182 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



The olfactory capsules, situated in front of the cranium, are 

 separated from one another by a vertical plate of cartilage con- 

 tinuous with the longitudinal partition of the sphenethmoid. The 

 walls of the capsules are mostly composed of cartilage. 



Membrane Bones. On the upper side of the olfactory capsules 

 are two triangular nasal bones, which form the posterior boundary 

 of the external nares. The cartilaginous floor of each olfactory 

 capsule is partly covered by a somewhat triangular vomer, which 

 is widely separated from its fellow. Each vomer bears a patch 

 of sharp vomerine teeth, and partly bounds the internal naris of its 

 side. 



Jaws. Both upper and lower jaws consist of a cartilaginous 

 basis and of several bones, which in the case of the former are all 

 membrane bones. 



(a) Upper Jaw. Owing to the projection of the sense-capsules 

 a sort of bay, the orbit, in which the eye lies, is left on each side 

 of the brain-case. It is bounded in front by a flat piece of 

 cartilage, the ethmo-palatine bar, which runs out transversely from 

 the floor of the olfactory capsule, sends a process forwards, and 

 becomes continuous with a slender pterygoid bar, which passes 

 back on the outer side of the orbit, and fuses behind with the 

 quadrate, cartilage (suspensorium). This is a cartilaginous rod 

 directed outwards, downwards, and backwards. Its proximal end 

 is forked, the short limbs of the fork being attached, above and 

 below, to the outside of the auditory capsule. The lower jaw is 

 " suspended " to its distal end, which presents an articular hollow. 

 These parts constitute the cartilaginous basis of the upper jaw 

 and its supports. Its actual margin is formed by a series of 

 membrane bones, of which the most anterior are the two pre- 

 maxillce, small tooth-bearing elements meeting in the middle line 

 in front of the olfactory capsules. On each side an elongated 

 curved maxilla, which bears most of the teeth, runs back from 

 the pre-maxilla and unites behind with a narrow quadrato-jugal 

 extending posteriorly to the quadrate cartilage. Three bones on 

 each side connect the upper jaw with the cranium and keep ib 

 firmly in position. These are the palatine, pterygoid, and squa- 

 mosal. 



The palatine is a slender transverse bone moulded on the hinder 

 edge of the ethmo-palatine bar, while the pterygoid bar is largely 

 ensheathed by the anterior ray of the three-rayed pterygoid bone, 



