THE CUTANEOUS GLANDS. 



1397 



probably accounts for the tint distinguishing the white area. Since the transformation of the 

 cells of the stratum germinativum into those of the nail-plate is confined to the matrix, it is evi- 

 dent that the continuous 



growth 'of the nail takes FIG. 1161. 



place along the floor and Nail-bed Nail-plate 



bottom of the nail-groove, 

 the last formed increment 

 of nail-substance pushing 

 forward the previously dif- 

 ferentiated material and 

 thus forcing the nail to- 

 wards the end of the digit. 

 The relation of the epi- 

 dermis of the nail-wall to 

 the substance of the plate 

 is one of apposition only, 

 production of the nail oc- 

 curring in no part of the 

 fold. Over the greater 

 extent of the latter all the 

 typical constituents of the 

 cuticle are represented, but 

 within the most proximal 

 portion the stratum germi- 

 nativum alone is present, 

 the stratum corneum fad- 

 ing away. Where the 

 horny layer exists, it rests 

 directly upon the nail, but 

 is differentiated from the 



latter by being less dense Transverse section of nail-wall and adjacent part of nail-plate and nail-bed. X'go. 



and by its response to 



stains. As the nail leaves the groove, a part of the stratum germinativum of the nail-wall is 

 prolonged distally for a variable distance over the dorsal surface of the nail-plate as a delicate 

 membranous sheet, the eponychium, which usually ends in a ragged abraded border. 



Stratum corneum 

 and 



Stratum 



germinativum 



of nail-wall 



Eponychium 

 Margin of nail 



Corium 



THE CUTANEOUS GLANDS. 



These structures include two chief varieties, the sebaceous and the sweat 

 glands^ together with certain modifications, as the ceruminous glands within the 

 external auditory canal, the circumanal glands, the tarsal and ciliary glands 

 within the eyelid and the mammary glands. In all the epithelial tissues the 

 secreting elements and the lining of the ducts are derivatives of the ectoblast 

 and, therefore, genetically related to the epidermis. 



THE SEBACEOUS GLANDS. 



Although these structures (glandulae sebacae) are chiefly associated with the 

 hair-follicles, in which relation they have been considered (page 1394), sebaceous 

 glands also occur, if less frequently, independently and in those parts of the skin in 

 which the hairs are wanting, as on the lips, angles of the mouth, prepuce and labia 

 minora. The size of these glands bears no relation to that of the hairs, since among 

 the smallest (.2-. 4 mm.) are those on the scalp. The largest, from .5-2.0 mm., 

 are found on the mons pubis, scrotum, external ear and nose. Conspicuous aggre- 

 gations, modified in form, occur in the eyelid as the Meibomian glands. 



Depending upon the size of the glands their form varies. The smallest ones are 

 each little more than a tubular diverticulum, dilated at its closed end. In those of 

 larger size the relatively short duct subdivides into several expanded compartments, 

 which, in the largest glands, may be replaced by groups of irregular alveoli, with 

 uncertain ducts that converge into a short but wide common excretory passage. 



Structure. The structural components of these glands include a fibrous 

 envelope, a membrana propria and the epithelium, the first two being continuous 

 with the corresponding coverings of the hair-follicle. The epithelium continued 



