248 MAMMALIAN ANATOMY 



the hiatus Fallopii, and is known as the small superficial petrosal nerve. 

 In the cat the tympanic branch passes from the jugular foramen, dorsal 

 to the entotympanic part of the bulla and the bullar septum, to gain 

 the tympanum, and the petrosal nerve escapes at the foramen between 

 the anterior border of the petrous and the alisphenoid where the Vidian 



nerve enters. 



THE SPHENOID BONE. 



General Description. The Sphenoid is a single and sym- 

 metrical bone, situated at the anterior end of the base of the cranium 

 (Fig. 180). It articulates with all the cranial bones except the inter- 

 parietal, and with three bones of the face, namely, the vomer and the 

 two palatines. It is visible externally at the back of the orbits and 

 at the lower part of the temporal and zygomatic fossse. It forms 

 the middle of the roof of the posterior nares, gives attachment to 

 the posterior wall of the pharynx, and assists in closing the nasal 

 chambers behind. On the inner surface of the skull it enters into 

 the formation of the anterior and middle cranial fossa? for the cerebral 

 lobes of the brain. It is perforated for the transmission of important 

 nerves and vessels. 



In shape it is irregularly five-sided (Fig. 181) ; with a long, trans- 

 verse, posterior base ; shorter, longitudinal sides ; and curved, anterior 

 sides which run inward and forward, but, inasmuch as the anterior 

 angle is transversely truncated, do not meet in front. The middle 

 portion of the bone, known as the body, is thick and narrow in front, 

 but flatter, shallower, and wider behind. On each side of the body 

 are two thin and curved outgrowths, known as the small and the great 

 wings of the sphenoid. The bone is easily separated into two parts 

 along a line which begins on each side on the anterior border of the 

 great wing some little distance within the tip or widest lateral projec- 

 tion, and then runs inward and backward to the body, which it crosses 

 transversely between the first and second pair of holes. The anterior, 

 subtriangular part may be called the anterior sphenoid ; the posterior, 

 quadrate part may be called the posterior sphenoid. The middle 

 region of the anterior sphenoid is formed by the presphenoid ; the lateral 

 masses or small wings are the orbitosphenoids. The portion of the 

 body of the bone belonging to the posterior sphenoid is the basisphe- 

 noid ; the large lateral parts, the great wings, are the alisphenoids. 1 



1 From ala, a wing, and sphenoid. 



