60 



ELASTICITY. 



the numerous folds or rugae into which it is 

 thrown seem 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, Sec., 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. By 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 ofthe 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 bones piled one 

 upon the other and connected by twenty- four 

 layers of fibro-cartilage, a tissue, as we have al- 



