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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



originates the breathing moment ; but if the 

 breathing is arrested (even voluntarily) for a few 

 moments the impression is at once acutely felt. 

 It follows that the stimulus conveyed to a mo- 

 tor ganglion by its afferent nerve may either 

 be reflected along the efferent nerve at once, and 

 may exhaust itself in motion, or it may pass on 

 to a sensory ganglion, and may excite sensation. 

 The sensory, like the motor, ganglia send out 

 efferent nerves to muscles, and are essentially 

 motor ganglia themselves, with the difference al- 

 ready mentioned, that the changes produced in 

 them are attended by sensation. Many of the 

 actions which they excite are as purely reflex, or 

 automatic, as those excited by the motor ganglia 

 themselves. The sudden closing of the eyelids 

 when any substance is seen to approach the eye, 

 the yawning which is produced by the sight of 

 yawning in others, the start which follows an 

 unexpected sound, are familiar examples which 

 might be multiplied a thousand-fold. Such ac- 

 tions, which are automatic and involuntary, but 

 ■which differ from those formerly described as 

 " reflex " in being accomplished by the agency of 

 a sensory ganglion, are called " sensori-motor," 

 and in most of them the sensation appears to ful- 

 fill no other purpose than that of being con- 

 ducive, under ordinary circumstances, to the 

 pleasure of the animal. The two groups together, 

 the reflex and the sensori-motor actions, contrib- 

 ute largely to form the sum of what is properly 

 called instinct — they are impulses "prior to ex- 

 perience and independent of instruction." Many 

 of them present a character of adaptiveness which 

 is curiously deceptive as regards the intelligence 

 or knowledge of the animals engaged in perform- 

 ing them. These animals are alike unconscious 

 of a purpose, and unable to act otherwise than 

 they do, but the original or true instincts, prob- 

 ably in every case, are directed to the mainte- 

 nance of life, to the capture of prey, to escape from 

 anger, or to the perpetuation of the species. 



The growth and nutrition of the nervous cen- 

 tres, like that of other organs of the body, are 

 increased by active employment ; and hence a 

 good many capacities for action, which are natu- 

 rally dormant, are capable of being developed by 

 use until they become acquired or secondary in- 

 stincts ; and these, or an increased aptitude for 

 acquiring them, become hereditary in the course 

 of a few generations. The "pointing" of dogs, 

 which is manifestly an instinct modified by train- 

 ing, and which is now often perfectly well done 

 by puppies the first time they are taken out in 

 quest of game, is a familiar example furnished 



by a lower animal; but the acquired instincts 

 are seen most frequently in mankind, who are 

 more exposed to the necessary training. Any 

 kind of muscular action, such as the fingering of 

 a musical instrument, the working of a treadle, 

 or even the act of writing from dictation or from 

 a copy, may assume a purely sensori-motor char- 

 acter, so that it can be performed automatically 

 and with perfect correctness when the thoughts 

 are far away. Dr. Carpenter relates some very 

 curious illustrations of this. 



In the higher mammalia, and in the greatest 

 degree in man, the sensorium is crowned by yet 

 another pair or assemblage of ganglia, constitut- 

 ing the cerebrum, which has for its office the 

 formation of ideas. An impression made upon 

 the sensorium, whether by the passing upward 

 of a message from a motor ganglion or directly 

 through an organ of sense, may either discharge 

 itself in sensori-motor action or it may pass on 

 to the cerebrum and lead to the formation of an 

 idea about the impression. The idea is not itself 

 an object of consciousness ; but it becomes so as 

 soon as it is carried back to the sensorium by 

 the efferent nerves of the cerebrum. It is then 

 recognized ; and at this stage of its history it 

 may go out in purely automatic action, which is 

 called ideo-motor. Thus the idea of a disgusting 

 object may occasion the muscular effort of sick- 

 ness, just as perfectly as the sight or smell of the 

 object itself. An idea which excites anger may 

 lead to a sudden blow ; or one which excites fear 

 may lead to sudden efforts to secure safety. An 

 officer was once breakfasting alone, in an Indian 

 jungle, in a tent supported by a tall central pole 

 of smooth and slippery bamboo. A tiger walked 

 into the tent. The officer climbed to the top of 

 the pole, and remained there until his uninvited 

 visitor had departed. The stimulus to this ex- 

 traordinary feat was not the sensation, not the 

 sight of the tiger, but the idea of the danger aris- 

 ing from its proximity; and all panic-stricken 

 flight is in like manner ideo-motor. 



There is great reason to believe that the nerve- 

 changes already mentioned, coupled with emo- 

 tions — which may be defined as ideas with which 

 feelings of pleasure or pain are associated — and 

 with memory, or the faculty of remembering past 

 ideas as matters of personal experience, are suf- 

 ficient to explain all the particulars of the con- 

 duct of the lower animals. It is probable that 

 the actions of a dog, when they are neither reflex 

 nor sensori-motor, neither original nor acquired 

 instincts, are always ideo-motor, and always the 

 inevitable results of the balance of his past ex- 



