TACTILE HAIR OF THE WHITE RAT 6 



the hair shaft is continuous with one of the layers of the inner 

 root sheath and as its plates are turned in an opposite direction 

 they interlock with the cuticle plates of the sheath. The cortical 

 substance is a continuation of the stratum spinosum or middle 

 Malpighian layer, while the cells of the medulla are formed by the 

 proliferation of the cylindrical cells of the outer root sheath. 



The outer root sheath is a continuation of the stratum germi- 

 nativum and consists of three layers, a basement layer of cylin- 

 drical cells, the Malpighian layer of large prickle cells, and a 

 granular layer of flattened cells which are often lacking. It is 

 the cells of the second layer which increase so greatly in some 

 parts of the follicle and are associated with the nerve endings. 

 The dermal sheath also has three layers. Naming these from 

 without inward they are a layer of longitudinal connective tissue 

 fibers, a layer of circular fibers and the glassy layer. The last 

 of these corresponds to the basement membrane of the derma. 

 It is a clear, thick, highly refractive layer and the inner portion 

 is said to be an exoplasmic product of the adjacent epithelium 

 (Kolliker '02). The papilla has a great growth in the rat and 

 often reaches the neck of the follicle. In its lower part it has a 

 rich plexus of nerves and blood vessels. While the above account 

 of the sheath layers is generally true, they have a far greater 

 thickness in some places than in others and merge into indis- 

 tinctness both in the region of the papilla and in the conical 

 body. 



From this description of its development it will be seen that 

 the tactile hair of mammals is similar to the ordinary hair, from 

 which it differs only in its greater development and higher spe- 

 cialization. It is a cell structure arising by differentiation from 

 the epidermal cutaneous cell layers and thus is essentially unlike 

 the invertebrate organs which resemble it but which are formed 

 by a chitinous secretion. 



The superior and iriferior enlargements in the follicle have 

 hitherto attracted much attention. They are the result of a thick- 

 ening in the Malpighian layer of the outer root sheath. These 

 cells not only multiply so as to form a greater number of layers 

 but the cells themselves increase greatly in size. This growth 



