192 ZOOTOMY. 



aperture, which communicates with the nasal cavity, and 

 serves for the passage to that cavity of the olfactory nerve. 



47. In the membrane filling the lower interorbital 

 fenestra there is a median aperture, the optic foramen 

 (n), for the transmission of the optic nerves from the brain 

 to the eyes. 



48. The tympanic cavity {ty), an irregular depression, 

 just behind the postero-inferior angles of the orbit, its 

 opening looking upwards and forwards : it is bounded by 

 the squamosal above, by the exoccipital behind, and by the 

 basitemporal (§ 50), beneath. 



49. The parasphenoid {pa.s), or basisphenoidal 

 rostrum, an elongated thickened rod of bone, pointed in 

 front, underlying the interorbital septum with which it is 

 completely ankylosed in the adult : in the young bird it is a 

 separate bone. At its base, it bears, on each side, a small 

 facetted basipterygoid process, for the articulation of 

 the pterygoid (§ 62). 



50. The basis cranii, or floor of the skull, extending 

 from the lower margin of the foramen magnum to the base 

 of the parasphenoid, and bounded externally by the lower 

 margin of the tympanic cavity : in the young skull the 

 hinder part of the basis cranii is formed, as already seen 

 (§ 37)) by the basioccipital ; in front of this is a large median 

 bone, the basisphenoid, underlaid and concealed in the 

 entire skull by the basitemporal, a large flat plate of 

 membrane bone, which extends forwards to the base of the 

 parasphenoid and outwards so as to form the ventral walls 

 of the tympanic cavities. 



51. The anterior edge of the basitemporal projects over the base of 

 the rostrum in the form of a delicate lamina of bone, between which 

 and the rostrum is a small space into which the two Eustachian tubes 

 open ; a bristle may therefore be passed from this common Eustachian 

 aperture (eu) into either tympanic cavity (§§ 298, 31S). 



