382 EXCRETION. 



ment of the little muscles around the hair-follicles, forming little bands attached to the 

 surface of the true skin and the base of the follicles, explains fully the manner in which 

 the " goose-flesh " is produced. (See Fig. 107, page 387.) Contraction of the skin, in 

 obedience to the stimulus of electricity, has been repeatedly demonstrated, both in the 

 living subject and in executed criminals immediately after death. 



The papillary layer of the skin passes insensibly into the subjacent structure and 

 presents no well-marked line of division. It is composed chiefly of amorphous mat- 

 ter like that which exists in the reticulated layer. The papilla? themselves appear to 

 be simple elevations of this amorphous matter, although they may contain a few fibres. 

 In this layer, we find a number of fibro-plastic nuclei, with a few little corpuscular bodies 

 called by Robin, cytoblastions. 



As regards their form, the papilla? may be divided into two varieties ; the simple and 

 the compound. The simple papilla? are conical, rounded, or club-shaped elevations of 

 the amorphous matter and are irregularly distributed on the general surface. The 

 smallest are from ^-^ to ^ of an inch in length and are found chiefly upon the face. 

 The largest are on the palms of the hands, the soles of the feet, and the nipple. These 

 measure from ^-f^ to ^ f an inch. Large papillae, regularly arranged in a longitudinal 

 direction, are found beneath the nails. The regular, curved lines observed upon the 

 palms of the hands and the soles of the feet, particularly the palmar surfaces of the last 

 phalanges, are formed by double rows of compound papilla?, which present two, three, 

 or four points attached to a single base. In the centre of each of these double rows of 

 papilla?, is an excessively fine and shallow groove, in which are found the orifices of the 

 sudoriferous ducts. 



The papilla? are abundantly supplied with blood-vessels, terminating in looped capil- 

 lary plexuses, and with nerves. The termination of the nerves is peculiar and will be fully 

 described in connection with the organs of touch. The arrangement of the lymphatics, 

 which are very numerous in the skin, has already been indicated in the general descrip- 

 tion of the lymphatic system. 



The Epidermis and its Appendages. The epidermis, or external layer of the skin, is 

 a membrane composed exclusively of cells, containing neither blood-vessels, nerves, nor 

 lymphatics. Its external surface is marked by exceedingly shallow grooves, which cor- 

 respond to the deep furrows between the papilla? of the derma. Its internal surface is 

 applied directly to the papillary layer of the true skin and follows closely all its 

 inequalities. This portion of the skin is subdivided into two tolerably-distinct layers. 

 The internal layer is called the rete mucosum, or the Malpighian layer, and the external 

 is called the horny layer. These two layers present certain important distinctive char- 

 acters. 



The Malpighian layer is composed of a single stratum of prismoidal, nucleated cells, 

 containing a greater or less amount of pigmentary matter, which are applied directly to 

 all the inequalities of the derma, and of a number of layers of rounded cells containing no 

 pigment. The upper layers of cells, with the scales of the horny layer, are semitransparent 

 and nearly colorless ; and it is the pigmentary layer chiefly which gives to the skin its 

 characteristic color and the peculiarities in the complexion of different races and of differ- 

 ent individuals. In the negro, this layer is nearly black ; and, when the epidermis is re- 

 moved, the true skin does not present any marked difference from the skin of the white 

 race. All the epidermic cells are somewhat colored in the dark races, but the upper 

 layers contain no pigmentary granules. The cells of the pigmentary layer are from ^Vo 

 to WTHT of an inch in length and from -^-^ to ^Vfr of an inch in their short diameter. 

 The rounded cells in the upper layers are from :nJ Vo to in>Vo of an inch in diameter. 

 The absolute thickness of the rete mucosum is from T^ to T V of an inch. 



The horny layer is composed of numerous strata of hard, flattened cells, irregularly 

 polygonal in shape, generally without nuclei, and measuring from -^-^ to T ^ of an inch 



