AVES. 233 



The upper is the fenestra avails, the lower the fenestra rotunda. 

 Into the front of the tympanic cavity the Eustachian tube opens. 

 Further forwards tlie brain-case is bounded laterally by an 

 orbital plate, which also forms the hinder and upper parts of the 

 wall of the orbit, and is made up below by the ali-sphenoid, and 

 above by the orbital process of the frontal. The floor of the brain- 

 case in front of the basi-occipital is formed by the basi-sphenoid. 

 From this a pointed rod, the para-sphenoid (basi-sphenoidal rostrum), 

 projects forwards. It is fused with the lower edge of a thin 

 bony plate, the interorbital septum, continuous behind with the 

 ali-sphenoids. The posterior part of the septum represents three 

 distinct bones, a median pre-sphenoid, and two lateral orbito- 

 sphenoids. The upper and anterior parts of the septum are, in 

 the young bird a distinct bone, the mesethmoid. The anterior 

 margin of the orbit is formed at this point by the lachrymal. 

 The basi-sphenoid is overlapped by a thin plate of bone, the 

 bast-temporal, which forms the lower boundary of the tympanic 

 cavity on either side. It tapers somewhat in front, and between 

 it and the basi-sphenoidal rostrum is a median Eustachian open- 

 ing, from which an Eustachian tube leads back on either side to 

 the corresponding tympanic cavity. 



The basi-temporal is probably equivalent to the posterior part of the 

 frog's para-sphenoid. 



The cavity of the brain-case closely corresponds to the shape 

 of the brain. Its floor is very steep, rapidly ascending towards 

 the front. The foramen magnum looks downwards in accordance 

 with the upright position of the neck. 



Nerve Exits. The olfactory foramen (I.) is in the front of the 

 brain-case. A vacuity in the dry skull at the upper margin 

 of the inter-orbital septum marks the course of the olfactory 

 nerves to the nasal capsules. Below the olfactory foramen is an 

 o/itic foramen (II.), and at this point, in the dried skull, the two 

 orbits are placed in communication by a hole. The oculomotor and 

 pathetic nerves have exit by special small foramina near the optic, 

 the fifth and sixth by a larger aperture behind the ali-sphenoid. 

 The eighth nerve pierces the inner side of the auditory capsule, 

 and, just in front of it, there is a small foramen for the seventh. 

 Behind the tympanic cavity there is a small opening on each 

 side at the posterior angle of the basi-temporal for the ninth, 



