184 



THE SKELETON. 



the Tensor palati, and above which is seen the posterior orifice of the Vidian 

 canal. Below and to the inner side of the Vidian canal, on the posterior surface 

 of the base of this plate, is a little prominence, which is known by the name of 

 the pterygoid tubercle. 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. The Superior constrictor of the pharynx is attached to its posterior 

 edge. The two pterygoid plates are separated below by an angular interval, in 

 which the pterygoid process, or tuberosity, of the palate bone is received. 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 

 presents, 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 one 

 on each side for the lingula. The six for 

 the anterior sphenoid are one for each 

 lesser wing, two for the anterior part of 

 the body, and one for each sphenoidal 

 turbinated bone. 



Post-sphenoid Division. The first 

 nuclei to appear are those for the greater 

 wings. They make their appearance 

 between the foramen rotundum and fora- 

 men ovale about the eighth week, and 

 from them the external pterygoid plates 

 are also formed. Soon after, the nuclei 

 for the posterior part of the body appear, 

 one on either side of the sella turcica, and 

 become blended together about the middle of foetal life. About the fourth month 

 the remaining four centres appear, those for the internal pterygoid plates being 

 ossified in membrane and becoming joined to the external pterygoid plate about 

 the sixth month. The centres for the lingulse speedily become joined to the rest 

 of the bone. 



Pre-sphenoid Division. The first nuclei to appear are those for the lesser 

 wings. They make their appearance about the ninth week, at the outer borders 

 of the optic foramina. A second pair of nuclei appear on the inner side of the 



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. 



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 Zesser* icrintf jtctrf of J>ocLy 



FIG. 146. Plan of the development of sphenoid. 

 By fourteen centres. 



