50 NUTRITIONAL PHYSIOLOGY 



cause and the response. The number of nerve-fibers 

 which can be excited by the slender proboscis of a mosquito 

 as it pierces the skin of a sleeper must be very small. The 

 reflex movement which results may involve a very large 

 share of his skeletal muscles. 



The fifth and final occurrence completing the reflex is 

 the reaction on the part of the muscles or the gland-tissue 

 in which the efferent fibers end. As already indicated, this 

 may be a movement, an outpouring of secretion, or it may 

 have a negative character, the suppression of movements 

 that would naturally have occurred, or possibly the with- 

 holding of some secretion which would otherwise have been 

 discharged. The illustrations of reflex action most often 

 chosen are those in which an immediate, even abrupt, 

 response is seen. Yet it is quite easy to find examples 

 of gradual adjustment to the new external condition. 

 Changes of color, the outward sign of changes in the 

 blood-supply of the skin, when they occur on account of 

 warming or cooling of the surface, are reflexes of this 

 prolonged and gentle order. 



If there is any doubt as to whether a certain action is to 

 be classed as a reflex, it may be tested according to the 

 foregoing analysis. There must be an assignable stimulus, 

 external at least as regards the central nervous system, 

 there must be an afferent flow of the impulses resulting 

 from the stimulation, a process within the bounds of the 

 central axis, a return flow of impulses in multiplied volume, 

 and the action itself. The more one thinks of the common 

 course of events, the larger the number of actions which 

 he finds he can place in this class. It becomes appropriate 

 to ask what kinds of bodily activity are outside this depart- 

 ment. To this question it can be replied that automatic 

 actions, such as the beating of the heart, are to be dis- 

 tinguished from reflexes. The nervous system is not re- 

 quired to maintain the heart-beat. There are cases also 

 in which the chemical composition of the blood reaching 

 the centers modifies their behavior and causes them to send 

 out certain impulses. Such cases do not fit our descrip- 



