ANIMALIA VERTEBRATA— VERTEBRATED ANIMALS. 



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rexy be understood from the following observations of the learned Dr Paley: — 

 ■* There is provided in the central part of the body a, hollovr muscle, invested with 

 spiral fibres, running in both directions, the layers intersecting one another : in some 

 animals, however, appearing to be circidar rather than spiral. By the contraction of 

 these fibres, the sides of the muscular cavities are necessarily squeezed together, so 

 as 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 which bring it back. This is a general accotmt of the apparatus ; and thd 

 simplest idea of its action is, that> by each contraction, a portion of blood is forced 

 as by a syringe into the arteries ; 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 fuU-grovni human 

 heart* I understand is about an ounce, or two table-spoons full. How quickly these 

 changes succeed one another, and by this succession, how sufficient 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 heai't 

 contracts four thousand times in one hour; from which it follows, that 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 be 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 afiair this is when we come to very large animals. The aorta of a 

 vhale is larger in the bore than the main pipe of the water-works at London bridge ; 

 tnd 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 a whale :— The aorta measured a foot in diameter. Ten or fifteen 

 gallons of blood are thrown out of the heart at a stroke, with an immense velo- 

 city, through a tube of a foot in diameter. Tiie whole idea fills the mind with 

 wonder." 



The circulation through a single heart may be seen xn 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 

 tiranch, and is thence transported through the ramified extremities of the arteries 

 throughout the body. Returning 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 blood 

 was transported to the lungs through the pulmonary arteries, and again brought back, 

 through the pulmonary veins, after having been purified. This partial aeration of the 

 blood imparts to the Batrachia or frogs a cold and sluggish character. 



The Fishes have 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 which is dissolved or 

 mixed in the water. From this circumstance it follows that the degree 

 of their respiration is still less than that of the Reptiles. 



The gills of Fishes are 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, resembling the pile 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 

 in the animal economy. By means of these organs, the Fish is enabled to absorb the 

 oxygen dissolved in the water ; and after yielding this substance, the water is dis- 

 charged through the branchial openings. The Fishes form a contrast with animals of 

 the other divisions, in this respect, that they do not inspire by the same opening through 

 which they expire. 



In the Mammalia, the circulation is double, and the aerial respiration 

 is 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 

 of the Mammalia, because they not only have a double circulation, with 

 a direct aerial respiration, but they also breathe through many other 

 cavities besides the lungs. The air penetrating into the cells distributed 

 all over the body, acts upon the branches of the aorta, or arteries of the 

 body, as well as upon the ramifications of the pulmonary artery. 



From these circumstances are derived the four kinds of motion for 

 which each of the four classes of Vertebrated animals seems particularly 

 designed. 



1. The Quadrupeds, in which the quantity of respiration is mode- 

 rate, are generally formed for walking, for eunning, and for developing 

 these motions with vigour and precision. 



2. The Bieds, wherein respiration is much more perfect, possess 

 that muscular vigour and that lightness of construction necessary for 



FLIGHT, 



3. The Reptiles, endowed with a more feeble respiration, are con- 

 demned to CRAWL upon the earth, and many of them pass a part of their 

 life in a continued state of torpor. 



4. The Fishes, in orde" to execute their less vigorous motions, require 



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to be supported ih a fluid of nearly the same specific gravity with 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 VERTEBRATED ANIMALS. 



IValhing — Leaping — Running — Trotting — Galloping — Climbing -Flging-^ 



Darting — Paddling — Diving — Swimming. 



To perfornl 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 particulal- direction, to the centre of gravity of the animal body, or that point 

 in the body around which all the parts balance and remain at rest. A certain number 

 of joints must exist, capable of a greater or less degree of flexure. Their relative 

 position must be so adapted that it may be comparatively easy to extend them on the 

 side to which the centre of gravity is made to incline, and difficult on the opposite 

 side, so that the general movement may tend in the former direction. 



The mechanical part of Animal motion may be understood from the following 

 illustration : — If we imagine a spring divided into two branches, one of which rests 

 upon a firm resisting base, and then suppose that the branches are compressed by 

 some external force, their elasticity will cause them to recede as soon as the compres- 

 sing force has been removed, and the two branches will be 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 tho 

 resisting body with more or less velocity. Accordingly, in any animal, while the 

 muscles QlexorsJ, which bend the part employed in effecting the movement, represent 

 the external compressing force of the spring, and those muscles (extensorsj, which 

 stretch it out, correspond to tho elasticity that makes the branches of the spring fly 

 asunder, the ground supporting the animal, or fluid in which it moves, forms th« 

 resisting basis. 



In Walking^ 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 tbo 

 ground. It differs essentially from Leaping, whore the entire body is projected into 

 the air; and from Running, which consists of a number of short leaps. 



In general, it is less painful to walk than to stand, because the same muscles are 

 not continued in action for so long a period ; and it is much easier to counteract thoso 

 unsteady motions which occur in walking by contrary and alternate actions than it ig 

 to prevent them entirely. Thus it follows, that though all animals which stand 

 erect on two legs, such as Man and Birds, can also walk on two legs, yet many 

 moving in an upright position with sufficient ease, cannot stand on two feet for any 

 time without very great fatigue and exertion. 



Wlien IVIan intends to walk on level ground, ho first extends one foot. His body 

 then rests equally upon both legs, the advanced leg making an obtuse angle with the 

 tarsus or instep, and the other an acute angle. As the ground does not yield to tho 

 point of the foot, the heel and the remainder of the leg must necessarily be raised, 

 otherwise the heel could not be extended. The pelvis and trunk are consequently 

 thrown upwards, forwards, and somewhat in a lateral direction. In this manner they 

 move round the fixed foot as a centre, with a radius consisting of the leg belonging to 

 that foot, which, during tho movement, continually diminishes the angle formed with 

 the tarsus. The leg communicating this impulse is then thrown forward and rests its 

 foot upon the ground; while the other, which now forms an acuto angle with its foot, 

 has the heel extended in its turn, and in like manner makes the pelvis and trunk turn 

 round upon the former leg. The centre of gravity is thus carried forward by these 

 movements at each progressive step, inclining, however, at the same time to the right 

 and left alternately, go as to be supported by each leg in its turn. It will also be 

 seen that each leg, immediately on extending its 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 the body,— and then 

 extends its heel again in order to transfer this weight to the other leg. 



In this manner, each leg supports the body in its turn; but it is also necessary that 

 the extensors of the thigh and knee should be brought into action, to prevent their 

 articulations firom giving way; and this motion is followed by a corresponding action 

 of the flexors of the same articulations. It will be observed, that tho three principal 

 articulations of each leg are situate in opposite directions to each other, that the foot 

 should be raised by their flexion immediately over the place which it occupied during 

 their extension. It would otherwise be impossible to bend them without throwing 

 the foot backwards or forwards. 



In consequence of the impossibihty of regulating the undulatory motion, in a man- 

 ner perfectly equal on both sides, a man cannot walk in a straight line with 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 advanced leg 

 is placed lower than that remaining behind; and the body would fall upon it with a 

 fatiguing and dangerous jerk, were it not carefully checked by the extensors of th* 

 hip. By this means, the body is compelled to descend gradually; but the muscles 

 of the loins soon become fatigued by the exertion. 



On the contrary, in ascending a stair-case, or an inclined plane, it is requisite at each 

 step, not only to transport the body horizontally, as on a level surface, but also to 

 bear it up against its own weight, by means of the extensors belonging to the knee of 

 the advanced leg, and to the heel of the leg remaining behind. The knee and calf of 

 the leg are therefore fatigued in ascending. A mechanical advantage is gained by lean- 

 ing the body forward in ascending, becaubo the lever, by which its weight retards the 



