278 AN INTRODUCTION TO ZOOLOGY 



almost at right angles. This passes inwards towards the mid-ventral 

 line The first seven pairs of these directly join the sternum, the 

 next two almost reach it, and are each connected with the rib in 

 front. The remaining pairs are not so complete and not so attached, 

 and in consequence sometimes termed the floating ribs. 



The sternum is a moderately wide flattened rod lying in the mid 

 ventral line and divided into six segments, or sternebrse. The first, 

 termed the manubrium, is larger than the rest and possesses a marked 

 ventral ridge-like keel ; to it is joined the first pair of ribs. The 

 posterior segments, the xiphisternum, is a long slender one, ter- 

 minating in a thin flattened rounded cartilaginous plate. It has 

 been found that in the embryo the sternebrae develop from the 

 ventral ends of the ribs, and it is a matter of doubt whether they 

 are to be regarded as strictly homologous with the segments of the 

 sternum in Rana. 



Skull. 



The skull of a mammal is modified to adapt it for the acqui- 

 sition of its food, and in the case of the rabbit, a Rodent or gnawing 

 animal, this process of modification has been carried so far that 

 it has resulted in a highly specialised structure that is considerably 

 removed from what may be considered a generalised mammalian 

 skull. For that reason it is better to study another form, say 

 the dog, in order to get a general idea of the structure of a skull, 

 and so it is only necessary here to refer to certain of its main features 

 in the rabbit. 



For the purposes of description we can divide the skull into a 

 larger posterior cranial region and a smaller anterior facial region. 

 At each side of the former, towards the front, lies a large hollow, the 

 orbit, in which the eye is situated, while behind this the auditory 

 capsule is actually fused with the cranium proper. The actual 

 portion occupied by the brain case is relatively small, since in their 

 middle regions the orbits are only separated by a thin bony partition, 

 the interorbital septum, perforated by the optic foramen, and the 

 brain does not extend directly in front of or below this. The various 

 cartilage and membrane bones comprising the skull remain more or 

 less distinct throughout life, being united along their edges by wavy 

 junctions, the sutures. In the following account the cartilage and 

 membrane bones will be distinguished by placing the letters c or 

 m in brackets after their names. 



The facial region consists mainly of an elongated nasal capsule. 

 This is roofed by the paired somewhat long nasal bones (m) and its 

 antero-lateral and ventro-lateral wells formed by the premaxillse 

 (m). Each premaxilla carries two incisor teeth, the anterior of 



