116 



LOCOMOTION. 



[CHAP. v. 



same general shape before as after their ossification ; and as this 

 process is slow, and not finally completed until adult age, they 

 share during a considerable period in the functions of the bony 

 skeleton. 



Until the completion of the process of ossification, the temporary 

 cartilages increase in bulk by an interstitial development of new 

 cells. A few vessels, also, shoot into them at an early period, 

 occupying small tortuous canals, which subsequently become obli- 

 terated. 



Ossification commences in the interior of the cartilage at deter- 

 minate points, hence called points or centres of ossification. From 

 these the process advances into the surrounding substance. The 

 period at which these points appear varies much in the different 

 bones, and in different parts of them. The first is the clavicle, in 

 which the primitive point appears during the fourth week : next is the 

 lower jaw; the ribs, too, appear very early, and are completed early; 

 next the femur, humeris, tibia, and upper jaw. The vertebrae and 

 pelvic bones are late, as well as those of the tarsus and metatarsus. 

 Some bones do not begin to ossify till after birth, as the patella. 



Fig, 28. In most bones ossification begins 



at more than one point. Thus, in 

 the long bones (fig. 28) there is a 

 middle point, to form the future 

 shaft ; and one at each extremity, 

 to form the articular surface and 

 eminences. That in the shaft is 

 the first to appear, and the others 

 succeed it at a variable interval. 

 The central part is termed the 

 diaphysis, and for a long period 

 after birth there remains a layer 

 of unossified cartilage between 

 this and the epiphyses, as the ex- 

 tremities are then styled. Pro- 

 cesses of bone have usually th< 

 own centres of ossification, an( 

 are termed epiphyses until they 

 are finally joined to the main 

 Im^f P ar tj after which they receive the 

 King's college. name of apophyses. 



Ossification generally extends in the direction that the future 

 laminae and Haversian rods are to assume, and which corresponds ii 



Vertical section of the knee-joint of an infant; 



