THE HUMAN STERNUM 7 



process of ossification which sets in presents somewhat difficult characteristics. 

 In man the process is endochondral. In the rat it is perichondral. In 

 man the ossification of the sternum proceeds in the way characteristic of the 

 development of epiphyses of long bones, the bodies of the vertebrae 

 and the basis cranii. The bone is deposited as separate nuclei in the interior 

 of the cartilage, and replaces it by gradual absorption of the latter, only a 

 thin layer on the surface being formed as perichondral or periosteal bone. 

 In the rat, on the other hand, each part (or so-called segment) of the sternum 

 is produced like a digit, by a cylindrical growth of perichondral bone, which 

 encloses the primitive cartilage and replaces it. 



Much stress — probably too much— has been laid upon the occurrence 

 of centres of ossification. At the lower end of the humerus there are four ; 

 at the lower end of the femur only one. One inclines to the belief that in 

 such cases — and the sternum appears to be an exactly parallel case — the 

 diflFerence in the two bones named is due to the difference in the forces 

 applied. In the case of the lower epiphysis of the humerus there are four 

 forces acting upon it ; the masses of muscles attached to the two epicondyles 

 and the contact of radius and ulna. In the case of the femur, a single 

 mechanical force is at work in the pressure of the upper end of the tibia. 



So in the case of the sternum. While it has been the subject of con- 

 siderable research and the most conflicting statements, we cannot regard 

 the centres of ossification in the sternum as of essential morphological 

 importance. There are three reasons which may be named in support of 

 this opinion, (i) They are subject to enormous variation ; (2) the ossifying 

 process is one which is delayed till a comparatively late period ; and (3) it is 

 not inconceivable that mechanical forces, such as the expansion of the 

 chest, associated with the growth of the contained viscera, and pressure and 

 traction due to the attachment of the costal . cartilages, may be the cause of 

 the excitement which induces ossification. 



Another important consideration of a general kind may be mentioned 

 here. In the attempt to form a clear idea of the individuality of a particular 

 structure, such as the sternum, what features are to be looked upon as 

 typical and characteristic .'' At what stage in its growth are we to compare 

 it with the corresponding structure in another organism ? Is it the adult 



