ANATOMY OF THE HORSE. 25 



middle, and just beneatli this an irregular aperture, havino- 

 interiorly two distinct passages, one terminated by a cul-de-sac, 

 including a foramen, which leads to the cochlea and communi- 

 cates through some small pores with the labyrinth ; the other, 

 the internal auditory foramen, is for the conduit of the auditory 

 nerves, and communicates with the aqueduct of the vestibule. 

 Another surface, the anterior, contributes in a small degree to 

 the formation of the concavity for the posterior lobe of the cere- 

 brum. The inferior surface, parted from the superior by a fis- 

 sure, is convex and prominent, but irregularly so : it constitutes 

 the exterior wall of the labyrinth. 



The petrous portion is received between the squamous and 

 the occipital bone, and though it is rarely found united with 

 either of them by bony matter, yet, loose as it is, it is so locked 

 in that it cannot be disjointed but with considerable difficulty 

 and even fracture of some part. 



SPHENOID BONE. (OS SPHENOIDES.) 



Silualio?i. Inferior and middle parts of the cranium. 



Form. It bears a striking resemblance to a bird in flight, 

 with its wings and legs extended : a comparison that has given 

 rise to its 



Division — Into body, alae or wings, and pterygoid processes 

 or legs : altogether presenting for consideration two surfaces and 

 two borders. 



T/ie Infejnor Surface, irregularly convex, is distinguishable 

 into three parts: — a middle and two lateral portions. The mid- 

 dle, thick, prominent, cylindroid, and oblong from before back- 

 wards, by its union with the basilar portion of the occipital bone 

 and the body of the ethmoid, forms the base of the cranium : 

 its porousness denotes muscular attachment. Between the 

 middle and lateral parts, on either side, runs a narrow fissure 

 denominated the pterygoid, which leads into two small canals, 

 one entering the cavity of the nose, the other the orbital hiatus : 

 this fissure affords a passage to the pterygoid branch of the por- 

 tio dura. — The lateral divisions send forth by the sides of then 

 fissures the pterygoid processes, which project downwards and 

 forwards, form a union with the palate bones, and afford attach- 

 ment to the internal pterygoid muscles. At the bases of these 

 processes are the pterygoid foramina; above and internally to 

 the base is the orbital hiatus, a considerable aperture, obliquely 

 ovoid from above downwards, opening into the back of the 

 orbit, and including the supero-posterior and infero-posterior or- 

 bital foramina: the former transmitting the ophthalmic nerve 



