412 DISSECTION OF THE PIGEON 



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

 terior end is covered by the cerebellum, and pos- 

 teriorly it is continuous with the spinal cord. 



2. The ventral surface of the brain. 



a. The optic chiasma, 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, imme- 



diately behind the optic chiasma. It is connected 

 with the pituitary body, which is usually left 

 in the skull when the brain is removed. 



C. 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 thirl ventricle, or cavity of the fore-brain, is of 



considerable extent from before backwards, and 

 dorso-ventrally ; but it is very narrow from side 

 to side. It opens in front by lateral apertures, the 

 foramina of Monro, into the lateral ventricles of 

 the hemispheres. Its roof is produced upwards 

 into the stalk of the pineal body, and its floor is 

 depressed to form the infundibulum. 



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- 

 ventrally. 



IX. THE SENSE-ORGANS. 



A. The Eye. 

 1. The eyelids. 



The upper eyelid is but slightly movable; the 

 lower very freely so. 



The nictitating membrane, or third eyelid, is a 



