TACTILE HAIR OF THE WHITE RAT 17 



mouth parts. Human hair begins to develop around the cuta- 

 neous orifices between the first and third months. Krause saw 

 tactile hairs earlier than any other hairs in the mole foetus of 

 9.5 mm., and in rabbits in the second half of the embryological 

 period they were completely keratinized, the folHcle showed the 

 two swellings and contained many blood vessels which corre- 

 sponded to the blood sinuses. 



A number of investigators have said that the structure and 

 innervation of the hair differed in young and adult animals but 

 tissue which I have stained from the rat of a few days contained 

 perfectly formed follicles with the usual mantle of touch cells 

 and a nerve ring as well. The course of development of this 

 mantle and nerve ring is worth our further consideration. 



Alerkel (76) discovered some peculiar cells in the snout of pigs, 

 round glistening cells in the inter-papillary spaces, the 'epithelial 

 Einsenkungen' of the Germans, to which came terminal fibers 

 ending in flattened discs about the cell or, as he then thought, 

 within it. He concluded that these were ganglion cells. Later 

 studies showed that there was no real connection of the fiber 

 with the cell and also that these cells were not confined to the 

 mouth parts of animals but were found on the cutaneous epithe- 

 lial border of human skin, in tactile hairs and in other places. 

 Szymonowicz, in a long series of articles, has shown that these 

 are merely the usual epithelial cells whose differentiation has 

 been caused by the coming of a nerve fiber and the formation 

 of a fibrillar plexus about the cell. 



Eimer ('94) studied in the snouts of animals a peculiar arrange- 

 ment of cells in a cylindrical or hour-glass form. These have 

 since been studied by Jobert ('72), Krause, Szymonowicz ('95), 

 Botezat ('02) and others. The structure, which has been named 

 Elmer's organ, lies in the same skin layers as the touch cell and 

 to it come medullated nerves which lose their sheaths and run 

 up between the cells as a central bundle of fibers whose lateral 

 branches lie between the cells or as a cup-shaped plexus about 

 them. These are evidently of the same nature as the touch cells 

 of Merkel in the inter-papillary spaces and also of the touch cells 

 of the tactile hair. 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 23, NO. 1 



