286 



THE BIOLOGY OF THE FROG 



CHAP. 



into two lamellae. This layer is very closely applied to the 

 central nervous system, and is continued into various fissures 

 of the brain, and the ventral fissure of the spinal cord. 



A cross section of the cord shows it to be composed 

 mainly of ganglion cells and nerve fibers. The central part 



of the cord is formed 



dm. 

 Tr.pr. 



p.m. 

 grt. 



4 



of gray matter which 

 consists chiefly of gan- 

 glion cells and non- 

 medullated nerves. 

 Near the center of the 

 gray mass is a small 

 canal, the canalis cen- 

 tralis, lined by a single 

 layer of epithelial cells. 

 This canal is the rem- 

 nant of the lumen 



Fig. 8o. — Cross section through the verte- 

 bral column, and spinal cord showing the 

 origin of the spinal nerves, c.c, central 

 canal ; en, centrum ; d.f, dorsal fissure ; 

 d.7n., dura mater; i-r, dojsal root of 



nerve; g.vi, gray matter; gn, ganglion of i i • 



dorsal root ; n.a, neural arch ; n.sp, neural formed by the closmg 

 spine; p.m., pia mater (the reference line over of the edges of 

 should stop at the margin of the cord) ; j n 



/, nerve trunk ; T)-.//, transverse process; the medullary grOOVe 

 v.f, ventral fissure; w.m, white matter, during development; 

 (After Howes.) ^^ .^^ ^^^^^^^^ ^^^ j^ 



widens out into the ventricles of the brain. 



At the sides the g^ra y matter is produced both dorsally 

 and ventrally into the dorsal and ventral cornua or horns. 

 The gray matter on the two sides of the cord is connected 

 both above and below the central canal by means of the 

 dorsal and ventral gray comtnisstires, which consist chiefly 

 of non-meduUated nerve fibers. Just below the ventral gray 

 commissure is a conspicuous oblique crossing of meduUated 

 fibers in the white matter, the ventral white commissure. 

 Below the white commissure is the ventral fissure, which sepa- 

 rates the right and left columns of white matter. From the 



