48 NUTRITIONAL PHYSIOLOGY 



when they end in connection with the cells of glands they 

 may be secretory. Probably also they may inhibit secretion. 



Just as we have found the word " motor " inadequate 

 and have agreed to replace it by " efferent," so the word 

 " sensory " does not properly indicate the whole service of 

 the fibers which bear impulses toward the brain and cord. 

 Sensory implies " productive of sensation," and we cannot 

 assign such a property to all the two million fibers which 

 assail the centers with their communications. In the 

 great majority of cases we do not feel any consequences of 

 their activity. The term afferent is free from this objec- 

 tion and is the logical complement of efferent. If one 

 hastens to ask what is the significance of afferent fibers 

 which do not arouse sensation, the answer is simple and 

 definite: They produce reflexes. 



If the first element in the reflex process is the applica- 

 tion of an external stimulus, it is now clear that the next 

 element is the afferent transmission of the impulses. 

 What these impulses are cannot be discussed. It should 

 be recalled that they are not fluid pulses nor electric cur- 

 rents in the usual sense of the expression. They repre- 

 sent energy of some kind in rapid, but not immeasurably 

 rapid, motion. They pass along the nerves at rates in 

 excess of 100 feet in a second, so that the longest paths in 

 the human body are traversed almost instantaneously. 

 The time used in such transits might be quite appreciable 

 if we could observe it closely in a whale. 



When the afferent impulses reach the central nervous 

 system the third event in the development of the reflex 

 act occurs. This is localized in the brain or the spinal 

 cord, and we may speak of it as "a central process " 

 without committing ourselves as to its exact character. 

 What we actually observe is that the arrival of the afferent 

 impulses is followed by the appearance of efferent ones. 

 It is not necessary to decide whether these efferent im- 

 pulses are the same currents which just entered the in- 

 tricate fabric of the central organ and which have found a 

 path open through its mazes which has led them out 



