70 
OF MECHANICAL ARRANGEMENT 
signed to each part. The arm is an instrument of motion, 
principally, if not solely. Accordingly the shallowness of 
the socket at the shoulder, and yieldingness of the car¬ 
tilaginous substance with which its edge is set round, and 
which in fact composes a considerable part of its concavi¬ 
ty, are excellently adapted for the allowance of a free mo¬ 
tion and a wide range; both which the arm wants. 
Whereas, the lower limb, forming a part of the column of 
the body; having to support the body, as well as to be the 
means of its locomotion; firmness v 7 as to be consulted, as 
well as action. With a capacity for motion, in all direc¬ 
tions indeed, as at the shoulder, but not in any direction 
to the same extent as in the arm, was to be united stabili¬ 
ty, or resistance to dislocation. Hence the deeper excava¬ 
tion of the socket; and the presence of a less proportion 
of cartilage upon the edge. 
The suppleness and pliability of the joints we every 
moment experience; and the firmness of animal articu¬ 
lation, the property we have hitherto been considering, 
may be judged of from this single observation, that, at 
any given moment of time, there are millions of animal 
joints in complete repair and use, for one that is disloca¬ 
ted; and this, notwithstanding the contortions and wrench¬ 
es to which the limbs of animals are continually subject. 
II. The joints, or rather the ends of the bones which 
form them, display also, in their configuration, another use. 
The nerves, blood-vessels, and tendons, which are neces¬ 
sary to the life, or for the motion of the limbs, must, it is 
evident, in their way from the trunk of the body to the 
place of their destination, travel over the movable joints; 
and it is no less evident, that, in this part of their course, 
they will have, from sudden motions, and from abrupt 
changes of curvature, to encounter the danger of compres¬ 
sion, attrition, or laceration. To guard fibres so tender 
against consequences so injurious, -their path is in those 
parts protected with peculiar care; and that by a provision 
in the figure of the bones themselves. The nerves which 
supply the fore-arm, especially the inferior cubital nerves, 
are at the elbow conducted, by a kind of covered way, be¬ 
tween the condyles, or rather under the inner extuberances 
of the bone, which composes the upper part of the arm.* 
same; both are solid bone covered by cartilage, and both have a rim of 
a strong fibro-cartilaginous texture, not only for the purpose of rendering 
the socket deeper, but for preventing fractures of the rim in robust exer¬ 
cises, to which, were it bony, it would be very liable.— Paxton. 
Ches. An. o. 255, ed. 7th 
