THE INTERNAL BASIS OF THE CRANIUM. 127 



termination of the foramen caroticum, with the impressions 

 made by the carotid arteries on the sides of the sella turcica. 

 The petrous portions of the temporal bones are the posterior 

 boundaries of the middle fossae. Their oblique direction, 

 inwards and forwards, is particularly remarkable; being formed 

 like triangular pyramids. Two of their sides are in the cavity 

 of the cranium ; one, which is anterior, forms a portion of the 

 middle fossa ; and the other forms a part of the posterior fossa. 

 The edge between them is very prominent, and has the tento- 

 rium or horizontal process of the dura mater attached to it. On 

 the anterior surface, in the middle fossa, may be traced the 

 groove, and the foramen for the Vidian nerve. 



The posterior fossae are larger as well as deeper than the 

 other two. Their boundaries are well defined by the edges of 

 the petrous bones above mentioned, and by the grooves of the 

 horizontal parts of the lateral sinuses. These fossae are nearly 

 separated from the general cavity by the tentorium, which is 

 attached to the edge of the petrous bone and also to the edge 

 of the horizontal part of the groove for the lateral sinuses. On 

 the tentorium lie the posterior lobes of the cerebrum ; and under 

 it, in these fossae, is the cerebellum. 



These fossae may be considered as one great cavity, which is 

 circular behind, and somewhat angular before. The angular 

 surfaces are formed by the posterior sides of the petrous portions. 

 Between them, is the oblique surface of the cuneiform process of 

 the occipital bone, which descend to the great foramen. On the 

 surface of each petrous bone is the meatus auditorius internus, 

 and the orifice of the aqueduct of the vestibule. Behind the 

 petrous" portion, the groove for the lateral sinus is very conspicu- 

 ous ; it terminates in the posterior foramen lacerum, which is 

 evidently formed by the temporal and the occipital bones. At 



the temporal bone. 12. The petrous portion of the temporal. 13. The sella 

 Turcica. 14. The basilar portion of the sphenoid bone, surmounted by the pos- 

 terior clinoid processes. 15. The foramen rotundum. 16. 'the foramen ovale. 

 17. The foramen spinosum ; the small irregular opening between 17 and 12 is 

 the hiatus Fallopii. 18. The posterior fossa of the base of the skull. 19, 19. 

 The groove for the lateral sinus. 20. The ridge upon the occipital bone, which 

 gives attachment to the falx cerebelli. 21. The foramen magnum. 22. The 

 meatus auditorius internus, 23. The jugular foramen. 



