PART I. LABYRINTH EXPERIMENTS 



A. Introduction 



The immediate incentive to this study was a desire to supple- 

 ment an histological investigation of the tactile hair of the white 

 rat with some functional data which should serve to explain 

 its stmcture. The complexity of the organ and its marvelous 

 innervation forced the conclusion that its real nature and use 

 were of greater significance than had as yet been shown although 

 it had been the object of much study. 



The follicle of the tactile hair of the white rat, like those of the 

 usual body hairs, consists of invaginated skin layers which forai 

 an outer and an inner root sheath. Besides these it has a dermal 

 sheath which encloses within its layers large blood sinuses. Of 

 these sinuses the inferior one is filled with erectile tissue and is 

 separated from the superior ring sinus by an outgrowth of the 

 root sheath, composed of loops of capillaries and connective tissue 

 called the ringwulst. 



The sensory supply to the follicle is from the trigeminus. A 

 large nerve, consisting often of 150 fibres, enters from below and 

 spreads out over it in a thick plexus the main fibers of which lie 

 almost parallel to the long axis of the follicle. These fibers 

 finally terminate in a mantle of touch cells in the outer root 

 sheath. In addition to this there is a second innervation. 

 Branches run down from the dermal plexus of the skin and fonn 

 a nerve ring about the neck of the follicle. 



So large and so complicated a nerve supply would indicate a 

 high degree of sensitivity which would be still further enhanced 

 by the nature of the tissue in which it lies, the vibratory char- 

 acter of the stimulus and lever-like application of the hair itself. 

 The details of this innervation and discussion of the sti"ucture 

 are given in another paper.* 



B. Historical 



Hair is a peculiar characteristic of mammals and is found on 

 every mammalian species so far as known; even the whale has 

 some about the mouth. If only a few hairs are found on the 



*To appear in vol. 24, Jour, of Comp. Neurology. 



