PHYSIOLOGICAL ANATOMY OF THE SKIN. 381 



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 amount of press- 

 ure and friction to which the surface is habitually subjected. The true skin is from T ^ 

 to of an inch 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 ^ of 

 an inch. 



Layers of the Slcin. 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 epidermis, cuticle, or scarf-skin. The true skin is at- 

 tached to the subjacent structures, more or less closely, by a fibrous structure called the 

 subcutaneous areolar tissue, in the meshes of which we commonly find a certain quantity 

 of fatty tissue. This layer is sometimes described under the name of the panniculus adi- 

 posus. 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. 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 -fa of an inch in thickness. In other parts it usually 

 measures from | to ^ of an inch. In very fat persons it may measure one inch or more. 

 Upon 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 subcuta- 

 neous areolar tissue ; and the under surface of the skin is always irregular, from the 

 presence of numerous fibres which are necessarily divided in detaching it from the sub- 

 jacent 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 aveolaD, which generally contain a certain amount 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 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 Slcin. The reticulated and the papillary layer of the true skin are 

 quite distinct. The lower stratum, the reticulated layer, is much thicker than the papil- 

 lary layer and is dense, resisting, quite elastic, and slightly contractile. It is composed of 

 numerous bundles of white fibrous tissue interlacing with each other in every direction, 

 generally at acute angles. Distributed throughout this layer, are found numerous anas- 

 tomosing, elastic fibres of the small variety, and with them a number of non-striated 

 muscular fibres. This portion of the skin contains, in addition, a considerable quantity 

 of amorphous matter, which serves to hold the fibres together. The muscular fibres are 

 particularly abundant about the hair-follicles and the sebaceous glands connected with 

 them, and their arrangement is such that, when they are excited to contraction by cold 

 or by electricity, the follicles are drawn up, projecting upon the general surface and 

 producing the appearance known as " goose-flesh." Contraction of these fibres is par- 

 ticularly marked about the nipple, producing the so-called erection of this organ, and 

 about the scrotum and penis, wrinkling the skin of these parts. The peculiar arrange- 



