294 AN INTRODUCTION TO ZOOLOGY 



save at the hinder end where they are separated by the interparietal 

 portion of the supra-occipital. 



Encasing the anterior end of the brain is the frontal segment 

 whose median, basal cartilage bone is the pre-sphenoid. It is a narrow 

 bone flat ventrally, but of irregular outline within the cranium ; 

 inside it is cancellous. Internally it just touches the lower border 

 of the optic foramen. To the side of the pre-sphenoids are attached 

 the thin wing-like orbito-sphenoids, also cartilage bones. They 

 form part of the inner wall of the orbit and contribute the greater 

 part of the margin of the optic foramen. At their posterior corner 

 they also participate in forming the front border of the foramen 

 lacerum anterius. Dorsally the cranial roof is composed of the 

 large membrane bones, the frontals, which not only meet in the 

 middle line continuing the sagittal suture, but also pass forwards 

 and ventrally for some distance. Dorso-laterally the frontal 

 bone gives off a projection, the post-orbital process, extremely well 

 developed in some other mammals, which marks the hinder limit 

 of the orbit. When the skull is viewed from above a large arch, 

 the jugal or malar arch is seen to sweep out on each side of the 

 cranium. The space within this arch may be divided into two, the 

 so-called orbital fossa,lodging the eye in front, and the temporal fossa, 

 which is filled in life by the great temporal muscle whose origin 

 on the sagittal crest has already been noted. 



Finally, the space at the front end of the cranium between the 

 pre-sphenoid and the frontals is closed in by a median vertical 

 plate of spongy cartilage bone, the ethnoid. Lateral expansions of this 

 form the cribriform plates, which are vertical bones perforated by 

 a large number of holes for the passage of numerous olfactory 

 nerves. The bone itself is continued forward in the middle line 

 as a thin vertical lamina, the mesethmoidal plate or septum narium, 

 part of which always remains cartilaginous, and this separates the 

 two olfactory capsules. 



The front part of the skull, occupying less than half its 

 length, consists in the main of the olfactory capsules and the bones 

 connected with it. The capsule consists of an upper space, the 

 olfactory chamber, and a lower tube-like portion, the narial passage. 

 The roof of these capsules is made partly of prolongations of the 

 frontal bones, the nasal processes, but mainly of two long narrow 

 membrane bones, the nasals, whose front ends form the dorsal 

 limit of the anterior nares. Their floor in the mid- ventral line is 

 composed of a small elongated membrane bone, the vomer, 

 representing a pair of bones fused. The remaining bones 

 participating in the formation of the nasal capsule, namely, the 

 palatines, maxillae and pre-maxillae, are better dealt with in 



