ORIGIN OF THE NERVOUS SYSTEM 257 



The fibers in a nerve have no necessary functional relations one with 

 another, but are brought together chiefly by convenience of passage. 

 They are characteristic of those animals in which sense organs and 

 muscles have become well differentiated and widely separated from the 

 central organs, and are not to be confused with elongated bundles of 

 nervous elements such as are to be met with in some ccelenterates and 

 many echinoderms, for though these may represent early steps in the 

 evolution of nerves, they still retain so many evidences of functional 

 interrelation among their elements that they are to be classed rather 

 with nervous nets than with nerves. 



The differentiation of nerves as thus defined implies an increased 

 interrelation of neurones in the central apparatus as compared with the 

 condition in the more primitive nervous net. The nature of this grow- 

 ing interrelation has been well expressed by Sherrington (1906) in his 

 principle of the common path. This principle implies that each sense 

 organ may be connected through the central organ with every effector 

 and conversely any effector may receive through the central organ 

 impulses from any sense-organ. In consequence the central organ must 

 contain many common paths which are momentarily used, now for this, 

 now for that combination of particular receptors and effectors. This 

 condition without doubt obtains in earthworms as it does in higher 

 animals, and is a feature that can hardly be said to exist in the nervous 

 nets of the ccelenterates. 



It is also probable that the nervous mechanism in ccelenterates dif- 

 fers from that in the earthworm in its capacity as a nervous transmitter. 

 Attention has already been called to the fact that transmission in the 

 nervous net of a ccelenterate may occur in almost any direction and that 

 in the central nervous organs of vertebrates it is very definitely limited 

 and may in fact flow in only one of two apparently possible directions. 

 So definite a restriction can not be asserted for the earthworm but, as 

 Norman (1900) has shown, significant differences do obtain. If an 

 earthworm that is creeping forward over a smooth surface is suddenly 

 cut in two near the middle, the anterior portion will move onward 

 without much disturbance whereas the posterior part will wriggle as 

 though in convulsions. This reaction, which can be repeatedly obtained 

 on even fragments of worms, shows that a single cut involves a stimula- 

 tion which in a posterior direction gives rise to a wholly different form 

 of response to what it does anteriorly; in other words, transmission in 

 the nerve-cord of the worm is specialized as compared with transmission 

 in the nervous net of the ccelenterate. 



There is good reason to believe that the cerebral ganglion or brain 

 of the earthworm is in a measure degenerate. Certainly if we turn to 

 such an annelid as Nereis we find in place of the small mass of gangli- 

 onic cells and fibers that represent the brain in the earthworm a much 



vol. lxxv— 17. 



