1 56 



THE FROG 



bridge is traversed from end to end by a very narrow longi- 

 tudinal canal, the central canal {c.c), so that the cord is not 

 a solid cyUnder, but a tube with an extremely narrow cavity 

 and excessively thick walls. 



The section also shows that the cord is not homogeneous, 

 but is composed of two different substances. Its outer part 

 is pure white and shining in the fresh cord, and is hence 



ziA 



Fig. 48. — Tran.sverse section of spinal cord of frog. 

 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 ; nv. c'. nerve cells 

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

 matter ; v. r, fibres of ventral root of spinal nerve ; w. ?«. white matter. (After 

 Howes.) 



called the white matter {w.m). Its internal substance has 

 a pinkish colour when fresh, and is called the grey matter 

 {d.h, v./i). The grey matter has a squarish outline in trans- 

 verse section. It surrounds the central canal, and is con- 

 tinued upwards and downwards, forming what are called the 

 dorsal {d.h) and ventral (v.h) horns of the grey matter. 



The Brain. — In front the spinal cord passes insensibly 

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

 diameter than the cord, and is made up of several very dis- 



