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, 
