220 AN INTRODUCTION TO ZOOLOGY 



ridges respectively. The former continues into the nasal capsule 

 in front, and the auditory capsule behind. These capsules, together 

 with the ridges, constitute an efficient orbit within which lies the 

 eyeball attached to the lateral cranial wall by the optic nerve and a 

 series of muscles. 



The anterior end of the orbit is also in part formed by a pre- 

 orbital or lateral ethmoidal process continuous with the hinder wall 

 of the olfactory capsule. This with the remaining part of the skull 

 in front of it, composed almost entirely of the nasal capsules, is 

 known as the ethmoidal region, and into it the cranial cavity does 

 not enter. The olfactory capsules are thin-walled hollow spherical 

 structures, each with a large opening on the ventral side. They are 

 completely separated from one another by an extension of the cranial 

 floor known as the mesethmoidal plate or internasal septum. This, 

 again, is prolonged as a slender rod, the rostral cartilage, which 

 together with two similar cartilaginous bars arising from the antero- 

 dorsal walls of the capsules constitutes the rostrum or skeletal 

 support of the snout. 



Let us turn now to consider the perforations in the side walls 

 of the cranium. At the antero-ventral corner of the orbit, which is 

 occupied by a large blood sinus, is the orbito-nasal foramen, placing 

 the orbital sinus in communication with the nasal sinus. Midway 

 along the orbit in its lower portion is the conspicuous optic foramen, 

 through which the optic nerve passes. In the postero-ventral 

 corner is another large perforation for the exit of the main branches 

 of the fifth and seventh and the entire sixth cranial nerves. Between 

 this and the optic foramen are three smaller holes for blood-vessels. 

 The most anterior of these is for the hyoidean artery, the posterior 

 for the internal carotid artery, and the middle one, slightly more 

 dorsal than the others, is the interorbital canal, which puts the two 

 orbital sinuses in communication. The remaining foramina in the 

 orbit are nearer the dorsal side of the cranium. In the postero- 

 dorsal corner is a foramen for the ophthalmic branch of the seventh 

 cranial nerve which runs forward in a shallow groove. Just below, 

 and in front of this, is a similar aperture for the ophthalmic branch 

 of the fifth cranial nerve, from which also a groove passes forward 

 soon uniting with the former. Running forward to the front end 

 of the orbit, the common groove passes out on to the dorsal side of 

 the supra-orbital ridge by a well-marked hole. Antero-ventrally 

 of the foramen for the fifth ophthalmic nerve lies that of the third 

 cranial nerve, and just dorsal to and in front of this, again, is another 

 for the fourth nerve. From the posterior border of the orbit below 

 the ridge of the horizontal semicircular canal runs a well-marked 

 furrow in the wall of the optic capsule, the post-orbital groove, in 



