40 MQRPHOLOGY OF THE VERTEBRATA. 



The molars and premolars all grow from persistent pulps, and 

 do not form fangs. Their crowns are transversely ridged. In the 

 young rabbit there are in the upper jaw three incisors and three 

 milk molars on each side. The lower jaw has only two milk 

 molars on either side. 



THE PREMAXILLjE 



Are extremely large, and bear the upper incisors. The orbits are 

 not separated by bone from the temporal fossae. The optic nerves 

 have a common outlet, the optic foramina being confluent, and in 

 this manner the orbits communicate with one another behind. 

 Each orbit presents a wide opening at its front part, which leads 

 into the lateral nasal eavity. 



THE ZYGOMATIC ARCH 



Developes a small prominence, both from its hind and fore 

 extremities. 



THE NASAL BONES 



Are remarkable both in their length and breadth. They extend 

 further back than the long, slender nasal processes of the pre- 

 maxillse. The tympanic is expanded below to form a bulla. 



THE PALATE, 



On account of the large size of the anterior palatine canals, is 

 reduced to a narrow bridge of bone, extending across in a line 

 with the premolar teeth. 



Post-orbital processes project outwards and backwards from the 

 frontals. The glenoid cavity is elongated from before backwards. 

 The narrow olfactory fossa is continuous behind with the 

 cerebral fossa, which is separated from the cerebellar ^fossa by 

 an upright tentorial ridge. 



In the young rabbit an interparietal bone can be distinguished. 



The lower jaw or mandible consists of two separated halves 

 or rami, united in front by a short symphysis. The angle of 

 each ramus forms a broad compressed plate, with the lower border 

 rounded and thickened, so as to project beyond the posterior 

 border of the ascending plate. The ascending portion of' the 

 ramus is long; and the condyles are elongated from before back- 

 wards, so as to correspond with that of the direction of the 

 .glenoid cavity. The coronoid process is well developed. 



HYOID BONE. 



In this there are nodules, which represent the ceratohyals, the 

 epihyals, and thyrohyals. 



Digitized by Microsoft® 



