THE LIBRARY

OF

THE UNIVERSITY
OF CALIFORNIA

PRESENTED BY

PROF. CHARLES A. KOFOID AND
MRS. PRUDENCE W. KOFOID

THE PHYSIOLOGY^*-

OF THE

DOMESTIC ANIMALS

A TEXT-BOOK FOR VETERINARY AND MEDICAL STUDENTS
AND PRACTITIONERS.

BY

ROBERT MEADE SMITH, A.M., M.D.,

PROFESSOR OF COMPARATIVE PHYSIOLOGY IN THE UNIVERSITY OF PENNSYLVANIA

FELLOW OF THE - COLLEGE OF PHYSICIANS AND . ACADEMY OF THE

NATURAL SCIENCES, PHILADELPHIA; OF THE AMERICAN

PHYSIOLOGICAL SOCIETY; OF THE AMERICAN SOCIETY

OF NATURALISTS; ASSOCIE ETRANGER DE LA

SOCIETE FRANCAISE D' HYGIENE, ETC.

WITH OVER 400 ILLUSTRATIONS.

PHILADELPHIA AND LONDON :

F. A. DAVIS, PUBLISHER

1890.

Entered according to Act of Congress, in the year 1889, by

F. A. DAVIS,

In the Office of the Librarian of Congress, at Washington, D. C., U. S.
All rights reserved.

Philadelphia:

The Medical Bulletin Printing House,
1231 Filbert Street.

TO MY FRIEND AND TEACHER,

CARL LUDWIG,

IN

HUMBLE RECOGNITION OF THE MANY FAVORS
CONFERRED ON THE AUTHOR.

M351840

PREFACE.

IN lecturing in the Veterinary Department of the Univer-
sity of Pennsylvania the author has found it a serious disad-
vantage that the students are compelled to rely solely on
the notes that they may be able to take during the lectures.
While French students have access to the encyclopaedic work
of Colin, and those familiar with the German language to the
admirable works of Schmidt-Miilheim, Bruckmiiller, Munk,
Ellenberger, Ghirlt, Thanhoffer, Miiller, and others, English-
speaking students have absolutely no work to which they
can turn to obtain any application of the laws of physiology
to* the functions of the domestic animals. Commenced
originally as outline notes for the author's own use in
lecturing, this work has been published at the request of his
students, in the hope that it may supply them with an
exponent of the laws of modern physiology applied, as far
as possible, to the functions of the domestic animals, and
that a recognition of its shortcomings may stimulate inves-
tigation of this much-neglected branch of physiology.

It is surprising, in view of the ceaseless activity of
physiological students throughout all the world, that more
attention has not been devoted to the application of improved
methods of research to the study of the functions of animals
so important in the domestic economy. Unfortunately,
investigators in this domain may almost be counted on the
fingers, and the field which is yet untouched is almost
unbounded. The author, therefore, has been compelled to
assume that in many cases the laws of the physiology of
man, which, to be sure, have been deduced from experiments

(v)

vi PREFACE.

on animals, are applicable to the vital processes of the
domestic animals.

Modern physiology rests on the application through
experimental research of the laws of physics and chemistry.
The fundamental principles of these sciences in their relation
to biology have been, therefore, discussed somewhat at
length. Experience has taught that a comprehension of the
laws of life in the higher mammals is best attained after a
familiarization with the vital operation of lower forms. The
first part of this book, therefore, deals with the general laws
of life, while in the second part these principles are applied
to the study of the vital operations in the domestic animals,
the study of each function being introduced by a sketch of
the mode of development of the mechanism by which that
function, in passing from lower to higher forms, is accom-
plished.

As far as possible the author has acknowledged in the
text his indebtedness to various authorities for the matter
or manner of his subject, though references to publications,
as tending to confuse the student, have been omitted.

For illustrations the author is indebted to the liberality
of the publisher, Mr. Davis, and to Messrs. Blakiston and
H. C. Lea & Co., of Philadelphia ; Appleton, Win. Wood & Co.,
and Macmillan, of New York; Ferdinand Enke, Stuttgart;
Engelmann and F. C. W. Vogel, Leipsic ; Paul Parey, Berlin ;
W. Braumiiller, Vienna; Carl Winter, Heidelberg; Hachette &
Cie, Bailliere & Fils, Asselin & Cie, Paris; Simpkin, Marshall
& Co., London; Moritz Perles, Vienna, and Hirschwald, Berlin.

ROBERT MEADE SMITH.

PHILADELPHIA:

332 SOUTH TWENTY-FIRST STREET,
January 3, 1889.

TABLE OF CONTENTS.

PAGK

INTRODUCTION, 1

PART I.

GENERAL PHYSIOLOGY.
THE PHYSIOLOGY OF ANIMAL CELLS.

SECTION I.

THE STRUCTURE OF ORGANIZED BODIES.

I. THE GENERAL PROPERTIES OF CELLS, . . . . .12

II. THE ORIGIN OF CELLS, . .14

III. THE MODIFICATION IN THE FORM OF CELLS, .... 26
IT. THE DEVELOPMENT OF TISSUES AND ORGANS, . . , 31

SECTION II.

CELLULAR PHYSICS.

I. THE PHYSICAL PROCESSES IN CELLS, . . . ... 37

1. Cohesion, 38 ; 2. Adhesion, 40 ; 3. Capillarity, 40 ; 4. Solution, 43 ;
5. Imbibition, 44 ; 6. Filtration, 48 ; 7. Diffusion of Liquids, 49 ;
8. Osmosis, 51; 9. Diffusion of Gases, 56 ; 10. Absorption of Gases,
58.

II. THE PHYSICAL PROPERTIES OF THE TISSUES, . . . .61

1. Cohesion, 62 ; 2. Elasticity, 65 ; 3. Optical Characteristics, 68 ; 4.
Electrical Phenomena, 70.

III. MECHANICAL MOVEMENTS IN CELLS, .70

1. Motion Produced by Imbibition in Cells, 70 ; 2. Protoplasmic Move-
ments, 72 ; (1) Movements in Protoplasmic Contents of Cells,
73 ; (2) Ciliary Movement, 77 ; (3) Movement in Specialized
Contractile Tissues, 81.

GENERAL CONDITIONS GOVERNING PROTOPLASMIC MOVEMENT, . 82
1. Temperature, 82; 2. Degree of Imbibition, 82; 3. The Supply of
Oxygen, 83; 4. Various Chemical and Physical Agents, 84.

(vii)

Vlll TABLE OF CONTENTS,

SECTION III.

CELLULAK CHEMISTRY.

I- PAGE

THE CHEMICAL CONSTITUENTS OF ORGANIZED BODIES, ... 85

A. NITROGENOUS ORGANIC CEL-CONSTITUENTS PROTEIDS AND THEIR

DERIVATIVES, 88

Class I. Albumens, 92; (1) Serum-Albumen, 92; (2) Egg Albumen,
93; (3) Vegetable Albumens, 93.

Class II. Globulins, 96 ; (1) Vitellin, 97; (2) Myosin, 97 ; (3) Para-
globulin, 97 ; (4) Fibrinogen, 97; (5) Globulin or Crystallin, 97.

Class III. Fibrins, 98.

Class IV. Derived Albuminates, 98 ; (1) Acid Albumen, 98; (2)
Alkali Albumen, 101.

Class V. Coagulated Proteids, 102.

Class VI. Amyloid Substance or Lardacein, 102.

Class VII. Peptones, 103.

ALBUMINOIDS, . . . . . 104

1. Mucin, 104; 2. Collagenous Albuminoids, 105; (a) Collagen, 106;
(b) Gelatin, 106 ; (c) Chondrogen, 107 ; (d) Chondrin, 107 ; 3.
Elastin, 108 ; 4. Keratin, 109.

DECOMPOSITION OF ALBUMINOUS BODIES, . . *" . . .109
FERMENTS, '- V ' HO

B. NON-NlTROGENOUS ORGANIC CELL-CONSTITUENTS, . . . . 112

I. CARBOHYDRATES, - . .. . .112

(a) Starches, 112; (1) Starch, 113; (2) Cellulose, 115; (3) Dextrin,
116 ; (4) Glycogen, 116 ; (5) Inulin, 116.

(5) Glucoses, 117; (1) Grape-Sugar, 117; (2) Laevulose, 118; (3)
Inosite, 118.

(c) Saccharoses, 119 ; (1) Cane-Sugar, 119 ; (2) Maltose, 120; (3) Lac-
tose, 120 ; (4) Arabin, 120.

II. HYDROCARBONS OR FATS, 120

C. INORGANIC CELL-CONSTITUENTS, 123

1. Water, 124 ; 2. Sodium Chloride, 128 ; 3. Potassium Chloride, 129 ;
4. Sodium and Potassium Carbonates, 129; 5. Calcium Carbonate,
130; 6. Magnesium Carbonate, 130 ; 7. Alkaline Phosphates, 130 ;
8. Calcium Phosphate, 133 ; 9. Magnesium Phosphate, 134 ; 10.
Sodium and Potassium Sulphates, 135 ; 11. Hydrochloric Acid,
135.

II.

THE CHEMICAL PROCESSES IN CELLS, 136

1. The Vegetable Cell, 137 ; 2. The Animal Cell, 142 ; 3. Fermenta-
tions, 145 ; 4. The Consumption and Development of Force in
Cells, 147.

TABLE OF CONTENTS. IX

PART II.
SPECIAL PHYSIOLOGY.

BOOK FIRST.
THE NUTRITIVE FUNCTIONS.

SECTION I.

FOODS. PAGE

I. VEGETABLE FOODS, 161

1, The Cereals, 163 ; 2. The Leguminous Plants, 173 ; 3. Bulbs and
Roots, 174 ; 4. Grasses, 175.

II. ANIMAL FOODS, ... V 188

1. Milk, 188 ; 2. Meat, 188.

III. INORGANIC FOODS, . ..191

1. Water, 191 ; 2. Nutritive Salts, 192.

IV. THE DIET OF ANIMALS, *-./-, 193

SECTION II.

DIGESTION.
I. GENERAL CHARACTERISTICS OP THE DIGESTIVE APPARATUS, . 203

IT. PREHENSION OF FOOD, . .226

1. Prehension of Solids, 226 ; 2. Prehension of Liquids, 236.

III. MASTICATION, . 238

1. Movements of the Jaws, 241 ; 2. Action of the Teeth in Mastica-
tion, 245 ; 3. Determination of Age by the Teeth, 255 ; 4. Action
of Tongue, Lips, and Cheeks, 264.

IV. DIGESTION IN THE MOUTH, . . . ..".. . , . 268

The Salivary Secretion, 268 ; 1. The Parotid Secretion, 274; 2. The
Submaxillary Secretion, 279 ; 3. The Sublingual Secretion, 283 ;
4. General Characters of the Salivary Secretion, 284; 5. The
Quantity of Saliva, 286 ; 6. The Physiological Role of the Saliva,
287 ; 7. The Mechanism of Salivary Secretion, 293.

!_(

V. DEGLUTITION, 307

VI. RUMINATION, . . . 316

VII. VOMITING, ^ 331

X TABLE OF CONTENTS.

PAGE

VIII. GASTRIC DIGESTION, 337

1. Chemistry of the Gastric Juice, 342 ; (a} Pepsin, 346 ; (5) Milk-
Curdling Ferment, 347 ; (c) The Acid of Gastric Juice, 349 ; 2.
The Action of Gastric Juice on the Food, 351 ; 3. The Secretion of
Gastric Juice, 356 ; 4. Gastric Digestion in Carnivora, 360 ; 5.
Gastric Digestion in Omnivora, 363 ; 6. Gastric Digestion in Soli-
pecles, 368 ; 7. Gastric Digestion in Ruminants, 374 ; 8. Gastric
Digestion in Birds, 379,

IX. DIGESTION IN THE SMALL INTESTINE, 382

I. Bile, 382 ; 1. The Chemical Characteristics of the Bile, 383 ; (a)

Mucin, 384; (6) The Bile Acids, 384; (c) The Coloring Matters of
the Bile, 387 ; (d) Cholesterin, 389 ; O) The Inorganic Constitu-
ents of the Bile, 390 ; 2. The Secretion of the Bile, 391 ; 3. The
Physiological Action of the Bile, 393.

II. The Pancreatic Secretion, 396 ; 1. The Chemical Composition of

the Pancreatic Juice, 402; The Pancreatic Ferments, 404 ; 2. The
Action of the Pancreatic Juice on Food-Stuffs, 405 ; (a) Action
on Carbohydrates, 406 ; (&) Action on Fats, 406 ; (c) Action on
Proteids, 408 ; 3. The Secretion of Pancreatic Juice, 412.

III. The Intestinal Juice, 416.

IV. Fermentative Processes in the Small Intestine, 418.
V. Intestinal Digestion in Different Animals, 419.

X. DIGESTION IN THE LARGE INTESTINE, . . , i .. 423

1. The Functions of the Caecum, 423 ; 2. The Functions of the
Colon, 429.

XI. THE COMPARATIVE DIGESTIBILITY OF DIFFERENT FOOD-STUFFS, 432
XII. THE COMPOSITION OF FAECES, . . , . . . . 445

XIII. THE MOVEMENTS OF THE INTESTINES, ..... 448

XIV. DEFECATION, . .'451

SECTION III.
ABSORPTION, . 453

1. Venous Absorption, 453 ; 2. Absorption by the Lymphatics, 456.

SECTION IV.
THE CHYLE, . ' 459

SECTION V.
THE LYMPH, .....;. 463

TABLE OF CONTENTS. JO.

SECTION VI. PAGE

THE BLOOD, . 469

1. The Red Blood-Corpuscles, 471 ; 2. The White Blood-Corpuscles,
479 ; 3. Blood-Plasma and Blood Coagulation, 483 ; 4. The Blood-
Serum, 489.

SECTION VII.

THE CIRCULATION OF THE BLOOD, 491

1. General View of the Organs of Circulation, 491 ; 2. The Action
of the Heart, 499 ; 3. The Hydraulic Principles of the Circula-
tion, 516 ; 4. The Circulation in the Arteries, 523 ; Blood Pres-
sure, 525 ; Velocity of the Blood, 530 ; The Pulse, 533 ; 5. The
Circulation in the Capillaries, 536 ; 6. The Circulation in the
Veins, 539 ; 7. The Influence of the Nervous System on the
Heart, 540 ; The Inhibitory Nerves of the Heart, 549 ; The Ac-
celerator Nerves of the Heart, 551 ; 8. The Influence of the
Nervous System on the Arteries, 552.

SECTION VIII.

RESPIRATION, . ... 561

1. General View of the Organs of Respiration, 562 ;. 2. The Mechani-
cal Processes of Respiration, 574 ; 3. The Rhythm of Respiration,
579; 4. The Chemical Phenomena of Respiration,. 587 ; 5. The
Nervous Mechanism of Respiration, 598 ; 6. The Influence of
Respiration on the Circulation, 605.

SECTION IX.
THE MAMMARY SECRETION, 609

1. The Physical and Chemical Properties of Milk, 610 ; 2. Casein and
Milk Coagulation, 614 ; 3. Milk-Sugar, 616 ; 4. Fat and Cream,
617 ; 5. The Inorganic Constituents of Milk, 619 ; 6. Variations
in the Quantity and Composition of Milk, 619 ; 7. The Secretion
of Milk, 624 ; 8. Milk Analysis and Inspection, 631.

SECTION X.
THE RENAL SECRETION, x 635

1. The Physical and Chemical Properties of Urine, 635 ; 2. The
Mechanism of Renal Secretion, 640 ; 3. The Mechanism of Mic-
turition, 648.

SECTION XL
THE CUTANEOUS FUNCTIONS, 651

1. The Sweat Secretion, 652 ; 2. The Sebaceous Secretion of the -Skin,
655 ; 3. Cutaneous Absorption, 656 ; 4. Cutaneous Respiration,
656 ; 5. The Lachrymal Secretion, 658.

Xll TABLE OF CONTENTS.

SECTION XII, PAGE

NUTRITION, 659

I. THE FATE or THE ALBUMINOUS FOOD-CONSTITUENTS, . . 660
II. THE FATE OF THE FATTY FOOD-CONSTITUENTS, . . .664

III. THE FATE OF THE CARBOHYDRATE FOOD-CONSTITUENTS, . . 666

IV. THE STATISTICS OF NUTRITION, . ; . . . . .672

1. Tissue Changes in Starvation, 674 ; 2. The Nutritive Processes in
Feeding, 680; (a) Feeding with Meat, 680; (6) Feeding with
Fat, 682 ; (c) Feeding with Carbohydrates, 683.

V. THE FOOD REQUIRED BY THE HERBIVORA UNDER DIFFERENT

CONDITIONS, . . . . ; . .-.-. . ... 684
VI. HUNGER AND THIRST, . . . * . ' . . . 692

SECTION XIII.
ANIMAL HEAT, . . . . ". ^ - - ~ .- ^ ^ ^ ^ 693

BOOK SECOND.
THE ANIMAL FUNCTIONS.

SECTION I.
THE PHYSIOLOGY OF MOVEMENT, . . . .. . .701

1. The Contractile Tissues, 701 ; (a) Chemical Composition of Muscle,
704 ; (&) Muscular Irritability, 709 ; (c) The Phenomena of Mus-
cular Contraction, 710 ; (d) The Electrical Phenomena in Muscle,
721 ; 2. The Applications of Muscular Contractility, 722 ; 3. Ani-
mal Locomotion, 731; 4. The Gaits of the Horse, 739 ; (a) The
Walk, 744; (5) The Amble, 746; (c) The Trot, 748; (d) The
Gallop, 749 ; 5. Other Movements in the Horse, 750 ; (a) Rearing,
750 ; (&) Kicking, 753 ; (c) Lying Down and Rising Up, 754 ;
(d) Walking Backward, 754 ; (e) Swimming, 755 ; 6. Special Mus-
cular Mechanisms The Voice, 757.

SECTION II.

THE PHYSIOLOGY OF THE NERVOUS SYSTEM, . . . . .765

I. THE CHEMICAL AND PHYSICAL CHARACTERISTICS OF NERVOUS

TISSUES, . . 774

II. NERVOUS IRRITABILITY, . . 776

III. THE ELECTRICAL PHENOMENA IN NERVES, . . . .779

IV. GENERAL PHYSIOLOGY OF THE NERVE-CENTRES, . . . 781
1. Reflex Action, 782 ; 2. Automatism, 784 ; 3. Inhibition, 785 ; 4.

Augmentation, 785 ; 5. Co-ordination, 785.

TABLE OF CONTENTS. xiii

PAGE

Y. THE FUNCTIONS OF THE SPINAL CORD, 786

(a) The Spinal Cord as a Collection of Nerve-Centres, 789 ; (&) The
Spinal Cord as an Organ of Conduction, 795.

VI. THE FUNCTIONS OF THE BRAIN, 803

1. The Medulla Oblongata, 810 ; 2. The Course of the Fibres of the
Medulla Oblongata, 818 ; 3. The Pons Yarolii, 821 ; 4. The Cere-
bral Peduncles, 821 ; 5. The Corpora Quadrigemina, 822 ; 6. The
Functions of the Basal Ganglia, 822; 7. The Functions of the
Cerebral Lobes, 823 ; 8. The Functions of the Cerebellum, 825.

VII. THE CRANIAL NERVES, 832

VIII. THE SYMPATHETIC NERVOUS SYSTEM, . . . . 835

IX. GENERAL AND SPECIAL SENSIBILITY, . . . . .837

A. THE SENSE OF SMELL, 841

B. THE SENSE OF SIGHT, .846

1. The Dioptric Mechanisms of the Eye, 851; 2. Visual Sensations, 864.

C. THE SENSE OF HEARING, 875

D. THE SENSE OF TASTE, .893

E. THE SENSE OF TOUCH,. . . . . . . . . 897

PART III.

THE REPRODUCTIVE FUNCTIONS, .901

SECTION I.
THE REPRODUCTIVE PROCESSES, 903

1. The Reproductive Tissues of the Female, 908 ; 2. The Reproduc-
tive Tissues of the Male, 913.

INTRODUCTION.

PHYSIOLOGY treats of the functions or actions of living beings.

When these actions or functions occur in a disturbed or irregular
manner, they constitute disease, or abnormal life, and become the subject
of abnormal physiology or pathology. Normal physiology is the basis of
pathology, and a knowledge of the one must precede the intelligent study
of the other: just as an acquaintance with the functions of the com-
ponent parts of a machine must precede the recognition of disordered
movement and the provision of means of repair.

Since the functions of the animal body are resident in the various
tissues and organs of the body, an acquaintance with the forms and
structure of those organs and tissues must precede the study of their
functions. The study of anatomy and histology, or microscopic anat-
omy, must therefore precede the study of physiology.

GENERAL PHYSIOLOGY treats of the functions of organized beings in
an abstract manner, that which regards the general laws of life, whether
seen in the animal or vegetable world. Although for the purposes of
practical life physiology is divided into several provinces, yet the knowl-
edge of general physiology is essential even to special students, since
the relation between the different forms of life is very close.

VEGETABLE PHYSIOLOGY is concerned solely with the consideration
of the vital actions or functions of plants.

COMPARATIVE PHYSIOLOGY treats of the functions of animals below
man, with a consideration of the means by which different functions are
accomplished by different animal forms.

SPECIAL PHYSIOLOGY is confined to the consideration of the vital
phenomena of a single species, single genus, or it may deal with the
consideration of a special function. In this book special physiology
will refer mainly to the study of the vital phenomena of the domestic
animals.

HUMAN PHYSIOLOGY treats exclusively of the vital phenomena of
man. But, while this branch of physiology is of greater importance to
the physician than the other divisions, in consequence of its relations
to human pathology and therapeutics, it should not be made the exclu-
sive subject of study; for the physiology of man cannot be properly
understood without a previous acquaintance with the vital phenomena of

2 INTRODUCTION.

the lower animals and plants. For the veterinary physician the study
of life in the domestic animals must be of the greatest importance.

Every living body is organized, that is. composed of instruments
or organs each one of which is destined to fulfill some special office in
the organism called its function, the sum of which functions constitute
the life of the individual. Other bodies met with in nature, and not so
constituted, are called unorganized, or inorganic, e.g., the mineral.

DISTINCTIONS BETWEEN ORGANIZED AND UNORGANIZED BODIES. Organ-
ized and unorganized bodies have few or no correlative points, but stand
opposed to each other in almost every characteristic trait.

Unorganized matter is only subject to the forces whose generality
of action constitutes physical and chemical laws. Organized matter is
also controlled to a certain extent by the same laws, and, although there
are a great many actions manifested by living bodies which are not
readily explicable by the ordinary physical laws, and for which the term
" vital phenomena" is conveniently employed, it does not by any means
follow that we have here to deal with any entirely distinct series of laws.
The attempt to reduce the so-called vital phenomena to physical and
chemical laws has already succeeded in demonstrating the dependence, on
physical and chemical principles, of many functions previously regarded
as purely vital in nature, and the hope may be reasonably held for con-
tinued progress in this direction. The sciences of physics and chemistry
are therefore the foundation-stones of modern physiology.

Nevertheless, organized and unorganized matter differ to such an
extent that their consideration forms entirely distinct branches of study.
The forms, the forces, and the laws of unorganized matter are the sub-
jects embraced by physics and chemistry. The forms and forces of
living organized matter are the objects of physiological science, or
biology.

Organic bodies differ from inorganic

1. In their Origin. The former spring from a parent, or from
previously-existing living matter, either by splitting, budding, seeds, or
eggs. The latter have no such origin, but ma}^ arise from the combina-
tion, under the influence of chemical affinity, of the elements which com-
pose them. Spontaneous generation, though claimed by some, has not
been satisfactorily established.

2. In their Form. Organized bodies are usually determinate in
their form, rounded in their outline, and, in their simplest expression,
either spherical or spheroidal in shape. Unorganized bodies, on the
other hand, are irregular in their outline (amorphous), or, if determinate
in form, are bounded by plane surfaces and straight lines.

3. Duration of Existence. Organized bodies have a definite time
to live, pass through distinct stages of development and growth, and ulti-

INTEODUCTION. 6

mately die. But the inorganic body may continue to exist until some
disrupting force separates the inorganic elements of which it is com-
posed, and enables them to form new combinations ; but so long as
uninfluenced by such an agency it may remain unchanged for an indefi-
nite period.

4. Size. Organized bodies have a definite limit to which they may
attain, varying, however, among individuals of the same species. And
vhen they exceed the average size of the species it is not by the
increased size of the individual, but b}^ the continued production of new
individuals or a repletion of parts already existing. The unorganized
body, on the other hand, is as indeterminate in size as in duration, con-
tinuing to grow so long as fresh particles are brought together.

5. Chemical Constitution. Of the sixty-five simple elements found
in nature but about twenty enter into the composition of organized bodies,
and of these but four are to be regarded as essential, viz., C.O.H.N.,
of which at least two are found in every organic compound. The remain-
ing elements are called incidental. Unorganized bodies may be simple in
their composition, or binary, ternary, quaternary, or higher ; but binary
is the most usual combination.

The molecular constitution of the organic body is also different
from the inorganic in being much more complex, both in the number of
elements which it contains and the number of atoms, or combining
equivalents of those atoms, which exist in a combining equivalent of
the compound. Thus, albumen, which forms an important constituent
of nearly all organized bodies, may be represented as C^oHagaNggO.;,^
(Schutzenberger), while ammonium carbonate, an inorganic compound
containing the same elements, with the exception of sulphur, may be
written as follows: (NH 4 ) S CO S -J-H 3 0.

From the large number of elements which enter into the composition
of organic bodies, and the large number of atoms constituting an organic
molecule, arises the great tendency to decomposition by which they are
characterized ; for, " the greater the number of atoms of an element
which enters into the formation of a molecule of a compound, the less
is the stability of that compound."

Inorganic compounds are therefore stable ; organic bodies, unstable.

It was formerly supposed that organic compounds could only be
formed under the influence of vitality, and that they could be decom-
posed by the chemist, but not recomposed. But this has been shown to
be an error, some of the organic acids, alcohols, organic coloring matters,
and some of the secondary organic components, such as uric acid and
urea, having been synthetically prepared by the chemist. It is thought,
therefore, not to be impossible that some of the higher organic com-
pounds, such as albumen, may ultimately be also made in the same

4: INTRODUCTION.

manner, though thus far all attempts in this direction have been unavail-
ing. All those compounds which have as yet been made by synthesis
are allied to those which result from a long-continued series of chemical
changes in the organism, produced by the action of oxygen upon prod-
ucts of disintegration.

6. In their Mode of Growth. Organized bodies grow by assimila-
tion, the internal deposit of materials by which the unlike become the
like. Unorganized bodies grow, or increase in size, by external deposit
or accretion. The organized body is dying from the moment of its birth,
and requires new materials to repair those losses and for the increase in
size. The unorganized body, as the crystal or the stalactite, continues
to increase in size so long as fresh particles are deposited upon it.
Every part of an inorganic body is therefore alike and independent of
the rest, and exhibits the same properties as the whole. The organized
body, on the contrary, is made up of a number of dissimilar parts, each
of which is more or less dependent upon the others, and each of which
requires different materials for its growth and reparation. In the unor-
ganized body a small portion serves to determine by anatysis the consti-
tution of the whole ; in other words, it is homogeneous. In the organ-
ized body each part is more or less dependent on the remainder, and
differs from it in chemical composition ; in other words, it is hetero-
geneous. Organic compounds, moreover, from the large quantity of
fluid they contain, are usually soft and ductile, while the inorganic body
is hard, rigid, and inflexible, and when once the affinities of its chemical
elements are satisfied it remains an inert mass. Within the organized
living body all is change. Death and repair are ever taking place. From
the commencement of its existence its growth, its progress toward
maturity, its decline, decay, and death are all made up of an incessant
series of changes. It is the constant round of these actions which con-
stitutes life ; their study is the subject of physiology.

It is thus seen that organized are distinguished from unorganized
bodies by three cardinal characteristics : 1. Tlie law of nutrition, the
most fundamental of all vital laws ; since in virtue of it the organism
continues to exist as an active being, and increases from infancy to
maturity. 2. The law of development, or differentiation, which causes
the organism to pass through the definite cycles of change constituting
what we call ages, and leading inevitably to the final changes which we
call death. 3. The law of reproduction, another aspect of the first law,
in virtue of which the organism gives origin to similar organisms from
one generation to another.

In no example of inorganic matter can any of these characteristics
be found. When inorganic bodies are said to grow, their growth is a
process of mere aggregation, one part adhering to another similar part.

INTRODUCTION. 5

The growth arises from no internal necessity, as in organic bodies. The
bulk is not increased by a process of assimilation which converts the
unlike into the like. Minerals do not feed ; they cohere. Nor have they
any power of development. They pass through no definite cycles of
change ; they have no stages of growth, no ages, no power of repro-
duction.

The constant round of actions, therefore, in the organized structure
called life, in them is wanting. They occupy space, but have neither
birth nor death.

DISTINCTION BETWEEN PLANTS AND ANIMALS. Organized bodies are
divided into two classes, animals and vegetables, constituting two sep-
arate kingdoms, which, though capable of ready recognition when studied
in their higher members, seem almost to overlap in their lowest expres-
sion. Hence, while the differences between the higher animals and higher
plants are so striking as not to need mention, when we examine the lowest
forms of life the greatest difficulty will sometimes be met with in the
attempt to decide whether the organism is an animal or a vegetable. For
when the protozoa, or lowest animals, are compared with the protophyta,
or lowest plants, all the differences which are so striking between the
higher animals and plants are completely wanting ; yet the protozoa are
as truly animal as are the vertebrata, and the protophyta just as surely
plants. Consequently the definition of an animal or a plant, to be of any
scientific value, must include the lowest as well as the highest forms.

We found, in our comparison of organic and inorganic matter, that
differences in form could be clearly made out. The external charac-
teristics of plants and animals are, however, inadequate to distinguish
them. Many animal forms, such as the hydrozoa, are essentially plant-
like in their external form, growing from fixed points and even repro-
ducing themselves by " budding," a process almost universally holding
in the vegetable kingdom. So also the well-known coral polyps and the
sponge closely resemble plants in external configuration, and, though
undoubtedly animals, were long placed by naturalists in the vegetable
kingdom.

Then, on the other hand, many plants, examined in respect to their
external form alone, would often be confounded with animals. Thus,
the germs of many algae, the ciliated zoospores, are scarcel}* to be dis-
tinguished from infusorial animalcules.

It was at one time thought that the power of motion was a proof
of animality ; but many of the lowest plants, such as volvox and the
diatoms, possess the power of motion, of changing their location, the
instruments being the same as in many animals, viz., cilia. Nor is the
power of moving in response to an irritant peculiar to animal life :
witness the Mimosa pudica, the sensitive plant, which closes its leaflets

6 INTRODUCTION.

on irritation ; the Dionsea muscipula, the Venus' Fly-Trap, the extremities
of whose leaves have the power of closing on insects or other bodies
brought into contact with them. Plants are also possessed of internal
motion: witness the circulation of the sap and the circulatory motions
in the interior of many vegetable cells. They also turn spontaneously
to the light and extend their rootlets to the most nutritive soil.

Again, all animals are not possessed of the power of motion.
Sponges, coral polyps, hydroid zoophytes, sea-mats, etc., are entirely
destitute of locomotive power, and spend their entire existence rooted
fast to some immovable object. Hence, the possession of motor power
is not characteristic of animal life, and its absence does not prove the
organism to be a vegetable.

Chemical analysis helps us but little more in the attempt to dis-
tinguish animals from vegetables. Carbon and nitrogen compounds form
a large proportion of the constituents of each, and a large number of
complex combinations found in animal tissues are represented by entirely
similar compounds in vegetable matter. There is therefore no one chemi-
cal compound whose presence is characteristic of animality or vegetable
nature; for u cellulose," the substance out of which wood-fibre and .the
walls of plant-cells are formed, has been ascertained to form the greater
part of the external coverings of certain molluscous animals (ascidians).
So also chlorophyll, the green coloring matter of plants, is the cause of
the green color of many infusorial animalcules and of Hydra viridis,
while starch has been found in the ventricles of the brain of animals,
and is represented by glycogen, a body closely analogous to starch and
manufactured by the animal economy. Such examples, therefore, show
that chemical examination can give us no definite aid in separating plants
and animals.

The microscope is also powerless to give us an infallible rule which
will enable us to distinguish animal from vegetable tissue. In other
words, plants and animals are built up on the same general plan ; their
intimate structure closely coincides. Both originate in cells, consisting,
in their typical form, of a cell-wall, cell-contents, or protoplasm, nucleus
and nucleolus, and in both the parent cell undergoes subdivision and
results in the birth, growth, and development of myriads of other cells,
constituting the tissue of the plant or animal, and differing no more from
each other than almost any mature animal or vegetable cell does from
the germ from which it originated.

Nor is the possession of a digestive cavity, mouth, or alimentary
tube characteristic of animals ; for there are vegetables which possess a
stomach, as the Nepenthes, or Pitcher-Plant, which has a cavity cor-
responding to a stomach, in which digestive fluids are poured out, and
in which digestion and absorption take place. On the other hand, many

INTRODUCTION. 7

animals among the protozoa, such as the amoeba, have no stomach, the
general surface serving not only for the purpose of digestion, but also
for absorption, an extemporaneous stomach being formed by wrapping a
part of the external general body surface around the substance to be
digested.

So also in the tape-worms and other parasitic forms of animal life,
there is an entire absence of any special aperture for the entrance of
nutritive matter, such organisms living by the simple imbibition of
nutritive matter in solution.

When, however, we examine into the nature and mode of assimila-
tion of food, the nutritive processes occurring in the interior of the
organism, and the results of the conversion and assimilation of food,
then only have we any reliable scientific data for distinguishing animals
from plants. In the first place, the food of animals differs from that of
plants in its nature. Animals require organic food ; plants live on inor-
ganic or mineral matter. The nutritive processes in the two kingdoms
are also diametrically opposed : the plant absorbs water, ammonia, carbon
dioxide and certain salts, and out of these manufactures the albuminoids,
carbohydrates and hydrocarbons found in vegetable tissue. The
animal feeds on these complex vegetable compounds, and this holds
whether the animal be herbivorous or carnivorous, and returns to the
soil and atmosphere the inorganic matter from which they were manu-
factured by the plant; and in the same form, i.e., carbon dioxide, water,
ammonia, and certain salts. The plant therefore converts simple inor-
ganic compounds into complex organic compounds, while the animal
reduces complex organic matter to its simple inorganic constituents.

A further point of distinction between animals and vegetables, and
one closely connected with the nutritive processes, is their behavior to
the atmosphere. The animal requires for the processes of reduction
already mentioned as constituting its mode of nutrition a constant supply
of ox}-gen, which is withdrawn from the atmosphere and returned to it
in the form of CO,, representing one of the end products of oxidation
of the carbon of its tissues and food. Plants, on the other hand, absorb
CO,, and under the influence of sunlight, by the action of their chloro-
phyll, break up this CO,, fix the carbon in their tissues, and set free
ox}-gen into the air. The plant thus absorbs what the animal excretes,
and the animal absorbs what the plant excretes. We thus see that
animals and plants offer striking points of contrast as to the character
of their food and the nature of their nutritive processes, and, although
there are several apparent exceptions to the general outline here given,
their consideration may be deferred to the chapters on the Chemical
Processes in Cells.

We have found now that all objects in nature must be either organic

8 INTRODUCTION.

or inorganic, and we have considered the means by which these bodies
may be separated: we, therefore, here leave the inorganic world (the
domain of physics, chemistry, mineralog} r , etc.), to confine our studies
to the animal kingdom. But here, from the fact that there was great
difficulty in separating the lower forms of animal from vegetable life, it
must be recognized that animals and plants possess many vital functions
in common ; and as the simplest expression of these functions must be in
the simplest organisms, the study of those functions may best commence
in the simple, uncellular organisms, whether animal or vegetable.
General physiology will thus deal with the Animal Cell : its form, origin,
modifications, constitution, and the various chemical and physical proc-
esses concerned in its nutrition, growth, development and reproduction.

It will, then, be shown that the higher animals are mere associations
of such simple organisms, in which the modification in the characters
of the various constituent cells leads to a division of labor. In other
words, development of tissues leads to a specialization of function, and
Special Physiology will deal with the study of the development of func-
tion, especially as seen in our domestic animals. The functions of animals
are divided into the Vegetative Functions, the Animal Functions, or
the functions of relation, and the Reproductive Functions.

The Yegetative Functions include everything which relates to the
nutrition of the animal in its widest sense. As the blood in higher
animals is the organ of nutrition, under this head are included (1st)
the additions to the blood, therefore, the description and modes of
prehension of Food ; Digestion, or the preparation of food for absorption ;
and Absorption, or the means by which nutritive and other matters enter
the blood. The Blood will, then, be considered as a tissue of nutrition
<or as a carrier to and from the various organs of the body by means of
its Circulation. As a boundary between the additions and (2d) the losses
to the blood Respiration will demand attention, while under the latter
head come the functions of Secretion and Excretion. The means by
which the identit}^ of the individual is preserved concludes the subject
of Nutrition and deals with the nutritive value of different foods and
their combinations, the adaptment of foods to the different demands on
the animal economy, and the subject of Animal Heat. The Animal Func-
tions, or those by which the body is brought into relation with the
external world by means of sensation, power of movement, consciousness,
and volition, include the study of the Muscular and Nervous Systems,
while finally the Reproductive Functions lead to the preservation of the
species, and include the subjects of Generation and Development, or
Embryology.

PART I.

GENERAL PHYSIOLOGY.

THE PHYSIOLOGY OF ANIMAL CELLS.

(9)

SECTION I.
THE STRUCTURE OF ORGANIZED BODIES.

CHEMICAL ANALYSIS has shown that all organized bodies are capable
of resolution into simple chemical elements which in themselves do not
differ from the elements out of which all matter is composed : in other
words, that the simple elements of which organized bodies are built up
are universally distributed throughout nature, and that no one element
is peculiar to organized matter. The characteristic of organized bodies
is, therefore, not to be found in any peculiarity of the matter of which
they are composed, but in the manner in which the atoms composing
that matter are grouped. In an inorganic body we are accustomed to
attribute its chemical properties to the nature, number, and mode of
association of its constituent elements, while its physical properties are
attributable to the mode of arrangement of its molecules.

Anatysis of organized bodies shows that in them we have certain
elements constantly present in certain definite proportions: it is there-
fore warrantable to assume that the chemical properties of organized
bodies are, as in the case of inorganic matter, due to the number, nature,
and mode of association of their elements. Further, we find in all
organized living bodies a certain identity of physical properties : it is
therefore warrantable to assume that the physical processes seen in
organized bodies are dependent on the mode of arrangement of their
constituent molecules. The elements constantly associated in living
matter are carbon, nitrogen, oxygen, hydrogen, and sulphur, forming a
complex combination, to which the term protoplasm has been applied.
This matter, protoplasm, whether found in the tissues of the highest
animals or plants, or in the lowest unicellular members of either kingdom,
has always the same composition and is always possessed of nearly the
same attributes ; with the restriction that we have already referred to as
to the difference in functions possessed by animals and plants, differences
which will probably in the future be cleared up, and found not to be in
contradiction to the statement that protoplasm is the universal basis of
organization.

All organized bodies are built up of associations of masses of proto-
plasm, which from their appearance are termed cells, or, from the func-
tions which they fulfill, elementary organisms: and as the physical
properties of inorganic matter are dependent on the arrangement of

12 PHYSIOLOGY OF THE DOMESTIC ANIMALS.

their molecules, so the physiological peculiarities of organized bodies
are dependent on their cellular structure.

Physiology is therefore the study of the properties of cells. Cells
possess the properties of Nutrition, Reproduction, Growth, Develop-
ment, and in many cases their contents are capable of Motion and mani-
festing Irritability.

I. THE GENERAL PROPERTIES OF CELLS.

Microscopic examination teaches that every living object, from man
down to the smallest animalcule invisible to the naked eye, from the
largest tree down to the most microscopic plant, is built up on the
same general plan. In each the same element of organization is found,
and every living form is built up of associations of these microscopic
units, each of which, even in the most complex forms of life, may be
regarded as separate individual organisms.*

For even in complex organisms cells to a certain extent carry on a
separate and independent existence. We see
separate cells originate separately, grow, repro-
duce themselves, become diseased and die
without the entire organism as a whole taking
any part in these different stages of existence
of its component parts. The individual life of
each separate cell is recognized in the different
activities of different cells : the activity of the
FIG i TYPICAL ANIMAL organism is the result of the, sum of these
( mn V ) VUM F separate existences.

In tlieir typical form both animal and
vegetable cells consist of closed vesicles, with

homogeneous or striated walls, a viscid albuminous contents, termed
protoplasm, containing an aggregation of granules called -a nucleus,
within which again is a still denser formation called a nucleolus. The
cell-contents is frequently vacuolated, i.e., contains minute cavities filled
with a clear fluid.

The contents of the cells, which we shall find to be functionally the
most important, is called protoplasm. It is a transparent mass in which
numerous granules are suspended, and which possesses in all young cells
(the property of contractility. It is often seen to be reticulated. In
older cells the quantity of fluid diminishes and the cells become firmer
and drier, while vacuoles often form and contain fluid. This change in
.the physical properties of cells is often associated with a visible change
in their chemical nature, thus, with a deposit of coloring matter, starch

* Such units of organization are termed cells, from the resemblance which micro-
scopic sections of young tissues, whether plant or animal, bear to a honey-comb.

THE GENERAL PEOPEKTIES OF CELLS. ; 13.

granules, fat globules, or granular matter. All substances which coagu-
late proteids have the same effect on protoplasm. The vital properties
of protoplasm will be studied later.

A membrane is usually present in all mature cells, though always
absent in embryonic forms. It may therefore be assumed that the mem-
brane results from a condensation of the outer layers of the cell-contents.
The membrane is apparently homogeneous, or may be porous.

The nucleus, the size of which is generally in proportion to that of
the cell, and which is usually oval or spherical, is never absent in early
forms of active cells, though it may disappear when the cell reaches
maturity. In its mature stage it is generally reticulated, that is, com-
posed of an investing cuticle within which the contents are arranged in
the form of a fibrillar net-work. The presence of a nucleus, which is
often difficult of recognition on account of its minute size, may be
demonstrated through the action of certain reagents, especially dilute
acids and staining fluids. Dilute acids render the protoplasm of cells
transparent without affecting the nucleus, which consequently becomes
more prominent; while staining fluids, such as carmine, haematoxylin,
and the anilin dyes, color the nucleus deeper in tint than the cell-contents.
The nucleus appears to be especially important in the reproductive func-
tions of cells, since when cells multiply by division the division always
commences in the nucleus.

The nucleolus is simpty a closer aggregation of the granules which
constitute the nucleus, and is very frequently absent.

Both cell-wall and nucleus may be absent from the lowest elementary
organisms.

As our conception of the structure of the higher animals and plants
is an association of elementary organisms invariably taking origin from
a single cell, our definition of such a simple organism or cell must be
modified so as to apply to the description of the simplest conceivable
organism capable of carrying on an independent existence. And as we
have seen that of the constituents of a typical cell but one, the cell-con-
tents, or protoplasm, is essential ; and as we know that there are
organisms capable of carrying on an independent existence in whom
neither cell-wall, nucleus, or nucleolus is to be detected, a cell may be
defined as a more or less homogeneous mass of organized material,
protoplasm, possessing development, growth, reproduction, nutrition,
and automatism.

The best known of such undifferentiated forms of cell life is the
amoeba, one of the simplest examples of an animal organism.

In its lowest form the amoeba (Protamoeba primitiva, Haeckel) con-
sists of a mass of jelly-like, structureless, albuminoid substance (proto-
plasm), which, so far as its chemical composition and general attributes

14 PHYSIOLOGY OF THE DOMESTIC ANIMALS.

are concerned, cannot be distinguished from the contents of all active
forms of cells (Fig. 2).

The amoeba is capable of spontaneous motion, both as regards
change of external form and of progressing from place to place.
Motions may also be evoked by various stimuli ; hence, free protoplasm,
in common with muscular fibre and ciliated organisms, is contractile.
The peculiarity of protoplasmic motion, as seen in the amoeba, is that
motion does not occur around a fixed point, but rather is a flowing
motion, such as might occur in the particles of a fluid. Thus, in an
amoeba the changes in form and location are effected through the thrust-
ing out of lobe-like prolongations of the periphery (pseudopodia), and
their subsequent withdrawal or the flowing into these extensions of the
remainder of the body.

Occasionally one or more of these pseudopodia become gradually
more and more constricted, until, finally, a portion becomes entirely

separated from the original
mass, increases in size, and
itself possesses all the proper-
ties of the parent stock; hence,
protoplasm is reproductive,
and possesses the power of
growth. Moreover, the move-
ments of an amo3ba are not

FIG. 2. -A NON-NUCLEATED CELL, THE PROT- necessarily the consequences
AMCEBAPRIMITIVA, AFTER HAECKEL. (Wundt.) of externa i stimuli, but may

A, original condition ; B, commencement of reproduction by fission : , . .

C, after complete separatum. be Self-Originating ; llCHCC,

protoplasm is also automatic.

If watched for some time, an amoeba will often be seen to take into
its interior, by flowing around them, small vegetable organisms, of which
portions are dissolved and converted into the substance of its body,
while the undigested remainder is extruded ; therefore, protoplasm, even
in the absence of all digestive organs, possesses the power of nutrition.

The amoeba requires for its existence an atmosphere of ox3 r gen,
which is absorbed, and which it again partly exhales as carbon dioxide.
Protoplasm is therefore respiratory.

II. ORIGIN OF CELLS.

We iiave seen that in the amoeba a simple mass of undifferentiated
protoplasm possesses the powers of reproduction, contractility, respira-
tion, irritabilit}^ nutrition, and automatism. Every form of life com-
mences its existence in the form of just such a simple mass of proto-
plasm. Starting with the ovum, and ending with the nucleated elements
found in the organs and tissues of the embryo and adult ; there is one

OKIGIN OF CELLS. 15

uninterrupted series of generations of cells, each cell becoming so modi-
fied as to specialize certain functions which are together possessed by all
forms of undifferentiated protoplasm. Thus, in the higher forms certain
cells will be found to have become so modified as to have the function of
reproduction especially developed ; they will therefore constitute the
reproductive tissues. In other cells the nutritive functions will become
most prominent ; they will therefore form part of the tissues whose
function is to preserve the nutritive balance of the organism. Specializa-
tion of function is therefore the explanation of the development of
tissues; the result is a plr^siological division of labor. We will have to
return to this subject again.

The germ of every animal and vegetable organism is a cell which
owes its existence to some similar, previously-existing cell. Neglecting
the origin and development of cells in the vegetable kingdom, every
cell which forms part of the organs or tissues of all forms of animal
life originated in and developed from a germ-cell or ovum.

The ovum of man and other mammals is
a minute mass of protoplasm, corresponding in
its general appearance with the description of a
typical cell. The protoplasm, or cell-contents,
is surrounded by a delicate, striated membrane,
the Zona radiata, or vitelline membrane. Within
the cell-contents, in addition to numerous minute
particles, the so-called yelk- globules, is a
collection of denser particles of protoplasm, FIG. 3. TYPICAL ANIMAL
the nucleus, or germinal vesicle, and within uS?"cxjJ) VUM OF
that, again, one or more still more solid masses, A
the nucleoli, or germinal spots.

The cell-contents is identical in nature and properties with the sub-
stance of the amoaba, and before and immediately after fertilization may
even be the seat of spontaneous movements of contraction and expan-
sion. When mature, its diameter in the domestic animals and man varies
from the ^J^ to the T ^ of an inch (0.18-0.2 mm.).

Fertilization leads to a cleavage of the protoplasm into two parts,
the nucleus being first divided, so that two new elements originate from
the ovum, each consisting of protoplasm and each containing a nucleus.
Each of the two new cells thus formed again subdivide into four, these
into eight, and so on through many generations, until a large number of
new cells, the so-called "mulberry mass," results from the subdivision of
the original parent cell.

According to Schleiden and Schwann, the founders of the cell doc-
trine, organic forms of life may originate in one of two wa} r s, either by
the aggregation of granules in a previously-existing homogeneous fluid

16

PHYSIOLOGY OF THE DOMESTIC ANIMALS.

FIG. 4. BLOOD-CORPUSCLES
IN DIFFERENT STAGES OF
DEVELOPMENT, SHOWING
REPRODUCTION BY FIS-
SION. (Ranke.)

(the blastema), forming the so-called "free cell-formation," or by the
subdivision of a previously-existing cell, the " endogenous cell-forma-
tion."

According to the first of these views, which may be compared to the
formation of crystals in a saline solution, granules first develop in a fluid

which contains all the. chemical constituents of
the organism, forming the nucleolus of the
future cell. Around this other granules are
gradually deposited until the nucleus is formed,
and the cell-contents and membrane gradually
consolidate around this. The first objection
to this theory, which, it is seen, implies spon-
taneous generation, lies in the fact that no one-
has ever been able to demonstrate such a cell-
formation or to discover the so-called cyto-
blasts. It was then shown that all the cells of
the embryo originate in the segmentation

spheres of the ovum, and the falsity of this doctrine of free cell-
formation is further proved from analogy by the manner in which the
connective-tissue cells take part in the development of pathological new
formations. There is now no more firmly-established dictum in plrysi-
ology than the statement that every cell originates from a previously-
existing cell. (Omnis cellula e cellula.)

The other view, which was also to a certain extent advocated by
Schwann, as to the origin of cells by sub-
division of a parent cell, is exemplified in
the mode of reproduction of many of the
lower forms of life. Cells may reproduce
themselves by simple division of the parent
cell or by endogenous division.

Cell reproduction always starts in the
nucleus. In simple division the nucleus first
becomes marked with a furrow, which gradt
ually deepens until the nucleus is divided
into two. The protoplasm of the cells is
then modified in the same way, until two new
cells are formed by the division of the

parent cell. This process may be followed in the reproduction of
the nucleated red blood-corpuscles of the embryo of the chick and
even in mammals (Fig. 4). A modification of this form of cell
reproduction is sometimes described as " budding." This process also
starts with the nucleus. A number of nuclei are first formed by the
subdivision of the nucleus ; these gradually separate; the protoplasm

FIG. 5. GEMMATION. A BUD-
DING GERM-CELL OF GOR-

DIUS, AFTER MEISSNER.

(Wundt.)

ORIGIN OF CELLS. 17

collects around them so as to form projections from the periphery of the
cell, which become more and more constricted until they finally separate
(Fig. 5). During division the nuclear membrane disappears, and the
nucleus usually divides before the cell-protoplasm, but not directly, by
simple cleavage, as was formerly supposed, but indirectly, by karyo-
kitiesis (from movement of nuclear fibrils). This is an exceedingly com-
plicated process. Its different stages may be divided about as follows :

1. The nuclear fibrils become very distinct, while the nuclear mem-
brane disappears and the fibrils of the nuclear net-work become twisted
and bent into a more or less dense convolution, while the entire nucleus
enlarges.

2. The fibrils unravel into loops arranged around the centre as a
wreath or rosette.

3. The peripheral points of the loops become broken and a star-
shaped figure of single loops is obtained. This is termed the aster, or
star.

4. The loops separate into two groups or new centres. This is the
diaster, or double star.

5. The two groups of threads become farther apart, as if attracted
by opposite poles, but still remain connected by fine pole-threads, which
represent the interstitial nuclear substance. In this stage the figure
resembles a spindle.

6. The connection between the two sets of threads is broken.

7. The threads of each set become convoluted.

8. A membrane forms for each set, and thus new daughter nuclei
result. (See Fig. 6.)

The cell-protoplasm may commence to divide at any stage between
the one when the threads aggregate around the two centres and the one
when two distinct nuclei are present ; or the division of the nucleus may
be followed by division of the cell, so giving a cell with two nuclei. It
is not proved that this is the universal mode of divisions of nuclei,
though it has been observed in all kinds of cells in the embryo, and to a
limited degree in the adult. On the contrary, it is probable that amoeboid
cells divide by the direct method and that other nuclei may also undergo
direct division, or Remak's division, by simple cleavage, though all the
cases in which constriction of nuclei is observed need not be cases of
commencing division, as the change in- shape may be due to pressure of
cell-protoplasm, or to nuclear contractions. (Klein).

The second form of cell-formation is termed by Kolliker " endoge-
nous cell-formation," and consists in the formation of cells within the
membrane of the parent cell, which ultimately bursts and discharges its
progeny of j^oung cells. The division of the mammalian ovum, the
growth of cartilage, and many pathological cell-formations are types of

2

18

PHYSIOLOGY OF THE DOMESTIC ANIMALS.

this mode of cell reproduction. The latter example furnishes many
instances in which a number of cells with entirety different attributes
from the parent cell develop in the interior of a cell, such as the develop-

FIG. 6. KARYOKINESIS. (Klein.}

A, ordinary nucleus of a columnar epithelial cell ; B, C, the same nucleus in the stage of convolution :
D, the wreath, or rosette form ; E, the aster, or single star ; F, a nuclear spindle from the Descemet's
endothelium of the frog's cornea ; G, H, I, the diaster ; K, two daughter nuclei.

ment of pus-corpuscles in the interior of different tissue-cells in inflam-
mation (Fig. 7).

The best object for the study of cell reproduction by division, and

the one of most interest tor us, is
found in the fertilized ovum of
mammals.

As the ovum approaches maturity,
even before impregnation takes place,
the germinal vesicle becomes obscure
and more and more irregular in out-
line, its membrane and reticulum
disappear, and the germinal spot is
broken up. What remains of the
germinal vesicle becomes converted
into a striated, spindle-shaped body,
which moves to the surface of the
ovum, to there undergo division into
two parts. One part becomes ex-
truded from the ovum to form what
is known to embryologists as the polar cell, and is soon followed by a
second similar cell, while the portion of the spindle remaining within
the ovum forms a new nucleus, the female pronucleus, from which, in
conjunction with the male elements, the future embryo is developed.

FIG. 7. THE FORMATION OP PUS-COR-
PUSCLES IN THE INTERIOR OF EPITHE-
LIAL CELLS, SHOWING ENDOGENOUS
CELL-FORMATION. (Ranke.)

A, single cylindrical cell from the human bile-duct :
B, a similar cell containing two, C containing four, and
D numerous pus-cells: E, isolated pus-corpuscles;
F, a ciliated epithelial cell from the human respiratory
tract, containing a single pus-corpuscle : and G, a pave-
ment epithelial cell from the urinary bladder of man,
containing numerous pus-corpuscles.

OKIGIN OF CELLS.

19

111 the unimpregiiated ovule spontaneous contractions are generally
seen in the protoplasmic cell-contents. When the egg becomes fertilized
these contractions become so modified as to cause the germinal vesicle
(or the bod} r resulting from the union of the male and female pronuclei),
and then the cell-contents to split into two parts contained within the
cell-membrane, which takes no part in this division. These segmentation
spheres, as already mentioned, continue to subdivide until an immense
number of minute, nucleated, membraneless cells are contained within the
vitelline membrane (forming the so-called mulberry mass).

From these elements, which become progressively smaller as cleavage
goes on, all the tissues of the embryo are formed. At first the} T all
possess mobilit} T (amoeboid movements), showing their analogy to the
simple amoaba, but. at birth this property is only retained by certain cells.
Like amrebse, they also grow in size and divide into new individuals.

At first all the cells resulting from the segmentation of the ovum
are exactly alike : they then undergo certain modifications in arrange-

123 A-

FIG. 8. OVA OF THE DOG FROM THE FALLOPIAN TUBE, SURROUNDED BY THE
ZONA PELLUCIDA, IN WHICH ARE FOUND NUMEROUS SPERMATOZOA, AFTER
BISCIIOFF. (Rank<>.}
1, ovum with two segmentation spheres, the Zona pellucida being surrounded by the Membrana gran-

ulosa ; 2, ovum with four segmentation spheres ; 3, ovum with eight segmentation spheres ; 4, ovum with

innumerable segmentation spheres, forming the mulberry mass.

ment and form, different in different classes of animals, from which the
different tissues of the embryo are developed.

The ova of animals are divided into two classes, those in which the
entire yelk is concerned in the production of the embryo, and those in
^Yhich a part only serves for this purpose, while the remainder of the cell-
contents is drawn upon for the nutritive needs of the embr3 r o. The first
of these which undergoes total segmentation is termed a holoblastic egg;
the second undergoes only partial segmentation, and is termed a mero-
blastic egg. The ovum of mammals serves for a type of the former
class ; the ovum of birds is typical of the second class. As already
mentioned, the mammalian ovum represents a typical cell; the ovum of
the bird differs in many points. Beneath the yelk-membrane is a layer
of minute flattened cells, which gradually disappear during the maturing
of the egg ; while the yelk consists of two parts, one serving for the
development of the embryo, the other for its nutrition. The part from

20 PHYSIOLOGY OF THE DOMESTIC ANIMALS.

which the embryo is formed is a small, white disk lying directly beneath
the vitelline membrane and termed the tread, the blastoderm or cicatricula.
In the hen's egg this disk is about four millimeters in diameter, and is
always found in the upper surface of the yelk. If a hen's egg is hard-
ened by boiling, and then cut in two by a vertical section so as to bisect
the yelk, the latter will be found not to be perfectly homogeneous. The
yelk is clothed externally by a thin layer of different material, which at
the edge of the blastoderm passes beneath it and becomes thicker so as
to form a bed on which the blastoderm rests, to become connected by a
narrow neck with a mass of similar matter occupying the centre of the

WY
W.Y. W

N.P.
B.L.

Y.V

CH.L

CH.L.

W.

VT.

FIG. 9. DIAGRAMMATIC SECTION OF AN UNINCUBATEB FOWL'S EGG, AFTER
ALLEN THOMPSON. (Foster and Balf our.)

BLi, blastoderm ; WY, white yelk this consists of a central, flask-shaped mass and a number of
layers arranged concentrically around this; YY, yellow yelk; VT, vitelline membrane; X, layer of
more fluid albumen immediately surrounding the yelk ; W, albumen, consisting of alternate denser and
more fluid layers ; CHL, chalazae ; ACH, air-chamber at the broad end of the egg this chamber is
merely a space left between the two layers of the shell-membrane ; ISM, internal layer of shell-membrane :
SM, external layer of shell-membrane ; S, shell ; NP, nucleus of Pander.

yelk, which nearly always remains partially fluid in the hard-boiled egg.
Within the yelk again are several concentric layers of this white yelk,
separated from each other b}^ la} T ers of yellow } r elk. The 3 T ellow yelk is
composed of comparatively large, unnucleated cells filled with highly
refractive granules, and containing vitellin, lecithin, and various fatty
bodies. The cells which form the white yelk are much smaller, are
nucleated, and often a large cell will be seen to contain numerous similar
but smaller cells.

When the egg is laid by the hen it has already undergone changes
which result from fertilization. We will first describe the characters of

ORIGIN OF CELLS.

21

the blastoderm when the egg is first laid, and then the changes which
have preceded it.

The blastoderm of an unincubated fertilized egg may be recognized
by the naked eye, when viewed from above, to consist of two parts: an
opaque, white circumference, the area opaca, and a central transparent
portion, the area pellucida. In the unfertilized egg these divisions are
not marked. They are due simply to the way the blastoderm, which is
itself entirely transparent, rests on the white yelk. The opaque, circular
ring is where the blastoderm is directly in contact with the white yelk,
while the central clear portion is due to the fact that the blastoderm is
separated from the yelk by a layer of liquid. The white spot often seen
in the centre of the blastoderm is the central column of white yelk
shining through the transparent membrane (Nucleus of Pander).

When the blastoderm is hardened and cut into vertical sections, it
is found to be composed of two layers of cells : the upper, small, nucle-
ated, C3"lindrical cells adhering closely together in a single la3 T er and

FIG. 10. SECTION OF AN UNINCUBATED BT-ASTODERM OF CHICK. (Klein.)

A, cells forming the ectoderm ; B, cells forming the endodenn ; C, large, formative cells : F, segmen-
tation cavity.

resting on the white 3 T elk ; the lower, an irregular net-work of larger cells
which are not nucleated, apparently, but which contain numerous highly
refractive granules. These are probably identical with the white-yelk
spheres already referred to, and are spoken of as formative cells.

The processes which in the hen's bod} r result in the formation of
such an egg are about as follow :

In the capsule of the ovary the yelk alone constitutes the egg. It
then, just before bursting its capsule, consists of a minute, yellowish,
ellipsoidal, cellular body, with a delicate membrane, the vitelline mem-
brane, immediately below which in a granular cell-contents, the yelk, lies
a lenticular, mass of protoplasm, the germinal disk; within this again is
a nucleus, the germinal vesicle, containing a nucleolus, or germinal spot.

When the ovum becomes mature the ovarian capsule bursts, and
the ovum (representing the yelk of the egg as laid) escapes into the
oviduct, undergoes impregnation by the spermatozoa found in the upper
portion of the oviduct, and has deposited around it the accessory

22 PHYSIOLOGY OF THE DOMESTIC ANIMALS.

portions of the egg through secretions from the walls of the oviduct.
Thus, the layer of albumen surrounding the yelk is first deposited in
the passage of the ovum through the second, tubular portion of the
oviduct, the chalazse (see Fig. 9), or twisted, denser portions of the
albumen, being due to the rotatory motion of the egg against the spiral
ridges of the oviduct. The shell-membrane is formed by the organiza-
tion of the most external layers of albumen, and the shell is formed in
the third portion of the oviduct, or the uterus. The walls of this
portion of the tube secrete a viscid fluid which surrounds the egg, and in
which inorganic particles are deposited. The egg remains in the uterus
for from twelve to eighteen hours, and is then expelled through the
cloaca, narrow end downward, by its muscular contractions.

1 2 3

FIG. 11. SURFACE VIEW OF THE EARLY STAGES OF SEGMENTATION IN A

Fowi/s EGG, AFTER COSTE. (Foster and Balfour.)

1 represents the earliest stage. The first furrow, B, has begun to make its appearance in the centre
of the germinal disk, whose periphery is marked by the line A. In 2 the first furrow is completed across
the disk, and a second similar furrow at nearly right angles to the first has appeared. The disk thus
becomes divided somewhat irregularly into quadrants by four (half) furrows. In a later stage, 3, the
meridian furrows, B, have increased in number, from four, as in B, to nine, and cross-furrows have also
made their appearance. The disk is thus cut up into small central, C, and larger. D, peripheral segments.
Several new cross-furrows are seen just beginning, as ex. gr., close to the end of the line of reference, D.

About the time the shell is being formed, provided impregnation
has taken place, changes occur in the blastoderm, which, though analo-
gous to the process of segmentation already mentioned as taking place
in the mammalian ovum, yet differs from it. The germinal vesicle first
disappears, and a furrow is then seen to run across the germinal disk,
dividing it into two halves. This furrow is then met by a second run-
ning at right angles to the first ; this is then crossed by another, and
division of the segments proceeds rapidly by furrows running in all
directions until the germinal mass is cut up into an immense number of
minute masses of protoplasm, smaller toward the centre than at the
periphery of the disk.

The furrows thus formed are not merely superficial, but extend
through the entire thickness of the germinal disk: hence the germinal
disk is cut up into minute masses of protoplasm. In other words, a

ORIGIN OF CELLS.

23

large number of cells has resulted from the segmentation of the parent
cell.

These cells arrange themselves into an upper layer, with their long

FIG. 12. SURFACE VIKW OF THE GERMINAL DISK OF A HEN'S EGG DURING
THE LATER STAGES OF SEGMENTATION. (Foster and Bal four.)

At C, in the centre of the disk, the segmentation masses are very small and numerous ; at B, nearer
the edge, they are larger and fewer; while those at the extreme margin, A, are largest and fewest of all.
It will be noticed that the radiating furrows marking off the segments, A, do not reach to the extreme
margin, E, of the disk.

The drawing is complete in one quadrant only. It will, of course, be understood that the whole circle
should be filled up in a precisely similar manner.

axes vertical, their nuclei become distinct, while the lower cells remain
large and granular and irregularl}' placed, forming in this way the unin-
ciibated blastoderm already described. (See Fig. 10.)

FIG. 13. SECTION OF THE GERMINAL DISK OF A FOWL'S EGG DURING THE
LATER STAGES OF SEGMENTATION. (Foster and Bal/our.)

This section, which represents rather more than half the breadth of the blastoderm (the middle line
being shown at C), shows that the upper and central parts of the disk segment faster than those below and
toward the periphery. At the periphery the segments are still very large. One of the larger segments is
shown at A. In the majority of segments a nucleus can be seen ; and it seems probable that a nucleus is
present in them all. Most of the segments are filled with highly refracting spherules, but these are more
numerous in some cells (especially the larger cells near the yelk) than in others. In the central part of
the blastoderm the upper cells have commenced to form a distinct layer. No segmentation-cavity is present.

A, large peripheral cell ; B, larger cells of the lower parts of the blastoderm ; C, middle line of blasto-
derm; E, edge of the blastoderm adjoining the white yelk; W, white yelk.

As a result of incubation a third layer of cells makes its appearance
between the two layers of the blastoderm just described, forming an
upper, a middle, and a lower Ia3*er, or the epiblast, the mesoblast, and the

24 PHYSIOLOGY OF THE DOMESTIC ANIMALS.

hypoblast (Fig. 14). From these three layers of cells the embryo is
developed.

Leaving at this point the changes which occur in the egg of the bird,
we have now to follow the analogous changes in the mammalian ovum.

We have already seen that in the mammalian ovum one of the first
evidences of impregnation is the division of the protoplasm of the ovum
progressively into smaller and smaller segmentation spheres, until the
cell-membrane becomes filled with an immense number of minute masses
of protoplasm. The general character of this process in its earlier stages
is probably identical in all the mammalia. The ovum of the rabbit has
been most studied, and the sketch here given is based mainly on Balfour's
summary of the early stages of development in the rabbit's ovum.

The ovum first divides into two nearly equal spheres, of which one

BD

BD, BD. MC.

FIG. 14. SECTION OF A BLASTODERM OF CHICK, AT RIGHT ANGLES TO THE
LONG Axis OF THE EMBRYO, AFTER EIGHT HOURS' INCUBATION, ABOUT
MIDWAY BETWEEN FRONT AND HIND ENDS. (Foster and Balfvur.)

A, epiblast; B, mesoblast; C, hypoblast: PR, primitive groove; F, fold in the blastoderm produced
accidentally ; MC. mesoblast-cell, the line points to one of the peripheral mesoblast-cells lying between
epiblast and hypoblast; BD, formative cells.

The section shows: (1) the thickening of the mesoblast under the primitive groove, PR, even when it
is hardly present at the sides of the groove : (2) the hypoblast, C, early formed as a single layer of spindle-
shaped cells ; (3) the so-called segmentation cavity, in which coagulated albumen is present. On the
floor of this are the large formative cells, BD.

is slightly larger and more transparent than the other. The larger sphere
and its products will be spoken of as the epiblastic spheres ; the smaller
one and its products as the hypoblastic spheres. Both these original
spheres soon divide into two, and each of these into two more, thus
making eight. At first these spheres are spherical, and arranged in two
layers formed of four epiblastic and four hypoblastic spheres. Soon r
however, one of the hypoblastic spheres passes into the centre, and the
whole ovum becomes spherical again.

In the next stage each of the four epiblastic spheres divides into
two, followed b} r the division of the hypoblastic spheres into two. The
ovum is then made up of sixteen different spheres, nearly of the same
size. Of the eight hypoblastic spheres four soon pass to the centre, and are
surrounded by the eight epiblastic spheres, arranged in the form of a cup.
Division of both sets of spheres now continues, the epiblastic layer con-

ORIGIN OF CELLS. 25

tinning to surround the central kypoblastic spheres, both sets continuing
to subdivide, until finally the ovum consists of an almost solid mass of
hypoblastic spheres surrounded by a layer of epiblastic cells.

When the process of segmentation is complete the epiblastic cells
are clear and have an irregularly cubical form, while the hypoblastic
cells are polygonal and granular and somewhat larger than the epi-
blastic cells.

The blastodermic vesicle next forms. This results from the forma-
tion of a narrow cavity between the epiblast and h} T poblast, which
increases in size until it entirely separates these two layers, except at the
point where the blastoderm was last in forming (the blastopore). As
the cavity increases in size the ovum also enlarges, so that soon it exists
in the form of a large vesicle, formed of a thin wall of a single layer of

EP

FIG. 15. OPTICAL, SECTIONS OP A RABBIT'S OVUM AT Two STAGES CLOSELY
FOLLOWING UPON THE SEGMENTATION, AFTER E. VAN BENEDEN.
(Half our.')

EP, epiblast; HY, primary hypoblast ; BP, Van Beneden's blastopore. The shading of the epiblast
and hypoblast is diagrammatic.

cells, the epiblastic cells, with a large cavity, the hypoblastic cells
forming a small, ventricular mass attached to the inner side of the epi-
blastic cells (Fig. 16). The ovum of the rabbit has now increased in
size from 0.09 mm., its size at the close of segmentation, to about 0.28
mm. It is inclosed by the vitelline membrane and a mucous layer
deposited by the walls of the Fallopian tube.

As the vesicle continues to enlarge, the hypoblastic cells spread out
beneath the epiblast, though remaining thicker in the centre than at the
edges, where the cells still possess the power of amoeboid movement.
The central, thicker portion, which is the commencement of the embryonic
area, forms an opaque, circular spot on the blastoderm.

The primitive hypoblast now becomes divided into two layers, the
lower continuous with the peripheral hypoblast and formed of flattened
cells, while the upper is formed of small, rounded elements, the meso-

26

PHYSIOLOGY OF THE DOMESTIC ANIMALS.

blast. The superficial epiblast, again, is formed of flattened cells, which
soon become columnar and appear to unite with the rounded elements
below, except at the lower part of the embryonic area. Here the blasto-

FIG. 16. RABBIT'S OVUM BETWEEN SEVENTY TO NINETY HOURS AFTER
IMPREGNATION, AFTER E. .VAN BENEDEN. (Sal/our.)

BV, cavity of blastodermic vesicle (yelk-sac) ; EP, epiblast ; H Y, hypoblast ; ZP, mucous envelope
(Zona pellucida).

derm, as in the chick, is constituted by three layers, the epiblast, the
mesoblast, and the hypoblast.

P.R,

EP.

M

HY.

FIG. 17. SECTION THROUGH THE OVAL BLASTODERM OF A RABBIT IN THE
SEVENTH DAY, THROUGH THE FRONT PART OF THE PRIMITIVE STREAK.
(Balfour.)

EP, epiblast; M, mesoblast; HY, hypoblast; PR, primitive streak.

The subsequent changes in the development of the blastoderm form
the subject of Embryology, and for their consideration the reader is
referred to text-books on anatomy.

III. THE MODIFICATION IN THE FOKM OF CELLS.

We have seen that originally all the cells formed by cleavage in the
egg are absolutely alike. Like the original egg, they are typical cells,
consisting of a cell-membrane inclosing finely granular protoplasm, in
which a nucleus and nucleolus may be recognized. They only differ from

MODIFICATION IN THE FOKM OF CELLS.

27

the original cell in size and in as
vet unmarked individual char-
acteristics which in the speciali-
zation of function of the organ-
ism will cause them finally, for
the : most part, to lose all mor-
phological resemblance to the
parent cell.

These differences in cells
produced in the development of
the organism are very numer-
ous.

First, as regards their size,
we find cells varying from the red
blood-cell -g-g 1 ^ to the large
ganglion-cell, ^<y of an inch
(Figs. 18 and 19).

In nearly all instances where
a collection of cells develop into
an organ or tissue the original
spherical form is lost, often
merely from mutual pressure and
from alteration in the cell-con-
tents, by which the most varied
forms are produced. Thus,
instead of the spherical form,
cells may take on an oval, elon-
gated shape (Fig. 20), or may be
cylindrical (Fig. 21), or again
from mutual pressure may form
regular hexagons. Others may
have long, thread-like attach-
ments developed, as in the sperm-
cells (Fig. 22), or even a number
of such prolongations, which as
long as the cell is alive continue
in active movement (ciliated
cells). (See Fig. '21.)

Often the nucleus deviates
from its spherical form, and may
become oval or, irregular in out-
line,, or, as in certain cells of
the marrow of bone and in

FIG. 18. RED AND WHITE BLOOD-CORPUSCLES,
ENLARGED Six HUNDRED DIAMETERS.

(Ellenberger.)

A, surface view of red corpuscles ; B, profile view ; C, rou-
leaux of red corpuscles ; D, central depression in red corpuscles :
E, crenated red corpuscles ; F, small, and G, large white cor-
puscles.

FIG. 19. AN ISOLATED GANGLION-CELL OF
THE ANTERIOR HORN OF THE HUMAN
SPINAL CORD, AFTER GERLACH. (Klein.)

A, axis-cylinder ; B, pigment. The branched processes of
the ganglion-cell break up into the fine nerve net-work shown
in the upper part of the figure.

28

PHYSIOLOGY OF THE DOMESTIC ANIMALS.

FIG. 20. NON-STRIATED MUSCULAR FIBRES, ISOLATED. (Klein.)

The cross-markings indicate corrugations of the elastic sheath of the individual fibres.

FIG. 21. VARIOUS KINDS OF EPITHELIAL CELLS. (Klein.)

A, columnar cells of intestine ; B, polyhedral cells of the conjunctiva : C, ciliated conical cells of the
trachea : D, ciliated cell of frog's mouth ; E, inverted conical cell of trachea ; F, squamous cell of
the cavity of the mouth, seen from its broad surface ; G, squamous cell, seen edgeways.

FIG. 22. VARIOUS KINDS OF SPERMATOZOA. (Klein.)

A, spermatozoon of guinea-pig not yet matured ; B, the same seen sideways, the head being flattened
from side to side ; C, a spermatozoon of the horse ; D, a spermatozoon of the newt.

MODIFICATION IN THE FORM OF CELLS.

29

striped muscle, may undergo reduplication without division of the
cell (Fig. 23).

The cell-contents, or protoplasm, particularly as regards its granular
constituents, may undergo the greatest variation. Often true crystalline

FIG. 23. BONE-CORPUSCLES, WITH THEIR PROLONGATIONS, AFTER ROLLETT.

(Flint.)

formations are included in the cell-contents ; or vacuoles may form in
which different fluids, sometimes watery, sometimes fatty, may collect, to
again disappear in later stages of the life of the cell.

FIG. 24. DIAGRAM OF DIFFERENT FORMS OF CARTILAGE. (Ellenberger.)

A, hyaline ; B, fibro-cartilage ; C, elastic cartilage ; D, secondary cartilaginous capsule, containing
the primary capsule.

Another modification in the form of cells consists in the alteration
of the border laj-ers of protoplasm, so that the cell is surrounded by a
more or less chemically or morphologically different area, or intercellular

30

PHYSIOLOGY OF THE DOMESTIC ANIMALS.

substance, which may present the greatest variety as to quantity. The
cell-membrane and so-called cell-capsule belong to these forms of proto-
plasmic modification. In cartilage and loose connective tissue this
intercellular substance exists in such amount that the still actively
moving protoplasmic cells appear to be forced apart by it. (See Fig. 24.)
Since the more active vital movements can only originate in the
semi-fluid protoplasm of cells, it is evident that the more or less rigid
intercellular substance could only take a slight part in organic processes
if there were not some means by which it could be brought into close

FIG. 25. NERVE-FIBRES. (Thanhoffcr.)

1, a, tnedullated nerve-fibre: b. non-medullated nerve-fibre from the sympathetic of the ox (after
Schultze) ; 2, non-medullated nerve-fibre from Jacobson's organ in the sheep (after Schnltze) ; 3, nerve-
fibres with Ranvier's nodes (R), from the sciatic of the frog: 4, funnel-shaped arrangement of medulla
from sciatic of frog, treated with osmic acid ; 5, nerve-fibre from frog, with axis-cylinder (t) and so-called
horny mesh-work; 6, diagram of medullated nerve-fibre: n, neurilemma; v, medullary sheath : t, axis-
cylinder.

association with the active vital processes constantly occurring in the
interior of cells. Consequent!}' we find the entire intercellular substance
pierced of a mesh-work of fine canals, through which the cells send
prolongations of their outer layer, which, after numerous subdivisions,
serve to connect neighboring cells.

By means of these juice-canals interchange between the contents of
the various cells is possible and the intercellular substance receives its
necessary supply of nourishment, while the connection of cell with cell
is an illustration of the loss of individuality of cell-life. Often we find

DEVELOPMENT OF TISSUES AND OKGANS. 31

cells connected by branching prolongations : in other cases the exten-
sions of the cells which, to be sure, have a very different function and
structure from those alreacty alluded to, but which, nevertheless, serve as
connections between cells so overbalance the cells in extent and number
that the latter often appear only as rounded swellings in the extensions
(e.g., in the nerves). (See Figs. 25 and 26.)

FIQ. 26. NERVOUS GANGLIONIC CEL.L AND BRANCHING FIBRES, AFTER
KRAUSE. (Thanhoffer.)

xt, cell body; m, nucleus; me, nucleolus; pn, protoplasmic prolongations; tf, axis-cylinder fibres;
tn, axis-cylinder prolongations.

IV. THE DEVELOPMENT OF TISSUES AND ORGANS.

The final result of the metamorphosis of cells is the formation of
tissues out of which the various organs of the body are built up.

32 PHYSIOLOGY OF THE DOMESTIC ANIMALS.

The development of tissue starts in the earliest stages of develop-
ment of the egg.

We have seen already that, as a result of fecundation, the egg divides
into a number of minute segmentation spheres which at first form a
solid, mulberry -like mass, and we have now to consider the changes
occurring in the mass of simple, undifferentiated cells which result in the
development of the tissues and organs of the completed organism.

In their early stages, as in the amoeba, each of these cells possesses
the powers of development, reproduction, growth, assimilation, respira-
tion, and contractility. As the organism passes to a higher stage we
find that many of these cells lose these general properties possessed in
their entirety by undifferentiated protoplasm, while certain of them are
put aside to carry on specific individual functions. Thus, in the young
embryo, as in the amoeba, all the cells possess the power of contractilit}' ;
as the organism develops, this property becomes restricted to cells form-
ing constituents of certain tissues, the contractile tissues, or the muscular
system. The amoeba, which we have already seen maj^ be regarded as
representing one of the units of which the higher organisms are built up,
possesses the power of irritability and automatism. In higher forms
this property of undifferentiated protoplasm is restricted to a single
tissue, the nervous system. The amoeba has no part specialized for the
various processes of nutrition; any part of its substance may take in
nutritive matter, may digest out the portions capable of supplying its
nutritive needs, may remove the undigestible residue : any portion of
the amoeba is capable of carrying on the metabolic processes by which
the matter absorbed as food is converted into protoplasm like itself, and
any portion is capable of absorbing the gases necessary for these com-
plex chemical processes and of getting rid of the effete products of its
nutritive processes. In the higher organisms certain cells are set aside
to form the organs concerned in the prehension of food ; others have for
their sole function the secretion of solvent juices which will digest out
the nutritive matters of the food ; others are the carriers of the matters
absorbed to remote corners of the organisms ; and the sole business of
certain other cells is to get rid of the useless matter and the products
of the waste of the economy. In the amoeba any portion may divide off
from the parent stock and so originate a new individual; in the higher
animals certain tissues or collections of cells have for their sole office
the reproduction of cells which shall constitute the starting point of a
new organism. In the amoeba, therefore, specialization of function has
not commenced ; each minute particle of the protoplasm which constitutes
it is capable of carrying on all the vital functions. In the higher organ-
isms, however, the elementary organisms of which they are built up are
so arranged that there may be a division of labor. These collections of

DEVELOPMENT OF TISSUES AND OEGANS. 33

cells, marked by a more or less exclusiveness of function, are termed
tissues. It must not be overlooked, however, that, though certain func-
tions are accentuated in individual tissues, they all possess remnants of
all the functions originally seen in undifferentiated protoplasm. Thus,
maii}^ besides the muscular tissues possess the power of contractility :
it is not the nervous system alone which retains the property of
responding to irritants. All the tissues are capable of reproducing them-
selves in part, and all possess the power of carrying on their own
nutritive processes if suitable food is supplied to them.

We have seen that fecundation of the ovum leads to the develop-
ment of an immense number of new cells, and we have referred to the
modifications in form to which these segmentative spheres may be
subjected.

Tissues are formed from these segmentative spheres in three different
ways (Wundt) :

1. Through formation of layers of cells.

2. Through union of cells.

3. Through excretions by cells.

Often all of these methods are united in the formation of a compound
tissue which is functionally active as a unit. Such a tissue is called an
organ. The classification of tissues is based on anatomical grounds; of
organs, on ph}'siological grounds.

1. To the first group belong the epitheliums. In most of these the
only modifications which occur in the form of the cells are due to
mutual pressure from close contact. Such cells are therefore polygonal
or flattened, or, when growth is most marked in one axis, cylindrical.
The epitheliums, in series of layers or in single rows, cover the external
surfaces of the body as well as the coatings of the digestive, urinary,
genital and respiratory tracts which communicate with it, the ducts of
glands, and the closed serous sacs. In the latter localitj^ they are called
endothelia, in the former epithelia. The hair and nails are modifications
of these tissues formed of small elongated cells grown together into
almost homogeneous tissues. The terminal portions of the organs of
special sense are also epithelial in nature.

Epithelial cells are connected by a thin layer of an albuminous
cement substance, which during life is in a semi-fluid condition.
The shape of the cell may be columnar or squamous. Spindle-shaped and
club-shaped cells, as well as goblet cells, ciliated cells, epidermic cells,
and prickle cells, are all modifications of these shapes. Endothelial cells
are always of the squamous variety, arranged in single layers of flattened,
transparent cells with oval nuclei. When their form approaches the
columnar, as occasionally is the case in certain serous membranes, they
are then in an active state of division, are called germinating cells, and

3

34 PHYSIOLOGY OF THE DOMESTIC ANIMALS.

the small spherical lymphoid cells which result are carried to the circu-
lation to become white blood-corpuscles.

Glandular tissue is also epithelial in nature. Cells are the essential
secreting organs of glands, though numerous other tissues enter into
their composition. Such cells are usually rounded or polygonal, and are
soft in consistence. Frequently the cells rapidly partially break down
and are carried off as constituents of the secretion, as colostrum cells,
or mucoid corpuscles (moulting); or the destruction of the cell form may
be complete and the constituents of the cell enter the secretion in the
form of a solution.

Muscular tissue forms the third of the group. In the muscles the
muscular cells are always closely associated with connective tissue.
Such cells may be of two different kinds, different in structure and in
function, the striped and unstriped cells. Muscle-cells are contractile,
like amoeboid cells, but contractility is only possible in one definite direc-
tion, that of their long axis ; muscles, therefore, become shorter and
thicker during contraction.

2. To the second group of tissues formed by union of cells belong
two tissues in which, in their development, the cells have become greatly
elongated, and after absorption of the cell-wall become converted into
fibres or tubes ; these are the nerves and capillaries.

In the nervous tissues certain cells retain their primitive character,
the nerve-cells, and only nerve-fibres properly belong to this group of
tissues. Nevertheless the nerve-cells are in continuity with the nerve-
fibres.

The capillaries in a similar manner originate from the arrangement
of nucleated, spindle-shaped cells in rows, which become hollowed out
through the formation of vaeuoles. The formative cells manifest a ten-'
dency to send out sprouts which connect with other forming or mature
capillaries by which the net-work of capillaries is formed. Lymphatic
capillaries are developed in the same way as the blood-vessels.

3. Tissues formed by excretions from cells, forming the third group,
may also be called intercellular tissues. Connective tissue is a type of
this class. Connective tissues serve to bind together all the organs of
the body ; they exist in various forms, which are to a certain extent
mutual!}' convertible ; they all yield allied chemical products.

Connective tissue originates in spherical cells, with very soft proto-
plasmic nucleated contents, which ultimately may form a perfectly homo-
geneous, laminated, or fibrous intercellular substance.

The cells themselves may exist in various different forms, either as
tendon-cells, when they are flattened, oblong masses of protoplasm
arranged in rows, with round nuclei lying in bundles of fibrils of white
connective tissue ; as branched cells, found in the cornea, serous mem-

DEVELOPMENT OF TISSUES AND ORGANS. 35

branes, and subcutaneous tissues, which, under certain conditions, pre-
serve their power of movement ; as pigment-cells, which are branched
cells filled, with the exception of their nucleus, with granules as fat-
cells, in which the protoplasmic cell-contents are replaced by a drop of
oil, which forces the flattened nucleus against the cell-membrane; and as
migratory cells, which are formed in the spaces of fibrous tissue, and
which possess the power of amoeboid movement.

The form of connective tissue in which least change is produced in
development is the so-called mucoid or gelatinous tissue. In some
of the invertebrates, as in mollusks, this tissue forms the greater part of
the bod}\ It consists of a soft, semi-fluid, intercellular substance, in which
numerous granules and broken down membraneless cells are suspended.
Many of these cells possess the power of amoeboid movement.

From this tissue in vertebrates the fibrillar connective tissue is de-
veloped by the condensation of this intercellular substance into bundles
of fibres, held together by an albuminous cement substance, in which
the forms of the cells become much changed, becoming flattened and
elongated by mutual pressure until the diameter of the cell is not greater
than that of the nucleus.

The elastic tissues are formed out of the fibrillar connective tissues,
which become so modified chemically as to yield elastin and not gelatin.
The fibres of elastic tissue are bright j'ellow in color, usually anastomose,
and are sometimes straight, but more often coiled up in bundles.

The different "forms of development of connective tissue, especially
of the intercellular substance, depend upon its different chemical meta-
morphoses. Thus, the gelatinous tissues owe their properties to a semi-
fluid albuminous booty, the connective tissue proper to gelatin-forming
bodies, cartilage or chondrin, and the elastic tissues to elastin, etc.

Bone is characterized by the deposit of inorganic compounds in its
intercellular substance. This hardened tissue then incloses the partially
broken down cells, which form the lacunae or bone-corpuscles, in which
the thickened membrane and nucleus are often visible, though they dis-
appear in old bones and their place is then taken \)y serous fluids, etc.

The bones are also rich in vessels which traverse the Haversian
canals.

As the deposit of salts occurs partly from these and partly from
the external membrane, the bones become laminated in structure, some
layers being parallel- with the canals, others with the exterior.

Cartilage is characterized by a sparse intercellular substance which
yields chondrin, and by large cells which are often the seat of endoge-
nous multiplication. As in bone, so in cartilage, the intercellular sub-
stance becomes arranged in the form of capsules around the cells, and
may either remain homogeneous (hyaline) or become fibrillar (fibrp-

36 PHYSIOLOGY OF THE DOMESTIC ANIMALS.

cartilage or elastic cartilage, in which a dense net-work of elastic fibres
occupies the intercellular matrix). Most cartilages, except on articulating
surfaces, are covered by a fibrous membrane, the perichondrium, supplied
with blood-vessels, lymphatics, and nerves.

Bone is surrounded by a fibrous membrane, the periosteum, with an
inner layer of oblong nucleated cells, which, from the fact that the bone
is developed from them, are termed osteoblasts. Similar cells are also
found in the marrow of bones.

The organs of the animal body are always composed of several
tissues; they may be of three classes, in each one of which some one
tissue is especially prominent in function :

1. Organs whose chief function depends upon tissues of the first
class (cells without intercellular substance).

a. Glands, which always in addition to the epithelium contain nerves
and blood-vessels. The epithelium is the essential part of glands, skin,
mucous and serous membranes.

/;. Muscles. In these are found muscle-cells as the functionally
prominent tissues, though associated with them are connective tissue,
blood-vessels, and nerves.

2. Organs whose chief function is manifested through tissues of the
second class.

a. Compound vessels, arteries, veins, and rvmph-vess-els. Although
the capillaries are formed by union of cells, in larger trunks this mode
of formation only applies to the endothelium, on which the other tissues
elastic tissue, connective tissue, and pale muscular fibres subsequently
develop in layers.

b. The organs of the nervous system. Nerve-cells and nerve-fibres,
formed by union end to end of cells, here form the essential tissues,
though blood-vessels and connective tissue are associated with them.

3. Organs whose function is due to intercellular substance. The bony
skeleton is the only representative of this group, and the modification
of connective tissue known as bone, or its antecedent cartilage, is the
characteristic tissue; as secondary tissues, connective tissue and blood-
vessels and nerves, representatives of the second class, are also met with.

For the mode of development of the compound organs in the
embryo the reader must be referred to text-books on anatomy or
embryology.

SECTION II.
CELLULAR PHYSICS.

I. THE PHYSICAL PEOCESSES IN CELLS.

As the tissues and organs of the animal body originate in ceils, we
should expect that the functions of the higher organisms, which we know
to be identical with those of the most elementary forms of life, would to
a certain extent be accomplished by the same general processes. We
have already divided the functions of animal life into the vegetative, or
nutritive, functions and the functions of relation, and have called atten-
tion to the attempt which has been made to reduce the working of these
processes to physical and chemical laws. Although in many points this
endeavor fails, the operation of the ordinary physical and chemical laws
serves to explain many of the complex phenomena of animal life. This
is especially seen in the maintenance of the nutrition of the organism.

That cells may retain a nutritive balance it is requisite, in the first
place, that they be .supplied with a proper pabulum, which must pass
from the exterior to the interior of the cells. We have found that the
typical cell is surrounded by a homogeneous or striated membrane,
which, like all other organic tissues, contains a large amount of water
closely associated with the ultimate molecules of which that membrane
is made up. Hence, the cell-membrane may be regarded as a porous
partition whose pores are filled with water, and which separates the
cell-contents from the surrounding media. These media may be either
gaseous, as the atmosphere ; fluid, like the lymph and blood in higher
animal forms, or water in aquatic forms of life; or semi-fluid, like the
more or less solid intercellular substance.

This passage of nutriment from the exterior to the interior of cells
is mainly accomplished b}^ purely physical means, not only in simple
unicellular organisms but also in higher forms of life, where digestion,
or the preparation of food for absorption, has for its object the reduc-
tion of food into such forms that the operation of the physical laws of
imbibition, capillarity, filtration, diffusion, and osmosis, aided perhaps
by chemical affinity, will be sufficient to enable the nutritive matters to
pass to the interior of cells.

When once in the interior of the cells the raw food-products must
be transformed into protoplasm similar to that of which the cell-contents

(37)

38 PHYSIOLOGY OF THE DOMESTIC ANIMALS.

is composed. This is a purely chemical process, is accompanied by the
liberation of force, and requires for its performance the free access of
oxygen. Gases, also, must therefore pass from the exterior to the
interior, a process which the laws of absorption and diffusion of gases
are quite sufficient to explain. Again, the chemical processes concerned
in the assimilation of food result in the formation of carbon dioxide,
urea, kreatin, and other crystalline bodies which are no longer of use
and which must be removed ; or the cell activity may take on the form
of a secretion, that is, the cell-protoplasm ma} 7 , from the matter sup-
plied to it, manufacture certain substances which in the higher organisms
have to be used elsewhere. In either case the products of the proto-
plasmic operations must be removed. Here, also, physical laws are all
sufficient. Gases are removed under the laws of gaseous diffusion and
absorption. All ciystallized products are eliminated by equally
simple means. The absorption of fluid by the cell-contents through imbi-
bition leads to increase of volume, and hence to increased pressure on the
cell-membrane. Filtration thus comes into operation. Or, if the pres-
sures within and without the cell are equal, interchange of matters in
solution may take place through diffusion and osmosis. All these proc-
esses may occur equally well in membraneless cells ; for, in all cases, to
permit interchange the dividing membrane must be capable of absorbing
the fluids or solutions with which it is in contact. The conditions, then,
are analogous to those of a body containing fluid by imbibition in con-
tact with another fluid.

We have now to consider some of the physical processes concerned
in these operations. The chemical processes will subsequently receive
attention.

The explanation of the physical processes in cells is to be found in
the molecular forces which fluid molecules exert on each other and on
solids with which they may be in contact.

1. COHESION is the force which binds together adjacent molecules of
the same nature ; for example, two molecules of water or two molecules
of iron. This attractive force is strongly exerted in solids, less so in
liquid, and is absent in gases. It is measured by the force which is
required to tear a body asunder. The closer the molecules of a body
are together, the greater their cohesive force.

Cohesion varies inversely with the square of the distance which
separates the molecules ; anything, therefore, which drives the molecules
apart will tend to weaken their cohesive force. When bodies are heated
the expansion which they undergo is due to the separation of their mole-
cules. Hence, when solids are heated their molecules are further and
further separated until finally their cohesive force is balanced by the
repulsive force, and the bodiespass from a solid to a liquid state. This

PHYSICAL PEOCESSES IN CELLS. 39

occurs in all cases, provided the heat to which they are subjected does
not cause them to undergo chemical change. If a body which has been
fused by heat has its temperature still further raised its molecules
become so much further separated that the force of cohesion is not
sufficient to keep them in contact, and the body then becomes vaporized.

In liquids the force of cohesion is not great ; hence their molecules
are readily displaced and a mass of liquid assumes the shape of the vessel
which contains it ; in other words, the force of gravity overcomes the
force of cohesion. Cohesive force is, nevertheless, present in liquids, and
may be demonstrated by the difficulty with which a plate of glass placed
horizontally in contact with the surface of water is removed vertically
upward. This difficulty is due to the fact that the adhesion of the glass
to the water being greater than the cohesion of the water, the molecules
of water must be violently separated to permit of the removal of the
glass.

The spheroidal form assumed by small masses of liquid, as in a drop
of dew, a globule of mercury, is due to the working of the force of cohe-
sion of the molecules of the liquid. In a small drop of mercury placed
on a surface for which it has no adhesion, as wood or glass, the sum of
the mutual attractions of all its molecules being greater than the force
of gravity acting on them, the globule assumes the spherical form. If,
however, the drop of mercury is large, then the force of gravity increas-
ing with the mass of the body becomes greater than its cohesion and the
drop becomes flattened.

The molecules of all liquids attracting each other, it is evident that
the molecules in the free surface of a liquid will be attracted by and will
attract those below them, but will exert or will be subjected to no exter-
nal attraction. At the surface of liquids, therefore, there is always an
inward attraction, which is called surface tension. Of course, in these
considerations external accidental pressures and attractions to which the
liquid may be subjected are neglected.

The surface tension of liquids is well illustrated by blowing a soap-
bubble on the end of a glass tube ; as long as the other end of the tube
remains closed the elastic tension of the air in the bubble balances the sur-
face tension of the soap-film, but when the end of the glass tube is opened
the tension of the film leads to the contraction and final disappearance
of the bubble. So also insects can move on the surface of water without
sinking, for the water, not being able to wet their feet, forms a depression,
and the elastic reaction of the surface supports them. The case is simi-
lar when a sewing-needle is floated on water ; as the needle is coated
with a thin film of oil the water does not adhere to it, the surface becomes
depressed, and its increased tension serves to support the weight. Wash-
ing the needle first in alcohol, ether, or potash causes it at once to sink.

40

PHYSIOLOGY OF THE DOMESTIC ANIMALS.

The importance of these facts will be seen in the explanation of
capillary phenomena.

2. ADHESION is the molecular attraction exerted between the surfaces
of bodies in. contact ; it may be manifested between solids (as between
two freshly-cut surfaces of a leaden bullet, or two pieces of plate-glass),
between solids and liquids (as when a drop remains clinging to a glass
rod which has been dipped in water), and between solids and gases (as
shown by the bubbles of air which adhere to a glass or metal plate when
immersed in water).

The adhesion of liquids to solids, which alone of the above will
receive attention at present, is greater than the cohesion of liquids, as
already mentioned. Thus, when a drop of water is placed on a glass
surface it does not assume the spheroidal form, but becomes flattened
out, showing that the adhesion of the water to the glass is greater than
the cohesion of the water. If, however, the glass plate be greasy, then
the drop of water will exert no adhesion to the glass and will remain
spheroidal. This adhesion of liquids to solids is not universal, but

.FIG. 27. FIG. 28. FIG. 29. FIG. 30.

CAPILLARY PHENOMENA. (Ganot.)

depends on the nature of both the solid and the liquid. Certain liquids
are capable of adhering to, or wetting, certain solids and not others ; and
a solid to which one liquid will adhere will be inert, or even repulsive to
another. These facts also are of importance in the explanation of
capillarity.

3. CAPILLARITY. When a solid body is placed in contact with a liquid
the phenomena (attraction or repulsion) which result are termed capillary
phenomena from the fact that they are best seen when capillary tubes
(capillus, a hair) are immersed in liquids.

As already stated, water is capable of adhering to glass. When a
glass rod is dipped into water, the water is raised up against the sides
of the rod to form a concave surface above the level of the water, as if
no longer subject to the laws of gravity (Fig. 27). If, on the other
hand, a glass rod be dipped into mercury, which does not adhere to
it, the mercury is depressed around the rod, forming a convex surface
below the level of the surrounding fluid (Fig. 28). If glass tubes
with narrow bore are immersed in water and mercury, in the former

PHYSICAL PKOCESSES IN CELLS.

41

the water will rise within the tube considerably above the level of
the water outside, and the surface of the water in the tube will be
concave (concave meniscus); while the mercury will be depressed in
the glass tube below the level of the mercury on the outside and the
surface of the mercury within the tube will be convex (convex meniscus)
(Figs. 29 and 30). If any two bodies, such as two glass plates, are
immersed sufficiently near to each other in a liquid, the liquid will
rise or be depressed between them, according as the liquid has or
has not any adhesion to the plates (Fig. 31), the degree of elevation or
depression being one-half what it would be if a tube of glass whose
diameter equals the distance between the two plates were immersed in the
same liquids. If a drop of water be placed in a conical glass tube of
small angle and horizontal axis, each end of the drop will have a concave
meniscus and it will move from the large to the small end of the tube :
if the liquid be mercury, each end will have a coiiA*ex meniscus and it
will move in the reverse direction (Figs. 32 and 33).

In the explanation of capillary phenomena
two causes deserve attention : first, the cause of
the curvature of the surface, and, second, the cause
of the ascent or depression of the liquid within
the tube.

FIG. 31.

FIG. 32.

FIG. 33.

The form of the surface of a liquid in contact with a solid depends
on the relation between the attraction exerted by the solid on the liquid
and the mutual attractions of the molecules of the liquid. Any molecule
of a liquid in which a solid is immersed is acted on by three forces : 1st,
gravity; 2d, the attractive force of the solid for the liquid molecule ; and
3d, the cohesive attractions of the other molecules of the fluid.

According to the relative intensities of these forces their resultant
may occupy one of three positions :

First. If the attraction of the solid balances the cohesive attrac-
tion of the fluid, the resultant of these two forces will coincide with the
force of gravity and the surface of the fluid will be horizontal; for to
be in equilibrium the surface of a liquid must be at right angles to the
direction of the resultant of all the forces acting on that liquid
(Fig. 34).

Second. If the attractive force of the solid for the fluid increases,
or if the cohesive force of the liquid diminishes, the resultant will fall
outside of the line of gravity or between the line of attraction of the

42

PHYSIOLOGY OF THE DOMESTIC ANIMALS.

solid and the line of gravit.y, and the surface of the liquid being at right
angles to that resultant will be concave (Fig. 35).

Third. If the attraction of the solid for the liquid decreases, or
the cohesive attraction of the liquid increases, the resultant will fall to
the other side of the line of gravity, or between the line of cohesive
force of the liquid and the line of gravity, and the surface of the liquid,
being perpendicular to that resultant, will be convex (Fig. 38).

FIG. 34.

FIG. 35.

FIG.

DIAGRAMS ILLUSTRATING CAUSE OF CURVATURE OF LIQUID SURFACES IN
CONTACT WITH SOLIDS. (Ganot.)

The molecule m is acted on by gravity, in the vertical line m P : is attracted by the plate n. in the line
n m, and by the liquid F, in the line m F. The direction of the resultant m R will depend upon the relative
intensities of these forces. If n m and m F balance, the resultant is vertical, m R (Fig. 34), and the surface
is horizontal. If n m increases, or m F decreases, the resultant R is within the angle n m P, and the surface
is concave (Fig. 35). If m F increases, or n m decreases, the resultant R is within the angle I'm F, and the
surface is convex, for the surface of a liquid is always perpendicular to the resultant of forces acting on
its molecules (Fig. 36).

The ascent or descent of liquid within a capillary tube is dependent
on the manner in which the curvature of the surface modifies the prin-
ciples of hydrostatic equilibrium.

When a tube of large calibre is immersed in a vessel containing
liquid the conditions of equilibrium are the same as in two communicating
vessels containing the same fluid. Equilibrium is only possible when the

surface of the liquid in both vessels is
on the same horizontal plane. For, take
any molecule in the plane MN (Fig. 37).
It will be subjected to a downward pres-
sure equal to the weight of a column of
the same fluid, the height of which is equal
to the distance of that molecule from the
surface of the fluid within the tube. It
will also be subjected to an upward pres-
sure which is equal to the weight of a
column of liquid whose height is equal to
the distance of that plane from the surface

of the liquid without the tube. These weights are, however, equal.
Therefore every molecule in the plane MN will be subjected to equal and
contrary pressures, and will consequently be in equilibrium.

Suppose, however, the tube have a diameter less than one millimeter.
The concave surfaces produced by the adhesion of the fluid to the

FIG. 37. DIAGRAM ILLUSTRATING
LIQUID PRESSURES.

PHYSICAL PROCESSES IN CELLS. 43

walls of the tube will then intersect, and the surface of the fluid within
the tube will be a concave meniscus. In other words, every portion of
the surface of the liquid within the tube will be under the attractive influ-
ence of the walls of the tube. A certain portion of the fluid within the
tube will so be held up by adhesion to the tube, and will hence exert
no downward pressure. As a consequence the downward pressure within
the tube will be less than the upward pressure of a column of fluid of
the same height without the tube. Any molecule on any plane below
the surface of the fluid in the tube would so be subjected to two
unequal pressures, a greater upward pressure and a lesser downward
pressure ; the column of liquid will therefore rise within the tube until
these two pressures are equal.

When, however, the force of cohesion of the liquid is greater than
that of adhesion to the walls of the tube, as already explained, the sur-
face becomes convex and the surface tension is increased. Since the
molecular forces are greater than gravity, the downward pressure in the
tube is greater than the upward pressure to which any plane is subjected
by the weight of the liquid outside of the tube. The fluid then is de-
pressed in the tube until these two pressures are equal.

Capillarit}- partly explains the ascent of the sap in trees, the ascent,
of oil in a lamp-wick, to a certain extent the movement of the blood and
lymph in the capillaries, but more especially the entrance of fluid into
porous bodies, a fact of the greatest importance as underlying the expla-
nation of imbibition, filtration, and osmosis.

4. SOLUTION. That a substance may enter the interior of cells it
must, as a rule, be in a state of solution; though we shall find, when we
study the process of absorption, that there are several exceptions to this
statement.

The process of solution of solids in fluids is of very general occur-
rence in cell life. Almost all food-stuffs are solid, and to be of nutritive
value must first be reduced to the form of a solution ; even the con-
sumption of organic matter in the vital processes of a cell results in the
formation of a watery solution, as in the formation of urine, sweat, and
the various secretions.

When a solid dissolves in a liquid, the cohesion of the molecules
of the solid is broken by their adhesion for the molecules of the liquid.
When, therefore, the attraction of the liquid for the solid is greater
than the cohesion of the solid, the latter is said to be soluble and its
molecules separate. The limit of solubility is reached when the attrac-
tions of adhesion and cohesion are balanced.

Anything that reduces the cohesion of a solid favors its solution;
thus, heat accelerates solution by separating solid molecules through
the expansion which it produces, and, by increasing the distance between

44 PHYSIOLOGY OF THE DOMESTIC ANIMALS.

the molecules, thereby weakens their cohesive force. Pulverizing, by
mechanically separating the molecules to a certain extent, also assists
solution.

Heat is always essential to the conversion of a solid into a solution.
Ordinarily the heat is abstracted from the surrounding media, and is
rendered latent in the solution, thus explaining the mode of action of
freezing mixtures. The amount of heat so rendered latent in forming a
solution is nearly always equivalent to the amount required to melt the
body.

In certain cases, however, instead of the temperature being lowered
in the process of solution, it actually rises, as when caustic potash is dis-
solved in water. This depends upon the fact that two contrary proc-
esses are going on at the same time ; the solution, which tends always
to produce a reduction of temperature, and the chemical union of the
potash with the water, which, like many other chemical processes, tends to
cause an increase of temperature. Consequently, as one or the other of
these processes predominates the temperature will fall or rise ; or, if the
two balance, will remain unchanged.

Solubility varies greatly in different bodies and in different liquids.
Some solids are soluble only in hot media, and are deposited on cool-
ing ; others only in cold liquids, and are thrown out of solution when
the temperature of the liquid is raised. As a rule, bodies dissolve in
liquids which have similar properties ; thus crystalline bodies are soluble
in water, fats in oil, metals in mercury, and resins in alcohol.

u When two or more salts are dissolved in water without chemical
action on each other, three conditions result : 1st. The quantity of each
salt held in solution is less than when it alone is present, though the
combined quantity is greater than when only one salt is used. 2d. The
quantity of each is as great as when one only is used ; then the total
quantity dissolved is the sum of that taken up in each single solution.
3d. The quantity dissolved is greater than when one alone is used,
the addition of the second salt in this case increasing the solubility of
the first, and often the first increasing also the solubility of the second"
(Draper).

When the cohesion of the solid and its adhesion for the liquid mole-
cules balance, the solution is then said to be saturated. In the case of
certain fluids, like alcohol and water, there is no limit to solubility ; their
molecules will freely mingle with each other, and the resulting liquid is
said to be a mixture, or an emulsion. On the other hand, two liquids
may offer an example of true solution, one being only capable of passing
to a certain degree between the molecules of the other, as in the case of
volatile oils and water, where the limit of solubility is readily reached.

5. IMBIBITION. Every porous solid may be considered as formed of

PHYSICAL PKOCESSES IN CELLS. 45

a collection of capillary tubes. When such a solid (e.g., a piece of chalk)
is immersed in a fluid which is capable of wetting it, the fluid will enter
into the pores of the solid through capillarity, and will remain even after
the body is removed from the fluid. The solid is then said to have
absorbed fluid by imbibition. Organic bodies also are capable of absorb-
ing fluid by imbibition, but the process is somewhat different from that
of the inorganic porous body.

Every organic bod}^, no matter what its consistency, contains always
a large amount of water in its composition, to which fluid, as we shall
find later, many of the physical properties of the tissues are due. When
inorganic bodies contain fluid, that fluid is held in one of two ways,
either chemically united with the body, as in hydrates, or as water of
crystallization ; or mechanically in the pores of the solid. Organic bodies
occupy a mean between these two. The water in their composition is
not in a form of chemical combination, nor is it held mechanicalty in
pores, as in the porous inorganic body, though the conditions are some-
what similar to the latter case. That there is a difference, however, is
proved by the different effects of the abstraction of water from organic
and porous inorganic bodies. The physical characters of porous, inor-
ganic solids, such as baked clay, are not seriously altered by the removal
of water contained in their pores. The abstraction of water from semi-
solid organic bodies, on the other hand, entirely changes their physical
and physiological properties. A piece of connective tissue in its fresh
condition is soft, white and glistening, flexible, extensible and elastic.
When the water which it contains is removed by drying, it shrivels up,
becomes rigid, yellowish in color, brittle, and loses weight. If it be then
immersed in water its previous characters will be restored. This differ-
ence in the manner in which the water entering into the composition of
organic and inorganic bodies is held is explained by the assumption that
in the porous inorganic body the water occupies comparatively large
spaces between particles of solid, while in the organic body the water
surrounds the ultimate molecules of the body. Organic tissues, ma}'
therefore be defined as bodies whose intermolecular spaces are filled
with fluid.

As the fluids are held in a different manner in inorganic and organic
bodies, it is natural to find that the way in which the fluid is absorbed
differs in the two cases.

When a dry, porous, inorganic body, such as a piece of chalk, is
thrown into w r ater, the water enters the pores of the chalk by capillarity
and displaces the air which was contained in its pores. It increases in
weight by the addition of the weight of the absorbed water, but does
not increase in volume. When a dry organic body, such as a piece of
gelatin, is thrown into water no air is displaced, and yet many times

46 PHYSIOLOGY OF THE DOMESTIC ANIMALS.

its own volume of water may be absorbed. The gelatin must therefore
increase in volume. The fluid in organic imbibition passes into the inter-
molecular spaces and separates the molecules. That an organic body may
imbibe fluid it is consequently necessarj^ that its molecules be freely
movable.

The power of imbibition possessed by the organic tissues is espe-
cially due to their albuminoid constituents. Protoplasm is, therefore,
above all capable of imbibition, and the rapid formation or disappear-
ance of vacuoles in protoplasmic cells may be due to rapid changes in
imbibition.

Every organic substance has a limit be} r ond which imbibition is
impossible. This limit is lower when the water contains solids in solu-
tion, provided the solids are not chemically acted on by the tissues, from
the fact that imbibed water is held with greater tenacity by the tissues
than are the substances held in solution in the water. Thus, when a
tissue saturated with a salt solution is subjected to pressure, the solution
first pressed out is more concentrated than that which is forced out when
the pressure is subsequently increased ; and in general the fluids lose in
concentration in imbibition. Organic tissues therefore absorb water with
greater readiness than saline solutions. The importance of this fact will
be seen in the explanation of osmosis.

The extent of imbibition depends, therefore, on the membrane and
the nature of the fluid with which it is in contact. Thus it has been
found by Liebig that one hundred parts of ox-bladder absorb, of

Water, 268 volumes.

Salt solution (1.204 sp. gr.), . . . .133
Alcohol (84 per cent.), . . . . .38 "
Marrow oil, . . . . . 17 - "

Membrane, therefore, has less affinity for a salt solution than for
water. This may also be shown by soaking a bladder in water, wiping
it dry, and then sprinkling it with common salt. The salt comes in
contact with some of the water in the bladder, dissolves, and forms a
salt solution. But as the membrane can contain less salt solution than
water, some of the solution is expelled and the bladder shrivels up.
So also a moistened bladder thrown into alcohol shrivels up, because
the alcohol mixes with the water in the bladder ; and as the bladder,
as shown above, can only absorb one-seventh as much alcohol as
water, the solution is driven out. This is the explanation -of the use
of alcohol and various saline solutions for hardening tissues for making
microscopic sections.

In the nutrition of animal cells the process of imbibition is an
important factor. Every substance which is soluble in water is capable
of being appropriated by the protoplasm and maj^, through imbibition,

PHYSICAL PEOCESSES IN CELLS. 47

obtain access to the interior of cells, to there undergo the transformations
which the needs of the economy necessitate. In cells which possess a
closed membrane, capillarity may also be concerned ; for there is reason
to suppose that the striated appearance which is seen on examining most
cell-membranes with a high power under the microscope is in reality due
to the presence of minute apertures, or canaliculi. Fluid will therefore
enter the pores in the membranes of cells, and so obtain access to the
protoplasmic cell-contents. The passage of fluids, however, through the
cell-membrane is not necessarily dependent on capillarity. For the
cell-membrane, like other organic tissues, is capable of absorbing water
by imbibition in the same way in which water is absorbed b} T gelatin, i.e,
by entering into its intermolecular spaces. The state of affairs is thus
similar to the conditions described in the experiment with the bladder
and salt. We have an organic tissue soaked with a fluid in contact with
a substance (protoplasm) having an affinity for that fluid greater than
the affinit}^ of the membrane for the fluid; the fluid, therefore, leaves
the cell-membrane to enter the protoplasm by organic imbibition.

The affinity of protoplasm for water is never satisfied during life :
or, in other words, the maximum amount of water capable of being
absorbed by cell-contents is never reached. Cells will, therefore, always
absorb fluid when brought into contact with it, and by so doing will
tend to increase in volume. As, however, the extensibility of cell-
membranes is in most cases very limited, the increase in volume of the
cell-contents will tend to cause filtration of the fluid contained in the
meshes of the protoplasm back through the cell-membrane to the
exterior.

These facts which we have learned in reference to the imbibition of
fluids by organic tissues give but an imperfect idea of the processes of
imbibition which take place in living cells. Many fluids which are
absorbed by dead tissues are perfectly indifferent to living cells, and
there can be no doubt but that imbibition in living tissues is largely
governed by the nature of the chemical affinities caused by the chemical
processes continually taking place in the interior of active cells. Thus,
living tissues (muscles) are incapable of absorbing dilute solutions of
sodium salts ; the same tissues when dead will absorb it in large amounts.
Potassium salts, on the other hand, are rapidly absorbed by the same
tissue and almost instantly cause its death, though even in this case the
power of imbibition for the potassium salt is greatly increased after
death. Again, we shall find that the prolonged activity of many tis-
sues, especially the muscles, is manifested in the production of an acid
reaction in the cell-contents; under such circumstances, sodium solutions,
which are indifferent to these tissues at rest, will now be absorbed by
them.

48 PHYSIOLOGY OF THE DOMESTIC ANIMALS.

The following figures represent the behavior of the muscular tissue
to solutions of different salts (Ranke) :

MAXIMUM IMBIBITION.

1 PER CENT. Na'Cl. 1 PER CENT. K Cl.

Living, resting muscle, . Positive, but not capable of

estimation, as the muscle
instantly died.

Living, tetanized muscle, . 13 Positive : not to be estimated,

for the same reason as above.
Dead muscle, . . .35 136 per cent.

Perfectly analogous observations have also been made in the case of
the nervous tissues.

It would, therefore, appear that living tissues only absorb by
imbibition when their vital forces are diminished in energy. In the
organism the cells are continually bathed in a fluid which contains the
matters necessary for the nutrition of the cells. If the cells have been
at rest their nutritive equilibrium lias not been disturbed and imbibition
does not take place. If, however, they are exhausted by previous
activity, imbibition is then inaugurated, nutritive substances enter, the
results of cell activity are extruded (acids, etc.), nutritive equilibrium is
restored, and imbibition not only ceases, but from the increased volume
of the cell-contents pressure is produced on the cell-membrane and the
excess of fluid is forced through again to the exterior b} T filtration.
Consequently cells which are most weakened by prolonged activity are
the cells which carry on t as a direct result of the chemical affinities
created by that activity, the most active imbibition. By these peculi-
arities in the conditions which govern imbibition in living cells is to be
explained the peculiar distribution of t