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.ni. 

 grt. 



4 



of gray matter which 

 consists chiefly of gan- 

 ghon cells and non- 

 meduUated nerves. 

 Near the center of the 

 gray mass is a small 

 canal, the canalis cen- 

 FiG. 80. — Cross section through the verte- tralis, lined by a single 



bral column, and spinal cord showing the 

 origin of the spinal nerves, ex, central 

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

 d.m, dura mater; d.r, dorsal root of 

 nerve; g.m, gray matter; gn, ganglion of 

 dorsal root ; n.a, neural arch ; v.sp, neural 

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

 should stop at the margin of the cord) ; 

 /, nerve trunk ; Tr.pr, transverse process ; 

 v.f, ventral fissure ; w.in, white matter. 

 (After Howes.) 



layer of epithelial cells. 

 This canal is the rem- 

 nant of the lumen 

 formed by the closing 

 over of the edges of 

 the medullary groove 

 during development ; 

 at its anterior end it 

 widens out into the ventricles of the brain. 



At the sides the gray matter is produced both dorsally 

 and ventrally into the dorsal and ventral coniiia 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 commissures, which consist chiefly 

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

 commissure is a conspicuous oblique crossing of medullated 

 fibers in the white matter, the ventral white cominissure. 

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

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



