CRANIUM. 43 



mentary crest, to support the membrane which divides the 

 compartment into two lateral sections, one for each hemisphere 

 of the cerebrum. Folds of membrane attached to the protuber- 

 ance, and lateral ridges, also render the transverse partitioning 

 of the cavity more perfect. 



The Lateral surfaces are concave, and. like the roof, marked 

 with numerous smooth grooves and impressions. They assist m 

 the division into cerebral and cerebellar compartments by means 

 of the parieto-temporal ridges, which are prolonged obliquely as 

 far as the sphenoid bone, the first portion being formed by the 

 occipital and internal face of the petrosal, and the second by the 

 squamosal bone. 



The Floor or inferior surface is very irregular, and presents, 

 from behind forwards, in the median line, the bones which 



Fig. 13. 

 Longitudinal section of a Horse's skuU. 1, Supraoccipital bone and 

 crest ; 2, Parietal bone : 3, Frontal boue ; 4, Frontal sinuc: 5, Nasal bone ; 

 6, Superior turbinal : 12. Inferior turbinal : 14. Superior maxiUa, and 13. 

 its palatine plate : 15. Premaiilla : 11. Palatine bone : 7. Ethmoid bone and 

 volutes : 8. Basi and pre-sphenoid bones : 9. Basi-occipital bone : 10. 

 Petrosal bone. The craniii:ai is the cavity under 1 and 2. 



represent the three posterior centra of the cranial vertebrae — viz.. 

 the basi-occipital, basi-sphenoid. and pre-sphenoid bones ; and 

 laterally we find the foramina lacera basis cranii, the remaining 

 portion of the floor being formed by the ali-sphenoid and orbito- 

 sphenoid bones the former presenting on each side of the centrum 

 two grooves running forwards, and terminating in foramina. On 

 the pre-sphenoid is a transverse slit, the cptlc hiatus, and the 

 raised ledge posterior to it is the olivary process, behind which 

 we find the sella turcica, a depression in the basi-sphenoid bone. 

 Laterally, the bony floor coalesces with the walls so as to form 

 deep ovoid cavities, which receive the mastoid or great lobes of 

 the cerebrum. 



The Posterior end contains the foramen magnum, by means of 

 which the cavity communicates with the spinal canal. 



