STERNUM 



9ir 



'P. obi. 



calcareous matter appear in the body of the Sternum, at the base 

 and in the anterior part of the keel ; but the whole does not become 

 bone until about the 20th Aveek, and even then the posterior rim 

 may yet remain unossified. In the Chick an unpaired Jophosteon 

 in the anterior basal portion of the keel and a pair of metostea 

 appear a few days Ijefore emerging from the shell ; and on 

 the day of hatching a pair of jjro-ostea is added, all these five 

 centres extending so as to coalesce 

 about 9 days later. On the ISth 

 day half of the keel is ossified, 

 a,fter Avhich follows the spina ; but 

 ossification is not complete before 

 the bird is 5 or 6 months old. A 

 comparison of a Duck's Breastbone 

 with that of a Fowl shews at a 

 glance that in the latter the 

 consolidation is much less, more 

 than two-thirds of it being formed 

 by the metasternum and the j^os- 

 terior lateral as well as the oblique 

 processes — all of them persisting 

 as outgrowths and being connected 

 only by non - cartilaginous mem- 

 branaceous tissue. In the macer- 

 ated skeleton the spaces between 

 these outgrowths appear as deep 

 " notches " ; or, if distally closed 

 by bone or cartilage, as fenestra. 

 Moreover, in many Birds an addi- 

 tional process appears between 

 the metasternum and the posterior lateral processes : the presence 

 of such a processus intermcdms divides each posterior " notch " into 

 two, and when a processus obliquus is wanting it is often hard to 

 determine whether there is such an intermediate process. These 

 posterior " notches " and fenestra} have for many years been used by 

 the hunter for neat " characters," and undue value has been attri- 

 buted to them, notAvithstanding that authorities such as Parker and 

 Prof. Selenka have insisted on the far greater taxonomic importance 

 of the configuration of the anterior portion of the Sternum.^ The 

 A'ariable mode of connexion of the Furcula Avith the keel of the 

 Sternum has already been dealt with (Skeleton, p. 858). 



The spina sterni often consists of an inner (dorsal) and an outer 

 (ventral) portion ; but sometimes they are confluent, or one of 

 them may be absent. The shape of the anterior free margin is 



^ This view lias also beeu strongly urged in Phil, Trans. 1S69, p. 337, as 

 well as by Prof. Fiirbriugei". 



Sternum of a Young Fowl. 

 F.c. coracoidal facet ; K. Keel ; Sp.e. and Sp.i. 

 Spina externa and interna (other letters 

 as before). 



