394 THE SKIN AND ITS APPENDAGES. 



follicle, loses its medullary sheath and divides into two branches, 

 which surround it in the form of a ring. From this complete or 

 partial ring of nerve -fibers numerous varicose fibers proceed upward 

 parallel to the axis of the follicle for a distance about equal to the 

 cross-diameter of the follicle, to terminate, it would seem, largely 

 outside of the glassy layer (Retzius). In certain mammalia the 

 nerve-fibers end in tactile discs, found in the external root-sheaths 

 of the so-called tactile hairs. The muscles of the hairs receive 

 their innervation through the neuraxes of sympathetic neurones, 

 which reach the periphery from the chain ganglia through the gray 

 rami communicantes. These nerves are known as pUomotor nerves, 

 and when stimulated, excite contraction of the erector muscles of the 

 hairs, causing these to assume an upright position and producing 

 the appearance termed goose skin, or cutis anserina. Langley 

 and Sherrington have made interesting and important observations 

 on the course and distribution of the pilomotor nerves. 



C THE NAILS. 



The nails are a peculiar modification of the epidermis. The 

 external arched portion is called the body of the nail ; that area upon 



Nail wall. 



Stratum- 

 Malpighii. 



Stratum cor-- 

 neum of the 

 nail groove. 



Fig. 317. Longitudinal section through human nail and its nail groove 

 (sulcus) ; X 34- 



which the latter rests, the nail bed, or matrix ; and the two folds of 

 epidermis which overlap the nail, the nail walls. The groove which 

 exists between the nail wall and nail bed is known as the sulcus of 

 the matrix, and the proximal imbedded portion of the nail as the 

 nail root, since all growth of the nail takes place in this region. 

 The nail bed consists of the corium, which is here made up of a 

 dense felt-work of coarse connective-tissue fibers. Immediately 

 beneath the nail the corium is raised into a number of more 

 or less symmetric longitudinal ridges, which again become con- 

 tinuous with the connective-tissue papillae of the skin at the line 

 where the nail projects beyond its bed. 



The depressions between the ridges are occupied by epidermal 

 cells, which also form a thin covering over the ridges themselves. 



