Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 2: Dia-Ins
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit25760/69/
ELASTICITY. 
61 
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. When, for 
instance, from long and severe exercise the fibro- 
cartilages have become somewhat pressed down 
by the superincumbent 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. 
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 size, 
and in whom, owing to the horizontal position 
of the trunk, these organs tend 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 integuments 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- 
mentum nuchse, which in these animals sup¬ 
ports the very weighty head, is a simple but 
complete substitute for the great mass of 
muscle which should have existed on the back 
part of the neck, in order to effect the same 
end. So obviously in this instance is elasticity 
a substitute for muscularity, that upon com¬ 
paring the structure in various animals we find 
the strength and elasticity of the ligament 
always proportionate to the weight of the head 
which it has to support. In the carnivora an 
interesting application of this property is seen 
in the retractde ligament passing between the 
claw and the phalangeal 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 mollusca 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 soft 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 alæ 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 edges 
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 in 
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. But 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
        

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