TACTILE HAIR OF THE WHITE RAT 7 



of sinus hairs, those with a ringwulst and a ring sinus, as in the 

 cat and rat, and those without the ringwulst with only one sinus 

 and that filled with trabeculae, as in the horse, cow, etc. Still 

 another cause of the apparent difference between authors is the 

 matter of terminology, of inexact definitions of particular histo- 

 logical parts. This causes considerable confusion, for example in 

 determining in which of the layers of the follicle the nerve end- 

 ings are found. Some who speak of the outer sheath mean 

 merely the fibrous part beyond the blood sinus, but others mean 

 the coats up to and including the glassy layer within the blood 

 sinus. The glassy layer has been called a dermal and an epider- 

 mal structure; it has been thought by some to consist of one 

 homogenous layer and by others to be made up of two layers; 

 and still another view recognizes in it two layers of different 

 origin, a dermal and an epidermal. For this reason when a 

 writer says that the nerves go through the outer root sheath or 

 through the dermal sheath one must stop to inquire what is 

 meant. 



The literature of this form goes back to Haller in the eighteenth 

 century and the subject has been a most fruitful one ever since. 

 Bonnet made an exhaustive study of the literature in 1878, 

 Botezat in 1897, Szymonowicz ('09) gives some of the more recent 

 references, and a rather extensive bibliography will be found in 

 connection with the general bibliography on hair given by Frieden- 

 thal ('08). This paper therefore will not attempt to deal with 

 the historical side of the question in any thorough way. 



Gegenbaur ('51) should be mentioned, who studied the tactile 

 hairs of nine mammals, and Ley dig ('59) who secured specimens 

 of skin from every mammalian family and gives a wealth of 

 detail considering the methods and means at his disposal, and 

 Odenius ('66) who thought that the development of the ringwulst 

 had to do with nocturnal habits. Ten years later Merkel ('76) 

 found the touch cells in this organ and about the same time 

 Ranvier ('75) described the menisques. Bonnet made one of 

 the most complete studies. He devotes twenty-eight pages to a 

 description o.f the innervation of this hair. Retzius published a 

 series of articles in 1892, 1893 and 1894 on the subject and 



