CHAP, in.] SKELETON OF THE HEAD AND TRUNK 69 



the internal organ of hearing. Continuing our enumeration, we have 

 (5) an ossification which gives rise to the upper part of the petrous 

 portion (that which is visible inside the skull), and to part of the 

 mastoid. It forms the upper margin of the fenestra ovalis, and is 

 especially related to the anterior vertical semicircular canal. It is 

 called the Pro-otic. We have next one (6) which gives rise to the 

 lower part of the petrosal (that concealed by the auditory bulla), 

 which forms the lower part of the fenestra ovalis, and surrounds 

 entirely the fenestra rotunda. It is called the opisthotic. Lastly, 

 we have an ossification (7) which gives rise to the mastoid process, 

 and which is developed upon the hinder part of what will be here- 

 after described as the posterior vertical semicircular canal of the 

 internal ear. This ossification is named the epiotic. The whole of 

 these, i.e., the petrous and mastoid portions taken together, are 

 known as the "periotic." The temporal bone articulates with the 

 occipital behind, with the parietal above, in front, with the 

 sphenoid, and (through the zygomatic process) with the malar. 

 The apex of the petrosal is wedged in between the basi- occipital and 

 the sphenoid. 



27. The SPHENOID is also a very complex bone, and consists of 

 two distinct parts,* one anterior, the other posterior. The posterior 

 sphenoid may be first described. This is the central bone of the 

 base of the skull, and its median part, or body, called the basi- 

 sphenoid (bs), joins the basi-occipital behind. It has on the middle 

 of its upper surface a shallow pit called the setta turcica, or pituitary 

 fossa, because it receives and supports an appendage of the brain, 

 called the " pituitary body." This fossa is bounded behind by a 

 small plate of bone (cp), which is inclined forwards as well as 

 upwards, and the hinder surface of which is continuous with the 

 upper surface (or basilar groove) of the basi-occipital. The plate is 

 called the clinoid plate, and its two upper angles are produced 

 outwards into prominences termed the posterior clinoid processes. 

 Beneath, the basi-sphenoid is nearly flat, and becomes narrower ^as 

 it advances forwards. Its structure is solid, not containing air- 

 cavities. 



On each side of the basi-sphenoid there projects outwards a large 

 crescentic plate of bone, concave above from behind forwards (as), 

 which is its longest dimension. This is the ali-sphenoid, or great wing 

 of the sphenoid, and it forms the side wall of the cranium, imme- 

 diately in front of the squamosal and auditory bulla. Its upper 

 margin is concave, the lower margin of the squama of the temporal 

 bone being received into its concavity. Its hinder end overlaps the 

 petrosal (helping to close the large aperture in which the Eustachian 

 tube ends), and meets the anterior end of the ossified tentorial plate 

 of the parietal. Its anterior end ascends behind the frontal, towards 

 or to, the parietal. The ali-sphenoid is perforated on each side 



* It is described here as a single bone, because it is so considered in human 

 anatomy. 



