420 EXCRETION 



to the papillae of the true skin, being, as it were, molded on them. It 

 is very constant in thickness in all parts of the skin. The cells of the middle 

 layers of the stratum Malpighii are connected by processes, and thus form 

 prickle cells, figure 28. The pigment of the skin, in the deeper cells of the rete 

 mucosum, causes the various tints observed in different individuals and differ- 

 ent races. The epidermis maintains its thickness in spite of the constant wear 

 and tear to which it is subjected. The columnar cells of the deepest layer 

 of the rete mucosum elongate, multiply by division, the new cells produced 

 being pushed toward the free surface of the skin. There is thus a constant 



production of fresh cells in the deeper 

 layers, and a constant throwing off of 

 old ones from the free surface. When 

 these two processes are accurately 

 balanced, the epidermis maintains its 

 thickness. When by intermittent 

 pressure a more active cell growth is 

 stimulated, the production of cells ex- 

 ceeds their waste and the epidermis 

 increases in thickness, as we see in 

 the horny hands of the laborer. 



FIG. 3 o6.-Terminal Tubules of The dermis > or cutis vera r ^ 



Sudoriferous Glands, Cut in Various skin, is a dense and tough, but yielding 



he Skln f thC Pig ' S 



D. rris.) and highly elastic structure support- 



ing the epidermis. It is composed of 



areolar connective tissue interwoven in all directions and forming numerous 

 spaces by its interlacements. These areolae in the deeper layers of the cutis 

 are usually filled with masses of fat cells, figure 305. Unstriped muscu- 

 lar fibers are also abundantly present, especially in the skin of animals which 

 erect the hairs with greater ease than is usually the case with man. 



There is a rich network of blood vessels to the dermis. In the dermal 

 papillae and about the sweat glands there are special loops of capillaries. 

 Nerve fibers are also distributed to the papillae. 



The special nerve terminations in the skin have been described on page 75. 



Glands of the Skin. The skin possesses glands of two kinds: Sudor- 

 iferous or sweat glands, and the sebaceous or oil glands. 



A sudoriferous or sweat gland consists of a small lobular mass, formed 

 of a coil of a simple tubular gland, surrounded by blood vessels, and em- 

 bedded in the subcutaneous adipose tissue, figure 305, C. The duct as- 

 cends from this coiled mass for a short distance in a spiral manner through 

 the cutis and the epidermis, and then opens on the surface of the skin. In 

 the parts where the epidermis is thin, the ducts themselves are thinner 

 and more nearly straight in their course. 



The duct is lined w r ith a layer of columnar epithelium continuous with 



