70 ZOOTOMY. 



169. The spinal cord (Fig. 22, my) lying in the neural 

 canal of the vertebral column, and, at the junction of the 

 latter with the skull, passing insensibly into the brain. It is 

 divided into a pair of dorsal and a pair of ventral 

 columns by deep dorsal and ventral fissures, and 

 is traversed through its centre by the minute central 



canal. 



170. The medulla oblongata or myelencephalon 



(Figs. 22- 25, m.o\ the hindmost division of the brain : 

 passing forwards from its junction with the spinal cord, it 

 undergoes a gradual increase in diameter, and is produced 

 dorsally, on each side, into a greatly convoluted body, the 

 corpus restiforme (Fig. 22, c.r\ Both dorsal and 

 ventral surfaces of the medulla are marked with obscure 

 median grooves, continuous respectively with the dorsal and 

 ventral fissures of the spinal cord : the dorsal groove separates 

 the dorsal pyramids, the ventral groove the ventral 

 pyramids of the medulla. 



171. The cerebellum (Figs. 22, 24, 25, cb}, a median 

 structure, situated to the dorsal side of the myelencephalon, 

 and composed of two distinct lobes, a posterior, having the 

 form of an isosceles triangle with the apex directed back- 

 wards, situated between the restiform bodies, and a squarish 

 anterior, partly overlapping the optic lobes ( 173). Both 

 lobes are marked on the surface by ridges (gyri) with inter- 

 vening depressions (sulci). 



172. A small transparent area behind the cerebellum 

 (Fig. 22, ?'. 4), roofing over the fourth ventricle ( 180). 



173. The mesencephalon (Figs. 22 25, <?./, c.c) con- 

 sisting on the upper surface of the two optic lobes 

 (<?./), ovoidal bodies, lying just in front of, and partly 

 overlapped by the cerebellum. The under surface of the 

 mesencephalon is formed by the crura cerebri 



