Solid bodies are never perfectly elastic ; for although some, when acted upon by forces within a certain range, are as completely elastic as the gases themselves, yet if the disturbing force be carried beyond a certain degree, they will never resume their original condition. Thus, a harp-string gently drawn by the finger is thrown by its elasticity into vibratory mo tions, returning when these have ceased to its exact original state : this may be frequently repeated and always with the same effect, as proved by the same note being repeatedly ob tained. If, however, it be once drawn with too great a force, it no longer returns to its original condition, a different tone is now produced by it: in other words, the solid substance of which it is composed exhibits a perfect elasticity, not, as the gases, under every degree of force, but only within a certain limit. Heat pro duces very different effects upon the elasticity of gaseous and solid bodies; we have just seen that we can increase the elastic power of the former to any extent by means of heat, but the elasticity of solids is, on the contrary, usually diminished by it ; very high tempera tures completely destroy it even in the most elastic metals. The design of this article does not permit us to enter more fully into the con sideration of those laws, or of the experiments by which they are demonstrated. We must refer for the further investigation of this sub ject to works which treat expressly upon physics.
The various hypotheses which have been put forth to explain the nature of elasticity, though many of them extremely ingenious, do not however properly come within the pro vince of the physical much less of the phy siological enquirer. Indeed, while men di rected their attention to such speculations little or no progress was made in real knowledge. The cause of elasticity, like that of life, is probably beyond the sphere of human un derstanding; and hence, in both sciences, the method of investigation should be the same— to study the laws or conditions under which the phenomena present themselves, and to lay aside all speculations as to their causes. But in abandoning these inquiries into the nature of elasticity we must particularly advert to the necessity of the physiologist possessing a clear and definite idea of this property of matter, so as to be enabled to recognize it under every circumstance, and to distinguish it from other physical and vital forces. Ignorance upon this point has been at all times a fruitful source of error in physiological investigations. The pro perty with which it is especially liable to be confounded is contractility : when it is re membered that at one period of medical his tory these two properties were looked upon as identical ; that even the illustrious Cullen has scarcely distinguished them, and that some of our most eminent living physiologists have fallen into manifest errors upon the same sub ject, it becomes plain that we cannot be too particular in familiarizing ourselves with the distinctions between these totally independent forces.
It is not enough to say that contractility is a vital and elasticity a physical property; for as we are ignorant alike of the nature of life and of elasticity, a distinction founded upon any such assumption must necessarily be futile. It is only by a diligent comparison of their respective laws that we can assign to each its proper limits. Let us then observe in con trasting them, first, that elasticity can never act as a prime mover; it is never a source of power, but merely the reaetion of a force pre viously applied : thus, the elasticity of the spring will never of itself set the watch in motion unless some external force shall, in the first instance, have acted upon or bent it. But contractility can of itself originate motiun, at least it is not essential that any mechanical force with which we are acquainted should precede its action. Again, the force of elas ticity can never exceed that other power which has called it into existence; if, fur instance, a weight of one pound be required to depress an elastic spring, the force of reaction upon the removal of that weight can never exceed the measure of a pound. But, in the case of muscular contraction, there is no such limit ; there is no fixed ratio between the cause and the effect; the slightest touch of a sharp in strument will, in an irritable muscle, such as the heart, excite the most violent contractions. Elasticity cannot manifest itself except by the removal or suspension of the cause which has called it into action : muscularity requires no such suspension of its exciting cause. The exciting cause of elasticity is always of a phy sical nature; but many other causes no ways allied to physical ones may excite the muscular power. Lastly, elasticity is not destroyed by death nor affected by opium or other narcotics, while contractility presents a very striking con trast in both these respects.
These facts are quite conclusive in proving that muscular and elastic contraction are go verned by distinct laws, and cannot conse quently be referred to the same source. But if some physiologists have erred in overlooking the distinctions between these two properties, if they have not analysed with sufficient care, others have unquestionably erred in an oppo site direction, and by pushing analysis too far, have attributed to imaginary forces effects which are the result of elasticity alone. We
feel much diffidence in controverting any doc trine supported by the genius and authority of Bichat, but we confess that the distinction which that celebrated anatomist is so anxious throughout his various works to establish be tween what he terms" contradilily of lissne" and elasticity, appears to us unfounded. Elas ticity according to him is a purely physical property. Contractility of tissue, though not actually a vital one, is however found only in the animal tissues; it does not depend directly upon life, but results merely from the texture and organization of those particles which con stitute the vital organs. The following passage from his work upon " Lire and Death" may, perhaps, assist us in understanding his views upon this subject. " Most organs of our bodies are held in a state of tension by various causes ; the voluntary muscles by their anta gonists; the hollow muscles by the substances contained within them; the vessels by means of their circulating fluids ; the skin of one portion of the body by that which covers the neighbouring part; the alveolar walls by the teeth contained within them. Now, upon the suspension of the distending causes, contrac tion takes place: divide a long muscle,—its antagonist becomes shortened; empty a hol low muscle, it shrinks upon itself: prevent the blood from entering an artery, the vessel be comes a ligament : cut through the integu ments, the divided edges are separated from each other by the contraction of the adjoining skin : extract a tooth from its alveolus, that channel becomes obliterated. • • • In all these cases it is the removal of a tension naturally inherent in the tissue which determines its contraction :—in other instances it is the re moval of a tension which does not naturally reside in the part. Thus we see the abdomen contract after parturition ; the maxillary sinus after the extirpation of a fungous growth ; the cellular tissue after the removal of an abscess; the tunics vaginalis after the operation for hydrocele ; the integument of the scrotum after the removal of ao enlarged testicle; the aneurismal sac upon the emptying of its fluid." Ile remarks in another place that motion when the result of elasticity is quick and sud den, and ceases as abruptly as it has been pro duced; but the motions which result from contractility of tissue are slow and impercep tible, lasting frequently for hours and even days, as are seen in the retraction of muscles after amputation. The distinction laid down in these passages appears to us totally un supported: to say, for example, that even in a dead artery there are two principles of con traction which, though their mode of action is literally the same, should nevertheless he con sidered distinct and referred to different sources, appears contrary to every rule of philosophic reasoning. As to the distinction drawn from the comparative quickness of these motions, it is only necessary to say that upon this view of the subject even the movement of the watch-spring itself cannot be attributed to elas ticity. We must then conclude that there are two and only two forces to which all the various movements of living bodies can be referred ; the one a vital force regulated by its own proper laws, the other a general physical property, whose mode of action is essentially the same in organized and unorganized bodies: the phenomena above enumerated by Bichat are certainly not the result of vital action (for lie admits that the contractility of tissue to which he ascribes them is not destroyed by death): they must then be owing to a physical force, and amongst the various physical agen cies we are acquainted with, elasticity is the only one to which they can be referred.
The " vis tnortua" of Haller appears like wise to differ little if at all from elasticity. Speaking of this force he observes, that, as i indeed the very name implies, it is totally n dependent of life, and adds—" llaec vis in partibus animalism perpetuo agere videtur, etiamsi non perpetuus effectus adparet. Vi deter enim contractio cuique particulm proprire a contraria contractione duorum elementorum vicinorum impugnari et distrahi, ut qum breviores fieri non possunt, quin mediam par ticulam distrahant. Id dum fit in omnibus, quies videtur, qure est summa virium contra riarum se destruentium. Quam primum vero aliqua particula a sodalibus separatur, inflicto vulnere, tune utique labium vulneris, nuns liberum, nec a contraria potestate retentum, se ad earn vicinam, a qua trahitur, integramque incise membranm partem retrahit." The facts so accurately described in this passage are easily explained by the operation of elasticity. Why then multiply causes ? Why assume the existence of another principle in order to ac count for them ? The phenomena ascribed by Cullen and others to what he terms " tonicity," are also, at least in many instances, the effects of the same physical force. (See CONTRAC