1 6 THE HUMAN STERNUM 



sternum projects into the abdominal wall. The ribs secondarily join the 

 sternal anlage, and are distinctly cartilaginous when they fuse with a cellular 

 sternum. After fusion, lines of separation occur at the junctions to produce 

 the costo-sternal joints. 



One would not venture to criticize in this way the results of so 

 eminent an observer if one had not had opportunities of largely repeating 

 the observations made by him, or if one were not convinced that Minot's 

 criticism*' is a suggestive one, and that a stage earlier than that at which Ruge 

 commenced his observations must be examined before conclusions such as 

 he advanced can be arrived at. 



From an examination of embryos, both chick and mammalian, and 

 from an examination of the observations of others on the development of 

 the vertebrate sternum, one concludes that the preponderating weight of 

 evidence is in favour of an independent sternum, associated in its early 

 development with the shoulder-girdle and acquiring secondary and variable 

 connexions with the ribs. 



C. Ossification of the Sternum 



The process of ossification of the sternum occurs late in foetal life, 

 and is characterized in man by deposits of endochondral bone. Such 

 diverse statements and so many observations have, at different times, been 

 made upon the occurrence of those centres, that it will be best to summarize 

 one's personal observations first, and then to compare them with those of 

 others. 



If these centres of ossification possess a morphological value, and 

 are, in any sense, indicative of a metameric formation of the sternum, it is 

 of prime importance to demonstrate their segmental character. 



It is to be noted that when ossification commences, the sternum is 

 an exact cartilaginous model of the future bone ; the presternum may or 

 may not be separated from the mesosternum by a transverse, fibrous 

 lamina ; and the cartilaginous metasternum is continuous with the meso- 

 sternum. 



The process is most conveniently studied in two stages — before and 

 after birth. 



