664 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



direct control, the normal individual has some power over many of these 

 reactions ; for example, the impulse to micturition or defecation 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 neurous 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 an infant, and 

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

 enabled to hang from a bar, 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 mouth of life (Robinson, Nineteenth Century, 1891). 



The Nervous Background. We return now to the conditions which 

 modify the spread of the impulses within the central system, when this 

 system is represented by the spinal cord of a reflex frog. Admittedly, there 

 is here present but a fraction of the central system. It has been shown that 

 all incoming impulses tend to spread over a large part of the central system. 

 In a reflex frog, therefore, the cord is cut off from the remote eifects of 

 impulses which normally enter the system by way of cells located in the por- 

 tion removed. Moreover, in the complete nervous system, the incoming 

 impulses tend to be transmitted to the cephalic end, and in some measure 

 give rise to impulses returning within the central system and affecting the 

 efferent cells. In a fragment of the central system like the cord, such im- 

 pulses taken up by the central cells must pass so far as the neurons are intact, 

 but as these end at the level of the section, such impulses are lost, in the 

 physiological sense, at that point. 



The fact, therefore, that the experiments with spinal reflexes are conducted 

 on a portion of the central system has two important physiological conse- 

 quences. In the first place, there are wanting incoming impulses, direct or 

 indirect, from the portion removed ; on the other hand, through the section 

 of the afferent neurons, in their course within the central system, there is a 

 direct diminution in the number of the pathways by which the impulses arriv- 

 ing at the cord may be there distributed. It is most probable that in the frog, 

 at least, the reduction of the central mass does not so much diminish the num- 

 ber of pathways by which the impulses may be immediately distributed by way 

 of the afferent and central elements, as it diminishes the number of impulses 

 which by way of the portion removed arrive at the efferent cells and modify 

 their responsiveness. 



The modification of the responsive cells under more than one impulse is 

 well illustrated by an experiment of Exner : l A rabbit was so prepared that an 



1 Archiv fiir die gesammte Physiologic, Bd. xxvii. 



