152 ANIMAL BIOLOGY. [Part I. 



on each side of it there is an optic fossa for the large optic lobe. 

 The cerebellar fossa is well marked, and there is a strong 

 tentorial ridge. The floor is hollowed out for the reception of 

 the medulla oblongata. 



(2.) The Olfactory Chamber. On either side of the beak are large 

 anterior nasal fossce (a. n. /.), so that the nasal chambers are less 

 completely enclosed than in the rabbit. Each fossa is bounded 

 by the nasals behind (na.) and the pre-maxillse (p. mx.) in 

 front. The mesethmoid is continued forwards in the fresh 

 skull by a cartilaginous plate, the septum nasi, which forms a 

 median vertical partition between the nasal cavities. Beneath 

 it lies the vomer. It is produced on each side into scroll-like 

 masses of cartilage, the turbinals. The anterior nares are far 

 back, just in front of the eyes ; the posterior nares open 

 almost directly beneath them in a longitudinal slit in the palate. 

 There are thus no narial passages like those in the rabbit. 



(3.) The Auditory Chamber. The auditory capsule is seen on the 

 inner side of the brain-case, lying just external to the foramen 

 magnum and adjoining the optic and cerebellar fossae. It is 

 limited above by a well-marked semi-circular ridge indicating 

 the position of the anterior semi-circular canal. In the chick 

 there are three distinct ossifications : (1) the prootic, which 

 forms a distinct bone for some time and constitutes the main 

 part of the capsule ; (2) the epiotic, which fuses early with the 

 supra-occipital; and (3) the opisthotic, which fuses with the 

 ex-occipital. 



No tympanic bulla covers over the tympanic chamber, which 

 is an irregular hemispherical depression. In the inner wall of 

 this cavity is a largish aperture, the fenestra ovalis, and some- 

 what behind and below this a smaller one, the fenestra rotunda. 

 In place of the chain of auditory ossicles there is a partly cartila- 

 ginous, partly osseous rod, the columella auris. This is expanded 

 at its proximal end into a plate-like stapes which fits into the 

 fenestra ovalis. The cartilaginous distal end is affixed to the 

 tympanic membrane, which is stretched across the aperture of 

 the cavity. 



(4.) The Optic Chamber, The capsule of the eye within which 



