329 



sometimes articulate with one auother in front. Paterson (5) found from 

 the examination of a large number (236) of foetal sterna that the 7th 

 costal cartilages articulated with one another in front in 14,4^0 of the 

 cases. Again Musgrove (8) has pointed out that a true articular 

 cavity is frequently wanting in the case of the 7th chondrosternal arti- 

 culation — its absence having been noted in 55% on the right side 

 and 50% on the left in his series. 



The 8th rib normally is in the condition of the most degenerate 

 form of the 7th rib. It is subject to variation however and when it 

 comes into relation with the sternum may present one of the following 

 forms : 



1) The cartilages may articulate directly with the end of the meso- 

 sternum. 



2) The cartilages of opposite sides may be fused with one an- 

 other across the metasternum. 



3) The cartilages of opposite sides may articulate with one an- 

 other in front of the metasternum. 



4) The cartilages may be attached to the sternum by ligamentous 

 bands. 



The condition of fusion of the 8th ribs in front of the metasternum 

 is found normally in the Macaci and Cercopitheci — the articulation 

 of the rib cartilages with one another in front in the higher apes. 



We are therefore entitled to assume that the 8th rib in man is a 

 degenerated sternal rib and further that the 7th rib shows evidence of 

 undergoing a similar degeneration. 



The fact however that the series of changes related to this de- 

 generation takes place invariably in front of the metasternum and is 

 associated with a fusion of the cartilages of the ribs undergoing de- 

 generation presents a further question for consideration. This question 

 involves the developmental history of the sternum in its relation to 

 the ribs — a subject on which there is some difference of opinion. 



Three views have been advocated as to the origin of the sternum. 

 According to Parker (9), Götte (1 1), Koelltker (9), Hoffmann (9) 

 and RuGE (10) the sternum arises in the form of two lateral carti- 

 laginous plates derived from the ends of the ribs. These two plates 

 in the course of development fuse to form the single median sternum. 

 Brucu (9) takes the view that the sternum arises independently as 

 two lateral cartilaginous bars which join with one another — first at 

 their ends and afterwards in the middle. The cartilages of the true 

 ribs according to this author arise as separate cartilaginous nuclei and 

 join later with the sternum on the one hand and the ribs on the other. 



