CHAPTER VIII. 



Tab. X; — THE CHEST, PATELLA, AND SHOULDER-BLADE. 



Fig. 1. The spine, ribs, and sternum, constitute the frame work of 

 the chest or thorax. Referring, however, to the plate, or to nature, 

 we observe that the ribs are not continued throughout from the spine 

 to the sternum, but intervening cartilages complete the form of the 

 chest, by connecting the end of the rib to the breast bone. This is a 

 further provision, relative to the mechanical function of the lungs, de- 

 serving notice. The muscles of respiration enlarge the capacity of 

 the chest by elevating the ribs ; and during the momentary interval 

 of muscular action the cartilages, from their great elasticity, restore 

 the ribs to their former position. 



Fig. 2. Represents the true shape of the patella, the anterior sur- 

 face convex. Fig. 3, the posterior surface, has two concave depres- 

 sions adapted to the condyles of the thigh bone. The projection of the 

 patella, as a lever, or pulley, removes the acting force from the centre 

 of motion, by which means the muscles have a greater advantage in 

 extending the leg. That this bone is " unlike any other in the body," 

 is a mistake ; such bones are numerous, though less obvious, for they 

 do not exceed the size of a pea : these are called sesamoid hones, and 

 are formed in the flexor tendons of the thumb, and sometimes in the 

 fingers. They are frequently found under the tendons of some of the 

 muscles. Two of these sort of bones are constantly found under the 

 articulation of the great toe with the foot. 



Fig. 4. The shoulder-blade (scapula) is joined to the collar bone 

 by ligaments, and to the thorax by powerful muscles which are capa- 

 ble of sustaining immense weights, and whose action gives the vari- 

 ous directions to the arm, and enables it freely to revolve at the 

 shoulder joint. 



