CHORDATA 



351 



the Central Nervous System in Man and Vertebrates," Figs. 21, 

 107-112. 



366. That portion of the central nervous system not enclosed 

 in the skull is called the spinal cord. It is surrounded and pro- 

 tected by the dorsal arches of the vertebrae. The cord is nearly 

 circular in cross section, is somewhat enlarged in the regions of 

 the appendages, tapers toward the posterior end and is divided 

 into symmetrical right and left lobes by a dorsal and a ventral 

 longitudinal groove (see Fig. 174, df.). It possesses a central 



FIG. 174. Diagram of a cross-section of the spinal cord through the roots of spinal nerves. 

 Drawn by Folsom. c, central canal; d.f., dorsal fissure; d.r., dorsal root of spinal nerve arising from 

 the dorsal horn of the gray matter (g); gn., ganglion on the dorsal root; n, spinal nerve; r./., ventral 

 fissure; p.r., ventral root of the spinal nerve, arising from the ventral horn of the gray matter; w., 

 white matter. (The dorsal fissure in the diagram is broader than it should be.) 



Questions on the figure. What is the structural difference between the white 

 and gray matter in the cord? Describe their arrangement. How are the two 

 halves of the cord united? Which are sensory and which motor roots? What 

 structural differences do you notice in the roots? 



canal continuous with the cavities of the brain. The outer 

 part of the cord (Fig. 174, w.) is composed of the white matter 

 (longitudinal nerve fibres) and the interior portion, of gray 

 matter (a mixture of nerve-cells and fibres) . This is somewhat 

 the reverse of the condition found in the brain. 



367. Peripheral Nervous System Spinal Nerves. Groups 

 of nerve fibres spring from the gray matter of the cord and pass 

 to the organs of the body. These nerves arise in pairs one 

 pair to each body segment and pass out between the vertebrae. 

 Each nerve has two "roots," a dorsal and a ventral, from each 

 of which some of its fibres come (Fig. 174, d.r., v.r.). The roots 



