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 



dJt 



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

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

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

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

 matter; y. 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 



