156 NERVOUS CORRELATION 



adjoining joint, or may pass from the ganglion along the 

 whole length of the body, arousing the most distant ef- 

 fectors to action. 



Complex Reflex Action. — It will be clear that in a 

 jointed animal with its multiplicity of receptors and ef- 

 fectors and its numerous ganglia there is much more scope 

 for complexity of nervous action than in the simple un- 

 jointed forms. An additional feature of the situation 

 also contributes to the complexity, namely, that there is 

 nothing to prevent more than one receptor from being 

 stimulated simultaneously. In fact, the usual thing is 

 for a number of receptors to be stimulated at once. This 

 means that several streams of nerve impulses are pouring 

 into the same ganglion if the animal be of the simple 

 type with only one joint, or into various ganglia if the 

 animal is composed of several joints. In either case 

 proper adjustment requires that the streams of incoming 

 impulses be so correlated that the outgoing impulses 

 to which they give rise shall throw the effectors into 

 action in a way that is orderly and adjusted to the 

 requirements of the situation. Part of the function of 

 the ganglia is thus to correlate the streams of incoming 

 nerve impulses into properly organized outgoing streams 

 to the effectors. Between the simple and direct passage 

 of a nervous impulse from a given receptor to a particular 

 effector, and this organization of streams of impulses to 

 bring about the complicated yet orderly activities of 

 many effectors is a long step, and we must confess at once 

 that we do not know how the process of correlation is 

 actually carried out. It is not at all difficult to prove 

 that there is such a process and that it functions very 

 efficiently, but much beyond that the investigations of 

 biologists have not yet enabled them to go. 



Importance of the Front End of the Body. — If the 

 activities of a jointed animal be considered as it moves 

 from place to place, it will be realized that its front end 

 is ordinarily the part that comes first into a new environ- 



