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office of the reservoir containing air, is performed in the animal body by the elasticity of the coats of the arteries; by which means the blood flows uninterruptedly into the arteries, and has a continuous flow in the veins beyond them.
But as life advances, the arterial system loses much of its elasticity, and becomes rigid. This is so common an occurrence that we can no more call it a disease than the stiffened joints of an old man; it is the forerunner or the accompaniment of the decline of life. Sometimes this change takes place too early in life, and to an extreme degree; and from its effects we must call it morbid; for it not unfrequently happens that the muscular power of the heart being still entire and vigorous, the arteries can no longer withstand it. They have lost that power which, yielding to the heart's action, resists, recoils, and the more it gives way, the more it takes off the suddenness of the shock; which, in yielding, wastes no power, since the recoil gives as much force to the acceleration of the blood, as was lost of the heart's action. The artery, then, being rigid, yields indeed to the heart's impulse, but has no rebound. It becomes permanently dilated or enlarged; and is called aneurismal. A stronger beat of the heart, excited by inordinate action or passion, chips and bursts the now rigid coats of the artery. If the breach be gradual, a pouch forms—a true aneurism. And here is the proof we require; for this bag coming to pulsate upon the solid bones, they are absorbed. That action of the heart, which was so lightly and so easily borne whilst the vessels were elastic, now beating upon a solid structure, in a short time destroys it. Thus, from what takes place on a very slight derangement of the properties of the parts, we are led to a more accurate knowledge of the fine adjustment of the active and resisting properties in the circulating vessels, during youth and health.
A piece of rope, of a new patent, has been shown to us, which is said to be many times stronger than any other rope of a like diameter. It is so far formed upon the same principle as the tendon of a muscle, that the strands are plaited, instead of being twisted: but the tendon has still a superiority; for the lesser yarns of each strand in it, are interwoven with those of other strands. It may be asked, however, do not the tendons of the human body sometimes break? They do; and in circumstances which only add to the interest of the subject. By the
exercise of the tendons, (and their exercise is the act of being pulled upon by the muscles, or having a strain on them,) they get firmer and stronger; but in the failure of muscular activity, they become less capable of resisting the tug made upon them; and if, after a long confinement, a man has some powerful excitement to muscular exertion, then the tendon breaks. An old gentleman, whose habits have been staid and sedentary, and who is very guarded in his walk, is upon an annual festival tempted to join the young people in a dance; then he breaks his tendo Achillis. This reminds us that we are speaking of a living body; and that, in estimating the mechanical properties, we ought not to forget the influence of Life, and the law that the natural exercise of the parts, whether they be active or passive, is the stimulus to the circulation through them, and to their growth and perfection.]
ON THE POSITION OF THE HEAD OF ANIMALS, AND
To illustrate the proposition, that "all parts of the skeleton are correlated; and that the variations in their form depend on the functions."
IT has been shown in the text, when treating of the upper or anterior extremity, that the changes of form exhibited in different animals, may be referred to one principle-the adaptation of the parts to their proper uses. The head, in certain animals, may be considered as performing in some measure the office of hands. Now, if we adopt this view, we shall be able to judge more correctly how far it holds true that the centre of the skeleton, to which the head belongs, remains permanent in its form, compared with the extreme parts.
We have seen that it is the opinion of some naturalists, that all the varieties in the conformation of the skeleton admit of being explained according to a law, by which the central parts preserve an uniform shape, whilst the extremities are incident to change. That opinion I shall controvert, and show that although the spine and head, in retaining their office, common to all vertebrated animals, of protecting the brain and spinal marrow, are permanent in regard to them, yet they vary in the shapes of their processes, and in their relations to the adjacent parts. Pursuing that idea, we shall be able to account for the characteristic forms of animals.
The principle, then, which will guide us, both here and in a more universal survey of animal nature, is, that the organisation varies in correspondence with the condition in which the animal is placed, in reference to procuring food, and multiplying its species. If we consider any of the great functions on which life depends, we shall perceive that the apparatus is altered and adapted to every changing circumstance. Digestion, for example, is the same in all animals; but the organisation presents numerous interesting varieties. Whether it be in the quadruped, the bird, the fish, or the insect, the stomach
varies both in its form and the number of its cavities, in accordance with the nature of the food which it receives. And the variation does not depend upon the size or form of the animal : it is adapted purely to the conversion of its particular food into nourishment: the gizzard of the fish, or of the insect, is as perfect as that of the fowl. So with the decarbonisation of the blood in breathing: the process of throwing off the carbon is the same in all living animals; but the mode in which respiration is performed varies according to circumstances; the apparatus is especially modified and adjusted, according as it is carried on in the atmosphere or the water.
But although the organs subservient to the grand functions, the heart and blood-vessels, the lungs, the stomach,be variously adapted to the different classes of animals, they change much less than the parts by which animals are enabled to pursue their prey or obtain their food. Their extremities, by which hey walk, or run, or creep, or cling, must vary infinitely. And so their teeth and horns, and the position of their head, and the strength of their neck, exhibit nearly as much variety as their proper extremities; because these parts likewise must be adapted to their different modes of obtaining food, or of combating their enemies.
Following this principle, therefore, let us observe the forms of some of the more remarkably-shaped animals, and endeavour to explain their meaning. When we look upon the boar's head, its form alone enables us to comprehend something of the habits of the animal; we see the direction in which he will employ his strength. He lives by digging up roots; and the instruments by which he feeds are also those by which he defends himself— the position of his tusks protects his eyes in rushing through the underwood; but the formation of the skull, and of the spine, and the mass of muscle in the neck, all show the intention of his configuration to be, that he may drive onward with his whole weight and strength, and rend with his tusks. Accordingly, the back part of the skull rises in remarkable spines or ridges for the attachment of muscles; and corresponding with them, the spinous processes of the vertebræ of the neck and back are of extraordinary length and strength. Processes of such dimensions as these distinctly indicate the immense power of the muscles which pass from the neck to the head. We now
understand the reason of the shortness and inflexibility of the neck of the boar; it is so formed because the strength of the shoulders is directed to the head, or, we may say, to the large tusks. An elongated and flexible neck would have rendered these implements useless. The characteristic form of the wild boar, then, consists in the height of the back, the shortness and thickness of the neck, the wedge shape of the head, the projection of the tusks, and the shortness of the fore-legs, which must always be in proportion to the neck.
Thus we perceive that the skull, unaffected in its office of containing and protecting the brain, is yet subject to variations in its form and place, according to its other functions; that it is adapted, just as the extremities are, to the animal's mode of life. In the same manner, we see the spine permanent in its office as a tube to protect the spinal marrow, but yet varying in its processes and articulations, as they bear a reference to the skull. In short, although these be the very central parts of all, they undergo changes of form in due accommodation to the whole skeleton.
What a complete contrast there is between the wild boar and any of the feline tribe! But it is a contrast of form and motion, at once referable to their spine. In the tiger or leopard, the perfect flexibility of the body, and the almost vermicular motion of the spine, correspond to the teeth and jaws, and the free use of the paws.
The peculiarity of the elephant's configuration has been happily illustrated by Cuvier: and the principle may be pursued in a manner interesting both to the naturalist and geologist.
In ourselves, we may feel between the shoulders a certain projection of the spine, which is called the vertebra prominens; and if we stoop forward, as in reading a book which lies upon the table, a ligament will be felt extending from that process to the back of the head. This ligament suspends the head, and relieves the muscles. But as man for the most part carries his head balanced on the extremity of the spine, or can vary its relation under fatigue, the strength of that suspensory ligament is not to be compared with the corresponding part in quadrupeds; where, from the horizontal position of the spine, the head always hangs: and where there would be a great waste of muscular exertion, but for the interposition of this elastic liga