720 STRUCTURE OF EPIDERMIS. [BOOK n. 



tendon-bundles, form a tough open network, the larger spaces of 

 which are frequently occupied by masses of fat cells of the sub- 

 cutaneous adipose tissue. Elastic fibres are very abundant in the 

 dermis proper, being very fine immediately beneath the epidermis 

 and becoming coarser in the deeper parts ; they are present also, 

 though to a less extent, in the subcutaneous connective tissue. The 

 skin as a whole is a very elastic structure. 



Blood vessels are very abundant, forming close set capillary 

 networks and loops immediately under the epidermis, especially 

 in the papillae, and more open networks elsewhere ; but no blood 

 vessel passes into the epidermis. Lymphatic vessels and lymphatic 

 capillaries are abundant in the dermis, being connected here as in 

 other regions of the body with smaller " lymph-spaces." 



The consideration of the nerves of the skin it will be advan- 

 tageous to defer until we come to deal with the skin as an organ of 

 sense ; for though some of the cutaneous nerve-fibres are efferent 

 fibres distributed to the blood vessels, and probably to the sweat- 

 glands and other structures not directly connected with the sense 

 of touch, by far the greater number are afferent fibres beginning in 

 distinct tactile organs, or otherwise serving as sensory structures. 



433. The epidermis consists of two parts, separated by a 

 fairly sharp line of demarcation : an inner soft layer, the Mal- 

 pighian layer, or stratum Malpighii, and an outer harder horny 

 layer, or stratum corneiim. The skin as is well known varies in 

 thickness in different regions of the body, and the differences are 

 due almost exclusively to variations in the thickness of the horny 

 layer which, as over the lips, may be extremely thin, or as on the 

 heel, excessively thick ; compared with the variations . in thickness 

 of the horny layer, the variations in thickness of the Malpighian 

 layer or of the dermis may be disregarded. 



The line of demarcation between the Malpighian and horny 

 layers follows the contour of the surface of the skin, not that of 

 the dermis, the papilla of which appear in sections as if imbedded 

 in the Malpighian layer. When the skin after death is macerated, 

 the horny layer is apt to peel off from the Malpighian layer below, 

 which, originally soft and rendered still softer by the maceration, 

 then appears as a layer of slimy tissue spread out between the 

 sides of and covering the summits of the papillas of the dermis, 

 somewhat after the fashion of a network ; hence this layer was in 

 old times spoken of as the rete mucosum. 



The lowermost, innermost portion of the Malpighian layer 

 resting upon the dermis, consists of a single layer of elongated, or 

 almost columnar cells placed vertically, that is with their long 

 axis perpendicular to the plane of the dermis. This layer, which 

 preserves the original features of the epiblast of the embryo, 

 and which may be followed over the papillae as well as along the 

 intervening valleys, presents a characteristic appearance in vertical 

 sections of the skin. Each cell, which is about as large as a 



