PHYSIOLOGICAL ANATOMY OF THE SKIN. 343 



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

 feet (2 square metres). In women of medium size, as the mean result of 

 threa observations, the surface was found to equal about twelve and a half 

 square feet (11,500 square centimetres). 



The thickness of the skin varies very much in different parts. Where it 

 is exposed to constant pressure and friction, as on the soles of the feet or the 

 palms of the hands, the epidermis becomes very much thickened, and in this 

 way the more delicate structure of the true skin is protected. It is well 

 known that the development of the epidermis, under these conditions, varies 

 in different persons, with the pressure and friction to which the surface is 

 habitually subjected. The true skin is T V to -J- of an inch (2-1 to 3-2 mm.) 

 in thickness ; but in certain parts, particularly in the external auditory mea- 

 tus, the lips and the glans penis, it frequently measures not more than y^- of 

 an inch (0-254 mm.). 



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

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

 are the true skin cutis vera, derma, or corium and the epidermis, cuticle, 

 or scarf-skin. The true skin is more or less closely attached to the subjacent 

 structures by a fibrous structure called the subcutaneous areolar tissue, in the 

 meshes of which there is usually 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 very much 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. Be- 

 neath 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 T ^- of an inch (2-1 

 mm.) in thickness. In other parts it usually measures -J- to \ of an inch (4-2 

 to 12'7 mm.). In very fat persons it may measure an inch (25-4 mm.) or 

 more. Upon the head and the neck, in the human subject, are muscles at- 

 tached 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 always ir- 

 regular, from the presence of fibres which are necessarily divided in detach- 

 ing it from the subjacent structures. The fibres which enter into the com- 

 position of the skin become looser in their arrangement near its under surface, 

 the change taking place rather abruptly, until they present large alveoli, which 

 generally 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 epi- 

 dermis is also divided into two layers, as follows : an external layer, called the 

 horny layer ; and an internal layer, called the Malpighian, 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 reticulated layer, is 



