Nervous System. 45 



These fibers from the skin carry nerve impulses inward, 

 as those going to the muscles carry impulses outward. 



Nerve Roots and their Functions. Observations made 

 on animals, and accidents in the case of man, show that 

 all the fibers of the nerves that carry currents to the mus- 

 cles pass out from the spinal cord into the ventral root, 

 and that all the fibers that carry currents inward enter the 

 spinal cord through the dorsal root. Hence, the dorsal 

 root is often called the afferent root, and the ventral the 

 efferent root. Since ingoing impulses produce sensation, 

 the dorsal root is called the sensory root, while the ventral 

 root, carrying currents outward to produce motion, is called 

 the motor root. 



Effect of Stimulating a Spinal Nerve. Experiments 

 have shown that if, in an uninjured animal, a nerve, or 

 more properly a nerve trunk as the sciatic nerve be 

 stimulated, for instance, by an electric shock, two effects 

 are produced : first, motion in the parts whose muscles are 

 supplied by the nerve ; second, sensation, which is referred 

 to the parts of the skin supplied by the branches of the 

 nerve. This double effect is because both sets of fibers in 

 the nerve have been stimulated, one set carrying currents 

 inward, the other outward. 



Cramp. Cramp is a spasmodic shortening of the mus- 

 cles, attended with pain. 



Tetanus. Tetanus (or locked jaw) is a spasmodic and 

 continuous shortening of the muscles, causing rigidity of 

 the parts they supply. It is due to the disordered and 

 excessive stimulation of the muscles through the nerves. 



Crossing of the Fibers from the Brain to the Spinal 

 Cord. Both the brain and the spinal cord consist of two 

 lateral halves connected by cross fibers. Each half of the 



