PHYSIOLOGICAL INTERPRETATIONS 75 



tion of all movements. Some parts of this intricate fabric, generally 

 witli thicker fibers, more or less well fasciculated, activate mass 

 movements of primitive type, and other parts control local reflexes 

 as these are individuated. But these systems of fibers are not segre- 

 gated in comjjletely insulated reflex arcs. They are interconnected by 

 collateral branches with one another and with the interstitial neu- 

 ropil. There are lines of preferential discharge, but whether any one 

 of them is actually fired depends on numberless factors of peripheral 

 stimulation and central excitatory state. 



The phylogenetic history is parallel. The further down we go to- 

 ward the primitive ancestral vertebrates, the less clear evidence do 

 we find of definitely localized reflex arcs, and the overt behavior 

 tends more toward mass movements of total-pattern type. 



It must be borne in mind that the development of the individual 

 does not exactly recapitulate the phylogenetic development (Hooker, 

 '44, pp. 15, 33). The pattern of the sequence of structural changes' 

 which take place during prefunctional stages of growth is determined 

 by the organization of the germ plasm and the interaction of the 

 genetic factors with one another. This organization, in turn, has been 

 determined during preceding evolutionary history in adaptation to 

 the environment and habitus of the species in question. In broad 

 lines the history of ancestral development is repeated in the growth 

 of the embryo, but cenogenetic modifications of it may appear in 

 adaptation to changing conditions, as illustrated, for instance, by the 

 appearance of some local reflexes earlier in mammals than in am- 

 phibians. 



The structural organization of the brain sets off in sharp relief a 

 few important general physiological principles. First, it is to be noted 

 that the "resting" nervous system is not inert. The body acts before 

 it reacts. There is always some spontaneous — that is, centrally ex- 

 cited — activity, and the importance of this factor increases as we 

 ascend the phylogenetic scale. There is always intrinsic activity, as 

 demonstrated, for instance, by the Berger rhythms, and it is always 

 acted upon by numberless extrinsic agencies. When an excitation is 

 received from the periphery, there results a change in the central 

 excitatory state both locally and diffusely, which involves both acti- 

 vation and inhibition. 



Another general principle may be mentioned here. The flow of 

 nervous impulses from receptor to effector is not one-way traflSc. 



