102 HUMAN PHYSIOLOGY 



They are determined by external stimuli some of which 

 may have been brought to bear a long time before the 

 occasion. A man somewhat suddenly decides to walk 

 up a hill to enjoy a prospect which he has seen the 

 previous year. We describe the act as voluntary but 

 at least one of the factors concerned in causing it is 

 the impression made upon his nervous system at the 

 time of his earlier visit. So the discussion of reflexes 

 leads not only to the problems of habit but to those of 

 memory. This subject will be more advantageously 

 considered in connection with the cerebrum. 



Examples of Reflex Action. We have spent a good 

 deal of time in treating the topic in general terms. 

 Let us now turn to some specific illustrations. What 

 are some of the reflexes exhibited by a baby and how 

 do they make for its welfare? One thinks imme- 

 diately of the sucking reflex. The infant sucks vigor- 

 ously whatever is put between its lips. In the natural 

 course of events this reaction insures a supply of food. 

 As it eats it frequently chokes and coughs; the cough is 

 a reflex which prevents the entrance of foreign materials 

 into the breathing passages. A similar reflex sneez- 

 ing is calculated to expel obstructing substances from 

 the nose. Vomiting is a reflex which relieves the over- 

 filled stomach. Crying has a less obvious function 

 unless we assume that it secures the attention of the 

 parents to needs which the child by itself cannot satisfy. 



Some of the reflexes which are prominent in the baby 

 are disguised or replaced by others during the period of 

 growth. Sucking ceases to be a predictable perform- 

 ance. Crying is less and less readily induced. The 

 other responses which we have mentioned continue to 

 occur. So, too, the reactions of the pupils and the 

 withdrawal of parts of the body from objects that 

 threaten injury are retained. In the course of a year 

 or two the child has the capacity to keep its feet and to 

 walk, attainments based largely upon the development 

 of reflex mechanisms. 



