SPHENOID BONE. 71 



bone, are the two pterygoid or Vidian canals, which give passage to the 

 Yidian nerve and artery at each side. 



The posterior surface is flat and rough, and articulates with the basilar 

 process of the occipital bone. In the adult this union is usually completed 

 by bone; from which circumstance the sphenoid, in conjunction with the 

 occipital, is described by Soemmering and Meckel as a single bone, under 

 the name of spheno-occipital. The posterior surface is continuous on 

 each side with the spinous process, and at the angle of union is the termi- 

 nation of the Vidian canal. 



The lesser wings (processes of Ingrassias) are thin and triangular, the 

 base being attached to the upper and anterior part of the body of the sphe- 

 noid, and the apex extended outwards, and terminating in an acute point. 

 The anterior border is irregularly serrated, the posterior being free and 

 rounded, and received into the fissure of Sylvius of the cerebrum. The 

 inner extremity of this border is the anterior clinoid process, which is sup.- 

 ported by a short pillar of bone, giving attachment to a part of the com- 

 mon tendon of the muscles of the orbit. The lesser wing forms the pos- 

 terior part of the roof of the orbit, and its base is traversed by the optic 

 foramen. 



The greater wings present three surfaces ; a superior or cerebral, which 

 forms part of the middle fossa of the base of the skull, an anterior surface 

 which assists in forming the outer wall of the orbit, and an external sur- 

 face divided into two parts by the pterygoid ridge. The superior part of 

 the external surface enters into the formation of the temporal fossa, and the 

 inferior portion forms part of the zygomatic fossa. The pterygoid ridge, 

 dividing the two, gives attachment to the upper origin of the pterygoideus 

 externus muscle. 



The spinous processes project backwards at each side from the base of 

 the greater wings of the sphenoid, and are received into the angular inter- 

 vals between the squamous and petrous portions of the temporal bones. 

 Piercing the base of each process is a large oval opening, the foramen 

 ovale; nearer its apex a smaller opening, the foramen spinosum; and 

 extending downwards from the apex a short spine, which gives attachment 

 to the internal lateral ligament of the lower jaw and to the laxator tym- 

 pani muscle. The external border of the spinous process is rough, to 

 articulate with the lower border of the squamous portion of the temporal 

 bone ; the internal forms the anterior boundary of the foramen lacerum 

 basis cranii, and is somewhat grooved for the reception of the Eustachian 

 tube. 



The pterygoid processes descend perpendicularly from the base of the 

 greater wings, and form in the articulated skull the lateral boundaries of 

 the posterior nares. Each process consists of an external and internal 

 plate, and an anterior surface. The external plate is broad and thin, 

 giving attachment, by its external surface, to the external pterygoid 

 muscle, and by its internal surface to the internal pterygoid. This plate 

 is sometimes pierced by a foramen, which is not unfrequently formed by 

 a process of communication passing between it and the spinous process. 

 The internal pterygoid plate is long and narrow, and terminated at its 

 extremity by a curved hook, the hamular process, around which plays the 

 tendon of the tensor palati muscle. At the base of the internal pterygoid 

 plate is a small oblong depression, the scaphoid fossa, from which arises 

 the circumflexus or tensor palati muscle. The interval between the two 



