78 INTRODUCTION TO NEUROLOGY 



I 



with reference to its environment. This is a very fundamental 

 distinction. These two functions are quite diverse and the 

 organization of these two parts of the nervous system shows cor- 

 responding structural differences. 



The internal adjustments of the visceral systems are effected 

 by a nicely balanced mechanism of local and general reflexes so 

 arranged that most of their work is done quite mechanically and 

 unconsciously. The taking of food and its preliminary mastica- 

 tion are generally voluntary acts whose various processes are 

 or may be controlled at will. But once the food has passed into 

 the esophagus, the further work of swallowing, digestion, and 

 assimilation is no longer under direct control. The presence of 

 a morsel of food in the upper part of the esophagus excites the 

 muscular movements necessary for the completion of the act of 

 swallowing, which no act of will can prevent or modify. In fact, 

 any attempt at conscious interference or regulation is apt to 

 result in an incoordination of the movements involved, and 

 sputtering or gagging may result. 



The mechanisms involved in these processes are inborn and 

 require no practice for their perfect performance. They are 

 innate, invariable, and essentially similar in all members of a 

 race or species. They are, moreover, nicely adapted to the 

 mode of life characteristic of the species. In a carnivorous ani- 

 mal the whole physiological machinery of nutrition is different 

 from that of a herbivorous animal. These physiological and 

 structural peculiarities by which each species of animal is 

 adapted to its mode of life have been brought about by natural 

 selection and other evolutionary factors. This is not absolutely 

 true of all visceral actions; some are acquired and modifiable. 

 But as a general rule this is their type. 



Some of the somatic actions are likewise innate and relatively 

 fixed in character. This is true of most of the proprioceptive 

 reactions and of many of the exte receptive as well. Fish can 

 swim as soon as they are hatched ; chicks just out of the shell have 

 an instinctive tendency to peck at all small objects on the ground. 

 But in most of these cases (of which innumerable instances 

 might be cited) some practice is necessary before perfect re- 

 sponses are attained ; and a very large proportion of the extero- 

 ceptive acts are not innate, but acquired by long and often ardu- 



