38 A MANUAL OF ANATOMY. 



Now study the parts at the base of the skull. Fig. 10. 



The base of the skull, internally, is divided into three 

 fossae the anterior, middle, and posterior. 



The anterior fossa has for its floor the orbital plate of 

 the frontal, the cribriform plate of the ethmoid, and the les- 

 ser wings of the sphenoid bones. 



The posterior margin of the last and the optic groove 

 bounds the fossa posteriorly. 



The Middle Fossa. This is the central hollow, and its 

 floor is composed of the greater wings and body of the 

 sphenoid and the front surfaces of the petrous portions of 

 the temporal bones. The fossa is limited behind by the 

 superior margin of the petrous portions of the temporal 

 bones and the dorsum ephippii. Anteriorly, by the lesser 

 wings of the sphenoid and optic groove. 



The posterior fossa of the skull is all that portion of the 

 base internally which extends behind the superior margins 

 of the petrosal bones and the dorsum ephippii. 



The contents of these various fossae will be apparent as 

 the dissections of the floor of the cranial cavity proceeds. 



The Tentorium Cerebelli. Fig. 9. 



This is a portion of the dura which lies between the 

 cerebrum and the cerebellum. It is attached externally to 

 the dura (and so to the bone) along the line of the lateral 

 sinuses (which it forms by the separation of its layers). 

 Internally it is attached to the superior border of the pe- 

 trous portion of the temporal bone (enclosing the superior 

 petrosal sinuses at this point), and to the posterior and 

 anterior clinoidal processes. 



An opening is left between the petrosal attachments 

 through which the crura cerebri, basilar artery, and third 

 and fourth nerves pass. 



