436 ZOOLOGY SECT, xm 



The sternum consists of six segments or sternebrce: the first, 

 the manubrium sterni or presternum, is larger than the rest, and 

 has a ventral keel. With the last is connected a rounded 

 cartilaginous plate, the xiphisternum. 



The skull (Figs. 1093, 1094), if we leave the jaws out of account, 

 is not at all unlike that of the Pigeon in general shape. The 

 length is great as compared with either the breadth or the depth ; 

 the maxillary region, or region of the snout (corresponding to 

 the beak of the Pigeon), is long in proportion to the rest, the orbits 

 closely approximated, being separated only by a thin inter-orbital 

 partition, and the optic foramina united into one. But certain 

 important differences are to be recognised at once. One of these 

 is in the mode of union of the constituent bones. In the Pigeon, 

 as we have seen, long before maturity is attained, the bony 

 elements of the skull, originally distinct, become completely fused 

 together so that their limits are no longer distinguishable. In 

 the Rabbit, on the other hand, such fusion between elements 

 only takes place in a few instances, the majority of the bones re- 

 maining more or less distinct throughout life. The lines along 

 which the edges of contiguous bones are united the sutures as they 

 are termed are sometimes straight, sometimes wavy, sometimes 

 zig-zagged serrations of the edges of the two bones interlocking ; 

 in some cases the edges of the bones are bevelled off and the 

 bevelled edges overlap, forming what is termed a squamous suture. 



Another conspicuous difference between the skull of the Rabbit 

 and that of the Pigeon is in the mode of connection of the lower 

 jaw, which in the former articulates directly with the skull the 

 quadrate, through which the union is effected in the Pigeon, being 

 apparently absent. Certain large apertures which are distinguish- 

 able are readily identified with the large openings in the skull of 

 the Pigeon. In the posterior wall of the skull is a large rounded 

 opening, the foramen magnum, flanked with a pair of smooth 

 rounded elevations or condyles for articulation with the first 

 vertebra, these obviously corresponding to the single condyle 

 situated in the middle below the foramen in the Pigeon. A large 

 opening, situated at the end of the snout and looking forwards, 

 obviously takes the place of the external nares of the Pigeon ; 

 and a large opening in the roof of the mouth, leading forward to 

 the external nasal opening, plainly represents, though much wider 

 and situated further back, the internal or posterior nares of the 

 Pigeon ; while the rounded tubular opening (and. me.} situated at 

 the side of the posterior part of the skull, some distance behind 

 the orbit, is evidently the same as the auditory aperture of the 

 Pigeon. 



Surrounding the large opening of the foramen magnum are the 

 bones of the occipital region of the skull, the supra-, ex- and basi- 

 occipitals. The first of these (s. oc.) is a large plate of bone whose 



