RICFLKXES 53 



window-washer crawled back into the room and lay for 

 a while upon the floor. 



As we are constantly speaking of nerve-centers, it is 

 time to define what is meant by that expression. A center 

 is usually assumed to be a collection of perikarya mediat- 

 ing a definite action which may be either motor or secre- 

 tory. The center is a station to which impulses flow from 

 different sources at different times, but which transmits 

 impulses always to a common path. When mention is 

 made of a center one thinks naturally of a circumscribed 

 spot in the nervous system an anatomic conception. 

 But it is necessary to bear in mind at the same time 

 that widely separated neurons may have such connec- 

 tions that they will be unified in their activities. In 

 other words, centers may have a physiologic existence 

 where there is no focal assembling of the cooperating 

 units. 



When we try to draw the line between those actions 

 of the nervous system which can unhesitatingly be called 

 reflexes and others not to be called so, we find ourselves in 

 the midst of difficulties. In elementary presentations of 

 the subject it is usual to distinguish between reflex and 

 "voluntary" action, but the more we observe the facts, 

 the less satisfactory this attempted distinction appears. 

 It is certainly true that in most of those movements 

 which we are accustomed to call voluntary we can still 

 recognize the directing influence of external conditions. 

 This is a way of saying that such acts partake of the 

 reflex character. We find it most clearly the case in 

 monotonous serial movements like those of walking. We 

 say that we walk because we will to do so, but we can 

 hardly claim that the formation of a conscious purpose 

 precedes the taking of each step. At another time we 

 may analyze more fully what can be baldly stated here: 

 that each position assumed by the body and limbs of the 

 walker establishes stresses within and contacts without of 

 such a nature as to dictate the appropriate succeeding 

 contraction. 



