452 GENERAL BIOLOGY 



autonomy, there are abundant paths leading to adjoining 

 segments, ultimately connecting all the segments together. 

 If one imagine the two ganglia shown in figure 109 (page 

 173) to be packed (as in fact they are packed) with cells of 

 the several types that are shown singly in that figure, he will 

 readily conceive how numerous are the paths of possible 

 communication by contact between the nerve cells. 



But the existence of a bypath does not mean that a nerve 

 impulse must always follow it. There are mam-travelled 

 roads, as well as bypaths in the nervous system. A stimu- 

 lus at any given point has its accustomed path which it 

 always follows; whether it shall overflow into its possible 

 bypaths is apparently determined largely by its intensity. 

 A finger prick if slight enough may merely cause the finger to 

 be lifted in response; thus but a few muscles will be called 

 into action. But a more vigorous stimulus at the same 

 point may cause the arm to be jerked back, involving the 

 action of many muscles: and a deep puncture at the same 

 point may initiate nervous impulses vigorous enough to over- 

 flow into most of the motor circuits of the body, calling the 

 whole muscular system into action. This also may be 

 likened by analogy to the distribution of stimulating news 

 by telegraph. A slight accident excites but a little local 

 interest among the few people who are most directly involv- 

 ed, but a war scare may set the wires going in every impor- 

 tant telegraph office in the country. There is a mechanism 

 at hand for communicating with all the motor circuits in 

 the organism ; but only so much of it is used as is warranted 

 by the nature of the stimulus. 



Doubtless these paths of nervous communication have 

 had a gradual evolution, and have been formed to meet the 

 needs of the organism during its long past history. For 

 they lead in the direction of harmonious and well 

 co-ordinated action. All the muscles that may be called 



