408 HISTORY OF THE HUMAN BODY 



ing a dell or trough, in the bottom of which lay the sensitive 

 surface. Such a method of protection, once inaugurated, could 

 have but one logical outcome, the gradual formation of a tube 

 through the increase in height and the approximation of the 

 protecting lateral folds, until in this way the form was at- 

 tained with which all the present-day vertebrates are equipped. 

 We must here not lose sight of the fact that the original ex- 

 ternal, and hence the sensitive, surface is not that of the ex- 

 terior, but that of the lumen of the tube, which explains the 

 fact, to be developed later, that in all lower vertebrates the 

 central or ganglion cells, which form the " gray matter," are 

 situated along the lumen, and not along the external surface, 

 a condition retained throughout in the more conservative 

 spinal cord, although secondarily in the higher forms large 

 masses of gray matter develop also over the external surface 

 of parts of the brain. 



A central nervous system, by thus sinking into the interior 

 and becoming, entirely covered up by a much less sensitive 

 surface, gains the protection which it seeks, but, in order to 

 retain its function as a receiver of external stimuli, a function 

 upon which its very existence as a nervous system depends, 

 it must retain its connection with the exterior through sets of 

 secondary cells which remain external and are yet continuous 

 with the central organ. 



These are the sensory cells, which, when grouped over a 

 certain area and specialized to receive a certain form of 

 stimulus, become definite sense-organs. These are connected 

 with the central system by sensory nerves, in which the direc- 

 tion of the impulse is always from without inward, that is, 

 afferent or centripetal. As the sensory cells become themselves 

 more specialized and hence more sensitive as well as more 

 vital to the organism, they themselves need protection, which 

 they obtain either by the formation of an external non-sensi- 

 tive horny layer, the epidermis, which protects the sensory 

 cells scattered over the general surface while it still allows the 

 transmission of the stimuli ; or, in the case of such special 

 sense organs as the patches of sensory cells that form the es- 



