izo THE POPULAR SCIENCE MONTHLY 



organs or receptors are, for the most part, the distal ends of sensory 

 neurones. The central organs or adjusters include the proximal ends of 

 these elements, all the association neurones, and the proximal ends 

 of the efferent neurones. The effectors are not neurones at all, but 

 muscle fibers, gland cells or other types of cells under the control of 

 nerves. Thus the ordinary reflex may be said to involve in sequence the 

 activity of a receptor, adjustor and an effector, to use modern terminol- 

 ogy, and these three elements are recognizable in every complete 

 reflex arc. 



Our own reflexes are sometimes associated with consciousness and 

 sometimes not. When we pass from a region of dim light to one of 

 bright light the pupils of our eyes contract without our being conscious 

 of the fact. In a similar way, when food is introduced into the digestive 

 tract, a whole succession of reflex movements is called forth without 

 any direct relation to our consciousness. On the other hand, if we burn 

 a finger, it is usually withdrawn with full recognition of the sensation 

 and the response. Thus a reflex may or may not be association with 

 a conscious state. 



From this standpoint, what is the condition in the lower animals? 

 Have they nervous systems composed of neurones and exhibiting reflexes 

 which in some instances are associated with consciousness, and in others 

 not ? In other words, what have been the steps by which has developed 

 that mechanism which serves us at once as the means of our simplest 

 reflexes and the material basis for our intellectual life ? 



As an example of the lower animals whose nervous activities are 

 worthy of consideration we may take the earthworm. This animal has 

 at its anterior end a small brain from which a ventral ganglionic chain 

 extends posteriorly through the rest of its body. It possesses sensory 

 neurones which extend from the skin into the central nervous organ and 

 motor neurones reaching from the central organs to the muscles. The 

 central organ itself contains association neurones. Thus the three 

 classes of nervous cells which occur in man are also represented in the 

 earthworm but with this difference. The association neurones, which in 

 man are relatively very numerous, are in the earthworm comparatively 

 few. Otherwise the essential composition of the nervous organs in 

 these two forms has much in common. 



Not only is the nervous system of the earthworm composed of ele- 

 ments essentially similar to those of the higher animals, but it exhibits 

 similar functional relations. The earthworm responds to a large range 

 of stimuli by appropriate and characteristic reactions, and its move- 

 ments justify the conclusion that its reflex arcs, like those of the higher 

 animals, involve receptors, an adjustor, and effectors. 



Whether certain of the reflexes of the earthworm are associated with 

 consciousness or not is a question that can not be answered definitely, 



