THE SKULL AS A WHOLE 175 



rimal bone, the lateral margin of which projects from the orbital rim 

 as a blunt subcutaneous process (processus subcutaneus) . This bone 

 is frequently missing from the dried skull unless care has been taken 

 to keep it in place. On the ventral side of the base of the subcuta- 

 neous process is the orbital opening of the nasolacrimal canal (canalis 

 nasolacrimalis), the bony enclosure of the nasolacrimal duct, which 

 in the natural condition leads from the corneal surface of the eye to 

 the anterior portion of the nasal fossa. A smaller projection 

 forming the ventral boundary of the nasolacrimal aperture is the 

 hamulus lacrimalis. Finally, in the ventral anterior angle of the 

 orbit, the bases of the three posterior cheek-teeth encroach to a 

 considerable extent on the orbital space. They are separated from 

 the orbital wall, by a deep infraorbital groove (sulcus infraorbitalis), 

 which leads forward into the canal of the same name. They partly 

 conceal two important apertures of this region, the orbital opening 

 of the pterygopalatine canal (canalis pterygopalatinus), leading to 

 the palatal surface, and the sphenopalatine foramen (foramen 

 sphenopalatinum), leading to the nasal fossa. The pterygopalatine 

 canal opens ventrally in the palato-maxillary suture of the hard 

 palate by a rounded aperture, the greater palatine foramen (fora- 

 men palatinum majus). 



The nasal cavity (cavum nasi) is enclosed by the maxilla and 

 premaxilla, with the assistance of paired roofing elements, the 

 nasal bones. Apart from the incisive foramina, which are closed 

 by soft tissue in the natural condition, the cavity is open at two 

 points. Posteriorly it communicates with the ventral surface of 

 the skull by the choanae, which, in the rabbit, are incompletely 

 divided. Anteriorly it opens to the outside by the piriform aperture 

 (apertura piriformis). The cavity is divided into right and left 

 portions, the nasal fossae. 



In a divided skull may be examined the space enclosed by the 

 cranial portion, the cranial cavity (cavum cranii). Its form 

 depends on that of the brain. It is divisible into three portions, 

 known as the cranial fossae. The anterior cranial fossa (fossa 

 cranii anterior) is a small division lodging in the natural condition 

 the olfactory bulbs of the brain. The middle cranial fossa, the 

 largest division of the cavity, accommodates the enlarged cerebral 

 hemispheres. The posterior cranial fossa is a small division 



