u S THE SKULL [CHAP. 



When the skull remains in connection with the vertebral 

 column, it will be seen that its axis is a continuation for- 

 wards of the axis of that column, consisting of the bodies of 

 the vertebrae ; and that its hinder termination is placed in 

 the same line with the odontoid process of the second cer- 

 vical vertebra, the anterior termination of the axis of the 

 spinal column. 



The large cavity above the axis of the skull (cerebral 

 cavity) is in direct continuity with the spinal canal above 

 the axis of the vertebral column. 



Beginning at the posterior end of the axis, the section 

 will be seen to have passed through a flat, elongated bone 

 (Fig. 46, BO\ terminating freely behind at the inferior 

 margin of the great opening (foramen magnum] at the hinder 

 end of the cerebral cavity, by which this cavity is continued 

 into the vertebral canal, and through which the backward 

 prolongation of the brain (the medulla spinalis) passes. This 

 bone is the basioccipital. 



Immediately in front of this is a bone (the basisphenoid, 

 BS] not quite so elongated from before backwards, but of 

 greater vertical depth ; the interior being more or less 

 cellular in structure. The under surface is flat, but the 

 upper surface is hollowed in the middle, and raised at each 

 extremity. This hollow corresponds to the part called 

 " sella turcica " in the human skull, and lodges the pituitary 

 body of the brain. 



Further forwards, and likewise separated by a vertical 

 fissure, is a bone (PS) of about the same length as the last, 

 but still more elevated, and very cellular within. Its in- 

 ferior contour is perfectly straight, but above it is somewhat 

 irregular. This is the presphenoid. 



So far the cranio-facial axis consists of bones placed in 

 a continuous line, more or less depressed, and broad from 



