LEPUS 137 



all round the nerve or blood-vessel, and the foramen then 

 pierces that bone. The most important foramina are those in 

 the orbit and hinder region of the skull, and they are most 

 conveniently studied in the skull of a young dog, in which the 

 sutures between the bones are still plainly visible. 



The optic nerve (II) emerges through the optic foramen in 

 the orbitosphenoid bone. Immediately behind this is the 

 foramen lacerum anterius between the orbito-sphenoid and 

 the alisphenoid. Through it pass the oculomotor (III), 

 trochlear (IV), abducens (VI), and the ophthalmic branch of 

 the trigeminal nerve (Vi). The maxillary branch of the tri- 

 geminal (V2) emerges through the foramen rotundum in the 

 alisphenoid, while the mandibular branch of the same nerve 

 (V3) passes through the foramen ovale between the alisphenoid 

 and the periotic. The Eustachian tube passes into the tympanic 

 bulla through an opening between the latter and the basi- 

 sphenoid. The foramen just median to this is the lacerum 

 medius (likewise between the tympanic bulla and the basi- 

 sphenoid), through which the internal carotid artery pierces 

 the skull. The so-called external carotid artery which is 

 given off from the internal carotid before the latter enters the 

 skull, runs forwards through the alisphenoid canal. This is 

 not a true foramen for it does not lead into the skull, but is a 

 short tunnel in the alisphenoid. Its hind entrance is below the 

 foramen ovale, and its anterior exit is confluent with the 

 foramen rotundum. The facial nerve (VII) emerges through 

 the stylo-mastoid foramen, between the hind face of the 

 tympanic bulla and the periotic. Between the periotic and 

 the exoccipital is a large elongated opening, the foramen 

 lacerum posterius. Through this, pass the glossopharyngeal 

 (IX), the vagus (X), and the spinal accessory (XI) nerves, and 

 the internal jugular vein. The hypoglossal nerve (XII) 

 passes through the condylar foramen in the exoccipital. The 

 large hole at the back of the skull for the spinal cord is the 

 foramen magnum. 



The lower jaw consists of a single bone : the dentary, 

 which articulates with the skull by means of the squamosal. 

 This method of articulation is characteristic of mammals, and 



