STEUCTURE OF THE SKIN. 



857 



of the joints, and it can be thrown into wrinkles by the contraction of the sub- 



cutaneous muscles, where those exist. Over the greater part of the body it is freely 



movable ; but on the scalp and lateral surfaces of the auriculae, as well as on the 



palms and soles, it is bound down to the subjacent tissues. 



The skin consists of two strata, viz. : a deep, termed the corium, and a superficial, 



the epidermis (Fig. 734). 



The corium or cutis vera is derived from the embryonic mesoderm, and consists 



essentially of a felted interlacement of connective tissue and elastic fibres. In its 



deeper part, or stratum reticulare, the fibrous bundles are coarse and form an open 



network, in the meshes of which are vessels, nerves, pellets of fat, hair follicles, 



and glands. This reticular stratum passes, as a rule, without any line of demarca- 



tion, into the panniculus adiposus or subcutaneous fatty tissue, but in some parts it 



rests upon a layer of 



striped or unstriped 



muscular fibres the 



latter in the case of the 



scrotum. In the super- 



ficial layer, or stratum 



papillare, of the corium, 



the connective tissue- 



bundles are finer and 



form a close network. 



Projecting from its 



superficial surface are 



numerous finger -like, 



single, or branched ele- 



vations, termed papillae 



(Fig. 734), which are 



received into corre- 



spending depressions 

 . on the under surface of 



the epidermis. These 



papillae vary in size, 



being small on the eye- 

 ! lids, but large on the 



palms and soles, where 



they may attain a 



length of 225 u, and 



Stratum lucidum 



Stratum 



granulosum 



Blood-vessels 

 and nerves 



EPIDERMIS AND PAPILLA OF CORIUM 



Where they produce the Fia ^.-VERTICAL SECTION 

 permanent curved 

 ridges already referred' to. Each ridge usually contains two rows of papillae, between 

 which the ducts of the sudoriferous glands pass to reach the surface. The papillae 

 consist of fine connective tissue and elastic fibres, mostly arranged parallel to the 

 long axis of the papillae ; the majority contain capillary loops, but some contain the 

 terminations of nerves. The superficial surface of the corium is covered with a 

 thin, homogeneous basement membrane. 



The epidermis or cuticle is derived from the embryonic ectoderm and covers 

 the corium. Its thickness varies in different parts of the body and ranges from 

 3 mm. to 1 mm. or more ; it is thickest on the palms of the hands and soles of 

 the feet, and thinnest on the eyelids and penis. It is non-vascular and consists of 

 stratified epithelium ; its superficial layers are modified to. form the stratum corneum, 

 which may be separated by maceration or blistering from the deeper, softer portion, 

 or stratum mucosum (Malpighi). The epidermis consists from within' outwards of 

 the following five strata (Fig. 734) : 



The stratum germinativum is a single stratum of nucleated columnar cells 

 planted by denticulated extremities on the basement membrane of the corium. 



. The stratum mucosum consists of six or eight layers of polygonal, nucleated 

 " prickle " or " finger " cells, the processes of which join those of adjacent cells. 

 Between the cells of this layer are minute channels, in which leucocytes or pigment 



