BY W. J. McKAY. 901 



Os Parasphenoideum. 



Parasphenoid, Huxley, Hoffmann, Parker, Parker and Bettany ; 

 Presphenoid, Owen. 



The basisphenoid and parasphenoid when detached from the 

 skull together make up a triangular-shaped bone, the apex of 

 which is anterior. 



The anterior portion of the inferior surface constituted by the 

 parasphenoid is deeply excavated, differing much from the corres- 

 ponding surface in Python, in which there is a very prominent 

 ridge, giving attachment to the dense fascia of the roof of the 

 mouth. On each side of the anterior portion the spheno-vomerine 

 muscles are placed. The unossified trabeculse can be seen running 

 forward from a point just below the optic foramen in a small 

 groove on either side of the bone and just beneath the inferior 

 portions of the frontals. The trabeculse when traced forward are 

 seen to " unite underneath the fore part of the frontals and become 

 compressed into a vertical ethmoidal plate passing on into the 

 nasal septum " (Parker). The posterior portion of the inferior 

 surface is convex. A small ridge exists in the mid-line which 

 gives attachment to the strong fascia of the region. On either 

 side of the ridge is an excavated surface from which the spheno- 

 pterygoid muscle arises. In Python this portion of the bone is 

 very different. There is a very prominent median ridge, and on 

 either side of the ridge is a large wing-like process which corres- 

 ponds to the basipterygoid process of Lacertilians. A similar 

 process occurs in Pseudechis. It gives origin to the spheno- 

 pterygoid muscle. The superior surface is convex in front, but 

 deeply excavated posteriorly to form a hollow " which contains the 

 pituitary body, a quantity of fibrous tissue, and the internal 

 carotid arteries which pass into it laterally beneath the parietal 

 shelf having previously perforated the basisphenoid" (Parker). 

 " There is a posterior clinoid wall, arching over the hinder part of 

 the pituitary body " (Parker). Posterior to this pituitary fossa 

 the bone is concave to receive the mid-brain. "The anterior 



