COORDINATION 85 



15. The acquisition of reflexes ; conditioned and uncondi- 

 tioned reflexes. There can be no doubt that many of these 

 reflex mechanisms are born with us. A newborn baby, for 

 example, like the adult, winks and secretes tears when the 

 cornea dries ; it secretes saliva when a sapid substance is 

 placed in the mouth ; it swallows when something touches 

 the throat; if a cane is brought in contact with the palm 

 of the hand, it is grasped firmly. These and many other 

 reflex actions take place from the first because the baby in- 

 herits and hence is born with the complete reflex mechanism 

 for their execution upon the application of the appropriate 

 stimuli. 



On the other hand, new involuntary reactions can be 

 acquired in adult life, even reactions which are useless to 

 the body. The extent to which this is true is illustrated by 

 the following extreme case : if a piece of ice is applied to a 

 definite spot of the skin, the amount of blood flowing through 

 that part of the skin is greatly diminished and the skin be- 

 comes pale. This is an inherited reflex which (Chap. XII) 

 protects the body from exposure to cold. A morsel of food 

 placed on the tongue (where it stimulates the afferent nerves 

 of taste) will reflexly excite the flow of saliva. In both cases 

 we see the obvious purposeful relation between the stimulus 

 and the reaction and in both cases we are dealing with in- 

 herited reflexes. Moreover, these two reflex mechanisms as 

 inherited are entirely independent of each other, for the 

 stimulation of the skin by ice does not excite the flow of 

 saliva nor does the stimulation of the sense of taste influence 

 the blood flow through the skin. If, however, every time 

 that one eats, a piece of ice is applied to the same region of 

 skin, so that loth these reflexes are simultaneously excited, in 

 the course of two weeks or more it will be found that the 

 application of ice * to the skin excites a flow of saliva even 

 though no food is taken into the mouth. In other words, these 

 two reflex mechanisms have become associated, so that activity 



