214 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



It thus appears that the reflex responses, namely, simple reactions unac- 

 companied by consciousness, are in man mainly given by the unstriped 

 muscle-tissue and by glands, and only in a minor degree by the striped 

 muscles. Moreover, while the typical reflex is a reaction over which we 

 cannot exercise direct control, the normal individual has some power over 

 many of these reactions; tor example, the impulse to micturition or defeca- 

 tion can be thus delayed, respiration arrested, and, in some instances, so 

 remote a reaction as the beat of the heart either accelerated or slowed at will. 



It is of interest to note that many reflexes which in the young are not 

 controlled, as micturition, for instance, become so gradually, a change most 

 probably dependent on the growth of axones from the cephalic centres into 

 the cord, thus subjecting the cord-cells to a new set of impulses which modify 

 their reactions. That such is the case is indicated by the fact that extreme 

 fright or anaesthetics, which diminish the activities of the higher centres, often 

 cause these reactions to take place involuntarily. Other reflexes are present 

 in early life, but disappear later; such are the sucking reflex of the infant, 

 and the remarkable clinging power of the hands, by which a young child is 

 enabled to hang from a liar, thus supporting the weight of its entire body, 

 often for several minutes. This last capacity soon begins to wane, and usually 

 disappears by the second month of life. 1 



Co-ordination of the Efferent Impulses. — Incessantly the efferent im- 

 pulses pass out from the cord to the muscles and glands. With each fresh 

 afferent impulse those which go out are modified in strength and in their 

 order, but just how they shall be co-ordinated is dependent on so many and 

 such delicate conditions that even in the simplest case the results are to be 

 predicted only in a general way. 



The attempt to determine the spread of the impulse in the cord by 

 observing the order in which the various muscles of the thigh and leg con- 

 tract in response to thermal stimuli was made by Lombard. 2 In a reflex 

 frog the tendons of the leg and thigh muscles were exposed at the knee, and 

 each attached to n writing-rod in so ingenious a manner that simultaneous 

 records of fifteen muscles could sometimes be obtained. The stimulus was a 

 metal tube, filled with water at 47°-61° C, which was applied to the skin. 

 Under these conditions, it was remarkable that a continuous stimulus was 

 often followed, not by a single contraction of the muscles, but by a series of 

 contractions, suggesting that in the central system the cell- were roused to a 

 discharge and then for a time concerned with the preparation for sending out 

 new impulses, and that during this latter period the muscles were relaxed. 



Apparently a high degree of uniformity in the conditions was obtained in 

 these experiments, but at the same time the reactions were far from uniform, 

 in either the latent time of contraction or the order in which the contraction 

 of the several muscles followed, although certain muscles tended to contract 

 first, and certain series of contractions to reappear. The co-ordination of the 



1 Kobinson : Nineteenth Century, 1891. 



1 Archiv Jiir Anatomie und Fhysiologie, 1885. 



