NERVOUS SYSTEM. 79 



and posterior commissures, the middle being absent. Olfac- 

 tory lobes large, hollow, possessing chambers which com- 

 municate with the ventricles of the cerebral hemispheres. 

 The brain relatively larger than that of the fish. In the frog 

 it forms ^\^ part of the whole tract. But in the tortoise is 

 much less, viz., ^ 5 ^ part. It does not fill brain case, and, as 

 in fishes, is on the same plane with spinal cord. 



Aves. Spinal cord smaller in mass than brain. It com- 

 pletely fills spinal canal. A narrow ventricle runs through 

 its middle. Conspicuous swellings are noticed at the points 

 of origin of brachial and sacral plexuses. At the latter 

 position a distinct ventricle (sinus rhomboidalis) is seen, 

 lined with ependyma and containing a fold of pia mater. 



Medulla oblongata without pyramids or olivary bodies. 

 Cerebellum joining with cerebral hemispheres to conceal the 

 ganglia beneath. It is without lateral lobes and folded. 

 Transverse section displays the arbor vitae. Optic lobes hol- 

 low, united into a single rounded mass of small size and 

 concealed by junction of the cerebellum and cerebral hemi- 

 spheres. Cerebral hemispheres relatively longer than in 

 reptiles and fishes; heart-shaped with the apex directed for- 

 ward ; lobes not convoluted. Corpus callosum present as a 

 rudiment; fornix small or wanting. Bodies, so called, cor- 

 pora striata and optic thalami present; former not striated; 

 latter not united by commissure, and joining with optic lobes 

 in origin of optic nerves. Olfactory lobes large, apparently 

 appendages to anterior lobes of cerebral hemispheres. Olfac- 

 tory nerve escapes from cranium through a single opening. 



The brain is relatively larger than that of either fishes 

 or reptiles, is placed at an obtuse angle to spinal cord, and 

 entirely fills brain case. In Fnngilla canaria (canary) it com- 

 prises 2 1 - of whole tract, in Anas (duck) 5 ^. 



Mammalia. Spinal cord, with but few exceptions, fur- 

 nished with ventricle. It is swollen at points of origin of 

 brachial and sacral plexuses, and frequently shorter than its 

 enclosing canal, the intervening space being occupied by 

 the obliquely-placed nerves (cauda equina). 



Medulla oblongata is divided into pyramids and lateral 



