332 THE BODY AT WORK 



system determine the disposition and degree of concentration 

 of its various business centres. It shows, when followed 

 throughout the whole animal kingdom, extreme variability of 

 its constituent organs, with absolute uniformity of plan. In- 

 deed, from the physiological point of view the term " organ " is 

 scarce admissible. It implies diversity of function in too high 

 a degree. The several parts into which the central nervous 

 system is obviously divisible co-operate so intimately as to 

 preclude us from thinking of them as separate organs. 



If the citadel of the central nervous system is to be captured, 

 all lines of approach must be tried. Its outward form must 

 be studied, its minute structure examined with the micro- 

 scope, its modifications in various animals compared, its de- 

 velopment followed, its reactions to artificial stimuli tested, 

 its pathological deficiencies and vagaries watched. Yet, of all 

 the means which have been made use of in attempting to 

 penetrate its secrets, the study of its history, by the methods 

 of comparative anatomy and embryology, has probably con- 

 tributed most to the development of sound ideas regarding 

 the manner of its working. The first differentiation visible 

 in the blastoderm the globe of cells into which the ovum 

 divides and out of which the embryo is built has relation 

 to the formation of the nervous system. If the earliest 

 stages of its growth are followed, and the different phases 

 through which it passes are compared with the forms which it 

 assumes permanently in lower animals, the plan or type upon 

 which it is constructed shows up distinctly. Looking down 

 the line to the earliest vertebrata, we can discern clearly the 

 form of nervous system possessed by their prototype. Not 

 that this " ideal ancestor " ever existed. Experience teaches 

 that it is unlikely that any animal that ever lived was abso- 

 lutely regular and symmetrical in all its parts ; nevertheless, 

 the type can be presented in a perfectly regular scheme. The 

 ideal ancestor of the vertebrata was segmented, like a cater- 

 pillar or a worm. Its mouth was not at the anterior extremity 

 of the body, but two (or more) segments behind it. Every 

 segment bore a sense-organ (at one period two sense-organs) on 

 either side. Beneath each sense- or gian there was a clump of 

 " gray matter." Each segment also contained (although not at 

 the earliest epoch) two clumps of nerve-cells and neuropil in 



