122 THE SKULL [CHAP. 



tentorium cerebelli), is called the cercbellar fossa, as it 

 lodges the division of the brain named "cerebellum." The 

 most anterior and smallest compartment is marked off by a. 

 vertical ridge on the orbitosphenoid and the frontal. Its 

 walls are chiefly formed by the cribriform plate. This is the 

 olfactory fossa (r/iinencephalic fossa, Owen), for the lodgment 

 of the olfactory lobe. Between these two is the great cere- 

 bral fossa, in which the hemisphere of the cerebrum lies. 

 This is very imperfectly divided below into two compart- 

 ments, by a slight ridge at the hinder edge of the orbito- 

 sphenoid and continued thence outwards at the junction of 

 the frontal and alisphenoid. This ridge corresponds with 

 the Sylvian fissure of the brain ; the part of the cerebral 

 fossa in front of it lodges the frontal lobe of the cerebrum, 

 that behind it the temporal lobe. 



Through the lateral parts of the floor of the cranial cavity 

 are various perforations, or foramina, either holes passing 

 directly through the bones, or vacuities occasioned by want 

 of contact, for a limited space, of contiguous bones. These 

 are mainly for the purpose of allowing of the exit of the 

 various nerves which take origin from the brain ; and as 

 they are extremely constant in their position, and offer useful 

 landmarks for determining the homologies of the bones 

 throughout the vertebrate series, it is important that they 

 should be well known. (See diagram at p. 126.) 



1. The most anterior is the space, before spoken of, in 

 front of the anterior segment, occupied by the hinder part 

 of the ethmo-turbinal, commonly called the "cribriform 

 plate." The numerous perforations in this plate transmit 

 the olfactory nerves arising from the olfactory lobes. 



2. Near the hinder border of the orbitosphenoid is a con- 

 spicuous, nearly round hole, through which the optic nerve 

 passes, and hence called optic foramen. 



