I select as a point of departure the human body, and the bodies of
domestic animals, because my readers are best acquainted with these.
The limbs consist, beginning on the outside, of skin, flesh, and bone.
The same parts can also be distinguished in the head, neck, and trunk;
but in these divisions of the body they enclose a cavity, the
_body-cavity_, which, again, contains various parts (“organs”), which
are not everywhere attached to the body-wall. Fig. 1 represents a
longitudinal section through the body. The skin is represented by a
line, flesh and internal lining are shaded, while the bones are black.
These parts form together the _body-wall_. In front the body-wall
encloses a cavity, the body-cavity (_Kh._), which in Mammals is divided
into two sections (thoracic cavity, _Bz.h_, and abdominal cavity,
_B.h._) by the _midriff_ (diaphragm). In the thoracic cavity are found
the lungs and heart (_H_), also most of the gullet or upper part of the
gut; the abdominal cavity contains the remainder of the often
much-coiled gut, which in one place widens into the stomach (_M_), also
the kidneys, spleen, and parts connected with the gut (_e.g._ the
liver). The cavities are bounded behind by the backbone (vertebral
column), which is made up of many flattened vertebræ. The uppermost
vertebra supports the skull, which encloses a _cranial cavity_
(_Sch.h._) continuous with a _vertebral canal_ bounded by the vertebræ.
Cranial cavity and vertebral canal form together a second body-space, in
which are contained the brain and spinal cord.
[Illustration:
FIG. 1.—Schematic Longitudinal Section of the Human Body.
]
We will now consider the individual parts of the body, beginning with
the _skeleton_. The axis of the skeleton is formed by the vertebral
column (spine), which is composed of flat bones, the _vertebræ_. A
vertebra usually consists of (1) the body, which occupies the front; (2)
the arch, which possesses several projections or processes (neural
spine, transverse processes, articular processes) and encloses the
vertebral canal (_W.h._). All mammals have seven neck or cervical
_vertebræ_ (Fig. 2, 1); while the number of the remaining vertebræ
varies according to the species. The cervical vertebræ, which support
the head, are followed by the dorsal or _thoracic vertebræ_ (12 in man,
Fig. 2, 2), and these by the strong loin or _lumbar vertebræ_ (5 in man,
Fig. 2, 3). Cervical, thoracic, and lumbar vertebræ are movable, but, in
man, the last-named are followed by five vertebræ immovably united
together to make up the _sacrum_, and these again by tail- or _caudal
vertebræ_. Man has four such vertebræ, all poorly developed, and fused
with one another (Fig. 2, 5); but in many animals there are a large
number, movably united to make up a tail.
The ribs, which in mammals bound the chest, are jointed to the thoracic
vertebræ. Man has 12 pairs of ribs; each rib consists of a bony part
behind and a gristly (cartilaginous) part in front. The so-called true
ribs (Fig. 2, 14) [the upper pairs] are movably united with the
breast-bone, but this is not the case with the false ribs (Fig. 2, 15).
In the head we distinguish the brain-case or _cranium_, and the skeleton
of the _face_. The first contains the cranial cavity in which the brain
is enclosed. We distinguish—2 frontal bones (fused together in man, Fig.
2, 6); 2 parietal bones (7); 2 temporal bones (8); an occipital bone (9)
composed of several pieces fused together, perforated by the foramen
magnum [where brain and spinal cord unite], and bearing two elevations
or condyles [for effecting union with the backbone]; and the sphenoid
and ethmoid bones which make up the base of the cranium. The facial
skeleton consists of the framework of the jaws and palate, and, together
with some of the cranial bones, bounds the cavities in which the eyes
are contained (orbits), and the nasal cavities. It consists of the
maxillary bones (Fig. 2, 12), the premaxillary bones (Fig. 3, 7,—in man
these 4 bones are fused together into one piece), the nasal bones, the
lachrymal bones, the ploughshare bone (vomer), the turbinated bones, the
cheek-bones (or malars, Fig. 2, 11), the palate-bones, and the lower jaw
(Fig. 2, 13). (The last originally consists of two symmetrical halves.)
The upper and lower limbs are built on the same type, and therefore
consist of corresponding parts (cp. Fig. 2). The more similar the
functions of the two pairs, the closer their resemblance. In the ox they
are much more alike than in man; in the bird, on the contrary, the
similarity is much less. A distinction can be drawn in both limbs
between the bony girdles (shoulder-girdle and hip-girdle), which serve
for union with the trunk-skeleton, and the different subdivisions of the
limbs themselves. I place side by side the parts of the arm and leg of
man.
ARM. │ LEG.
I. Shoulder-girdle, consisting │ I. Hip-girdle, consisting of:
of: │
Shoulder-blade (Scapula) (Fig.│ Hip-bone (Ilium) (24).
2, 17). │
Collar-bone (Clavicle). │ Pubis.
Coracoid process (of Scapula).│ Rump-bone (Ischium).
II. Upper arm: │ II. Thigh:
Upper arm-bone (Humerus) (18).│ Thigh-bone (Femur) (25).
III. Fore arm: │III. Leg:
Radius (19). │ Shin-bone (Tibia) (26).
Ulna (20). │ Clasp-bone (Fibula) (27).
IV. Hand: │ IV. Foot:
Two rows of wrist-bones │ Two rows of ankle-bones
(Carpal bones) (21). │ (Tarsal bones) (28).
Metacarpal bones (22). │ Metatarsal bones (29).
Finger-bones (Phalanges) (23).│ Toe-bones (Phalanges) (30).
The differences between arm and leg are explained by their different
uses. The bones of the leg, used to support the human body, are firmer
and thicker, but less movable than those of the arm, which is employed
in grasping. Consequently the union between the hip-girdle and the
trunk-skeleton is firmer than that of the shoulder-girdle. The radius
can rotate upon the ulna, so as to completely turn the hand over; a
similar twisting of the foot would not be of use, and cannot be
effected. The leg has a knee-pan (patella) (Fig. 31), with which there
is no bone in the arm to correspond. In the foot the toes are short, and
the remaining parts long; for instance, one of the tarsal bones, the
calcaneum (heel-bone), is strongly developed and projects behind (28*).
In the hand, the digits are relatively long, and since the tip of the
thumb can be made to touch the tips of all the fingers, are admirably
adapted for grasping.
[Illustration:
FIG. 2.—The Human Skeleton.
]
The number of fingers or toes is at most five, but may be less. The
horse has a single digit to each limb; the ox, two well developed and
two remaining as rudiments; the pig, two large and two small; while the
dog has four toes in the hind foot, five in the fore foot.
Man walks on the sole of the foot. Some other animals (dog, cat) on the
toes; others again (horse, ox, pig), on the tips of the toes. In the
last case there is not simply a horny structure (nail or claw) on the
upper side of the toe, but a hoof sheathing the whole of its tip. In
many animals the thigh and upper arm are drawn close up to the body, so
that the limbs appear quite different from those of man. (Compare Fig. 2
with Fig. 3.)
[Illustration:
FIG. 3.—Skeleton of an Ox. I. _Skull_: 1, Frontal bone, with horn
cores, _a_; 2, temporal bone; 3, malar or cheek-bone; 4, maxillary
bone; 5, lachrymal bone; 6, nasal bone; 7, premaxillary bone; 8,
lower jaw; 9, orbit; 10, occipital bone. II. _Neck and Trunk_: _H_,
7 cervical vertebræ; _R_, 13 thoracic vertebræ; _L_, 6 lumbar
vertebræ; _K_, sacrum; _Su_, caudal vertebræ; _C_, 13 pairs of ribs;
_D_, sternum. III. _Fore Limbs_: _Sc._, scapula; _A_, humerus; _S_,
radius; _E_, ulna; _U_, carpus; _M_, metacarpals; i., ii., iii.,
phalanges. IV. _Hind Limbs_: _B_, hip-girdle, _a_, ilium, _b_,
ischium; _F_, femur; _P_, patella; _T_, tibia; _Sp_, tarsus; _M_,
metatarsals; i., ii., iii., phalanges.
]
The bones are usually surrounded by flesh. This consists of a number of
different pieces united together by a delicate, elastic, fibrous mass
(connective tissue). The different pieces are termed _muscles_, each of
which is again made up of a large number of muscle-fibres, all taking a
longitudinal direction. Each fibre can contract, and a muscle becomes
shorter and thicker by simultaneous contraction of all its fibres. The
contraction and subsequent relaxation of muscles move other parts. There
are some muscles, the _hollow muscles_, which surround a cavity, and by
their contraction propel the liquid or solid substances found in their
cavity. The heart, for example, is a large muscle of this sort, serving
to propel the blood, while the hollow muscular coat of the gut moves on
the contained food. Other muscles are fixed by their ends to other parts
of the body, which they move by their contraction. We distinguish
between _dermal muscles_ and _skeletal muscles_, attached respectively
to the skin or by one end to an integumentary structure (hair, feather,
scale), and to parts of the skeleton. The animals which are devoid of
any internal skeleton, the _invertebrates_ (_i.e._ all animals except
vertebrates), naturally possess no skeletal muscles. Examples of _dermal
muscles_ are those by means of which a bird erects its feathers
(tail-coverts of peacock!), and those which enable a hedgehog to roll
itself into a ball and stick out its spines. Each end of a _skeletal
muscle_ is connected with a bone. If such a muscle contracts the more
easily movable bone is drawn towards the less easily movable one (Fig.
4). In order that the bones may be movable upon one another they are
united together by joints.
According as muscular movements are, or are not, under the influence of
the will, they are distinguished as _voluntary_ and _involuntary_. To
the latter kind belong the movement of the heart, and the movements of
the muscles in the wall of the gut by which the food is made to
progress.
[Illustration:
FIG. 4.—Bending of the Arm by Contraction of the Biceps Muscle. _a_,
humerus; _b_, ulna; _c_, elbow-joint; _d_, biceps muscle; _e_,
origin; _f_, insertion of the same. In the right-hand figure of the
muscle _d_ is contracted; in the left-hand figure it is slackened.
]
To destroy the contractile power of a muscle it is not necessary to
injure the muscle itself. Every muscle is related to a nerve, which
sends its fine branches to the fibres making up the muscle. If we cut
the nerve, the corresponding muscle loses its power of contraction. But
the nerve arises from the _central nervous system_, which in vertebrates
principally consists of the brain and spinal cord. The muscle will
therefore lose its contractile power if the connection with these
central parts is broken. The true cause of movement resides in these
parts. A sort of change, the essential nature of which is unknown to us,
takes place in them, and is propagated along the nerve to the muscle,
causing it to contract. The central nervous system is, therefore, the
origin, the centre from which the order to contract proceeds; hence its
name. The nerves which run from these central parts to the muscles are
known as the _nerves of movement_ (motor nerves).
[Illustration:
FIG. 5.—Diagram to explain the Action of the Motor and Sensory Nerves.
]
There is still, however, a second group of nerves, the _nerves of
sensation_ (sensory nerves), which arise in the sense-organs (skin,
mucous membrane of tongue, nose, ear, eye), and convey to the central
nervous system the impressions they receive from the outer world by the
aid of these sense-organs. In the appended diagram (Fig. 5), _C_
represents the central nervous system; _B.N._, a motor nerve, branching
in the muscle _M_; _G.N._, a sensory nerve, which runs from the
blood-bathed inner skin or dermis (_L.h._), underlying the outer skin or
epidermis (_O.h._), to the central system. (The arrows indicate the
direction in which impulses are conveyed along the corresponding nerve.)
Men or animals lose in weight if they take no food. The reason for this
is that certain substances leave the body either as gases (through the
lungs), or as liquids (by the kidneys and sweat-glands), without a
corresponding compensation. An animal or human being could not live
without taking in fresh substances, which, according as they are solid
or liquid, are known as food or drink. The different kinds of food and
drink, which, with few exceptions (salts, water), are taken from the
animal and plant kingdoms, cannot, however, _as such_, replace the
gradually diminishing body substance, for, to begin with, they contain
useless matters, which pass out of the body in the _fæces_ (dung). And
even the nutritious parts of the animal and vegetable substances taken
into the stomach, are not always in a form in which they can be used _at
once_. Digestion, which in all the higher animals takes place in a
_food-tube_ (gut), serves to reduce them to a suitable condition, at the
same time separating the useless matters. The action of several fluids
(saliva, gastric juice, bile, etc.) secreted by glands, extracts the
useful (nutritious) substances from the food and drink, converting them
also into a suitable form. The smaller the pieces into which the food is
separated, the better can this purpose be effected. In mammals the teeth
serve to break down the food; in birds and many Invertebrates the same
part is played by special secretions of the stomach or intestine
provided with hard ridges.
So long as the nutritious food-stuffs remain in the food-canal, even
though in a completely suitable form, they cannot nourish the body. And
since waste of the substance of the body everywhere takes place, it is
absolutely necessary that the food-stuffs should pass after digestion
into a system of organs going to all parts of the body. This system is
the _circulatory_, or _vascular system_. Food-stuffs enter it from the
gut directly or indirectly, reaching it in the latter case through the
_lymphatic_ (lacteal) system.
The _blood_ is the fluid into which the food-stuffs are taken up. It
consists of an almost colourless liquid, together with an innumerable
number of exceedingly minute blood-corpuscles.
The blood flows through the body in a system of tubes, or
_blood-vessels_, which branch repeatedly, and at last become merged in
the microscopic _capillary blood-vessels_. These capillaries are present
in nearly all parts of the body except the epidermis and epidermal
structures (hairs, feathers, scales, etc.). They have exceedingly thin
walls, which present no resistance to the passage of the nutritious
substances contained in the blood, so that these can be absorbed by
those parts of the body which lie between the individual capillary
vessels. The central organ of the circulation is the _heart_, an
enlarged part of the vascular system, possessing thick muscular walls.
By contraction of these, the blood is driven out of the heart (Fig. 6,
_H_); and its exit is possible on one side only (_a_), as at the other
side (_b_) there is a valve, which closes when the heart contracts. The
vessel into which the blood leaving the heart enters is termed an
_artery_ (_S.A._) It divides into several branches, also known as
arteries, and the smallest arteries pass into capillaries, which again
are connected with _veins_, which join larger and larger veins, until
finally one or a few open into the heart (_A_).
[Illustration:
FIG. 6.—Diagram of the Course of the Circulation.
]
Since the blood in the course of its circulation gives up some of its
nutriment to the various parts of the body, it would in the end become
useless for the purposes of nutrition if it did not receive a fresh
supply of food-stuffs from the gut, either directly or indirectly
(through the lacteal system). But apart from this, the blood would
ultimately become useless, and that very quickly, if it did not traverse
the lungs, kidneys, and sweat-glands. It is well known to every one that
a man or animal cannot live without air, or at any rate without a
certain gas, _oxygen_, that is contained in air. This oxygen must be
able to penetrate into the minutest particles of the body, and the
blood, in the corpuscles of which it is contained, carries it
everywhere. In the smallest particles (molecules) of the body an
oxidation (combustion) of body substance takes place, which not only
causes an evolution of heat, but also renders the body capable of doing
work. But if now the blood passes from the capillaries into the veins,
it contains too little oxygen. And besides, it has taken up from the
molecules of the body several substances, developed in those molecules,
which would be fatal to the animal if they were not removed from the
body. Now, when the blood streams through the lungs, it gets rid of the
poisonous gaseous matter, and when it traverses the kidneys and
sweat-glands it parts with the injurious liquid and solid substances.
But in the lungs the blood takes up at the same time fresh oxygen; and
since in this way the air in the lungs becomes poor in oxygen, the
_movements of breathing_ (respiration) provide for the passage of a
fresh supply of oxygen into the lungs. Only the higher Vertebrates
breathe by means of lungs; fishes and numerous aquatic Invertebrates
breathe by gills, and insects by air-tubes (tracheæ).
[Illustration:
FIG. 7.—The Small-winged Gall-fly, _d_ (_Andricus terminalis_), lays
its eggs separately in the rootlets of oak. Root-galls (_a_) result
from this, and inside of each of them a larva develops which, after
a metamorphosis, becomes a relatively large, wingless gall-fly (_c_)
known as _Biorhiza aptera_. This pierces the oak-buds in early
spring, and lays a large number of eggs in them; from part of the
bud is formed a large juicy gall (_b_), containing several larvæ,
from which the small-winged gall flies (_d_) develop. The species
here represented exist, therefore, in two forms, _e_ and _d_
(Heterogeny).
]
While _Nutrition_ is the life-process which shields the _individual_
from death, Reproduction serves to maintain the _species_. It is
familiarly known that the offspring generally resemble their parents.
But it is also a fact recognized by the stock-breeder, that a particular
animal will not only transmit several of its _own_ characteristics to
its offspring, but perhaps also various characteristics of the
grandparents or of animals belonging to still more remote generations,
although these characteristics are not visible in the animal which is
actually breeding (_Reversion_, _Atavism_). Among insects and the lower
animals there are species which, as adult animals, appear not in _one_
form, but two or several. In this case, as a regular thing, the
offspring does not resemble the parents, but the grandparents,
great-grandparents, or a still earlier generation. The older observers
have placed the offspring and the parents, and sometimes the
grandparents too, of the same animal species in different species, or
even genera or families, until newer researches on the reproduction and
development of these animals have proved them to belong to one and the
same species (see Fig. 7 and explanation). The method of reproduction by
which a species appears in two or several forms is distinguished as
_heterogeny_ and _metagenesis_, or _alternation of generations_. In the
first (Fig. 7) sexually reproducing animals alternate with other sexual
animals. It may be that these are of separate sexes, or else they may
possess both male and female organs (_hermaphrodite_). In metagenesis a
sexual generation regularly alternates with one or several generations
reproducing asexually.
The animal kingdom falls (cf. p. 2) into sub-kingdoms or main divisions.
Seven of these are commonly distinguished: I. Backboned animals; II.
Jointed-limbed animals; III. Worms; IV. Molluscs; V. Echinoderms; VI.
Cœlenterates; VII. Protozoa.
First Sub-Kingdom: VERTEBRATA (BACKBONED ANIMALS).
The Vertebrate body possesses a bilateral or twosided symmetry; _i.e._
it can be separated into two exactly corresponding halves, by a plane of
division. The bilateral symmetry is strictly carried out as regards the
external parts of the body, a single exception to this being flat-fish
(plaice, flounder, etc.); but, on the other hand, it is more or less
obliterated in the arrangement of the internal organs. In the Vertebrate
body we find, as an axis, a vertebral column (backbone) made up of
vertebræ, and traversed by the vertebral canal. As soon as this canal
widens out in the skull to the cranial cavity, the spinal cord, which it
contains, merges into the brain. In addition to the cavity containing
the central nervous system, and placed on the upper side (= dorsal side)
of the animal, a cavity, the _body-cavity_, is found in the under side
(= ventral side). It contains for the most part the organs of
respiration, circulation, digestion, and excretion (Fig. 1), and in
Mammals is divided by the diaphragm into _thoracic_ and _abdominal
cavities_. In all the other subdivisions of the animal kingdom the
central nervous system is situated in the same cavity as the
above-mentioned organs.
Various bones are connected with the vertebral column, and they serve
for the attachment of muscles. The bones collectively constitute the
skeleton, which is one of the most distinctive features of a Vertebrate.
The animals of this sub-kingdom never have more than four limbs, and
their blood is red, while that of most other animal groups is
colourless.
The structure of the heart in the various Vertebrates must also be
noticed. In no Vertebrate is this organ so simple in structure as in the
scheme given in Fig. 6; such an arrangement, moreover, would involve
great difficulties. One great difficulty would be that while the blood
was leaving the heart at _a_ (Fig. 6), no fresh blood could enter, so
that the blood in the veins would stand still. Even in the lowest
Vertebrates (the Fishes) this difficulty is obviated, for where the main
vein (or veins) opens into the heart an enlargement of this vein is
found, where the blood can collect as long as the heart continues to
contract. This expansion is also reckoned as part of the heart, and
named the _auricle_ (Fig. 8, _V.K._), while the heart proper is termed
the _ventricle_ (_K._). It is also easy to see that there must be a
tolerably wide opening between the two chambers, so that as soon as the
ventricle becomes flaccid the auricle can force blood into it. But there
being such a wide aperture between auricle and ventricle, one valve is
not enough to make it impossible for the blood to pass back into the
auricle during the contraction of the ventricular walls. There are two
or three valves there (Fig. 8, _Kl._) fixed by fibres to the wall of the
ventricle. In order that the blood which is forced into the artery
(_S.A._) may not pass into the ventricle during its relaxation, there is
another valve (not indicated in Fig. 8), at the base of the artery.
[Illustration:
FIG. 8.—Diagram of the Heart in a Fish.
]
An arrangement like that so far described is found in fishes. The heart
consists in them of an auricle, into which is returned the blood that
has traversed the body, and of a ventricle which moves it on again. But
the blood that has traversed the body is on that account poor in oxygen,
and consequently unfit to be circulated again when it is returned to the
heart. It is necessary for it to take up fresh oxygen before being
circulated again. In fishes this difficulty is met by the blood, poor in
oxygen, which flows out of the ventricle, first going to the _gills_ and
streaming through them. The gills consist of a very large number of
small, thin-walled outgrowths arranged in regular rows on the firm
gill-arches. The blood, poor in oxygen, passing out of the ventricle and
through various arterial branches to the gill-filaments, takes up fresh
oxygen as it streams through these from the oxygen dissolved in the
water which constantly surrounds them. For this purpose a stream of pure
water is regularly taken in by the mouth and expelled again, right and
left, through the gill-slits. The blood, having become rich in oxygen in
the gills, is now once more fit for circulation through the body, and
therefore flows out of the gill-capillaries into larger vessels, which
finally unite into a single large vessel that carries the purified blood
to the various parts of the body. In the arrangement of the heart here
described there is the disadvantageous condition that the blood is
obliged to traverse two sets of capillaries (gill and body capillaries).
This is not an easy matter, for there is a great deal of friction
between the blood and the walls of the capillaries, constituting a
hindrance to its progress. The circulation of the blood in fishes is
consequently very slow, and since the blood contains the oxygen which is
used by the various parts of the body, oxidation goes on slowly in the
body of a fish; hence the small amount of heat developed there. Since
fishes almost immediately give off to their surroundings the small
amount of heat which they develop, they have no constant body
temperature, varying in this respect with the temperature of the
surrounding water. Such animals are termed _cold-blooded_.
In all other Vertebrates a more rapid movement of the blood is rendered
possible by the insertion of a second heart, quite similar in every
respect to the other heart, in the course of the blood between the
respiratory organs and the body. The first heart drives the blood
through the lung capillaries, and therefore corresponds to the
fish-heart; from these capillaries the blood returns to the auricle of
the second heart, and from the ventricle of that heart travels to the
various parts of the body. When it has completed this course, it returns
to the auricle of the first heart. Although these two structures work
independently, they lie close together and make up a single organ. We do
not therefore speak of two individual hearts, but of one heart with two
halves. The first half, which receives the blood, poor in oxygen, that
is returned from the body, and sends it on to the lungs, lies on the
right, and is termed the _right_ half. The second half, which receives
the richly oxygenated blood from the lungs, and pumps it to the various
parts of the body, is termed the _left_ half (Fig. 9 and explanation).
In the arrangement just described, which is found in Mammals and Birds,
the blood returning from the lungs is propelled with new force through
the body, and therefore circulates very quickly, so that the various
parts receive a relatively large amount of oxygen in a short time. It is
therefore intelligible that Birds and Mammals develop more warmth than
Fish. They possess a special, constant body temperature, somewhat
different in different species, but usually lying between 98° and 104°
Fhr., and they are called _warm-blooded_ animals.
[Illustration:
FIG. 9.—Diagram of the Mammalian Heart. 1, right, 2, left ventricle;
3, right, 4, left auricle; 5, superior, 6, inferior vena cava; 7,
pulmonary artery forking into branches for right and left lungs; 8,
the four pulmonary veins; 9, the great body-artery (aorta); the
arrows indicate the direction of the blood-stream.
]
[Illustration:
FIG. 10.—Diagram of the Heart of a Reptile. Between the right (_r.K._)
and left (_l.K._) ventricles is a perforated partition. _r.V.K._,
right auricle; _l.V.K._, left auricle; _H.v._, vena cava, carrying
back the blood which has traversed the body into the right auricle;
_L.art._, pulmonary artery; _L.v._, pulmonary vein; _Ao._, aorta.
]
In Reptiles (snakes, lizards, etc.), the two halves of the heart are not
entirely distinct, since there is an opening in the partition-wall
between the two ventricles. As a consequence of this, the poorly
oxygenated blood of the right half of the heart mixes with the richly
oxygenated blood of the left half, the extent to which this mixing takes
place being proportional to the size of the aperture. In Reptiles,
therefore, the blood supplied to the lungs is not so poor in oxygen as
it might be, nor, on the other hand, is the blood supplied to the other
parts of the body completely oxygenated. As consequences of this: (1)
respiration is feebler, and (2) the development of heat less than in
Mammals and Birds (_i.e._ reptiles are cold-blooded), and (3) the
chemical changes taking place in the body (the metabolism) go on more
slowly than in warm-blooded animals, and we can understand why reptiles
execute fewer movements in a given space of time.
[Illustration:
FIG. 11.—Diagram of a Frog’s Heart. (The ventricle _K._ is quite
undivided: compare Figs. 9 and 10.) Other letters as in Fig. 10.
]
In Amphibians (_e.g._ frog) the two ventricles are similarly connected,
but the opening is still larger than in Reptiles, and the partition-wall
may even be altogether absent. It follows, therefore, that the mixing of
the two kinds of blood is still more complete, and that Amphibians, too,
are cold-blooded.
The vertebrate sub-kingdom embraces the classes of I. Mammals; II.
Birds; III. Reptiles; IV. Amphibians; V. Fishes.