ELASTICITY. 



Gl 



ready 'icon, possessed of extraordinary elasticity. 

 The chord, instead of filling the whole cavity, is 

 suspended within it by means of an elastic liga- 

 nicnt; and thus this delicate cylinder of nervous 

 matter is hung loosely upon a series of clastic 

 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 recover-; its proper condition. When, for 

 instance, from long and severe exercise the fibro- 

 cartilages have become somewhat pressed down 

 by the superincumbent weight, a few hours' 

 repose iii the horizontal position is sufficient to 

 restore the spine to its proper length. This fact 

 has not escaped the shrewd practical obsena- 

 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 safely in a -.Teat degree to the same me- 

 chanism. 'Hie 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. 



2. Elasticity is often had recourse to as a 

 substitute for muscular contraction, and, as it 

 would appear, with a view to economize that 

 more important property. We find, for ex- 

 ample, that in most animals the abdominal 

 viscera are supported in their position chiefly 

 by the muscles of the abdomen, and that on 

 being forced downwards in inspiration by the 

 descent of the diaphragm, they are again 

 pressed upwards by the contraction of these 

 muscles. In the large ruminating quadrupeds 

 whose abdominal viscera are of so great a si/e, 

 and in whom, owing to the horizontal position 

 of the trunk, these organs lend directly down- 

 wards, the quantity of muscular power requi- 

 site to support and move them should neces- 

 sarily have been of great amount ; but instead 

 of increasing the quantity of muscle to such 

 an extent, nature has effected her purposes 

 by much more simple means. Beneath the 

 abdominal internments there exists a mem- 

 brane of great strength and elasticity, which 

 not only supports the viscera but also helps 

 to elevate them after they have been forced 

 downwards in inspiration. The elastic liga- 

 mentuin micha-, which in these animals sup- 

 ports t!ie very weighty head, is a simple but 

 complete substitute for the great mass of 

 muscle which should hate existed on the back 



part of the nock, in order to effect the same 

 end. So obviously in this in-tanee is elasticity 

 a substitute for muscularity, that upon com- 

 paring the structure in various animal-, we tind 

 the strength and elasticity of the ligament 

 always proportionate to the weight of the head 

 which it has to support. In the carnivora an 

 inteicsting application of this property is seen 

 in the retractile ligament passing between the 

 claw and the phalangea! bone; as the claw in 

 many genera is the chief weapon of attack, it 

 must not be suffered to come into contact with 

 the ground in progression, for otherwise it 

 would become blunted, as seen in those which 

 do not use it for the purposes mentioned; it is 

 consequently suspended by the retractile liga- 

 ment until drawn down at the will of the animal 

 by means of the flexor muscles. Elasticity is 

 here used as the means of suspension in order 

 to save the effort of a constant muscular exer- 

 tion. In the mo'.lusca we see this property 

 again employed to economize muscularity : 

 the shell of the oyster admits of being opened 

 as well as closed at the will of the animal; 

 but muscularity is the source of the one ac- 

 tion; elasticity residing in a strong ligament is 

 the means of effecting the other. 



3. Elasticity frequently preserves the patu- 

 lous condition of certain outlets in the animal 

 body, as, for example, those of the eyes and 

 nostrils. This object is attained by the inser- 

 tion of a rim of highly elastic cartilage into 

 the sort parts which bound these openings. A 

 material of greater rigidity, such as bone, 

 would, it may be objected, have answered the 

 purpose still better: but the rigidity of that 

 substance would have greatly interfered with 

 the free movements necessary for the functions 

 of the lids, and in the nose would not only 

 have increased the risk of injury from external 

 violence, but would have prevented the ap- 

 proximation of the alae which must take place 

 in order to expel the nasal mucus. Neither 

 would a soft and inelastic material have an- 

 swered the purpose, for then the first effect of 

 inspiration would be to approximate the < k'es 

 of the opening, and thus to prevent the further 

 entrance of air. The tracheal and bronchial 

 canals are likewise preserved patulous by the 

 same elastic material ; and we again meet with 

 it performing a like office in the Eustachian 

 tube and the external meatus of the ear. 



4. Elasticity is sometimes rendered subser- 

 vient to locomotion, or the general movement 

 of the body. The elastic pad placed beneath 

 the foot of the dromedary and many other ani- 

 mals is no doubt intended to facilitate progres- 

 sion, and to compensate in some degree for 

 the yielding looseness of the sands upon which 

 they tread. The same apparatus is found yi 

 very great perfection in the feet of the carni- 

 vora, and must be of great use in enabling 

 them to make those enormous bounds by which 

 they spring upon their prey. Hut perhaps one 

 of the most interesting examples of elasticity 

 being rendered subservient to locomotion is 

 met with in certain fish. The salmon, during 

 its annual ascent to fresh-water streams for the 



