

LEPUS CUNICULUS 293 



are the ex-occipital bones, which form the larger part of the circum- 

 ference of the foramen. Each has its hinder margin produced 

 into a well-marked curved occipital condyle, which serves for articula- 

 tion with the atlas or first vertebra. A large occipital sinus lies 

 within the condyle, and its anterior and posterior openings show 

 clearly inside the brain case. At its antero-lateral margin the bone 

 is produced ventrally into a strong par-occipital process, giving a 

 surface for muscle attachment, and internal to this it is perforated 

 by a small hole, the condylar foramen, which is the exit for the 

 hypogossal nerve. The line of junction between the basi- and ex- 

 occipital bones is obliterated in the adult skull, but clearly visible 

 in young examples. The dorsal border of the foramen magnum 

 is formed by a median bone, the supra-occipital, constituting the 

 flat, almost vertical plate at the hinder end of the cranium. In 

 an old dog it is completely fused with a forward prolongation that 

 appears as a median dorsal tongue of bone which, however, ossifies 

 separately, and in some animals remains throughout life a distinct 

 membrane bone, the interparietal. 



The mid- ventral unit of the parietal segment is the basi-sphenoid, 

 a flat plate of somewhat thicker cartilage bone more or less cancellous 

 within. Ventrally it is fairly level, but inside the cranium it is 

 hollowed in the middle to form a depression, the sella turica, in 

 which lies the pituitary body. In some mammals its floor is per- 

 forated by a small opening, the pituitary foramen, which is not present 

 in Canis. From the sides of this, extending upwards and outwards, 

 arise the wing-like alisphenoids, also cartilage bones. Ventrally 

 this bone gives off a vertical bony plate, the external pterygoid 

 process, to the inner edge of which is attached the palatine bone. 

 The alisphenoid itself is important because of the perforations or 

 foramina connected with it. The base of the pterygoid process is 

 pierced by the alisphenoid canal, through which the external 

 carotid artery runs. The lower anterior margin of the portion 

 of the alisphenoid in the orbit forms the posterior limit 

 of a well-marked aperture running vertically, the foramen 

 lacerum anterius. Through this pass the third, fourth and sixth 

 cranial nerves supplying the muscles of the eye and also the 

 ophthalmic branch of the fifth nerve. In its basal region the 

 alisphenoid is perforated by two openings ; one is approximately 

 round, the foramen rotundum, serving for the exit of the maxillary 

 branch of the fifth nerve, and the other a slightly larger, oval, more 

 posterior hole, the foramen ovale, for the mandibular branch of the 

 trigeminal. The dorsal portion of the cranium in this region is 

 formed by the large square-shaped membrane bones, the parietals, 

 meeting in the middle line in a junction termed the sagittal suture, 



