TACTILE HAIR OF THE WHITE RAT 25 



from the trigeminus, as studies in degeneration show. The touch 

 cells are similar to those which Merkel found on the cutis border 

 and have invaginated with the skin layers and reached a higher 

 development here. They are found especially well developed in 

 the mouth parts of many animals and are described by Szymono- 

 wicz on the beard hairs of men. The size of the follicle and the 

 richness of its innervation do not depend on the size of the animal 

 but upon the tactual functional significance of the mouth parts. 

 Histological and structural studies show that the tactile hair is a 

 powerful organ of touch and that this is due: 



(a) To its great innervation. 



(b) To the increase in the area stimulated. 



(c) To the leverage which magnifies the stimulus. 



(d) To the vibratory nature of the stimulus, which is the only 

 adequate stimulus for some reflexes, which summates subliminal 

 stimuli, which prolongs the initial stimulus. 



(e) To its muscular connection, which transmits stimulus over 

 large areas. 



(f) To its haemostatic apparatus, which permits free vibration 

 of the hair to the depth of the follicle, which may increase or 

 modify pressure, which may raise or lower the nervous threshold, 

 and which may possibly have some chemical significance. 



Experimental studies with living animals show that : 



(a) It is an aid in locomotion. 



(b) It functions in equilibration. 



(c) It aids in determining nearness or position of edges or 

 corners. 



(d) It is an aid in the discrimination of inequalities of surface. 



(e) It is a supplement to poor vision. 



This work was done under the direction of Prof. J. G. Wilson 

 in the Otological Department of the Northwestern University 

 Medical School. The expense of the research was in part defrayed 

 by a grant from the Patten fund. 



I should like to acknowledge my indebtedness to Professor 

 Wilson for constant advice, to Professor Herrick of the University 

 of Chicago for suggestions during the course of the investigation 

 and to Miss Hill for her careful drawings. 



