OF THE NERVOUS SYSTEM. 



383 



diverge laterally in order to form the medulla oblongata, leave be- 

 tween them an open 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 pos- 

 teriorly, 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 cord. In the brain of the 

 alligator this fissure appears to be interrupted at the cerebellum ; 

 but in the higher classes, where the lateral portions of the cerebel- 

 lum are more highly developed, as in the human subject (Fig. 126), 

 they are also separated from each other posteriorly on the median 

 line, and the longitudinal median fissure is complete throughout. 



In birds, the hemispheres are of much larger size than in rep- 

 tiles, and partially conceal the optic ganglia. The cerebellum, 

 also, is very well developed in this class, and presents on its sur- 

 face a number of transverse foldings or convolutions by which 

 the quantity of gray matter which 

 it contains is considerably in- 

 creased. The cerebellum here 

 extends so far backward as almost 

 completely to conceal the medulla 

 oblongata and the fourth ven- 

 tricle. 



In the quadrupeds, the hemis- 

 pheres and cerebellum attain a 

 still greater size in proportion to 

 the remaining parts of the brain. 

 There are also two other pairs of 

 ganglia, situated beneath the he- 

 mispheres, and between them and 

 the tubercula quadrigemina. 

 These are the corpora striata in 

 front and the optic thalami behind. 

 In Fig. 129 is shown the brain of 

 the rabbit, with the hemispheres 

 laid open and turned aside, so as to show the internal parts in their 

 natural situation. The olfactory ganglia are seen in front ( i ) con- 



Fig. 129. 



BRATX OFRABBIT. viewed from above. 

 1. Olfactory ganglia. 2 Hemispheres, turned 

 aside. 3. Corpora striata. 4 Optic thalami. 

 5. Tubercula quadrigemina. 6. Cerebellum. 



