GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 163 



from his flank, and if unable to reach it with one foot will bring 

 the other into service. All his acts, however, are purely mechan- 

 ical, and are determined by the spread of impulses over reflex 

 paths of less or greater complexity. 



The grading of the synaptic resistances in the spinal cord of any 

 animal, including man, is established with the development of the 

 cord itself. The organism is born with paths of least resistance 

 from the different receptors to adaptive muscles laid down. These 

 paths are as much part of the hereditary equipment of the indi- 

 vidual as is the spinal cord itself. Spinal cord reflexes apparently 

 do not require to be developed by training. They seem to be 

 performed as perfectly the first time as at any later time. An 

 excellent example of this class of reflexes in man is the sneezing 

 reflex, and we all know that the new-born infant does not have 

 to learn how to sneeze. He can do it from birth. Moreover, 

 and this is important as regards spinal reflexes as a class, if the 

 stimulus is strong enough he cannot help doing it. Although, as 

 we shall learn, we have a certain degree of voluntary control over 

 some spinal reflexes, their essentially automatic character should 

 be emphasized. 



In general the higher we look in the animal scale the less varied 

 and extensive are the spinal reflexes. A large proportion of all the 

 activities of such animals as fish and frogs are in this class, while in 

 man they are confined to a few relatively simple acts, such as 

 coughing, sneezing, winking, and simple withdrawal of an ex- 

 tremity from a source of irritation. 



Significance of the Head Senses in the Control of Reflexes. 

 We have noted how, in the lower animals, highly complicated acts 

 are performed automatically through the operation of the " spinal 

 cord" reflexes. When we study such activities in an animal whose 

 brain has been destroyed we note that on the sensory side they are 

 based exclusively on the body senses, touch, temperature, pain, etc. 

 (p. 172). The destruction of the brain has cut off all possibility of 

 any action on the part of the head senses, sight, hearing, taste, and 

 smell (p. 173). One result of this dependence on the body senses of 

 spinal cord reflexes is that they are, as a class, immediately pro- 

 tective. The adaptive response consists of the withdrawal from or 

 removal of a direct source of irritation. The chief significance of 

 the head senses is in their property of giving information of what is 



