THE NERVOUS SYSTEM 



385 



animal, but the destruction of the medulla is fatal. Of 

 the twelve pairs of nerves issuing from the contents of 

 the skull (encepJialoii), ten come from the medulla oblon- 

 gata. Among these are the nerves of hearing and taste, 

 and those that control the lungs and heart. Respiration 

 ceases immediately when the medulla is injured. 



The spinal cord is a center for originating involuntary 

 actions, and is also a conductor transmitting through 

 its cells and fibers to 

 the brain the impres- 

 sions received by the 

 sensory organs, and 

 taking back to the 

 motor organs the im- 

 pulses of the brain. 

 In man, thirty-one 

 pairs of nerves arise 

 from the cord to sup- 

 ply the whole body, 

 except the head. 

 Each nerve has an 

 anterior and a poste- 

 rior root. The fibers 

 of the former go to the 

 muscles, and carry 

 the impulses which 

 cause muscular con- 

 traction (hence called 

 motor fiber!) ; those 

 of the posterior root 

 convey sensations 

 from the exterior to 



the central organs (sensory). The fibers leading from 

 the brain to the cord cross one another in the medulla 

 oblongata, so that if the right cerebral hemisphere be 

 DODGE'S GEN. ZOOL. 25 



Fig. 343. Relation of the Sympathetic and Spinal 

 Nerves : c, fissure of spinal cord; , anterior root 

 of a dorsal spinal nerve ; /, posterior root, with 

 its ganglion ; a ' , anterior branch ; /', posterior 

 branch ; s, sympathetic ; f-, its double junction by 

 nerve filaments. 



