THE HUMAN FRAME. 77 



pie yet beautiful mechanical contrivance, referable to the 

 structure of the bones which enclose it. The ribs are artio 

 ulated to the backbone, or rather to its side projections, ob- 

 liquely : that is, in their natural position they bend or slope 

 from the place of articulation downwards. But the basis 

 upon which they rest at this end being fixed, the conscquenco 

 of the obliquity, or the inclination downwards is. that when 

 they come to move, whatever pulls the ribs upwards, neces« 

 sarily at the same time draws them out ; and that, while 

 the ribs are brought to a right angle with the spine behind, 

 the sternum, or part of the chest to which they are attached 

 in front, is thrust forward. The simple action, therefore, of 

 the elevating muscles does the business ; whereas, if the ribs 

 had been articulated with the bodies of the vertebree at right 

 angles, the cavity of the thorax could never have been fur- 

 ther enlarged by a change of their position. If each rib had 

 been a rigid bone, articulated at both ends to fixed bases, 

 the whole chest had been immovable. Keill has observed 

 that the breastbone, in an easy inspiration, is thrust out 

 one-tenth of an inch ; and he calculates that this, added to 

 what is gained to the space within the chest by the flatten- 

 ing or descent of the diaphragm, leaves room for forty-two 

 cubic inches of air to enter at every drawing-in of the breath. 

 AYhen there is a necessity for a deeper and more laborious 

 inspiration, the enlargement of the capacity of the chest may 

 be so increased by effort, as that the lungs may be distended 

 with seventy or a hundred such cubic inches.* The thorax, 

 says Schelhammer, forms a kind of bellows, such as never 

 have been, nor probably will be, made by any artificer. 



V. The patella, or kneepan,t is a curious little bone ; 

 in its form and office unlike any other bone in the body. It 

 is circular, the size of a crown-piece, pretty thick, a liltlo 

 convex on both sides, and covered with a smooth cartilage. 

 It Ues upon the front of the knee ; and the powerful tendons 

 by which the leg is brought forward, pass through it — oj 

 * Anat. p. 229. t See Fig. 4. 



