156 



THE FROG 



CHAP. 



into paired half-cylinders, right and left, joined in the 

 middle by a narrow bridge. This bridge is traversed 

 from end to end by a very narrow longitudinal canal, 

 the central canal (c. c), lined by epithelium, so that the 

 cord is not a solid cylinder, but a tube with an extremely 

 narrow cavity and excessively thick walls. 



The section also shows that the cord is not homo- 

 geneous, but is composed of two different substances. 



Its outer part is pure white and shining in the fresh 



p.W vj> 



FIG. 48. Transverse section of spinal cord of Frog. (X 35.) 

 c.c. central canal; d.f. dorsal fissure ; d. h. dorsal horn of grey matter ; d. r. fibres 

 of dorsal root of spinal nerve ; nv. c. nerve-cells of dorsal horn ; m>. c' nerve-cells 

 of ventral horn ; p.w. pia mater ; v.f. ventral fissure ; v. h. ventral horn of grey 

 matter ; v. r. fibres of ventral root of spinal nerve ; w. m. white matter. (After 

 Howes.) 



cord, and is hence called the white matter (w. m). Its 

 internal substance has a pinkish colour when fresh, and 

 is called the grey matter. The grey matter has a 

 squarish outline in transverse section. It surrounds 

 the central canal, and is continued upwards and down- 

 wards, forming what are called the dorsal (d. h) and 

 ventral (v. h) horns of the grey matter. 



The Brain. Anteriorly the spinal cord passes in- 

 sensibly into the brain (Fig. 49), which is of somewhat 



