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) 

 pm. 



grt. 



4 



This canal is the rem- 

 nant of the lumen 



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- 

 Fig. 80. — Cross section through the verte- traits, lined by a single 



bral column, and spinal cord showing the , , p •.!_,• 1 ,1 



origin of the spinal nerves. c.c, central la y er 0t e P ltrielial cells - 



canal; en, centrum; d./\ 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 ; n.sp, neural formed by the closing 



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



should stop at the margin of the cord) ; 



/, nerve trunk ; Tr.pr, transverse process; tne medullary groove 



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



(After Howes.) . ° . V , . 



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 coniua 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-medullated nerve fibers. Just below the ventral gray 

 commissure is a conspicuous oblique crossing of medullated 

 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 



