ANATOMY OF THE PIGEON. 533 
ureters. In neither sex in birds are the genital ducts pro- 
vided with accessory glands. 
As usual among birds, the head is approximately top- 
shaped. The eyes are very large and much exposed, as be- 
comes evident upon dissecting off the skin as in the figure. 
The external ear isa mere circular opening, entirely covered 
during life by the feathers. The side of the cranium may 
be removed soas to expose the brain, with the large smooth. 
cerebral hemispheres (C), the convoluted cerebellum (C%), 
and the much smaller medulla (Md). To study the brain 
satisfactorily, it must be removed from its case. A view of 
it from the side is given in Fig. 459, A, and a view from 
above in the same figure at B. The medulla oblongata 
(4) appears as hardly more than the enlarged upper end 
of the spinal cord ; upon its dorsal surface there is a trian- 
gular depression IV, the fourth ventricle, which is par- 
tially concealed by the cerebellum (Cb), a large mass mark- 
ed by transverse ridges and imperfectly divided into three 
lobes, thus exhibiting, both in its size and its complication 
of structure, a great advance over the reptiles. The corpora 
quadrigemina or bigemina* (Q) project as two large lobes far 
out on the sides and down the base of the brain ; their posi- 
tion and great size are characteristic for the whole class. 
The optic thalaini, which intervene between the bigemina 
and the hemispheres, are relatively small; they enclose the 
third ventricle and have a funnel-shaped downward exten- 
sion, to which the pituitary body is attached, as to a stalk. 
The cerebral hemispheres (He) form more than half of the 
whole brain ; their surfaces are entirely without convolu- 
tions, but each hemisphere has a small projection, the olfac- 
tory lobe (OZ), upon its anterior and inferior extremity. 
The cavities of the hemispheres or the lateral ventricles are 
very large and extend also into the olfactory lobes. The 
greatly thickened inferior walls of the hemispheres are 
termed the corpora striata. Birds differ from mammals in 
having only a rudimentary forniz and no corpus callosum. 
The description of the cranial nerves is purposely omitted. 
* Also called the optic lobes, middle brain, and mesencephalon. 
