384 HUMAN ANATOMY 



ology run together, and the clearer will be the conception of the 

 sort of structure which it will be most desirable to increase for the 

 attainment of our final purpose. On the other hand, if we follow 

 the path from the grosser to the finer anatomy, we are led to in- 

 quire whether there is any one part or system of the human body 

 which at the present moment is specially worthy of attention. 

 When we say that the nervous system is such a part, I think that 

 even those who are not engaged in the study of it will admit that 

 there are some grounds for the statement. The peculiar feature 

 which sets the nervous system apart is the fact that its enlarge- 

 ment, both in the animal series and during the development of the 

 individual, is in a very special way accompanied by changes in its 

 physiological and psychological reactions. To be sure, we think of 

 it as built up fundamentally by the union of a series of segments, 

 but the relationship established between these segments becomes ul- 

 timately so much more important than the constituent units that in 

 the end we find ourselves working with a single system of enormous 

 complexity, rather than a series of discrete units; a state of affairs 

 which is not paralleled in any other tissue. In addition to this, the 

 nervous system as a whole is par excellence the master system of 

 the body, and as such, the reactions of the organism are very largely 

 an expression of its complexity. Indeed, within the different classes 

 of vertebrates, the various species may be regarded as compound 

 bodies composed of four fundamental tissues, and a species could 

 well be defined by the quantitative relations found to exist be- 

 tween the nervous, muscular, connective, and epithelial constitu- 

 ents. Working from this standpoint, Dubois, 1 the Dutch anato- 

 mist, stimulated by the work of Snell, 2 has brought forward evi- 

 dence for the view that when, within the same order, several species 

 of mammals similar in form, but differing in size, are compared 

 with one another, the weight of the brain is found to be closely 

 correlated with the extension of the body surface, and by inference 

 with the development of the afferent system of neurones. This view 

 would seem to imply that in these cases there is the same density 

 of innervation of each unit- area of skin; but the correctness of this 

 inference can only be determined by the careful numerical study of 

 the afferent system of the animals compared. It will appear, how- 

 ever, that under the conditions imposed the relative weight of 

 the brain depends upon the fact that each unit-area of skin, repre- 

 sented by the nerves which supply it, calls for a correlated addi- 

 tion of elements to the central system, and thus the increase in 

 one part is followed by a corresponding increase in the other. When, 

 however, the large and small individuals within the same species 



1 Dubois, Archiv fur Anthropologie, 1898. 



2 Snell, Archiv fur Psychiatrie und Nervenkrankheiten, 1892. 



