86 THE NERVOUS SYSTEM OF THE FROG 



ii. The ventral or " anterior " fissure is a broad and 

 shallow median cleft : there is in the frog no 

 distinct dorsal fissure, 

 iii. The white matter forms the outer part of the cord, 

 and is chiefly composed of medullated nerve fibres, 

 iv. The grey matter forms the central part of the cord, 

 and is composed of a dense network of non- 

 medullated nerve fibres, in which are imbedded 

 numerous nerve cells. 

 V. The comua are the processes, ventral or " anterior,'' 

 and dorsal or " posterior," into which the grey- 

 matter is produced on either side, 

 vi. The central canal of the cord lies in the median 

 plane, nearer the ventral than the dorsal surface, 

 vii. The nerve roots are only seen if the section happens 

 to pass through their points of origin. 

 a. The dorsal or " posterior " root is connected with 

 the dorsal cornu of the grey matter ; it is a 

 single thick band of nerve fibres. 

 ^. The ventral or "anterior" root is connected 

 with the ventral cornu of the grey matter ; it 

 consists of a number of very slender bands of 

 nerve fibres. 



b. With the high power. 



i. The ganglion cells are large nucleated branched 

 cells lying in groups in the grey matter : they 

 are largest and most numerous in the ventral 

 cornua. 

 • ii. The neuroglia is a delicate network of connective 

 tissue fibres and cells, penetratiug and supporting 

 all parts of the cord, and continuous at the surface 

 with the pia mater. 



iii. The pia mater is the delicate connective tissue 

 membrane closely ensheathing the cord. 



iv. The bloodvessels of the cord are small and numerous : 

 they enter from the pia mater. 



V. The central canal is lined by a ciliated epithelium. 



