64 THE HUMAN STERNUM 



from coming to a conclusion diametrically opposed to the view which he takes, 

 and which has become crystallized into dogmatic assertion by succeeding 

 writers. As Minot suggests, an earlier stage in development must be 

 observed in order to ascertain whether a sternal anlage exists or not before 

 the elementary growth of the ribs is completed. This I believe has been 

 demonstrated as a cellular strand projecting from the shoulder-girdle, and 

 secondarily connected with the rib elements. 



2. The ' segmental ' ossification of the sternum and the bilateral 

 ossification of the mesosternum (in certain of its segments and in certain 

 animals) have been cited as arguments in favour of its costal origin. It has 

 already been conclusively demonstrated (i) that the centres of ossification 

 are not costal but inter-costal, (2) that they do not agree as a rule with the 

 number of associated ribs, and (3) that they are more usually, both in 

 individuals and in species, median than bilateral. 



3. The type of sternum characteristic of mammals (quadrupeds) is 

 segmented {^-g-, Edentata). It has been already shewn that this form of 

 sternum may be regarded rather as a modification than as the primitive 

 elemental type. 



4. Teratological conditions lend absolutely no support to the idea 

 of the costal origin of the bone. From abnormal conditions only tentative 

 conclusions can be arrived at as to what the procedure should be in normal 

 conditions. But in most cases either excess or arrest of development is 

 the cause of an abnormality. The most remarkable form of arrest of 

 development of the sternum is fissura stemi, in which an absence of the 

 bone altogether or its longitudinal fissure into two lateral halves may occur. 

 There is no doubt that the normal mode of development of the bone in 

 many lower forms is in two lateral strips separated by the heart. But there 

 is no instance of the occurrence of 'segments' of the sternum isolated from 

 one another, and connected with the ventral ends of the ribs, as one would 

 expect if the various parts of which the bone consists were originally derived 

 from the ribs. In other words, obvious proof of the existence of a sternum 

 without rib connexions has been adduced, but no evidence exists of the 

 persistent association of elements of costal origin arrested in the process of 

 development into a composite sternum. 



