MAMMALIA. 273 



basi-occipital by a thin plate of cartilage, and a narrow anterior end united 

 by cartilage to the pre-sphenoid. Upon its upper surface is a pit, the 

 stlla turcica (for the pituitary body), bounded behind by the posterior 

 clinoid processes. Firmly fused to each side of the basi-sphenoid is a thin 

 wing-like expansion, the ali-sphenoid, from the under side of which a trans- 

 verse lamella, the external pterygoid process, passes vertically downwards. 

 The pre-sphenoid is a vertical plate, the upper surface of which is produced 

 backwards into the anterior cUnoid processes which bound the sella turcica 

 in front. After limiting the optic joramen below, the pre-sphenoid ends in 

 two diverging lamellae which articulate above with the orbital processes of 

 the frontal, and help to form the inner wall of the orbit. A wing-like 

 orbito- sphenoid is firmly fused with the pre-sphenoid on either side, form- 

 ing the hinder and upper boundaries of the optic foramen, and uniting 

 with the orbital process of the frontal above, and the ali-sphenoid behind. 

 The narrow front end of the brain-case is filled in by the cribriform plate, 

 a lamella perforated by numerous holes and forming the party-wall between 

 the cranial and nasal cavities. It is the hinder part of the mesethmoid. 

 The side- wall of the skull is largely formed in its posterior region by the 

 squamosal, which tills up the gap between parietal and ali-sphenoid, unites 

 in front with the frontal and orbito-sphenoid, and behind overlaps the 

 descending process of the parietal. A delicate backward process from the 

 squamosal helps to keep the large, irregular periotic in place, and a large 

 zygomatic process runs outwards and forwards from it just behind the 

 orbit. 



The auditory capsule is formed by the periotic bone which 

 results from the early fusion of pro-, epi-, and opisthotic elements. 



The outer and hinder mastoid portion of this bone is porous, and a 

 tapering mastoid process runs down from it in front of the par-occipital 

 process. The inner side of the mastoid part presents a deep rounded pit, 

 the floccular fossa, for the flocculus of the cerebellum. The petrous portion 

 which forms the rest of the periotic is very dense, and contains the mem- 

 branous labyrinth. Upon its outer surface there is a smooth projection, the 

 promontory, above which is the elliptical fenestra ovalis, and behind it the 

 more irregular fenestra rotunda. 



Closely applied to the outer surface of the periotic is the 

 tympanic, a bone with a dilated lower part (bulla) and a short 

 part directed upwards and backwards. The inner side of both is 

 incomplete, for which the periotic makes up, forming the inner 

 wall of the tympanic cavity, which is contained in the bulla. The 

 tubular part supports the external auditory meatus, and, at its 

 junction with the bulla, the tympanic membrane is stretched over 

 a flattened rim which projects into the latter. Extending across 

 the tympanic cavity are four minute auditory ossicles. 



(1) The malleus, which possesses two slender processes (one, the man- 



ubrium, attached to the tympanic membrane), which are given off from a 



rounded head. This articulates by a saddle-shaped surface with the 



head of (2) the incus which gives off a small process backwards and a 



2 18 



