52 OSTEOLOGY. 



SPHENOID. 



(Fig. 13. 8.) 



This is a single bone situated in the middle and anterior parts 

 of the base of the cranium, between the occipital and ethmoid 

 bones, and tranversely between the two frontal and temporal 

 bones. Its shape is generally likened to that of a bird in flight, 

 with its legs and wings extended. In the young state this bone 

 separates readily into two parts, an anterior and a posterior. 

 Anatomically, it may be divided into two centre-pieces, and two 

 pairs of flattened processes or wings. The posterior centre-piece is 

 then termed the basi-sphenoid, articulating behind with the basi- 

 occipital, and before with the other centre, the pre-sphenoid, the 

 two constituting the body of the bone. The expanded parts, 

 collectively termed the wings, are formed of two plates from the 

 posterior centre called the ali-sphenoid, and two considerably 

 larger ones from the anterior centre, the orbito-sphenoid bones. 

 Although for descriptive purposes it may be sufficient to divide 

 the bone into a body and two wings, still it is well to bear in 

 mind the above divisions and names. 



The body is semi-cylindroid in form, convex inferiorly, where 

 it is roughened for muscular attachments, and on either side is 

 the long pterygoid process, continuous with the posterior wing. 

 On each side of the body, and winding round to the anterior part 

 of the pterygoid process, is a small groove, the Vidian fissure, 

 which the palatine bone converts anteriorly into the small Vidian 

 canal, through which the nerve of that name passes to join the 

 spheno-palatine ganglion. Above this fissure, and passing through 

 the base of the pterygoid process, is the large pterygoid foramen, 

 which bifurcates anteriorly, the larger and inferior branch giving 

 passage to the internal maxillary, while the smaller, which is 

 directed upwards, lodges the anterior deep temporal artery ; 

 and in front of the process is a large irregular opening, the orbital 

 hiatus, into which various foramina open. Still more anteriorly 

 is the optic foramen, and at the anterior border of the body, a 

 notch, which, with the frontal bone, forms the internal orbital 

 foramen, giving passage to branches of the ophthalmic artery and 

 nerve. 



The internal or superior surface is smooth, and presents, in 

 the posterior segment of the body, a shallow fossa, the pituitary 



