ANIMALIA VERTEBRATA— VEIITEBRATED ANIMALS. 
2.5 
may be understood from tbe following observations of the learned Dr Paley: 
** There is provided in the central part of the body a hollow muscle, invested with 
spiral fibres, running in both directions, the layers intersecting one another : in some 
animals, however, appearing to be circular rather than spiral. By the contraction of 
these fibres, the sides of the muscular cavities are necessarily squeezed together, so 
w to force out from them any fluid which they may at that time contain; by the 
relaxation of the same fibres, the cavities are in their turn dilated, and, of course, 
prepared to admit every fluid which may be poured into them. Into these cavities 
are inserted the great trunks, both of the arteries which carry out the blood, and of 
the veins wliicb bring it back. This is a general account of tiie apparatus ; and the 
simplest idea of its action is, that, by eacli contraction, a portion of blood is forced 
as by a syringe into the arteries j and at each dilatation, an equal portion is received 
from the veins. This produces at each pulse a motion and change in the mass of 
the blood, to the amount of what the cavity contains, which, in a full-grown human 
heai't, I understand is about an ounce, or two table-spoons full. IIow quickly these 
changes succeed one another, and by this succession, how suflicient they are to support 
a stream or circulation throughout the system, may be understood by the following 
computation: — Each ventricle will at least contain one ounce of blood. The heart 
contracts four thousand times in one hour; from which it follows, tiiat there pass 
through the heart, every hour, four thousand ounces, or three hundred and fifty 
pounds of blood. Now, the whole mass of blood is said to bo about twenty five 
pounds; so that a quantity of blood, equal to the whole mass of blood, passes through 
the heart fourteen times in one hour; which is about once in every four minutes. 
Consider what an affair this is when wo come to very large animals. The aorta of a 
whale is larger in the boro than the main pipe of the water-works at London bridge ; 
and the water roaring in its passage through that pipe is inferior, in impetus and velo- 
city, to the blood gushing from the whale’s heart. Hear Dr Hunter's account of the 
dissection of « whale : — The aorta measured a foot in diameter. Ten or fifteen 
gallons of blood are thrown out of the heart at a stroke, witli an immense velo- 
city, through a tube of a foot in diameter. The whole idea fills the mind with 
wonder.” 
The circulation through a single heart may be seen in the frog. The heart is 
composed of only one ventricle, and of one auricle. From the ventricle the blood 
is propelled through two divisions of the aorta, finally terminating in one large 
branch, niid is thence transported through the ramified extremities of the arteries 
throughout the body. Boturning by the vena cava, it is again carried to the auricle, 
and thence restored to the ventricle. But during its passage a part only of the bl<K>d 
was transported to the lungs through the pulmonary ai-leries, and again brought back, 
through the pulmonary veins, after having been purified. This partitd aeration of tho 
blood imj'arts to tho Batrachia or frogs a cold and sluggish character. 
The Fishes Imvc a double circulation, but their respiratory organ is 
formed for breathing through the medium of water; and their blood 
receives the action only of that portion of oxygen wbicli is dissolved or 
niixed in the water. From this circumstance it follows that the degree 
of their respiration is still less tlian that of the Reptiles. 
The gills of Fishes arc situate at each side of the throat, and immediately adjoining 
the heart. Dr Monro is of opinion, that they present an extent of surface to the 
action of the water equal to that of the entire human body. The fibres resemble 
the teeth of an exceedingly fine comb, and they are covered with minute protu- 
berances, rcscrabling the ])ile of velvet, while innumerable blood-vessels distribute 
their delicate fibres over the entire surface. The distribution of these vessels on the 
folds and divisions of the gills forms one of the most minute and delicate arrangements 
lu tho animal economy. By means of these organs, the Fish is enabled to absorb the 
oxygen dissolved in the water; and after yielding tins substance, the water is dis- 
charged through the branchial openings. 'I'he Fishes form a contrast with animals of 
the other divisions, in this respect, that they do not inspire by the same opening through 
w^hich tliey expire. 
lu the Mammalui, the circulation is double, and the aerial rcspii-iition 
'S simple, that is to say, it is performed only in the lungs. The quantity 
of their respiration is therefore greatly superior to that of Reptiles, in 
consequence of the form of the heart, or circulating organ, and also to 
that of the Fishes, from the nature of the surrounding element. 
The quantity of respiration possessed by Birds is yet greater than that 
^f the Mammalia, because they not only have a douljle circulation, with 
^ direct aerial respiration, but they also breathe through many other 
cavities besides the lungs. The air penetrating into the cells distributed 
over the body, acts upon the branches of the aorta, or arteries of the 
^edy, as well as upon the ramifications of tho pulmonary artery. 
From these circumstances arc derived the four kinds of motion for 
■^hich each of the four classes of AT'rtebratod animals seems particularly 
designed. 
The QuADEurEDs, in which the quantity of respiration is modc- 
^■ate, are generally formed for walking, for hunning, and for developing 
l^hese motions with vigour and precision. 
The Birds, wherein respiration is much more perfect, possess 
muscular vigour and that liglUncss of construction necessary for 
^Ugut. 
Tu'e Rei'tiles, endowed with a more feeble respiration, arc con- 
demned to CRAWL upon the earth, and many of them pass a part of their 
^fe in a continued state of torpor. 
The Fishes, in order to execute their less vigorous motions, require 
7 
to be supported in a fluid of nearly the same specific gravity W’ith their 
own bodies. 
All the other organic arrangements proper to each of these four classes, 
and especially those which are connected with motion and with external 
sensation, bear a necessary relation to these essential characters. 
MOTIONS OF THE VERTF.BRATED ANIMALS. 
Jralltinff — Leaping — lit/nninif — Trotting — Gallopmg — Climbing — Flying — 
Darting — Faddling — JDh'ing — iiwimmhxg. 
To perform all the different kinds of progressive motion which are enjoyed by Man 
and the lower animals, it is necessary that a certain velocity should be communicated, 
in one particular direction, to the ventre of gravity of the animal body, or that point 
in the body around w'hich all the parts balance and remain at rest. A certain number 
of joints mu5t exist, capable of a greater or less degree of flexure. Their relative 
position must be so adapted that it may he comparatively easy to extend tlicm on tho 
side to which the centre of gravity is made to incline, and difficult on tho opposite 
side, so that the general movement may tend in the former direction. 
The mech.mical part of Animal motion may be understood from the followinf»- 
illustration : — If we imagine a spring divided into two branches, one of which rests 
ujion a firm resisting base, and then suppose that the branches are compressed by 
some e.Kternal force, their elasticity will cause them to recede as soon as the compres- 
sing force has been removed, and the two branches will bo inclined at the same angle 
to each other as they were before the compression. But as that branch which rests 
upon the basis is unable to overcome its resistance, the movement takes place wholly 
in the opposite direction, and the centre of gravity of the spring is forced from the 
resisting body with more or loss velocity. Accordingly, in any animal, while tho 
muscles (Jlexors)^ which bend the part employed in effecting the movement, represent 
the external compressing force of the spring, and those muscles (cxteyinors)^ which 
slretcb it out, correspond to the elasticity that makes the branches of the spring fly 
asunder, tbs ground supporting the animal, or fluid in which it moves, forms the 
resisting basis. 
In IValhing., the centre of gravity is alternately moved by one part of the extremi- 
ties and supported by the other, the body never being completely separated from the 
ground. It differs essentially from Leaping^ where the entire body is projected into 
the air; and from Hunning, which consists of a number of short leaps. 
In general, it is less painful to walk than to stand, because the same muscles arc 
not continued in action for so long a period; and it is much easier to counteract those 
unsteady motions Avliich occur in walking by contrary and alternate actions than it is 
to prevent them entirely. Thus it follows, that though all animals which stand 
erect on two legs, such as l\I-iu and Birds, can also walk on two legs, yot many 
moving in an UjUight position with sufficient case, cannot stand on two foot for any 
time without very great fatigue and exertion. 
When I\Ian intends to walk on level ground, he first extends one foot. Ills body 
then rests equally npon both leg.s, tho advanced log making an obtuse angle witli the 
tarsus or instep, and tho other an acute angle. As tlio ground does not yield to the 
point of the foot, the heel and the remainder of the leg must necessarily be raised, 
otherwise the heel could not be oxiouJed. The pelvis and trunk are consequently 
thrown upwards, forwards, and somewhat in a lateral direction. In this manner they 
move round llie fixed foot as a centre, with a radius consisting of the leg belonging to 
that foot, which, during the movement, continually diminishos the angle formed with 
the tarsus. Tho log communicating this impulse is then thrown forw.ard and rests its 
foot upon tho ground; while tho other, which now forms an acute angle with its foot, 
h.is the heel extended in Us turn, and in like manner makes the pelvis and trunk tiwii 
round upon the former log. Tbe ceutro of gravity is thus carried forward by these 
movements at each progressive stop, inclining, however, at the same time to tlie right 
and loft alternately, so as to bo supported by each leg in its turn. It will also be 
seen tliat each leg, immediately on extending ita heel, bends and rises, in order to its 
being moved forward, — extends in order to rest its foot upon the ground, turns 
upon this foot as on a fixed centre, so as to support the weight of tlic body, and then 
extends its heel again in order to transfer this wnight to the other leg. 
In this manner, each leg supports the body in its turn; but it is also necessary that 
the extcnsois of the thigh and Unco should be brought into action, to prevent their 
articulations from giving way; and this motion is followed by a corresponding action 
of the flexors of the same articulations. It will be observed, tliat the three principal 
articulations of each leg are situate in opposite directions to each other, tliat tho foot 
should bo raised by their flexion iimiiediatcdy over the placa which it occupied during 
their extension. It would otherwise be impossible to bond them without throwing 
the foot backwards or forwards. 
In consequence of the impossibility of regulating the undulatory motion, in a man- 
ner perfectly equal on both sides, a man cannot walk in a straight line wdth his eyes 
shut; nor could he even preserve a uniform direction, did he not correct these devia- 
tions by the sense of sight. 
In descending a stair-case, or in walking down an inclined plane, the alvanced leg 
is placed lower than that remaining behind; and tho body would fail upon it with a 
fatiguing ami dangerous jerk, were it not carcrull}^checked by the extensors of the 
hip. By this racaiw, the body is compelled to descend gradually; but the muscles 
of tho loins soon become fatigued by tho exertion. 
On the contrary, in ascending a st-iir-case, or an inclined plane, it is requisite at each 
step, not only to transport the body horizontally, os on a level surface, but also to 
bear it up against its own weight, by means of the extensors belongingto the knee of 
the advanced leg, and to the heel of the leg reraahiing behind. I'ho kuoc and calf of 
the leg are therefore fatigued in ascending. A mechanical a.tvanlage is gained by lean- 
ing the body forward in ascending, because the lever, by which its weight retards the 
