488 MAMMALIA. 



f rentals of the lower types. The lacrymal bone (absent in the 

 Pinnipedia and some Cetacea) is placed in the anterior wall of 

 the orbit, and often also appears as a facial bone on the face 

 (Fig. 259, La). 



The most important foramina in the cranial wall are, typically, 

 as follows. (1) 1'he perforations in the cribriform plate which 

 transmit the fibres of the olfactory nerve. (2) The optic foramen 

 in the orbitosphenoid. (3) The foramen lacerum anterius 

 (sphenoidal fissure), a space between the orbitosphenoid and 

 alisphenoid, which transmits the 3rd, 4th, 6th cranial nerves 

 and the anterior division of the 5th. (4) The foramen rotundum 

 and (5) the foramen ovale, both perforations of the alisphenoid 

 which transmit respectively the second and third divisions of 

 the 5th nerve. (6) The foramen lacerum medium between the 

 alisphenoid and the periotic (just internal to the opening of the 

 bony eustachian passage) for the passage of the internal carotid 

 artery. (7) On the inner surface of the periotic is the opening 

 of the meatus auditorius internus which transmits the auditory 

 and facial nerves, the former to terminate in the walls of the 

 membranous labyrinth, the latter to traverse the bone and 

 emerge by (8) the stylomastoid foramen, which is placed imme- 

 diately behind the bull a and just anterior to the paroccipital pro- 

 cess. (9) The foramen lacerum posterius between the periotic 

 and exoccipital for the 9th 10th and llth nerves and the internal 

 jugular vein. (10) A foramen in the exoccipital just in front of 

 the condyle, called the condylar foramen, for the transmission 

 of the 12th nerve. (11) The foramen magnum by which the 

 spinal cord enters the skull. 



The alisphenoid canal, present in some mammals, is a hori- 

 zontal canal in the alisphenoid at the root of the pterygoid 

 process, through which the external carotid passes in part of 

 its course ; it begins behind just in front of the foramen ovale 

 and ends in front at the foramen rotundum. 



The brain so completely fills the cranial capsule in the Mam- 

 malia that the internal surface of the skull presents a relatively 

 accurate impression of its surface. Owing to the considerable 

 size of the brain the cranial capsule is far more spacious than in 

 any other class of Vertebrata ; but it presents great variations 

 in this respect in the different groups, being smallest in some 

 of the extinct orders. 



