160 MR. R. H. BtlRNE ON THE [Feb. 17, 



Ruge has pointed out that the xiphisternum (processus eiisiformis) 

 of" Man owes its origin to the eighth and ninth pairs of ribs ; for he 

 says, in the course of the paper above referred to, " das achte und 

 neunte Rippenpaar vom Sternum sich loslosend ihre medialea 

 Produkte noch im Processus ensiformis wiedererkennen lassen." 



The Leporine sternum usually consists of four segments, exclusive 

 of the omosternal and xiphisternal ones, and it therefore follows 

 that that pair of ribs which stand related and give origin to the last 

 prexiphisterual sternebra is the sixth pair ; and as this is so the 

 xiphisternum, on analogy to the human subject, might be expected 

 to arise from the seventh and eighth ribs, one or both. The seventh 

 pair of ribs ultimately break loose and approximate in the ventral 

 middle line. 



Not unfrequently, in both Rabbits and Hares, there is an extra 

 sternebra intercalated behind the sixth (= fourth sternebra as 

 ordinarily enumerated), and the occurrence of this structure (figs. 

 A-F, p. 161) has not previously been recorded. 



This extra sternebra is invariably displaced dorsally ; the lower 

 ends of the seventh pair of ribs pass over its ventral surface and 

 meet in the middle line, repeating as it were, in regard to it, those 

 relationships which they more normally bear to the head of the 

 xiphisternum. 



The sternebra in question always lies behind the sixth pair of 

 ribs ; these are attached to the synchondrosis between the sixth and 

 the extra sternebra (VII), wherefore the latter would appear to be 

 derived from the seventh pair of ribs. 



This seventh sternebra is very variable in size, presumably as the 

 result of variation in degree of absorption. It is probably not un- 

 common, for of thirteen fully ossified Leporine sterna, which I have 

 examined, six possessed it in some form, and although in one case 

 somewhat minute, it was yet distinctly recognizable. 



Figs. A-G represent a series of fully formed sterna. In figs. A, 

 B, C, D, E the extra sternebra (S. VII) will be seen to be represented 

 more or less by bone ; but in fig. F it is only present in cartilage, 

 and in fig. G it is not present at all. 



In rare cases the two portions of the primitive cartilages from 

 which this sternebra is derived may be found still more or less un- 

 united, a distinct groove being recognizable upon its dorsal surface 

 (fig. F). I have found this to be so in animals possessed of an 

 otherwise fully formed and ossified sternum. Be the individual 

 conditions what they may, the seventh pair of ribs invariably meet 

 in the ventral middle line, and are in no way attached at the sides 

 of their intersternebra as are the ribs in front of them. 



Turning now to the development of the seventh or " extra " 

 sternebra. The earhest stage in its formation which I have 

 observed was in an embryo Rabbit 3 cm. long. At this age (fig. I) 

 the median cartilages of the sternum had only fused at their 

 anterior ends ; the first seven pairs of ribs were joined to them, 

 but the rest floated free. The costal segments related to each pair 

 of ribs had manifestly been formed each in the same way, the only 



