THE NERVOUS SYSTEM 



121 



are situated in the central portion of each half, and the 

 two masses of gray matter thus formed are connected by a 

 narrow isthmus of gray matter. 

 The outer part of the cord 

 consists chiefly of white fibers. 

 The white matter is thus on the 

 outside of the cord (Fig. 107). 

 The brain, unlike the cord, has 

 the gray matter on the outside 

 and the white matter on the in- 

 side. For microscopic study of 

 the spinal cord, see Fig. 108. 



FIG. 107. CROSS-SECTION OF 

 SPINAL CORD, showing area 

 of gray matter (dark). 



The Work of the Spinal Cord.- -There are two functions 

 of the cord : reflex action and transmission of impulses 



from the body to the brain. 

 Reflex action is action that 

 takes place without the aid 

 of the will. 



Reflex action never begins 

 in the cord, but at the outer 

 end of a sensory fiber, usu- 

 ally located in the skin. 

 The impression goes to the 

 cord along a sensory fiber. 

 It is received in a sensory 

 cell and transferred by den- 

 drons to a motor cell which 

 sends back an impulse along 

 a motor fiber to a muscle ; 

 the muscle contracts and 

 the action is complete. At 

 least two nerve cells are necessary for reflex action. The 

 actions of the lowest animals are almost entirely reflex. 



..,.. 



*.""'*> ?*=. 



* ; 



. 



v. .- 



* ., .. 



^0:1, .-i. M ..,_- .-- ..-,. .:>.:; 



FIG. 108. SECTION OF SPINAL 

 CORD, showing nerve cells (large 

 black spots) with their branches 

 (black dots and lines). Five 

 bundles of nerve fibers are shown 

 near upper margin. (Peabody.) 



