THE NERVOUS SYSTEM. 



437 



Fig. 150. 



BUAIN OF PIGEON Profile view. 

 1. Cerebrum. 2. Optic tubercle. 3. Ce- 

 rebellum. 4. Optic nerve. 6. Medulla 

 oblongata. 



space, which is continuous with the posterior median fissure of the 

 cord. This space is known as the " fourth ventricle." It is partially 

 covered in by the backward projection of the cerebellum, but in the 

 alligator is still somewhat open posteriorly, presenting a kind of chasm 

 or gap between the two lateral halves of the medulla oblongata. 



The successive ganglia which compose the brain, being arranged in 

 pairs as above described, are separated from each other on the two 

 sides by a longitudinal median fissure, which is continuous with the 

 posterior median fissure of the spinal cord. In the brain of the alligator 

 this fissure appears to be interrupted at the cerebellum ; but this is due 

 to the incomplete development of the 

 lateral portions of this organ, as com- 

 pared with its middle. The same pe- 

 culiarity is to be seen in birds and in 

 most quadrupeds ; while in man the 

 lateral portions of the cerebellum are 

 so highly developed as to project, on 

 each side, above the level of its central 

 part, and the longitudinal median fis- 

 sure, accordingly, appears complete 

 throughout. 



In birds the hemispheres, or cere- 

 brum, are of comparatively larger size, and partially or completely con- 

 ceal the optic tubercles in a view taken from above. The cerebellum is 

 well developed in this class, and presents on its surface a number of 

 transverse foldings or convolutions 

 by which its gray substance is con- 

 siderably increased in quantity. The 

 cerebellum extends so far backward as 

 to completely cover the medulla ob- 

 longata and the fourth ventricle. 



In quadrupeds the cerebrum and 

 the cerebellum attain a still greater 

 size as compared with the remain- 

 ing parts of the brain. There are 

 also two other collections of gray 

 substance on each side, situated in the 

 inferior part of the hemispheres, ante- 

 riorly to the tubercula quadrigemina. 

 These are the "corpora striata" in 

 front, and the " optic thalami" behind. 

 These bodies are frequently designated 

 by the name of the " cerebral gan- 

 glia," Since they are collections of BRAIN OF RABB IT , viewed f.-om 

 ,, ... above. 1. Olfactory ganglia. 2. Hemi- 



gray matter Which OCCUpy the lower spheres of the cerebrum, turned aside. 



and central parts of the cerebrum, and 3 Cor P ra striata. 4 Optic thaiimi. 



, 5. Tubercula quadrigemina. 6. Cerebei- 



intervene between the tracts of white i um . 



Fig. 151. 



