172 ANATOMY OF THE RABBIT 



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, which is ventral and transmits 

 the internal carotid artery into the cranial cavity, the foramen 

 ovale of the typical mammalian skull, which is the antero-dorsal 

 part and serves to transmit the mandibular portion of the fifth 

 nerve. 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 (foramen caroticum internum). It is the anterior end 

 of a canal transmitting the internal carotid artery prior to the 

 entry of that vessel to the cranial cavity through the foramen 

 lacerum; the posterior end of this canal, the point at which the 

 internal carotid artery enters the tympanic bone, or the external 

 carotid foramen (foramen caroticum externum), being visible as 

 a rounded aperture lying on the posteromedial surface of the 

 tympanic bulla. The second, lateral aperture communicating 

 with the foramen lacerum is that of the auditory (Eustachian) 

 tube (tuba auditiva). It leads into the tympanic cavity, and in the 

 natural condition the tube places this in communication with the 

 nasal portion of the pharynx. Associated with the mastoid process 

 is a small a perture, the stylomastoid foramen (foramen stylomastoi- 

 deum), the external opening of the facial canal, which serves for the 

 passage of the facial nerve. This foramen is named from the fact 

 that in the human skull it is bounded in front by the styloid process, 

 which is absent in the rabbit, and behind by the mastoid process. 

 A slit-like aperture , the jugular foramen (foramen j ugulare) , lies in the 

 jugular fossa, between the posterior ventral margin of the tympanic 

 bulla and the occipital condyle. It transmits the first portion of 

 the internal jugular vein from the transverse sinus of the dura 

 mater, and also the ninth, tenth, and eleventh cranial nerves. 

 Finally, immediately in front of the dorsal portion of the condyle, 

 the occipital segment is perforated by several small apertures 

 together representing the hypoglossal canal (canalis hypoglossi), 

 and serving for the transmission of the hypoglossal nerve. 



The roof of the cranial portion is largely formed by two pairs 

 of thin membrane elements, the frontal and parietal bones. The 



