I02 THE SKULL [chap, 



tentorium cerehelli), is called the cerebellar fossa, as it 

 lodges that division of the brain. 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 

 {rhinencephalic fossa, Owen), for the lodgment of the olfac- 

 tory lobe. Between these two is the great cerebral fossa, in 

 which the hemisphere of the cerebrum lies. This is very 

 imperfectly divided below into two compartments, by a slight 

 ridge at the hinder edge of the orbitosphenoid and con- 

 tinued 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. 106.) 



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

 front of the anterior segment, occupied by the hinder part 

 of the ethmoturbinal, 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. 



