LEPUS CUNICULUS 281 



front and the alisphenoid behind. The squamosal is a large bone 

 whose anterior portion takes part in the formation of the hinder wall 

 of the orbit. Its lateral portion helps to form the side wall of the 

 hinder region of the cranial cavity. Laterally it gives off a con- 

 spicuous zygomatic process, on the postero- ventral surface of which is 

 a smooth facet for the articulation of the lower jaw. The alisphenoid 

 is a wing-shaped bone, part of which is embedded in the posterior 

 corner of the orbit. From its ventral border arises a small trans- 

 versely lying plate of bone, the external pterygoid process, perforated 

 by three holes ; the innermost is for the passage of the internal 

 maxillary artery and vein and the outermost for branches of the 

 mandibular trunk of the trigeminal nerve. At the innermost 

 ventral corner of the alisphenoid bone is a large vertical slit, the 

 foramen lacerum anterium or sphenoidal fissure, which permits the 

 exit of the third, fourth and sixth cranial nerves and the first two 

 branches of the fifth. The external lower border of the orbit is 

 composed of the zygomatic processes of the maxilla and the 

 squamosal and a thin strip of bone lying in the vertical plane, the 

 malar or jugal (m), which joins them. The three structures together 

 forming a curved zygomatic arch or zygoma. 



The extreme front end of the cranial cavity is bounded by a 

 vertical transverse plate of bone, the mesethmoid (c), which is not 

 visible externally and completely separates the cranial and nasal 

 cavities. The septum narium, already dealt with, is a median for- 

 ward projection of this bone and its lateral wings, termed the 

 cribriform plates, are perforated by a number of holes which transmit 

 the various branches of the olfactory nerve. 



The base of the cranial region between the palatines is formed by a 

 thin vertical plate, the presphenoid (c), which forms the lower border 

 of the orbital foramen. Within the skull, on the floor of the cranial 

 cavity, it is produced into a small transverse anterior clinoid process. 

 Immediately behind this bone is the basisphenoid (c), a median 

 triangular fairly thick bone which is bounded laterally by the ali- 

 sphenoids and posteriorly by the basioccipital. It is perforated in 

 the mid-ventral line by a small hole, the pituitary foramen. On 

 the inner floor of the cranial cavity it is produced upwards into a 

 transverse post-clinoid process, and between this and the anterior 

 process in a depression, the sella turcica, within which lies the pituitary 

 body. Just at the junction of the basi and alisphenoid is attached 

 a small, thin, vertical bone, the pterygoid, which is joined to the 

 palatine in front and passes downwards in a free hamular process. 



The auditory capsule is composed of a series of bones which in 

 the adults fall into two portions, the periotic and the tympanic. The 

 periotic is an irregular mass comprising developmentally three separate 



