424 DISSECTION OF THE PIGEON 



f. The medulla oblongata is thick and wide : its an- 

 terior end is covered by the cerebellum, and pos- 

 teriorlj' it is continuous with the spinal cord. 



2. The ventral surface of the brain. 



a. The optic cMasma, formed by the crossing of the 



optic nerves, lies between and slightly in front 

 of the optic lobes. 



b. The infundibulum is a small median process, im- 



mediately behind the optic chiasma. It is con- 

 nected with the pituitary body, which is usually 

 left in the skull when the brain is removed. 



G. Bisection of the Brain. 



Divide the brain into right and left halves by a median 

 longitudinal section, so as to expose its cavities. 



a. The third ventricle, or cavity of the fore-brain, is 



of considerable extent from before backwards, 

 and dorso-ventrally ; but is very narrow from 

 side to side. It opens in front by lateral aper- 

 tures, the foramina of Monro, into the lateral 

 ventricles of the hemispheres. Its roof is pro- 

 duced upwards into the stalk of the pineal body, 

 and its floor is depressed to orm the infundi- 

 bulum. 



b. The Sylvian aqueduct, or cavity of the mid-brain. 



is small, but communicates laterally with large 

 cavities in the optic lobes. 



c. The fourth ventricle, or cavity of the hind-brain, 



is wide from side to side, but shallow dorso- 

 ventraUy. 



XX. THE SENSE -OEGANS. 

 A. The Eye. 



1. The eyelids. 



The upper eyelid is but slightly movable; the 

 lower one very freely so. 



The nictitating membrane, or third eyelid, is a 

 fold at the anterior angle of the eye, lying within 



