22 S. B. VINCENT 



Poirier and Charpey ('07) give one of the best accounts of the 

 way the organ functions: 



When an excitation occurs which affects the tactile hair it produces 

 first a motor reflex. Under the influence of tlie muscles the hair straight- 

 ens itself again. In erecting it compresses the venous vessels in the 

 blood sinus. These do not delay in distending themselves — and permit 

 the blood again to enter. Also the nerve terminations are compressed 

 between the hair and the blood sinus. They are thus stimulated and 

 transmit to sensory and peripheral neurones the impressions which they 

 receive. 



There are many questions one would like to ask concerning the 

 relation of structure to function here. The following account is 

 as complete as we can make it at present. The hair is a power- 

 ful tactile organ. Its great innervation would imply this and it 

 has already been discussed. There are other factors, however, 

 which contribute to this end, among which we may mention 

 first the area stimulated. Comparing the area of the hair shaft 

 with the area of the invaginated inner follicle with its mantle 

 of touch cells we find that the actual surface stimulated by the 

 vibrations of the hair is 200 times that occupied by the hair 

 itself on the surface of the skin. This of itself would be enough 

 to justify our first point but there are still others. The hair 

 whose delicate tips extend beyond and to the side of the head is 

 a lever whose fixation in the follicle magnifies the effect produced 

 by the slightest touch. Sherrington ('00) says: 



The short hairs of the skin much enhance its tactual sensitivity. On 

 9 sq. mm. of skin from which the hairs had been shaved the liminal 

 stimulus was found to be 36 mgms., whereas, on the same surface, 

 before it was shaved, 2 mgms. was the liminal stimulus. The liminal 

 stimulus for the touch spots about a short hair is three to twelve times 

 greater than for the hair itself, i.e., the hair is three to twelve times 

 more sensitive than the ''spots." Each short hair is a lever, of which 

 the long arm outside the skin acts at an advantage upon the touch 

 organs at the root. The short hairs are probably the most sensitive 

 tactual organs of the body. 



The muscles of the conical body are such that by their con- 

 traction this hair is permitted to vibrate freely to the very depth 

 of the follicle, while on the other hand contraction of the fibers 



