132 Anatomy of the Rabbit. 



bone, where it is readily distinguishable by its pitted appearance. 

 Its ventral portion bears a slender projection, lying parallel to the 

 jugular process, the mastoid process (processus mastoideus). 



A series of foramina, lying partly within the orbit, and extend- 

 ing thence posteriorly along the boundary between the lateral and 

 ventral walls to the occiput, put the cranial cavity in communica- 

 tion with the outside, and serve for the passage of nerves and vessels. 

 The first and largest of these, the optic foramen (foramen opti- 

 cum), occupies the middle portion of the orbit, and transmits, in the 

 natural condition, the optic nerve. Following this, is a^ vertical 

 slit-like aperture — -not to be confused with the perforations of the 

 external lamina of the pterygoid process — -the superior orbital 

 fissure (fissura orbitalis superior). It represents both the superior 

 orbital fissure of the normal mammalian skull and the foramen 

 rotundum, and provides for the passage outward of the third, 

 fourth and sixth cranial nerves, together with the first and second 

 divisions of the fifth. The lateral lamina of the pterygoid process 

 presents three foramina, of which the largest, anterior, and medial 

 one, the anterior sphenoidal foramen (alar canal), serves for the 

 transmission of the internal maxillary artery, while the remaining 

 two, the middle and posterior sphenoidal foramina, transmit 

 muscular branches (masseterico-temporal and pterygobuccinnator) 

 of the mandibular nerve. On the medial side of the base of the 

 medial lamina of the pterygoid process there is a shallow longitu- 

 dinal groove, representing the pterygoid canal (canalis ptery- 

 goideus) of the human skull. Immediately in front of the tympanic 

 bulla, on the ventral surface of the skull, an irregular aperture, 

 the foramen lacerum, leads directly into the cranial cavity. It is 

 incompletely divided into two parts by a slender bony splint. It 

 contains, in addition to the foramen lacerum, the foramen ovale 

 of the typical mammalian skull, and serves to transmit the mandib- 

 ular portion of the fifth nerve and the internal carotid artery. 

 Looking into the aperture from the front, it is seen to communicate 

 not only with the cranial cavity, but also with two apertures in the 

 anterior portion of the auditory complex. One of these — that 

 toward the middle line — is the internal carotid foramen (fora- 

 men caroticum internum). It is the anterior end of a canal trans- 

 mitting the internal carotid' artery; the posterior end of this canal, 



