64 
OF MECHANICAL ARRANGEMENT 
V. The patella, or kneepan is a curious little bone; in 
its form and office, unlike any other bone of the body. [PI. 
X. fig. 2, 3.] It is circular: the size of a crown piece; 
pretty thick; a little convex on both sides, and covered with 
a smooth cartilage. It lies upon the front of the knee: and 
the powerful tendons, by which the leg is brought forward, 
pass into it, (or rather it makes a part of their continu¬ 
ation,) from their origin in the thigh to their insertion in 
But these evils are avoided by the interposition of the elastic cartilage. 
On their fore part the ribs are eked out, and joined to the breastbone by 
means of cartilages, of a form corresponding to that of the ribs, being, as 
it were, a completion of the arch of the rib, by a substance more adapted 
to yield in every shock or motion of the body. The elasticity of this 
portion subdues those shocks which would occasion the breaking of the 
ribs. We lean forward, or to one side, and the ribs accommodate 
themselves, not by a change of form in the bones, but by the bending or 
elasticity of the cartilages. A severe blow upon the ribs does not break 
them, because their extremities recoil and yield to the violence. It is only 
in youth, however, when the human frame is in perfection, that this pli¬ 
ancy and elasticity have full effect. When old age approaches, the car¬ 
tilages of the ribs become bony. They attach themselves firmly to the 
breastbone, and the extremities of the ribs are fixed, as if the whole arch 
were formed of bone unyielding and inelastic. Then every violent blow 
upon the side is attended with fracture of the rib, an accident seldom oc¬ 
curring in childhood, or in youth. 
But there is a purpose still more important to be accomplished by 
means of the elastic structure of the ribs, as partly formed of cartilage. 
This is in the action of breathing, or respiration; especially in the more 
highly-raised respiration which is necessary in great exertions of bodily 
strength, and in violent exercise. There are two acts of breathing— ex¬ 
piration, or the sending forth of the breath ; and inspiration, or the 
drawing in of the breath. When the chest is at rest, it is neither in a 
state of expiration nor in that of inspiration ; it is in an intermediate con¬ 
dition between these two acts. And the muscular effort by which either 
inspiration or expiration is produced, is an act in opposition to the elastic 
property of the ribs. The property of the ribs is to preserve the breast in 
the intermediate state between expiration and inspiration. The muscles 
of respiration are excited alternately, to dilate or to contract the cavity 
of the chest, and, in doing so, to raise or to depress the ribs. Hence it 
is, that both in inspiration and in expiration the elasticity of the ribs is 
called into play; and, were it within our province, it would be easy to 
show, that the dead power of the cartilages of the ribs preserve life by 
respiration, after the vital muscular power would, without such assistance, 
be too weak to continue life. 
It will at once be understood, from what has now been explained, how, 
in age, violent exercise or exertion, is under restraint, in so far as it de¬ 
pends on respiration. The elasticity of the cartilages is gone, the circle 
of the ribs is now unyielding, and will not allow that high breathing, that 
sudden and great dilating and contracting of the cavity of the chest, which 
is required for circulating the blood through the lungs, and relieving the 
heart amidst the more tumultuous flowing of the blood which exercise 
and exertion produce.— Bell's Treatise on Animal Mechanics. 
