8. Mucous membrane.—That this tissue is possessed of some degree of elasticity would appear from the well-known contraction which is found in the lower part of the intestinal canal after the establishment of an artificial anus; from the great variation of size which is observed in the stomach, and by means of which it can accomodate itself to the quantity of food con tained within it; and from many other simi lar instances. But in these cases it is often difficult to determine how far contraction de pends upon the mucous membrane, or upon the other tissues with which it is associated. We should also bear in mind that the contraction of the inner coat of the stomach is much less than might in the first instance be supposed ; the numerous folds or rugs into which it is thrown seeni destined to compensate for its im perfect elasticity.
9. Serous membrane is still lower in the scale. In those organs whose size is subjected to frequent variation, such as the stomach, in testines, urinary bladder, &c., we find an inter esting provision to permit enlargement without at all stretching their serous envelope. The organ, instead of possessing a simple serous capsule, is inserted between two loosely adher ent folds of peritoneum which permit its insinu ation between them as soon as distension takes place. I3y this simple contrivance the possibi lity of rupture or even tension of the serous coat is completely obviated, even in cases of extreme enlargement. The tunica vaginalis testis would appear to possess more elasticity than other membranes of this class:—after the ope ration for hydrocele, a disease in which it is distended far beyond its proper limits, a sudden contraction of its tissue evidently occurs.
10. Nervous matter.—Upon the division of a nerve little or no retraction of the divided ex tremities takes place. The brain however possesses an obscure elasticity, as may be seen upon making a horizontal section of its sub stance : the numerous red points which there present themselves are owing to the blood forced from the divided vessels by the surrounding pressure.
11. Fibrous membrane is remarkable for its very low degree of elasticity ; hence liga ments and tendons often give way rather than yield to a distending force. It is owing to the unyielding nature of the subcutaneous fascia in some situations that abscesses and other swel lings occurring beneath, produce but little swel ling upon the surface, and cause such severe pain to the patient ; hence too upon dividing this fascia, no enlargement of the wound occurs as in other tissues by the elastic retraction of its edges. When the distending force however is slowly applied, there appears to exist some degree of elasticity even in fibrous membranes ; thus in hydrops articuli the structures about the joint are frequently much distended by the ac cumulation of fluid within, upon the absorption of which they slowly resume their proper con dition.
III. We shall now proceed to point out some instances in which elasticity plays an im portant part in the mechanism of organized beings ; but it may be necessary to remark that in doing so we by no means profess to give an anatomical description of the various structures alluded to. We shall endeavour merely to bring into one general view some of the most inter esting cases in which elasticity plays a prominent part, and thus enable the reader to refer to the separate articles in which these details are fully discussed.
Nature avails herself of this physical property in the construction of organized bodies, for several distinct ends. It is sometimes employed as a means of protecting certain delicate and important organs by bearing off or decomposing the forces to which they are exposed. It is often used to economise muscular contraction, not only in supporting depending parts, but likewise in effecting the movement of one por tion of the body upon another. In some instan ces it is rendered subservient to the general movement of the body, or locomotion. By elas ticity the proper patulous condition of certain canals and outlets is secured ; and lastly, it very often serves to divide the power of particular muscles or sets of muscles, and thus to transfer the contractile force from one portion of an ap paratus to another.
1. Elasticity is employed by nature as a means of protecting the body generally, or some of its organs more particularly, against external violence. The great elasticity of the various
tissues in the young subject, and of the osseous system especially, affords at that period of life no inconsiderable security to the whole system : the bones themselves can yield in a very great degree to external impressions and thus prevent their bad effects. The frequent and apparently dangerous falls of children, and the perfect im punity with which they are encountered, are known to every one, and can easily be accounted for by the great elasticity of the tissues at that period of life. The opposite extreme of human existence, in which we meet with the reverse of these conditions, is equally illustrative of our subject ; for then the bones, owing to the pro gressive accumulation of earthy matter, have al most lost their power of yielding, and hence a very slight force is sufficient to fracture them. But elasticity plays a still more important part in protecting certain organs, such as the spinal chord, whose structure is so delicate that it may be torn by the slightest violence, and whose func tion is frequently deranged even by mere con cussion. The mechanism of the vertebral column exhibits at every step the most admirable appli cation of elasticity to the protection of its con tents. An unskilful mechanic who sought to afford the greatest security to this contained or gan might naturally enough suppose that its safety would be proportionate to the strength and density of the material which he should employ in incasing it ; he would probably have thrown around it a strong cylinder of solid bone, such as we see employed for a different object in the tibia or femur. But the condition of old age again affords us a complete refutation of such reasonings ; the spinal column by the successive consolidation of its component parts is then in fact converted into one long cylinder of extraordinary strength; it has become literally a single bone ; but now every touch upon the surface of the body, every application of the foot upon the ground, is conveyed by the solid and almost inelastic bones to the spinal cord, thus rendering even the movements of progression a source of pain ; hence repose is the natural con dition of this period of life, as restless activity is that of childhood. But looking at the spinal column in the active or adult age we perceive a totally different mechanism ; it now consists of no less than twenty-four distinct hones piled one upon the other and connected by twenty-four layers of fibro-cartilage, a tissue, as we have al ready seen, possessed of extraordinary elasticity. The chord, instead of filling the whole cavity, is suspended within it by means of an elastic liga ment; and thus this delicate cylinder of nervous matter is hung loosely upon a series of elastic springs which effectually break the many jolts and concussions incident to the frame in the various movements of active life. it is owing to this extreme elasticity of the spinal column, that even after very long-continued pressure, it soon recovers its proper condition. %Viten, for instance, from long and severe exercise the fibro cartilages have become somewhat pressed down by the superineumbent weight, a few hours' repose in the horizontal position is sufficient to restore the spine to its proper length. This fact has not escaped the shrewd practical observa tion of the lower classes ; when admission into the army can be obtained only by persons of a certain stature, the candidate who apprehends he can spare nothing in that particular, usually presents himself after his night's repose. The delicate viscera of the thoracic cavity owe like wise their safety in a great degree to the same me chanism. The cartilages which connect the ribs and sternum, and which, as we shall presently find, are destined to modify the movements of the thorax, tend likewise to its security by per mitting it to yield to external forces. The ob scure elasticity of the ribs themselves and of the ligaments connecting them to the spine contri bute to the same end ; hence we seldom find the thoracic viscera ruptured even by the greatest violence applied against their walls. It is this elasticity, aided no doubt by other still more efficient causes, which enables the mountebank to receive with impunity the blows of the weightiest sledge on an anvil laid upon his chest.