4. Skin.—The great elasticity of the cuta neous tissue is exhibited in innumenible in stances; the extension which it undergoes in pregnancy, in ascites, in cases of large fatty and other tumours, and the promptitude with which in these instances it returns to its proper state after the removal of the distending causes, are matters of every day observation, and are chiefly owing to its elasticity. The great re traction of the integuments in amputation depends likewise upon the same principle. There is perhaps no tissue in the body where elasticity is more impaired by advanced age : in the young or adult subject, when, owing to disease or other causes, the subcutaneous adi pose matter has become suddenly absorbed, the skin, owing to its great elasticity, is ena bled to contract, and thus accommodate itself to the diminished distention ; while in old age, under the same circumstances, the power of contraction is lost, and hence it hangs in loose folds or wrinkles, so characteristic of that period of life. These remarks are meant to apply chiefly to the true skin or eorion.
5. Cellular tissue ranks high among the elastic structures : many of the cases which we have just instanced as proving the elasticity of the cutaneous tissue, indicate likewise its existence in the cellular membrane; anasarca, wdema, and still more emphysema, can occur only in consequence of the distention of those filamentous threads which form the cells; and as recession occurs immediately upon the re moval of the distending force, it is plain that elasticity is the principle to which the change must be attributed. NVe may likewise remark that there is no tissue whose elasticity is so frequently and perhaps so usefully em as that which we are now considering; or it is by this property of cellular membrane that the motion of the several muscles is per mitted and even assisted : thus upon elevating the arm the yielding cellular tissue of the axilla permits the member to be drawn up wards, and when the arm is again depressed the elasticity of the same tense filaments as sists in some degree the muscles which bring it down.
6. Muscle.—Elasticity appears to belong to the muscular system in a very high degree ; it is, however, extremely difficult to estimate its extent in the muscular fibre itself, partly owing to its being the seat of two other contractile forces, the vis insita and vis nervea, and partly to the great quantity of cellular and other tis sues which enter into the structure of muscle, and thus impart to it their physical properties. There are however many instances in which we must concede elasticity to the muscular fibre ; the contraction which occurs in the abdominal muscles even long after death, upon removing the accumulation of air or fluid contained within the peritoneum; and the recession of the cut edges which takes place upon dividing a muscle under the same circumstances, cannot be ascribed either to the vis nervea or to the vis insita, (for they have ceased to exist,) and the contraction is evidently too extensive to be attributed wholly to the cellular tissue. But
we may observe the operation of this property even in the living muscles : on dividing the facial muscles of one side in a living animal the mouth is gradually drawn towards the opposite, and this takes place not by the effort, but solely by the elasticity of the un injured muscles, which have now no coun teracting force upon the other side to resist their contraction. So it is with all the other muscles during what is called their state of rest : the elasticity of one class is exactly ba lanced by the same property in their antago nists; and hence when the influence of the will is completely withdrawn, as in sleep, we may estimate the comparative quantity of elas ticity which antagonizing muscles are possessed of : those of the face for example are exactly equal upon opposite sides in this respect, and accordingly the mouth retains its proper central position; but in the limbs, as the elasticity of the flexors exceeds that of the extensors, we usually find these parts of the body during sleep in a semiflexed position.
7. Bone possesses considerable elasticity, though its degree is frequently underrated by the superficial observer. It is not easily demon strable in the larger bones, but upon cutting even these into thin plates its existence becomes at once evident. There are many phenomena both healthy and diseased which depend upon the elasticity of bone; the enlargement of the maxillary sinus from the growth of fungus within its cavity, and the collapse of its walls upon the removal of the distending matter ; the obliteration of the alveolus after the extraction of a tooth ; the narrowing of the optic hole which is found in cases of atrophy of the optic nerves, and of the carotid canal after tying the carotid artery ; the diminution of the orbital cavity which gradually takes place upon extir pation of the eye—all these changes depend in a great degree upon the elastic qualities of bone. The great elasticity of the osseous system in the young subject, and the almost entire absence of it in the bones of old persons, is at once ex plained by the fact that elasticity resides in the cartilaginous and not in the earthy ingredient; the great proportion of the former in the young bone, and the accumulating deposition of earthy matter as age advances, are known to every observer.