THE SENSE OKGANS 



THE sense organs have always been classified into lower 

 and higher, and that not without justification. Conspicuous 

 among the lower sense organs are those of the tactile sense lying 

 in the integument ; and by the higher sense organs are under- 

 stood the olfactory, visual, auditory, and gustatory apparatus, 

 which are located in special depressions or cavities of the 

 head. 



It may now be considered as certainly established that all 

 the latter may be traced back phylogenetically to tegumental 

 sense organs, and that their sensory epithelia are to be regarded 

 as modified epidermal derivatives. 



INTEGUMENTAL SENSE OKGANS 



It appears to me not improbable that the tactile bodies which 

 are profusely scattered throughout the integument of man are 

 genetically closely connected with his gradual loss of hair. I am 

 led to this conclusion by the fact that tactile bodies appear 

 in the lower Mammals principally, indeed, perhaps exclusively, 

 in places where there is no hair (proboscis, entrance to the mouth, 

 plantar surface of the paw). They appear unnecessary in hairy 

 parts of the body, because the hairs themselves, being richly 

 provided with nerves, are capable of exercising a delicate tactile 

 function. 



How far certain epithelial structures proved by Maurer to 

 exist in the hair germs are to be deduced from phylogenetically 

 older tegumental sense organs like those of the Anamnia, must 

 be established by further investigation (compare also the already- 

 mentioned temporary appearance of sense organs in the cephalic 

 region in embryos, ante, p. 133). 



