CHAP, x.] OF THOUGHT. 433 



to be a source of automatic nervous activity, an unconscious 

 nervous centre. It may then be the same with the nervous 

 ganglions of the arthropods, &c.,'in which, besides, the prepon- 

 derance of the cerebroidal ganglion is much less evident, and 

 surely much less absolute than in the brain of vertebrated 

 animals. 



In the latter, the brain is the principal, and very probably 

 the only conscious organ. In traversing the series, from lower 

 to higher, we see the cerebral hemispheres grow large in propor- 

 tion as the species is better endowed and more intelligent. 

 There are first simple nervous ampullae, scarcely to be distin- 

 guished from the other nervous intracranian expansions (see 

 Book I., ch. VI). Then gradually these cerebral vesicles grow, 

 and end by dominating and covering, more or less completely, 

 the other encephalic nervous ganglions ; their surface, almost 

 exclusively formed of nervous cells, becomes furrowed, is folded 

 in flexuous digitations called circumvolutions. The more in- 

 telligent the animal, the more numerous generally are these 

 circumvolutions. Their object, like that of the pecten, which 

 thrusts, in the eye, from back to front, the retina of certain 

 birds, is to multiply surface under a small volume. At the 

 same time that their number and ramifications augment, the 

 grey substance, the cellular cortex, which covers them, and is 

 the conscious part of the brain, augments in thickness. In man, 

 we must estimate at many thousands the number of nervous cells 

 superposed in strata in a square millimetre of cortical cerebral 

 substance, and the number of these strata is greater at the 

 anterior part of the brain, in the frontal lobes, which seem to 

 be the headquarters of intelligence. 1 



In a preceding chapter, we have summarised the beautiful 

 anatomical systematization of the cells and fibres of the human 

 nervous centres, for which we are indebted to M. Luys. We 

 have seen that thousands of sensitive and peripheric fibres of 



1 J. Luys, Etiides de Physiologic et de Pathologic Cirtlrales, p. 11. Paris, 

 1874. 



F F 



