64 THE ANATOMY OF VERTEBRATED ANIMALS. 



larger, more horizontal, and possesses more of a rostrum in 

 the Chimpanzee, it is still larger, somewhat deflexed, and very 

 thick posteriorly; and has a large rostrum. In proportion to 

 the hemispheres, the anterior commissure is largest in the 

 Rabbit and smallest in the Chimpanzee. The Rabbit and the 

 Pig have a single corpus niammillare, the Chimpanzee has 

 two. The cerebellum of the Rabbit is very large in proportion 

 to the hemispheres, and is left completely uncovered by them 

 in the dorsal view. Its median division, or vermis, is straight, 

 synmietrical, and large in proportion to the lateral lobes. The 

 floeculi, or accessory lobules developed from the latter, are 

 large, and project far beyond the margins of the lateral lobes. 

 The ventral face of the metencephalon presents on each side, 

 behind the posterior margin of the pons varolii, flattened rec- 

 tangular areae, the so-called corpora trapezoidea. 



In the Pig, the cerebellum is relatively smaller, and is par- 

 tially covered by the hemispheres ; the lateral lobes are larger 

 in proportion to the vermis and the flocouli, and extend over 

 the latter. The corpora trapezoidea are smaller. In the Chim- 

 panzee, the relatively still smaller cerebellum is completely 

 covered ; the vermis is ver^' small in relation to the lateral 

 lobes, which cover and hide the insignificant flooculi. There 

 are no corpora trapezoidea. 



In all the characters now mentioned, the brain of Man 

 differs far less from that of the Chimpanzee than that of the 

 latter does from the Pig's brain. 



The Myelon. — The spinal canal, and the cord which it con- 

 tains, are lined by continuations of the three membranes which 

 protect the encephalon. The cord is sub-cylindrical, and con- 

 tains a median longitudinal canal, the canalis centralis, the 

 remains of the primitive groove. It is divided by anterior and 

 posterior median fissures into two lateral halves, which are, 

 usually, connected only by the comparatively narrow isthmus, 

 which immediately surrounds the canalis centralis. The cord 

 may, in the adult, extend through the whole spinal canal, or it 

 rnay come to an end at any point between the caudal extrem- 

 ity and the anterior thoracic region. 



The distribution of the two essential constituents of ner- 

 vous tissue, ganglionic corpuscles and nerve-fibres, is very defi- 

 nite in the spinal cord, ganglionic corpuscles being confined 

 to the so-called " gray matter " -ivhich constitutes the isthmus, 

 and spreads out into two masses, each of which ends in an an- 

 terior (or ventral) and a posterior (or dorsal) horn. Nerve- 



