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Of The Animal Structure Regarded As A Mass

Contemplating an animal body in its collective capacity, we cannot forget to notice, what a number of instruments are brought together, and often within how small a composes its body, (but which seems to be all employed,) we have instruments for eating, for digesting, for nourishment, for breathing, for generation, for running, for flying, for seeing, for hearing, for smelling, each appropriate,—each entirely different from all the rest.

The human, or indeed the animal frame, considered as a mass or assemblage, exhibits in its composition three properties, which have long struck my mind as indubitable evidences, not only design, but of a great deal of attention and accuracy in prosecuting the design.

I. The first is, the exact correspondency of the two sides of the same animal; the right hand answering to the left, leg to leg, eye to eye, one side of the countenance to the other; and with a precision, to imitate which in any tolerable degree, forms one of the difficulties of statuary, and requires, on the part of the artist, a constant attention to this property of his work, distinct from every other.

It is the most difficult thing that can be to get a wig made even; yet how seldom is the face awry! And what care is taken that it should not be so, the anatomy of its bones demonstrates. The upper part of the face is composed of thirteenth, without a fellow, in the middle: the lower part of the face is in like manner composed of six bones, three on each side respectively corresponding and the lower jaw in the centre. In building an arch, could more be done in order to make the curve true, i.e. the parts equi-distant from the middle, alike in figure and position?

The exact resemblance of the eyes, considering how compounded this organ is in its structure, how various and how delicate are the shades of color with which its iris is tinged; how differently, as to effect upon appearance, the eye may be mounted in its socket, and how differently in different heads eyes actually are set,—is a property of animal bodies much to be admired. Of ten thousand eyes, I do not know that it would be possible to match one, except with its own fellow; or to distribute them into suitable pairs by any other selection than that which obtains.

This regularity of the animal structure is rendered more remarkable by the three following considerations:—First, the limbs, separately taken, have not this correlation of parts; but the contrary of it. A knife drawn down the chine, cuts the human body into two parts, externally equal and alike; you cannot draw a straight line which will divide a hand, a foot, the leg, the thigh, the cheek, the eye, the ear, into two parts equal and alike. Those parts which are placed upon the middle or partition line of the body, or which traverse that line, as the nose, the tongue, the lips, may be so divided, or, more properly speaking, are double organs; but other parts cannot. This shows that the correspondency which we have been describing, does not arise by any necessity in the nature of the subject: for, if necessary, it would be universal; whereas it is observed only in the system or assemblage: it is not true of the separate parts; that is to say, it is found where it conduces to beauty or utility; it is not found where it would subsist at the expense of both. The two wings of a bird always correspond: the two sides of a feather frequently do not. In centipedes, millepedes, and that whole tribe of insects, no two legs on the same side are alike; yet there is the most exact parity between the legs opposite to one another.

II. The next circumstance to be remarked is, that whilst the cavities of the body are so configurated, as externally to exhibit the most exact correspondency of the opposite sides, the contents of these cavities have no such correspondency. A line drawn down the middle of the breast, divides the thorax into two sides exactly similar; yet these two sides enclose very different contents. The heart lies on the left side; a lobe of the lungs on the right; balancing each other neither in size nor shape. The same thing holds of the abdomen. The liver lies on the right side55, without any similar viscus opposed to it on the left. The spleen indeed is situated over against the liver; but agreeing with the liver neither in bulk nor form. There is no equipollency between these. The stomach is a vessel, both irregular in its shape, and oblique in its position. The foldings and doublings of the intestines do not present a parity of sides. Yet that symmetry which depends upon the correlation of the sides, is externally preserved throughout the whole trunk; and is the more remarkable in the lower parts of it, as the integuments are soft; and the shape, consequently, is not, as the thorax is by its ribs, reduced by natural stays. It is evident, therefore, that the external proportion does not arise from any equality in the shape or pressure of the internal contents. What is it indeed but a correction of inequalities? an adjustment, by mutual compensation, of anomalous forms into a regular congeries? the effect, in a word, of artful, and, if we might be permitted so to speak, of studied collocation?

III. Similar also to this, is the third observation; that an internal inequality in the feeding vessels is so managed, as to produce no inequality in parts which were intended to correspond. The right arm answers accurately to the left, both in size and shape; but the arterial branches, which supply the two arms, do not go off from their trunk, in a pair, in the same manner, at the same place, or at the same angle. Under which want of similitude, it is very difficult to conceive how the same quantity of blood should be pushed through each artery: yet the result is right; the two limbs, which are nourished by them, perceive no difference of supply, no effects of excess or deficiency.

Concerning the difference of manner, in which the subclavian and carotid arteries, upon the different sides of body, separate themselves from the aorta, Cheselden seems to have thought, that the advantage which the left gain by going off at a much more acute angle than the right, is made up to the right by their going off together in one branch56. It is very possible that this may be the compensating contrivance; and if it be so, how curious, how hydrostatical?

IV. Another perfection of the animal mass is package. [Pl. XXII. fig. 1.] I know nothing which is so surprising. Examine the contents of the trunk of any large animal. Take notice how soft, how tender, how intricate they are, how constantly in action, how necessary to life! Reflect upon the danger of any injury to their substance, any derangement of their position, any obstruction to their office. Observe the heart pumping at the centre, at the rate of eighty strokes in a minute: one set of pipes carrying the stream away from it, another set bringing, in its course, the fluid back to it again; the lungs performing their elaborate office, viz. distending and contracting their many thousand vesicles, by a reciprocation which cannot cease for a minute; the stomach exercising its powerful chemistry; the bowels silently propelling the changed aliment; collecting from it, as it proceeds, and transmitting to the blood an incessant supply of prepared and assimilated nourishment; that blood pursuing its course; the liver, the kidneys the pancreas, the parotid, with many other known and distinguishable glands, drawing off from it, all the while, their proper secretions. These several operations, together with others more subtile but less capable of being investigated, are going on within us, at one and the same time. Think of this; and then observe how the body itself, the case which holds this machinery, is rolled, and jolted, and tossed about, the mechanism remaining unhurt, and with very even of its nicest motions. Observe a rope dancer, a tumbler, or a monkey: the sudden inversions and contortions which the internal parts sustain by the postures into which their bodies are thrown; or rather observe the shocks which these parts, even in ordinary subjects, sometimes receive from falls and bruises, or by abrupt jerks and twists, without sensible, or with soon-recovered damage. Observe this, and then reflect how firmly every part must be secured, how carefully surrounded, how well tied down and packed together.

This property of animal bodies has never, I think, been considered under a distinct head, or so fully as it deserves. I may be allowed, therefore, in order to verify my observation concerning it, to set forth a short anatomical detail, though it oblige me to use more technical language than I should wish to introduce into a work of this kind.

1. The heart (such care is taken of the centre of life) is placed between the soft lobes of the lungs; tied to the mediastinum and to the pericardium; which pericardium is not only itself an exceedingly strong membrane, but adheres firmly to the duplicate of the mediastinum, and, by its point, to the middle tendon of the diaphragm. The heart is also sustained in its place by the great blood-vessels which issue from it57.

2. The lungs are tied to the sternum by the mediastinum, before; to the vertebrae by the pleura, behind. It seems indeed to be the very use of the mediastinum (which is a membrane that goes straight through the middle of the thorax, from the breast to the back) to keep the contents of the thorax in their places; in particular to hinder one lobe of the lungs from incommoding another, or the parts of the lungs from pressing upon each other when we lie on one side58.

55The principal lobe of the liver is on the right, but a smaller is extended into the left side. See Plate XXII.
56Ches. Anat. p. 184. ed. 7.
57Keill’s Anat. p. 107. ed. [e.pk].
58Keill’s Anat. p.119 ed. 3.

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