by another,
356
Sense-organs, origin in vertebrata, 336
Sensory areas in cortex of the great
brain, 348
Sensory nerves, their connection with
cerebro-spinal axis, 304
Shell-fish, poisonous extract of, 41
Shivering due to loss of heat from skin,
257
Sight. Cf. Vision
Skate, electric organs of, 289
Skilled movements, dependent upon
kinesthetic sensations, 357
Skin, experiment of cutting nerve, 424
variety of sensations from, 423
Sleep, condition of neurones in, 362
Sleeping sickness, 33
Smallpox, protection against, 78
Smell, disappearance of sense of, in later
life, 370
dog’s dependence upon sense of, 366
reason for mental associations with
sensations of, 371
sensitiveness to mercaptan, 365
Smells, nice and nasty, 369
447
Smoking, mental effect of, 371
Sneezing on looking at bright light, 317
Sore-throat, cause of deafness, 412
Soul, Aristotle’s definition, 32
Sound, mode of conduction, 404
rapidity of vibrations of, 406, 418
Sounds of the heart, 228
periodic and aperiodic, 409
Spectacles, defects of eyeball which call
for, 392
Speech, derangements of, due to dissase
of the brain, 353
mechanism of, 437
Sphygmographs for recording pulse, 245
Spinal dog, reflex action in, 330
frog, reflex action in, 307
ganglia, development of cells, 299
Splanchnic nerves, regulation of blood-
pressure by, 236
Spleen, destruction of blood-corpuscles
in, 80
structure, 79
Squint, correction of double vision in,
397
Starch, formula, 15
Star-shapes due to puckering of crys-
talline lens, 393
Starvation, statistics of, 156
Stiffness of muscles, cause of, 45, 271,
Stimuli to muscles and nerves, 248
Stokes, discovery of spectrum of blood,
68
Stomach, digestion in, 120
glands of, 123
referred pains from, 316
shape and size, 99
Stone in the bladder, its cause, 213
Subconscious self, 355
Sugars, digestion of, 120, 136
formule, 15
Sun, apparent size near horizon, 399
Suprarenal capsules, their structure and
function, 91
Sweetbread as article of diet, 215
Sympathetic system of nerves, 243, 325
diameter of fibres, 325
Synapses of nerve-cells, resistance inter-
posed at, 306
Synaptases, constructive ferments, 18
Synthesis by plants, 15
Tapeworms, resist digestion in the in-
testines, 21
Taste, confusion with sense of smell, 364
localization on tongue, 367
sense of, in fishes, 365
sensitiveness to quinine, 369
Taste-bulbs, their structure, 368
Tattooing, removal of pigment by leuco-
cytes, 55
Tea, its dietetic value, 122
ye Beate Oi ee a oy
« in a a)
, .
448
- Teeth, 96
Tendon, the growth of, from cells, 28
Tension of gases in the lungs, 190
Tetanus, the vibratile contraction of
mustle, 279
Thoracic duct, discharges lymph into
veins, 43, 131
Thorax, negative pressure in, 222
Thorns on
Thyroid body or gland, forms an in-
ternal secretion, 86
relation to goitre, 85
structure of, 85
Tight-lacing, deformation of organs
which it causes, 220
Tigroids, in nerve-cells, stores of energy,
320
Tissues, respiration in, 165, 193
Tone of muscles, 272
Tongue, as organ of taste, 367
Tonsils, function as guardians of the
fauces, 53
structure, 52
Torpedo, electric organs of, 290
Touch, sensations of, 426
Toxins produced by microbes, 20
Urea, amount relatively to proteins
consumed, 155
antecedents of, 146, 212
chemical formula, 211
secreted during period of starva-
tion, 156
Uric acid, amount secreted daily, 213
artificial production of, 13
chemical formula, 13, 214
diathesis, its relation to diet,
140
due to metabolism of leuco-
cytes, 53, 216
form in which excreted, 207
made in the liver of birds, 13
Urticaria due to abnormal composition
of lymph, 41
Vaccination, protective value of, 22
Valves of heart, their mechanism, 226
THE
THE BODY AT ‘WORK
endrites of nerve-cells, 300 -
Vascular svat tone of, 236, 240
Vaso-constrictor nerves, 236
Vaso-dilator nerves, 236.
Vegetables, dietetic value of, 139
digestion of, 125, 137
Vermiform appendix, 88
Villi of intestine, absorption of food
by, 130
fat seen in, during active di-
gestion, 134
Viscera, their insensitiveness to injury.
316, 426
Vision, colour contrasts, 382
duration of images, 382
judgment of distance and size, 411
solidity, 401
stereoscopic, doctrine of corre-
sponding points, 397
Visual purple, 381
Vital action, definition of expression,
205
Vivisection, 4
Vocal cords, structure, 431
how modified in singing, 435
Voice, breaking of, in boys, 434
falsetto, how produced, 435
range of human, 435
registers, 436
Vomiting, 105
Vowels, synthesis by tuning-forks, 439
Wandering cells, 33
Warmth, appreciation of, by skin, 429
Waste substances, classification, 194
how eliminated from body, 59
Waterfall, negative after-image of, 384
Water-weed, experiment proving that
it respires, 24
Wear and tear of bioplasm, 145
Wisdom-tooth, tending to disappear, 96
Yawning, beneficial effect on circula-
tion, 222
nervous mechanism of, 180
Young’s theory of colour-vision, 385
Zymogen, 110
END
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a Mr. Pawar Arnold’s List of |
Technical & Scientific Publications
Extract from the LIVERPOOL POST of Dec. 4, 1907:—
“During recent years Mr. Edward Arnold has placed in the hands of
engineers and others interested in applied science a large number of
volumes which, independently altogether of their intrinsic merits as
scientific works, are very fine examples of the printers’ and engravers’
art, and from their appearance alone would be an ornament to any scien
tific student’s library. Fortunately for the purchaser, the publisher has
shown a wise discrimination in the technical books he has added to his
list, with the result that the contents of the volumes are almost without
exception as worthy of perusal and study as their appearance is
attractive.”
:
The Dressing of Minerals.
By HENRY LOUIS, M.A,
Professor of Mining and Lecturer on Surveying, Armstrong College, Newcastle-on-Tyne.
With about 4oo Illustrations. Royal 8vo., 30s. net.
The object of this book is to fill a gap in technological literature which exists
between works on Mining and works on Metallurgy. On the interme-
diate processes, by which the minerals unearthed by the miner are
prepared for the smelter and for their use in arts and manufactures, no
English text-book has yet appeared. The present work should, therefore,
be very welcome to students, as well as to miners and metallurgists,
ARNOLD’S GEOLOGICAL SERIES.
GENERAL EDITOR: DR. J. E. MARR, F.R.S.
The economic aspect of geology is yearly receiving more attention in our
great educational centres, and the books of this series are designed in the
first place for students of economic geology. It is believed, however,
that they will be found useful to the-student of general geology, and
also to surveyors and others who are concerned wiih the practical appli-
cations of the science.
The Geology of Coal and Coal-Mining.
By WALCOT GIBSON, D.Sc., F.G.S.
352 pages. With 45 Illustrations. Crown 8vo., 7s. 6d. net (post free 7s. rod.).
Many years’ professional experience among the coalfields of this country
and of South Africa have enabled the author to treat his subject in a
thoroughly original manner. The book therefore contains a great
amount of valuable information, as well as many criticisms and suggestions
which have not hitherto appeared in any text-book on the subject.
IN PREPARATION.
The Geology of Ore Deposits.
By H. H. THOMAS anp D. A. MACALISTER,
Of the Geological Survey.
Illustrated. Crown 8vo., 7s, 6d, net (post free 7s, 1od.).
LONDON: EDWARD ARNOLD, 41 & 43 MADDOX STREET, W.
bah “Electrical Trackow } ih
By ERNEST WILSON, Wut. Scu., M.I. E. ee a
Professor of Electrical Engineering in the Sieh ena Laboratory, King’s College, London, —
AND FRANCIS LYDALL, B.A., B.Sc.
NEW EDITION. REWRITTEN AND GREATLY ENLARGED.
Two volumes, sold separately. Demy 8vo., cloth.
Vol. 1., with about 270 Illustrations and Index.
Vol. II., with about 170 Illustrations and Index.
I5s. net each volume (post free 15s. 5d.).
‘‘ We are most decidedly of the opinion that both of these volumes will prove of
great value to engineers, and that the last volume, in view of the present great interest
in the question of single-phase traction, is of the utmost importance, for in it for the
first time is published a great amount of data with reference to which, hitherto, the
manufacturing companies concerned have observed great secrecy.”"—7" he Times
(Engineering Supplement).
A Text-Book of Electrical Engineering.
By Dr. ADOLF THOMALEN.
Translated by G. W. O. HOWE, M.Sc., Wut. Scu., A.M.LE.E.,
Lecturer in Electrical Engineering at the Central Technical College, South Kensington.
With 454 Illustrations. Royal 8vo., cloth, 15s. net (post free 15s. 6d.).
This translation of the “ Kurze Lehrbuch der Electrotechnik ” is intended
to fill the gap which appears to exist between the elementary
text-books and the specialized works on various branches of electrical
engineering.
Alternating Currents.
A Text-Book for Students of Engineering.
By C. G. LAMB, M.A., B.Sc.,
Clare College, Cambridge ; Associate Member a the Institution of Electrical Engineers ; :
Associate of the City and Guilds of London Institute.
vili+ 325 pages. With upwards of 230 Illustrations. Demy 8vo., cloth,
Ios. 6d. net (post free 10, I1d.).
Electric and Magnetic Circuits.
By ELLIS H. CRAPPER, M.1I.E.E.,
meread of the Electrical Engineering Department in the University College, Sheffield.
viii+380 pages. Demy 8vo., cloth, Ios. 6d. net (post free 1os. 11d.).
Applied Electricity.
A Text-Book of Electrical Engineering for ‘‘ Second Year” Students.
By J. PALEY YORKE,
Head of the Physics and Electrical Engineering Department at the London County Council
chool of Engineering and Navigation, Poplar.
xii+ 420 pages. Crown 8vo., cloth, 7s. 6d.
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nie ssw he F :
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_ By’F. C. LEA, B.Sc., A.M.INST.C.E.,
vorth Scholar, A.R.C.S.; Lecturer in Applied Mechanics and Engineering Design,
_ City and Guilds of London Central Technical College, London.
‘ , With 367 Diagrams. Demy 8vo., 18s. net (post free 18s. 5d.).
_ This book is intendedyto supply the want felt by students and teachers
q alike for a text-book of Hydraulics to practically cover the syllabuses
of London and other Universities, and of the Institution of Civil
Engineers.
Hydraulics.
By RAYMOND BUSQUET,
Professeur & I’Ecole Industrielle de Lyon.-
AUTHORIZED ENGLISH EDITION.
Translated by A. H. PEAKE, M.A.,
Demonstrator in Mechanism and Applied Mechanics in the University of Cambridge.
vilit+ 312 pages. With 49 Illustrations. Demy 8vo., cloth, 7s. 6d. net
; (post free 7s, 10d.).
sz The Balancing of Engines.
By W. E. DALBY, M.A., B.Sc., M.INST.C.E., M.I.M.E.,
Professor of Engineering, City and Guilds of London Central Technical College.
SECOND EDITION, REVISED AND ENLARGED.
xli+ 283 pages. With upwards of 180 Illustrations.
Demy 8vo., cloth, Ios. 6d. net (post free ros. Iod.).
Valves and Valve Gear Mechanisms.
BY W. . DALBY, M.A.;. B.Sc. M.INST.C.E....M.1M.E:.
Professor of Engineering, City and Guilds of London Central Technical College.
xvili+ 366 pages. With upwards of 200 Illustrations.
Royal 8vo., cloth, 21s. net (post free £1 Is. 5d.).
Power Gas Producers.
Their Design and Application.
By PHILIP W. ROBSON,
Of the National Gas Engine Co., Ltd. ; sometime Vice-Principal of the Municipal Schoul of
Technology, Manchester.
Demy 8vo., cloth, 10s. 6d. net (post free 10s. 11d.).
The recent enormous increase in the use of gas power is largely due to
the improvements in gas producers. This book, which is written
by a well-known expert, goes thoroughly into the theory, design,
general management.
and application of all kinds of plants, with chapters on working and»
|The Mudcat hd { miasdeny: of Stru . 3 L
z Members. | oe
By R. J. WOODS, M.E., M.INsT.C.E.,
_ Formerly Fellow and Assistant Professor of Roginseciag, Royal Indian Engineering College,
Cooper’s Hill.
SECOND EDITION, REVISED.
xii+310 pages. With 292 Illustrations. Demy 8vo., cloth, tos. 6d. net
(post free ros. tod.).
Calculus for Engineers.
By JOHN PERRY, M.E., D.Sc., F.R.S.,
Professor of Mechanics and Mathematics in the Royal College of Science, London ;
Vice-President of the Physical Society; Vice-President of the Institution
of Electrical Engineers.
NINTH IMPRESSION.
viiit+382 pages. With 106 Illustrations. Crown 8vo., cloth, 7s. 6d.
Mathematical Drawing.
Including the Graphic Solution of Equations.
By G. M. MINCHIN, M.A,, F.RS.,
Formerly Professor of Applied Mathematics at the Royal Indian Engineering mes
Cooper’s Hill ;
AND JOHN BORTHWICK DALE, M.A.,
Assistant Professor of Mathematics at King’s College, London.
Crown 8vo., cloth, 7s. 6d. net (post free 7s. 1od.).
Five-Figure Tables of Mathematical Functions.
Comprising Tables of Logarithms, Powers of Numbers, Trigonometric,
Elliptic, and other Transcendental Functions.
By JOHN BORTHWICK DALE, M.A.,
Assistant Professor of Mathematics at King’s College, London.
vi+92 pages. Demy 8vo., cloth, 3s. 6d. net (post free 3s. 1od.).
This collection of Tables has been selected for use in the examinations
of the University of London.
Logarithmic and Trigonometric Tables (To
Five Places of Decimals) By JOHN BORTHWICK DALE, M.A.,
Assistant Professor of Mathematics at King’s College, London.
Demy 8vo., cloth, 2s. net (post free 2s. 3d.).
Traverse Tables. With an Introductory Chapter
on Co-ordinate Surveying. By HENRY LOUIS, M.A., Professor
of Mining and Lecturer on Surveying, Armstrong College, New-
castle-on-Tyne ; and G. W. CAUNT, M.A., Lecturer in Mathematics,
Armstrong College, Newcastle-on- Tyne. XXvill + 92 pages. Demy
8vo., flexible cloth, rounded corners, 4s. 6d net (post free 4s. 9d.).
’
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4
Technical and Scientific Publications — 5
By JULIUS B. COHEN, PuH.D., B.Sc.,
Professor of Organic Chemistry in the University of Leeds, and Associate of Owens
é College, Manchester.
Demy 8vo., cloth, 21s. net (post free 21s. 6d.).
The book is written for students who have already completed an
elementary course of Organic Chemistry, and is intended largely to
take the place of the advanced text-book. For it has long been the
opinion of the author that, when the principles of classification and
synthesis and the properties of fundamental groups have been
acquired, the object of the teacher should be, not to multiply
facts of a similar kind, but rather to present to the student a broad
and general outline of the more important branches of the subject.
This method of treatment, whilst it avoids the dictionary arrange-
ment which the text-book requires, leaves the writer the free disposal
of his materials, so that he can bring together related substances,
irrespective of their nature, and deal thoroughly with important
theoretical questions which are often inadequately treated in the
text-book.
The Chemical Synthesis of Vital Products and
the Inter-relations between Organic
Compounds.
BY RATHAEL MELDOLA,. FR, V:P:C.5.50.0G.. ete,
Professor of Chemistry in the City and Guilds of London Technical College, Finsbury.
Vol. I., xvit+338 pages. Super Royal 8vo., cloth, 21s. net
(post free 21s. 5d.).
The great achievements of modern Organic Chemistry in the domain of
the synthesis or artificial production of compounds which are known
to be formed as the result of the vital activities of plants and animals
have not of late years been systematically recorded. The object of the
present book is to set forth a statement, as complete as possible, of the
existing state of knowledge in this most important branch of science.
The Chemistry of the Diazo-Compounds.
By JOHN CANNELL CAIN, D.Sc. (Manchester and Tibingen),
Editor of the Publications of the Chemical Society.
176 pages. Demy 8vo., Ios. 6d. net (post free ros. 1od.).
Lectures on Theoreticaland Physical Chemistry.
By Dr. J. H. VAN ’T HOFF,
Professor of Chemistry at the University of Berlin.
Translated by R. A. LEHFELDT, D.Sc,
Professor of Physics at the Transvaal Technical Institute, Johannesburg.
In three volumes, demy 8vo., cloth, 28s. net (post free 28s. 7d.),
or separately as follows :
PARTI. CHEMICAL DYNAMICS. 254 pages, with 63 Illustra-
tions. 12s. net (post free 12s. 5d.).
PaRT I]. CHEMICAL STATICS. 156 pages, with 33 Illustrations.
8s. 6d. net (post free 8s. 1od.).
PART III. RELATIONS BETWEEN PROPERTIES AND
COMPOSITION. 143 pages, 7s. 6d. net (post free 7s. 10d.)
on
with the -
_Experimental Researches.
Furnace.
Membre de I’Institut ; Professor of Chemistry at the Sorbonne.
AUTHORIZED ENGLISH EDITION. _ ’
Translated by A. T. de MOUILPIED, M.Sc., Ph.D., on
Assistant Lecturer in Chemistry in the University of Liverpool.
xii+ 307 pages, with Illustrations. Demy $8vo., cloth, Ios. 6d. net
(post free 10s. 10d.). j
“There is hardly a page of it which is not crowded with interest, and hardly a
section which does not teem with suggestion ; and if the coming of this English edition
of the book has been so long delayed, we may still be thankful that it has come at last,
and come in a form which it is a pleasure to handle and a delight to read,” —/ectrical
Review,
Electrolytic Preparations.
Exercises for use in the Laboratory by Chemists and
Electro-Chemists. 3
By Dr. KARL ELBS,
Professor of Organic and Physical Chemistry at the University of Giessen.
Translated by R. S. HUTTON, M.Sc.,
Demonstrator and Lecturer on Electro-Chemistry at the University of Manchester.
xii+100 pages. Demy 8vo., cloth, 4s. 6d. net (post free 4s. 1od.).
The book contains a complete course of examples on the application of
electrolysis to the preparation of both inorganic and organic sub- |
stances. It will be found useful as filling a distinct gap in the text-
book literature suitable for use in chemical laboratories, and should
enable the chemist to make use of the many valuable and elegant
methods of preparation which have been worked out during recent
years, the advantages and ease of application of which he cannot
appreciate without such a guide.
Introduction to Metallurgical Chemistry for
Technical Students.
By J. H.. STANSBIE, .B-Sc..(LOND.),..F..1.G.,
Associate of Mason University College, and Lecturer in the Birmingham University Technical
School.
SECOND EDITION.
xli+252 pages. Crown 8vo., cloth, 4s. 6d.
An Experimental Course of Chemistry for Agri-
cultural Students. By T. S. Dymonp, F.I.C., Lately Principal Lecturer
in the Agricultural Department, County Technical Laboratories, Chelmsford.
New Impression. 192 pages, with 50 Illustrations. Crown 8vo., cloth,
2s. 6d.
A History of Chemistry.
By Dr. HUGO BAUER,
Royal Technical Institute, Stuttgart.
Translated by R. V. STANFORD, B.Sc. (LOND.),
Priestley Research Scholar in the University of Birmingham.
Crown 8vo., cloth, 3s. 6d. net.
- By HENRI MOISSAN, | —
is da . STRUTT, F.R.S., .
— + Fellow of Trinity College, Cambridge.
_ SECOND EDITION, REVISED AND ENLARGED.
: bd ‘ . .
___—_viii++222 pages, with Diagrams. Demy 8vo., cloth, 8s. 6d. net
| (post free 8s. Iod.).
nf
-_ ‘Tf only a few more books of this type were written, there might be some hope of
a general appreciation of the methods, aims, and results of science, which would go
far to promote its study. . . . A book for which no praise can be excessive.” —
_ Atheneum.
_ Astronomical Discovery.
By HERBERT HALL TURNER, D.Sc, F.R.S.,
Savilian Professor of Astronomy in the University of Oxford.
xli+225 pages, with Plates and Diagrams. Demy 8vo., cloth, ros. 6d. net
. (post free Ios. 11d.).
An Introduction to the Theory of Optics.
By ARTHUR SCHUSTER, Ph.D., Sc.D., F.R.S.,
Recently Professor of Physics at the University of Manchester.
xvi+ 340 pages, with Illustrations. Demy 8vo., cloth, 15s. net
(post free 15s. 5d.).
‘We know of no book written with a set purpose better adapted to serve the
purpose for which it was written, nor any that the earnest student of optics will find
more interesting and profitable. The work itself, without the confession of the preface,
shows that Professor Schuster is a teacher, and every page bears evidence that he is a
master of his subject. ... We heartily recommend the book to our readers.’”’—
Ophthalmic Review.
Wood.
A Manual of the Natural History and Industrial Applications of the
Timbers of Commerce.
BY GC: S-BOULGER FF LS- FCS. A6 1)
Professor of Botany and Lecturer on Forestry in the City of London College, and formerly in the
Royal Agricultural College.
NEW EDITION. Revised and Enlarged and profusely illustrated.
Demy 8vo., 12s. 6d. net (post free 12s. 11d.).
“It is just the book that has long been wanted by land agents, foresters, and wood-
men, and it should find a place in all technical school libraries.” —Fie/d.
Manual of Alcoholic Fermentation and the
Allied Industries.
By CHARLES G. MATTHEWS, F.L.C., F.C.S., ETC.
xvi+295 pages, with 8 Plates and 40 Illustrations. Crown 8vo., cloth,
7s. 6d. net (post free 7s. 10d.).
; “This is a book worthy of its author, and well worth perusing by every student.
a . . . The student, both old and young, as well as the practical brewer, will find this
7 book gives him some very useful information.”—Brewers’' Guardian.
The Evolution Theory. By D:
MANN, Professor of Zoology in the University of |
Translated, with the Author’s co-operation, by J. ARTHUR TH
_. Regius Professor of Natural History in the University of Aberdeen
MARGARET THOMSON. Two vols., xvi+ 416 and viii + 396 pages, wi
130 Illustrations. Royal 8vo., cloth, 32s. net. .
‘« The subject has never been so fully and comprehensively expounded before ; and
i ca
and the absorbing interest of his exposition, with its prodigious wealth of illustration,
its vast store of zoological knowledge, its ingenious interpretations and far-reaching _
it is not necessary to subscribe to all the author’s tenets in order to recognise the a
theories. English readers have reason to be grateful to Professor and Mrs. Thomson ©
for their admirable translation.” — 7he Times.
The Chances of Death and Other Studies in
Evolution. By Kart Prarson, M.A., F.R.S., Professor of Applied
Mathematics in University College, London, and formerly Fellow of King’s
College, Cambridge. 2 vols., xii + 388 and 460 pages, with numerous
Illustrations. Demy 8vo., cloth, 25s. net (post free 25s. 6d.).
The Life of the Salmon. With reference more
especially to the Fish in Scotland. By W. L. CALDERWOOD, F.R.S.E.,
Inspector of Salmon Fisheries for Scotland. Illustrated. Demy $vo.,
7s. 6d. net.”
- « We have no hesitation whatever in advising al] persons interested in the salmon,
whether as fishermen, naturalists, or legislators, to add this book to their libraries.”’
—Nature.
Animal Behaviour. By Professor C. Ltioyp
MorcGan, LL.D., F.R.S., Principal of University College, Bristol.
vili+ 344 pages, with 26 Illustrations. Crown 8vo., cloth, 7s. 6d. net (post
free 7s. I1d.).
This important contribution to the fascinating subject of animal psycho-
logy covers the whole ground from the behaviour of cells up to that
of the most highly developed animals.
BY THE SAME AUTHOR. .
Habit and Instinct. viii+ 352 pages, with Photo-
gravure Frontispiece. Demy 8vo., cloth, 16s.
Professor ALFRED RUSSEL WALLACE :—‘“‘ An admirable introduction to the study
of a most important and fascinating branch of biology, now for the first time based
upon a substantial foundation of carefully observed facts and logical induction from
oo BY THE SAME AUTHOR. :
The Springs of Conduct. Cheaper Edition.
viii+317 pages. Large crown 8vo., cloth, 3s. 6d. This volume deals
with the Source and Limits of Knowledge, the Study of Nature, the Evolution
of Scientific Knowledge, Body, and Mind, Choice, Feeling, and Conduct.
' BY THE SAME AUTHOR.
Psychology for Teachers. New Edition, entirely
rewritten. xii+308 pages. Crown 8vo., cloth, 4s. 6d.
An Introduction to Child Study. By Dr. W. B.
DRUMMOND. Crown 8vo., cloth, 6s. net.
The Child’s Mind: Its Growth and Training. By
W. E. Urwick, University of Leeds. Crown 8vo., cloth, 4s. 6d. net.
LONDON: EDWARD ARNOLD, 41 & 43 MADDOX STREET, W.
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