486 MAMMALIAN ANATOMY 



over the middle of the antero-posterior length of the posterior fossa 

 (Fig. 392). The middle fossa appears to slope in the direction oppo- 

 site to that assumed by the anterior fossa, or upward and backward. 

 It is denned in front by the posterior boundaries of the middle fossa, 

 and behind by the tentorium. 



The floor (Fig. 390) is short, but extends across the skull and 

 turns sharply up at the sides into the lateral walls. It is formed by 

 the basisphenoid, the alisphenoids, and the squamous of the temporals. 

 At its posterior part in the middle line is the dorsum sellae, in front of 

 which is the sella turcica, or pituitary fossa, bounded in front by the 

 elevated middle part of the anterior fossa. On each side of the sella 

 turcica is an oval flattened area which leads forward and outward into 

 the sphenoidal fissure, the foramen rotundum, and the foramen ovale. 

 The lateral fourths of the floor are on a slightly higher plane than the 

 middle portion, overhanging it and the foramina by sharp edges con- 

 tinued upward and backward as the edges of the large central emargi- 

 nation in the tentorium. These elevated portions are concave from 

 before backward and transversely, and are continuous behind with the 

 posterior or tentorial wall. Each exhibits in the line of the squamo- 

 sphenoidal suture a longitudinal ridge, which separates two longitudinal 

 impressions for cerebral convolutions. Each exhibits also a transverse 

 suture between the alisphenoidal and parietal parts of the tentorium. 



The concave lateral walls (Figs. 391, 394) are short below, long 

 above, and arch upward to form the roof. They are composed of the 

 upper parts of the squamous of the temporals for a short distance 

 below, but principally of the extensive parietals. The lateral walls 

 and the roof exhibit impressions for the cerebral convolutions. An 

 arborescent line or groove for the middle meningeal artery is seen 

 on the lower posterior part of each lateral wall. The falx cerebri is 

 attached along the middle line of the roof. 



The posterior wall, or tentorium, slopes backward and upward ; it 

 is strongly convex from side to side and from above downward. In 

 its lower part is a large median opening which converts the posterior 

 wall of the fossa into little more than a long shelf, and permits the 

 union of the cerebral and cerebellar portions of the brain. 



The Posterior Fossa (Figs. 390, 391, 392) is much smaller than 

 the middle fossa. Its antero-posterior length is shorter above and 

 below than in the middle. 



