76 



THE SKELETON. 



of the pterygoid fossa, and gives attachment to the Internal pterygoid. The 

 internal pterygoid plate is much narrower and longer, curving outward, at its 

 extremity, into a hook-like process of bone, the hamular process, around which 

 turns the tendon of the Tensor palati muscle. The outer surface of this plate 

 forms part of the pterygoid fossa, the inner surface forming the outer boundary 

 of the posterior aperture of the nares. On the posterior surface of the base of 

 the process, above the pterygoid fossa, is a small, oval, shallow depression, the 

 scaphoid fossa, from which arises the Tensor palati, and above which is seen the 



Fia. 39. Sphenoid bone. Posterior surface. 



posterior orifice of theVidian canal. Below and to the inner side of theVidian 

 canal, on the posterior surface of the base of the internal plate, is a little prom- 

 inence, which is known by the name of the pterygoid tubercle. The Superior 

 constrictor of the pharynx is attached to the posterior edge of the internal 

 plate. The anterior surface of the pterygoid process is very broad at its base, 

 and forms the posterior wall of the spheno-maxillary fossa. It supports Meckel's 

 ganglion. It preser.ts, above, the anterior orifice of the Vidian canal ; and below, 

 a rough margin, which articulates with the perpendicular plate of the palate 

 bone. 



The Sphenoidal Spongy Bones are two thin, curved plates of bones, which exist 

 as separate pieces until puberty, and occasionally are not joined to the sphenoid 

 in the adult. They are situated at the anterior and inferior part of the body of 

 the sphenoid, an aperture of variable size being left in their anterior wall, through 

 which the sphenoidal sinuses open into the nasal fossae. They are irregular in 

 form and taper to a point behind, being broader and thinner in front. Their 

 upper surface, which looks toward the cavity of the sinus, is concave ; their under 

 surface convex. Each bone articulates in front with the ethmoid, externally with 

 the palate ; its pointed posterior extremity is placed above the vomer, and is 

 received between the root of the pterygoid process on the outer side and the 

 rostrum of the sphenoid on the inner. 1 



Development. Up to about the eighth month of foetal life the sphenoid bone 

 consists of two distinct parts : posterior or post-sphenoid part, which comprises 

 the pituitary fossa, the greater wings, and the pterygoid processes ; and an 

 anterior or pre-sphenoid part, to which the anterior part of the body and lesser 

 wings belong. It is developed by fourteen centres : eight for the posterior 

 sphenoid division, and six for the anterior sphenoid. The eight centres for the 

 posterior sphenoid are one for each greater wing and external pterygoid plate, 

 one for each internal pterygoid plate, two for the posterior part of the body, and 



1 A small portion of the sphenoidal turbinated bone sometimes enters into the formation of the 

 inner wall of the orbit, between the os planum of the ethmoid in front, the orbital plate of the palate 

 below, and the frontal above Cleland, Roy. Soc. Trans., 1862. 



