C4 OF MECHANICAL ARRANGEMENT 



V. The patella, or knee-pan is a curious little bone ; in 

 its form and office unlike any other bone in 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 through it (or rather it makes a part of their continu- 

 ation ) from their origin in the thigh, to their insertion in 

 the tibia. It protects both the tendon and the joint from 

 any injury which either might suffer, by the rubbing of one 

 against the other, or by the pressure of unequal surfaces. 

 It also gives to the tendons a very considerable mechani- 

 cal advantage by altering the line of their direction, and 

 by advancing it further out from the centre of motion ; and 

 this upon the principles of the resolution of force, upon 

 which principles all machinery is founded. These are its 

 uses. But what is most observable in it is, that it appears 

 to be supplemental, as it were, to the frame ; added as it 

 should almost seem afterward ; not quite necessary, but 

 very convenient. It is separate from the other bones ; that 

 is, it is not connected with any other bones by the com- 

 mon mode of union. It is soft, or hardly formed, in infan- 

 cy ; and produced by an ossification, of the inception or 

 progress of which, no account can be given from the struct- 

 ure or exercise of the part. 



sjnration, 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 condition between these two acts. And the mus- 

 cular 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 al- 

 ternately, to dilate or to contract the cavity of the chest, and, in do- 

 ing so, to raise or to depress the ribs. Hence it is, that both in inspira- 

 tion 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 depends 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 

 ■dnimul Mechanics. 



