264 STUDIES IN PHYSIOLOGY 



nerve is allowed to grow again, the dog recovers the sense 

 of feeling in its leg and is able to move It at will. 



In a second animal we may cut only the dorsal roots of 

 the nerves to the front leg, severing these roots between 

 the cord and the spinal ganglia. The dog is still able to 

 move its leg, as usual. But if the paw is pinched, or even 

 burned, the animal shows no sign that it feels any pain. 

 When, on the other hand, only the ventral roots of these 

 spinal nerves are cut, the dog loses all control over the mus- 

 cles of its leg. Let the paw now be pinched, however, and 

 the animal at once gives unmistakable signs of discomfort. 



Functions of Nerve Fibers. Experiments like those just 

 described prove beyond a doubt that the nerve impulses that 

 result in sensation or in motion are carried by nerve fibers. 

 It is evident, too, that the dorsal and ventral roots of spinal 

 nerves differ in their function. We saw that sensation was 

 destroyed by cutting the nerve fibers that enter the dorsal 

 horns of the gray matter. Hence, we conclude that the 

 fibers in the dorsal roots carry messages to the spinal cord, 

 and, because they have this function, we call them af'fer-ent 

 fibers (Latin ad = to -\-ferre = to carry). By the last ex- 

 periment described above we proved that the ventral roots 

 conduct messages from the cord to the muscles; these nerve 

 fibers are therefore known as ef'fer-ent (Latin ex = from -f 

 ferre = to carry) (see Fig. 117). 



Nerve Impulses. We have liken }d nerve fibers to tele- 

 graph wires, and nerve impulses have been described as 

 messages that pass along the axis cylinders. But in mak- 

 ing these comparisons we must remember that telegraphy 

 and the action of the nervous system have, in all probabil- 

 ity, little real resemblance. We know that nerves transmit 

 impulses at the rate of about one hundred feet per second ; 

 electricity travels thousands of miles per second. Hence a 

 nerve impulse cannot very closely resemble what we call 

 a telegraph message. On the other hand, this nerve im- 

 pulse travels much too rapidly to be explained as a chem- 



