896 THE NERVES. 



The encephalic isthmus is not divided into two sections by the pons Varolii, 

 which is absent in birds ; the crura cerebelli are immediately connected with 

 the corpora restiformia. The lower face of the isthmus is very convex posteriorly ; 

 in front, the tubercula bigemina are united to each other by a comparatively 

 large transverse cord, formed by the optic nerves intercrossing in the median 

 line. The superior face is depressed above the cerebellum, so as to constitute a 

 fourth ventricle, the floor of which is also shaped like the point of a pen ; in 

 front of this ventricle are the tubercula bigemina {optic lobes) — two voluminous 

 tubercules separated from each other above, where they embrace the cerebellum, 

 and salient on the sides of the lower face. They are hollow internally, and 

 communicate with the aqueduct of Sylvius. The thalami optici are little developed. 



The cerebellum is almost reduced to the median lobe, the lateral lobes — situated 

 behind and below it — being very small and conical. By its anterior extremity, 

 this cerebral ganglion passes between the corpora bigemini, and touches the 

 cerebral hemispheres. The cerebellum is annulated transversely to its surface, 

 and between the principal fuiTows are secoj^dary ones, as in Mammals. The 

 white substance forms, in its interior, an arborization in relation, by the number 

 of its branches, with the simplicity observed on the surface of the organ. In 

 the centre of the cerebellum of birds is a small cavity communicating with the 

 fourth ventricle. 



The cerebrum, divided into two hemispheres by a shallow fissure, has the 

 shape of the heart on a playing-card, more particularly when viewed on its lower 

 face. The convolutions are absent on the upper and lateral faces of the organ, 

 and on the inferior is a vestige of the fissure of Sylvius, which is directed obliquely 

 forward and outward. The olfactory lobes are little developed, and are placed 

 together in the mesian line. 



The two ventricles are confounded, there being no corpus callosum nor 

 septum lucidum. There is no reflected portion in the ventricle ; consequently 

 the hippocampi and mastoid lobules are absent ; the corpora striata are, on the 

 contrary, large, and occupy nearly the whole floor of the ventricles. 



Cranial Nerves. — These are twelve pairs, as in Mammals ; and their origin 

 is analogous, if not identical ; the only trifling differences observed are due to 

 the absence of the pons VaroUi, and the convexity of the lower face of the 

 isthmus. 



Olfactory nerve.— We have mentioned above how this is formed at the anterior 

 portion of the cerebral hemispheres. 



Optic nerve.— It appears to be detached from the tubercula bigemini, and, 

 after a very short course, to intercross with that of the opposite side. In certain 

 Birds, and especially in the diurnal rapacious kind, the optic nerves are con- 

 stituted by fasciculi of undulating nerve-tubes. 



Common motores oculonm. Pathetici. External motores oculor urn. -There is 

 nothing particular to remark respecting these. 



Trifieminal nerve.— Th\^ nerve divides into three principal branches, as in 

 the domesticated animals. The op)hthahnic branch has a nasal ramuscule that 

 becomes superficial, and extends to the extremity of the beak, as well as a third 

 filament that is lost around the inferior orifice of the nasal cavities. 



The superior maxillary issues from the cranium by the opening through which 

 the lower maxillary nerve passes, creeps below the orbit, traverses the maxillary 

 bone, and terminates on the sides of the beak by filaments that resemble the 

 infra-orbital ramuscules of the Horse. 



