CHAPTER XCV 



THE MOTOR AREAS OF THE CEREBRUM AND THE EFFERENT 

 PATHWAY TO SKELETAL MUSCLE 



It is a debatable question whether motor acts are ever initiated by the 

 nervous system except in response to some stimulus which sends afferent 

 impulses into the brain or cord, or as the result of changes in the im- 

 mediate environment of the nerve centers due to abnormal conditions in 

 the circulation. There are, however, a large group of responses the nature 

 of which is conditioned not only by the immediate stimulus which calls 

 them forth, but by the previous experience of the organism. Situations 

 which have existed in the past leave their mark upon the nervous system 

 in the form of memories, associations and the like, and these determine 

 how the animal or man will behave when new groups of stimuli, or sit- 

 uations, arise. We shall see later, how previous experience may alter the 

 nature of even involuntary responses to simple stimuli, when we 

 take up the formation of conditioned reflexes (page 954). In the present 

 place it will suffice to point out that unless we have considerable knowl- 

 edge of the past experience of an animal it is impossible to predict how 

 it will respond to certain situations. Responses of this type consequently 

 are not obviously and invariably related to any particular stimulus, as 

 are the simpler reflex responses, and consequently appear to arise spon- 

 taneously in the nervous system. They are consequently called voluntary 

 acts, which we shall take to imply that their nature and occurrence is 

 related quite as closely to preexisting conditions in the nervous system 

 as to the immediate situation which brings them ijorth. 



In animals from which the cerebrum has been removed, motor responses 

 of a very perfect nature may still be carried out. A pigeon in this con- 

 dition can walk, fly, coo, etc., quite normally, and if fed may live indefi- 

 nitely. A dog from which the cerebral cortex is removed shows strik- 

 ingly little difference in its behavior from a normal animal. Its equilib- 

 rium is good, it moves easily, avoiding objects in its path, swims when 

 thrown into water, feeds himself if food is brought into contact with 

 his nose and rejects food of disagreeable taste. The reactions of such 

 animals become almost strictly predictable, because they show no signs 

 of being influenced by past experience. The pigeon is no longer fright- 

 ened by a loud noise or sudden movement, nor does it suddenly become 

 active for no obvious reason as a normal bird would. The dog shows no 



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