THE SPINAL NERVES 67 



eties of cutaneous sensation is mediated by a separate set of nerve-fibers. But 

 little progress has as yet been made toward identifying these various func- 

 tional groups. We know that both myelinated and unmyelinated fibers are 

 present in the cutaneous nerves (Ranson, 1915), but are not able to say with 

 certainty which subserve each of the varieties of cutaneous sensation. There 

 are many good reasons, however, for the belief that painful afferent impulses 

 and possibly also those of temperature are carried by the unmyelinated fibers, 

 and that those of the touch and pressure group are mediated by the myelinated 

 fibers. The evidence on which this statement is based has been briefly sum- 

 marized on pages 102-104. 



Fig. 41. Free nerve endings in the epidermis of a cat's paw: A, Stratum corneum; B, stratum 

 germinativum Malpighii, and C, its deepest portion; a, large nerve trunk; b, collateral fibers; c, 

 terminal branches; d, terminations among the epithelial cells. Golgi method. (Cajal.) 



All sensory nerve endings in the skin belong to the exteroceptive group, 

 but it is not so easy to say which ones are responsible for each of the several 

 varieties of cutaneous sensation, namely, touch, pain, heat, and cold. On 

 structural grounds we may recognize three principal groups: (1) endings in hair- 

 follicles, (2) encapsulated nerve endings, and (3) free terminations in the epi- 

 dermis. 



Free Nerve Endings. Some of the myelinated fibers as they approach 

 their terminations divide repeatedly. At first the branches retain their sheaths, 

 but after many divisions the myelin sheaths and finally the neurilemma are lost 

 and only the naked axis-cylinders remain. These enter the epidermis, where, 



