468 HISTORY OF THE HUMAN BODY 



and those formed like them. Thus, we are positive concerning 

 the sense furnished by the eye, since the variations from the 

 human structure are very slight in any case, even in fishes; 

 the ear, however, is somewhat more variable, and presents sev- 

 eral problems, since the lower types of ear lack certain parts, 

 like the cochlea, which in Man are essential to the complete- 

 ness of the sense of hearing as we understand it. In this 

 case we have two alternatives, either that in the different forms 

 the same function is subserved by different parts, as is possible 

 in the case of the brain (cf. the pallium of teleosts and the 

 cerebral hemispheres of mammals), or else that there are ele- 

 ments in the human sense of hearing not perceived by ears be- 

 longing to other types. 



Aside from the above, the sense of smell seems to be a com- 

 mon possession, and in terrestrial forms the sense of taste also 

 seems general if we are to judge from the similarity in the 

 location and' structure of certain specialized sense-organs and 

 the identity of their nerve supply. After these are excepted, 

 however, there remains a large number of types of terminal 

 sense-organs, more or less localized in different areas, the spe- 

 cial functions of which are practically unknown, but are in- 

 cluded within the comprehensive terms of touch or feeling. 

 That many distinct impressions are involved in this is shown 

 by this very dissimilarity in the structure of the terminal or- 

 gans, the complexity of which, in certain cases, suggests the 

 possibility of definite senses, at least as distinct as those of smell 

 or taste ; but as few of these types occur in Man, and as even 

 here the elementary sensations have not been wholly coordi- 

 nated with the various forms of nerve terminations, but little 

 can yet be stated on the subject, and the psychology of the 

 tactile sensations of the lower vertebrates remains an unex- 

 plored field. 



Probably the lowest form of vertebrate sense-organ, and one 

 that is universally distinguished among them, is that of simple 

 sensation, the contact sense, which resides in the epidermis 

 and is thus generally met with over the entire surface. The 

 vertebrate epidermis, which, in contrast to that of invertebrates, 

 is many cells thick, is supplied everywhere by sensory nerves 



