THE HUMAN STERNUM 31 



is not impossible that the single centre in the femur is produced by the 

 exercise of a single fundamental force — the direct relation of the femur to the 

 tibia ; while on the other hand, at the lower end of the humerus, causes of 

 ossification may be found in the pressure of the radius and ulna, and the 

 traction of the muscular masses attached to the epicondyles of the humerus. 



So in the case of the sternum. Its cartilaginous condition may 

 justifiably be taken as the basis of comparison in different groups of 

 animals ; and the endochondral centres of ossification may be regarded as 

 altogether secondary, and devoid of any special morphological significance. 

 As in the case of epiphyses, the centres of ossification in the sternum may 

 be explained by the exercise of traction or pressure on the part of the ribs 

 and costal cartilages. 



This view appears, on the whole, more satisfactory than a nebulous, 

 transcendental notion, of a hypothetical representation of sternebrae or sternal 

 segments. 



It affords an explanation of the intercostal and median deposit of bone 

 in the cartilaginous sternum, which produces ultimately an apparent, and it 

 may be permanent, segmentation of the sternum in such animals as the 

 Edentata, possessing a mobile sternum. Per contra^ it explains the condition 

 found in man, cheiroptera, birds, etc., in which, along with well-developed 

 clavicles and shoulder-girdles, the mesosternum remains a simple bone, and 

 even becomes more consolidated than in the embryonic condition. Lastly, 

 it explains the irregularity in the occurrence of the centres of ossification of 

 the sternum. Even leaving out of account the ossification of the meta- 

 sternum, the ' segments,' of which the mesosternum is composed, are 

 not (except in certain mammals, to which reference will be made later) 

 equal to or alternate with the number of costal cartilages associated 

 with it. 



In the human mesosternum there are, in my experience, never five 

 segmental centres of ossification ; at the most four, or still more commonly 

 only three are present. There is very rarely a centre in the interval between 

 the articulations of the sixth and seventh costal cartilages ; and there is 

 more often than not no centre in the space between the fifth and sixth. Their 

 absence needs no explanation, but may be suggested as due to the close. 



