306 EXCRETION 



than ordinary size, it may extend to twenty-one or twenty-two square 

 feet (2 square meters). In women of medium size, the surface is 

 equal to about twelve and a half square feet (i 1,500 square centimeters). 

 The true skin is -^ to -| of an inch (2.1 to 3.2 millimeters) in thickness ; 

 but in certain parts, particularly in the external auditory meatus, the lips 

 and the glans penis, it frequently measures not more than T J 7 of an inch 

 (0.254 millimeter). 



Layers of the Skin. The skin is naturally divided into two principal 

 layers, which may be readily separated from each other by maceration. 

 These are the true skin cutis vera, derma, or corium and the epi- 

 dermis, cuticle, or scarf-skin. The true skin is more or less closely 

 attached to subjacent parts by a fibrous structure called the subcuta- 

 neous areolar tissue, in the meshes of which there usually is a certain 

 quantity of adipose tissue. This layer is sometimes described under 

 the name of the panniculus adiposus. The thickness of the adipose 

 layer varies in different parts of the general surface and in different 

 persons. There is no fat beneath the skin of the eyelids, the upper and 

 outer part of the ear, the penis and the scrotum. Beneath the skin of 

 the cranium, the nose, the neck, the dorsum of the hand and foot, the 

 knee and the elbow, the fatty layer is about ^ of an inch (2.1 milli- 

 meters) in thickness. In other parts it usually measures \ to \ of an 

 inch (4.2 to 12.7 millimeters). In very fat persons it may measure an 

 inch (25.4 millimeters) or more. On the head and the neck, in the 

 human subject, are muscles attached more or less closely to the skin. 

 These are capable of moving the skin to a slight extent. Muscles of 

 this kind are largely developed and quite extensively distributed in some 

 of the lower animals. 



There is no sharply-defined line of demarcation between the cutis 

 and the subcutaneous areolar tissue ; and the under surface of the skin 

 is irregular, from the presence of fibres that are divided necessarily in 

 detaching it from the subjacent structures. The fibres which enter into 

 the composition of the skin become looser in their arrangement near its 

 under surface, the change taking place rather abruptly, until they present 

 large alveoli, which usually contain a certain quantity of adipose tissue. 



The layer called the true skin is subdivided into a deep, reticulated, 

 or fibrous layer, and a superficial portion, called the papillary layer. 

 The epidermis also is divided into two layers, as follows : an external 

 layer, called the horny layer; and an internal layer, called the Mal- 

 pighian, or the mucous layer, which is in contact with the papillary 

 layer of the corium. 



The Corium^ or True Skin. The reticulated and the papillary 

 layers of the true skin are quite distinct. The lower stratum the 



