2. Elasticity is often had recourse to as a substitute for muscular contraction. and, as it would appear, with a view to economize that more important property. We find, for ex ample, that in most animals the abdominal viscera are supported in their position chiefly by the muscles of the abdomen, and that on being forced downwards in inspiration by the descent of the diaphragm, they are again pressed upwards by the contraction of these muscles. In the large ruminating quadrupeds whose abdominal viscera are of so great a size, and in whom, owing to the horizontal position of the trunk, these organs tend directly down wards, the quantity of muscular power requi site to support and move them should neces sarily have been of great amount ; but instead of increasing the quantity of muscle to such an extent, nature has effected her purposes by much more simple means. lleneath the abdominal integuments there exists a mem brane of great strength and elasticity, which nut only supports the viscera hut also helps to elevate them alter they have been forced downwards in inspiration. The elastic liga mentin nuehw, which in these animals sup ports the very weighty head, is a simple hut complete substitute for the great mass of muscle should have existed on the hack part of the neck, in order to of the same end. So obviously in this instance is elasticity a substitute for muscularity, that upon com paring the structure in various animals we find the strength and elasticity of the ligament always proportionate to the weight of the head which it has to support. In the carnivore an interesting application of this property is seen in the retractile ligament passing between the claw and the phalangeal bone; as the claw in many genera is the chief weapon of attack, it must not be suffered to come into contact with the ground in progression, for otherwise it would become blunted, as seen in those which do not use it fur the purposes mentioned; it is consequently suspended by the retractile liga ment until drawn down at the will of the animal by means of the flexor muscles. Elasticity is here used as the means of suspension in order to save the effort of a constant muscular exer tion. In the mollusca we see this property again employed to economize muscularity : the shell of the oyster admits of being opened as well as closed at the will of the animal ; but muscularity is the source of the one ac tion; elasticity residing in a strong ligament is the means of effecting the other.
3. Elasticity frequently preserves the patu lous condition of certain outlets in the animal body, as, for example, those of the eyes and nostrils. This object is attained by the inser tion of a rim of highly elastic cartilage into the soft parts which bound these openings. A material of greater rigidity, such as bone, would, it may be objected, have answered the purpose still better: but the rigidity of that substance would have greatly interfered with the free movements necessary for the functions of the lids, and in the nose would not only have increased the risk of injury from external violence, but would have prevented the ap proximation of the alm which must take place in order to expel the nasal mucus. Neither would a soft and inelastic material have an swered the purpose, for then the first effect of inspiration would be to approximate the edges of the opening, and thus to prevent the further entrance of air. The tracheal and bronchial canals are likewise preserved patulous by the same elastic material; and we again meet with it performing a like office in the Eustachian tube and the external meatus of the ear.
4. Elasticity is sometimes rendered subser vient to locomotion, or the general movement of the body. The elastic pad placed beneath the foot of the dromedary and many other ani mals is no doubt intended to facilitate progres sion, and to compensate in some degree for the yielding looseness of the sands upon which they tread. The same apparatus is found yr very great perfection in the feet of the carni vore, and must be of great use in enabling them to make those enormous bounds by which they spring upon their prey. But perhaps one of the most interesting examples of elasticity being rendered subservient to locomotion is met with in certain fish. The salmon, during its annual ascent to fresh-water streams for the purpose of depositing its spawn, often encoun ters cataracts of great height, and which would seem to render farther progress impos sible. By means, however, of a powerfully muscular tail and elastic spine it is enabled to surmount those obstacles ; resting one side upon a solid fulcrum, it seizes its tail between its teeth, and thus draws itself into an arch of amazing tension ; then suddenly letting go its hold, and thus freeing the elastic spring which its body represented, it is thrown into the air, often, as Twiss has seen in Ballyshannon in Ireland, to a height of twelve or fifteen feet, and falls beyond the obstacle which had op posed it.
5. Elasticity becomes occasionally in the animal machine a means of dividing muscular force, and thus transferring it from one portion of an apparatus to another. The muscles of inspiration are, if we may use the word, too strong for their opponents, and hence it be comes necessary to transfer a portion of their superfluous strength to the weaker set. This is effected by means of the elastic cartilages which connect the ribs and sternum. The in spiratory muscles in enlarging the thorax act with such a force that they not only elevate the ribs, but even stretch and twist the cartilages, and hence no sooner is inspiration completed than elasticity comes into play, tending to depress the ribs and thus to assist the weaker muscles. But we must not fall into the error of suppo sing that elastic ity is in this case a substitute for muscularity, and much less that it is in itself a source of power. The only power exercised by it is that which it has just bor rowed from the inspiratory muscles: had not the elasticity of the cartilages been set in action by this external agency, it would, like the elas ticity of the watch-spring under the same cir cumstances, have remained for ever dormant. In those interesting discussions which have arisen of late years relative to what is termed the suction power of the heart, we apprehend that much error has arisen from overlooking this simple law of elasticity. That doctine will of course be fully stated and examined in its proper place; at present we shall merely observe that it was first regularly put forward in the admirable work of Dr. Wilson Philip, that it was followed up and explained by Dr. Carson, and that these views were regarded by Laennec with such respect that he pronounces their discovery the most important step made in this department of physiology since the time of Harvey. The heart, it is said, is not merely a forcing pump which by the contrac tion of its ventricle propels the blood through out the arteries ; it is likewise a suction pump, for by the expansion of the auricles it draws in Ore blood from the veins. Now this expansive force, if indeed it exist at all, is, we are quite satisfied, merely the effect of the heart's elas ticity; for the reasonings of those who attempt to prove it of a specific nature are evidently insufficient. In this point of view the heart's expansion cannot be regarded as a new and independent power; if that organ be really elastic, then the muscular force of its systole must be greater than it would otherwise have been, for it has not only to propel the blood through the arterial system, but likewise to overcome the resisting elasticity of its own structure: this suction power of the heart is then merely the recoil of the surplus force; what is gained upon the one hand is lost upon the other ; and hence elasticity in this instance cannot be regarded as an independent prin ciple contributing to the blood's motion, but merely as a means of dividing muscular power and transferring a portion of it from the begin ning of the arterial to the end of the venous system.
6. An interesting application of elasticity in the animal machine is to convert an occasional or intermitting force into a continued one. As human ingenuity has long since discovered the application of this principle, we may see it employed in many mechanical contrivances. In the common fire-engine, for instance, we observe that though it is worked by interrupted jerks, yet the water issues from its pipe, not per saltum as we should have expected, but in one uniform and continued stream. This is effected by causing the fluid to pass, in the first instance, into a hermetically sealed vessel con taining a portion of atmospheric air : the accu mulation of the water presses the air into a smaller space, but in doing so it is reacted upon by the elasticity of that gas, which may thus be considered as a powerfully elastic spring exerting upon the surface of the water an uniform and continual pressure. The very same principle is employed in the mechanism of the arterial system. Upon opening one of the small arteries we perceive that the blood does not flow per saltum as in those which are nearer to the heart, but issues in an uniform and uninterrupted stream. The intermitting action of the heart has in fact been converted into a continued one by means of the elasticity of the arterial tissue. We might indeed say with truth that the blood in these small arteries is not directly propelled by the heart at all; the force of that organ is expended in distend ing the larger elastic arteries, as the force in the fire-engine is expended in compressing the air. The immediate cause of motion is in the one case the reaction of the elastic air, and in the other the reaction of the elactic artery.
For the BIBLIOGRAPHY of this article, see that of Finaous Tissue and MUSCLE..
(John E. Brenan.)