THE MTELON. 69 



varolii, flattened rectangular areas, the so-called corpora 

 trapezoidea. 



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

 partially covered by the hemispheres ; the lateral lobes are 

 larger in proportion to the vermis and the flocculi, and 

 extend over the latter. The corpora trapezoidea are smaller. 

 In the Chimpanzee, the relatively still smaller cerebellum is 

 completely covered ; the vermis is very small in relation to 

 the lateral lobes, which cover and hide the insignificant 

 flocculi. There are no corpora trapezoidea. 



In all the characters now mentioned the brain o£ Man 

 differs far less from that of the Chimpanzee than that of 

 the latter does from the Pig's brain. 



The Mijelon.—The spinal canal, and the cord which it 

 contains, are lined by continuations of the three mem- 

 branes which protect the encephalon. The cord is sub- 

 cylindrical, and contains a median longitudinal canal, the 

 canalis centralis, the remains of the primitive groove. It is 

 divided by anterior and posterior median fissiires into two 

 lateral halves, which are, usually, connected only by the 

 comparatively naiTOw isthmus, which immediately sur- 

 rounds the canalis centralis. The cord may, in the adult, 

 extend through the whole spinal canal, or it may come to 

 an end at any point between the caudal extremity and the 

 anterior thoracic region. 



The distribution of the two essential constituents of 

 nei-vous tissue, ganglionic corpuscles and neiwe-fibres, is 

 very definite in the spinal cord, ganglionic coi-puscles being 

 confined to the so-called " grey matter " which constitutes 

 the isthmus, and spreads out into two masses, each of which 

 ends in an anterior (or ventral) and a posterior (or dorsal) 

 horn. Nerve-fibres also abound in the grey matter ; birt 

 the so-called " white matter," which constitutes the external 

 substance of the cord, contains only the fibrous nei-vous 

 matter, and has no ganglionic corpuscles. 



The spinal nei-ves arise in opposite pairs from the two 

 halves of the cord, and usually correspond in number with 



