ELASTICITY. 



59 



4. Skin. The great elasticity of the cuta- 

 neous tissue is exhibited in innumerable 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 corion. 



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, 

 oedema, 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. We may likewise 

 remark that there is no tissue whose elasticity 

 is so frequently and perhaps so usefully em- 

 ployed as that which we are now considering ; 

 for 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 otl.-er 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. 



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 n 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 ; 



