Volume 2

The cyclopaedia of anatomy and physiology / Edited by Robert B. Todd.

Date:
1836-59
Catalogue details

Licence: Public Domain Mark

Credit: The cyclopaedia of anatomy and physiology / Edited by Robert B. Todd. Source: Wellcome Collection.

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    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, tendingto depress the ribs and thus to assist the weaker muscles. But we must not fall into the error of suppo- sing that elasticity 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 the 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 oidy 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 Fibrous Tissue and Muscle. (John E. Brenan.) ELBOW, REGION OF THE; fold or bend of the arm. (Fr. pli duhras; coude.) The region of the elbow is situated at the angular union of the arm with the fore-arm, and con- tains the humero-cubital articulation and the various organs which surround it: the extent of this region may be determined, superiorly by a circular line at a finger’s breadth above the internal condyle, and inferiorly by a similar line at two fingers’ breadth below that process: its greatest extent is in the transverse direction, and it forms an angle salient posteriorly and