50 THE HUMAN STERNUM 



The only fish which is known to possess a sternum is Notidanus 

 indicus, the Perlon shark. This is described by Professor T. J. Parker" in 

 Nature. It had previously been described by Haswell." It is present in 

 the middle ventral line as a distinct, four-sided, lozenge-shaped cartilage, 

 ' let into the arch as it were in front.' Two elements are distinguishable 

 in it — anterior and posterior. Its remarkable feature from our present 

 point of view is its intimate association with the shoulder-girdle, and the 

 entire absence of connexion with the ribs. This sternum is an addition to 

 the shoulder-girdle, and is added without the intervention of the ribs at all. 



Attention may be directed in this place to an interesting and 

 remarkable paper by Professor T. J. Parker'* on the origin of the sternum. 

 Enquiring if there is any genetic connexion between the ' omosternum ' of 

 Notidanus and of Amphibia and the ' costal ' sternum of Amniota, he 

 inclines to the belief that the latter is derived from the former. He thinks 

 it possible that in early Amniota a ' post-omosternum ' existed, joined late 

 in development by the first pair of ribs. Later on the succeeding ribs 

 would join it. In Apteryx he found that in the earliest stage in which a 

 sternum was present, it extends back to the level of the third rib ; the first 

 and second pair are attached to it by articulation, the third by fibrous tissue. 

 In the next stage, the first three pairs are attached by joints, the fourth pair 

 by fibrous tissue, from which he concludes that the sternum undergoes a 

 backward growth. Retardation of chondrification of the sternum would 

 lead to the development of a sternum of indifferent tissue, subsequendy 

 chondrified from the ribs. 



He states certain hypothetical stages in the phylogeny of the sternum, 

 tracing it by gradual differentiation from an association with the shoulder- 

 girdle (probably bilateral), a secondary union with the ribs, a delay in 

 chondrification, and a subsequent extension of cartilage from the ribs. 



These views are valuable. The facts I have adduced regarding the 

 early development of the sternum give support to the hypothesis of Parker, 

 and indicate that a view different from that of Ruge and his followers is at 

 least tenable. 



The essential feature of the parts under consideration in fishes is the 

 union of the elements of the shoulder-girdle in the mid ventral line, for 

 the construction of the girdle itself and for the protection of the heart. 



