466 HISTORY OF THE HUMAN BODY 



the recognition of musical tones; indeed, the early aquatic 

 forms possess in the lateral line organs an entire system, no 

 trace of which seems to have survived the transition to land, 

 and yet, with no especial progress on the part of the sense- 

 organs, the central nervous system, and especially the brain, 

 the receiving organ of the special senses, has increased from 

 a simple condition to one showing a marvelous degree of 

 complexity. The cause of this extreme development must be 

 laid, then, not to the sense-organs, but to the direct and cumu- 

 lative influence of the impressions received. The motor cen- 

 ters, which have contributed not a little to the complexity of 

 the central nervous system, have also developed in response 

 to the external environment, though rather more indirectly, 

 perhaps, through the necessity of controlling the more spe- 

 cialized limbs and other parts, which, in their turn, were 

 directly influenced by external conditions. 



The morphological history of the sense-organs does not, 

 therefore, show the extensive progress exhibited in the case 

 of most of the other systems; but as certain definite changes 

 were necessitated by the transition from water to land, this 

 history is divided into two great stages, ( i ) that of the aquatic, 

 and (2) that of the terrestrial life. 



Throughout the animal kingdom, the elementary type of 

 sense-organ is a single epithelial cell, connected by a nerve 

 with some sensory center. From this as a starting point 

 higher efficiency is gained in three ways: (i) by the asso- 

 ciation of a number of these elementary units to form a larger 

 sensory area, (2) by the specialization of the cell itself, and 

 (3) by the development of accessory parts. 



The area over which a given form of sensory cell may occur 

 may be a general surface of indefinite limits, or it may be 

 restricted and form a definite sense-organ. Most generally 

 the epithelial cells composing such an area are not homogene- 

 ous, but are differentiated among themselves into two sorts, 

 sensory cells and supporting cells; the first are the receptive 

 units of the nervous system ; the latter are non-sensitive, and 

 are grouped about the sensory cells in such a way as to form 



