i S 6 



THE FROG 



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

 tudinal 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 homogeneous, 

 but is composed of two different substances. Its outer part 

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



t/.r 



r IG. 40. Iransverse section ot spinal cord ot trog. 



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

 of dorsal root of spinal nerve ; ni>. c. nerve cells of dorsal horn ; nz>. c'. nerve-cells 

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

 matter ; v. r. fibres of ventral root of spinal nerve ; iv. m. 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. 

 The grey matter has a squarish outline in transverse section. 

 It surrounds the central canal, and is continued upwards 

 and downwards, forming what are called the dorsal (d.ti) and 

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



The Brain. Anteriorly 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- 



