THE EPIDERMIS 



1285 



especial medical and surgical importance as indicating topographically the position of the joints, 

 their relation to which has been recently made clearer by means of the X-ray. 



The folds and furrows brought about through the action of the skin muscles run at right 

 angles to the muscle fibres and are more or less transitory at first but become more permanent 

 through repeated or long-continued action. They are represented by the wrinkles of the fore- 

 head, the lines of expression of the face, the transverse wrinkles of the scrotum and the radiating 

 folds around the anus. The more superficial cristae cutis and sulci cutis are arranged in groups 

 within and around the touch pads, on the volar surface of the hands and the plantar surface of 

 the feet (figs. 1042, 1043). The cristse of each group are parallel. They correspond to the rows 

 of papillae of the corium. 



Because the patterns of the crista) and sulci are characteristic for the individual, and per- 

 manent from youth to old age, they have been classified in a number of types and are important 

 medicolegally as a means of identification. The various systems of classification are based 

 upon the arrangement over the distal phalanges of the fingers and make use of (1) a transverse 

 ridge which is parallel with the articular plicae (2) a curved ridge with its convexity distally 

 and more or less closely meeting the first, medially and laterally, and (3) the curved and concen- 

 tric ridges between these two (fig. 1043). 



There are also a great number of minute depressions which mark the points where the hairs 

 pierce the surface and where the glands open. These are popularly knowTi as pores. Under 



Fig. 1046. 



-Papilla op the Corium after Maceration. From Retouched Photograph. 

 Epithelium Removed by Maceration. ( X 25.) 



Crista cutis 



Sulcus cutis 



Papilla 



^>- Papillae corii 



the influence of cold and emotion the hair muscles contract and cause a slight elevation of the 

 skin at the point where the hair emerges. This roughened appearance of the skin is popularly 

 known as "goose-flesh." 



A complex wrinkUng of the skin appears in old age, or in the course of exhausting diseases,''as 

 a result of loss of elasticity and from absorption of the cutaneous and subcutaneous fat. Rounded 

 depressions called dimples are produced by the attachment of muscle-fibres to the deep surface 

 of the skin, as on the chin and cheek, and are made more evident by the contraction of these 

 fibres. Others are produced bj^ the attachment of the skin by fibrous bands to bony eminences, 

 as the elbow, shoulder, vertebrae, and posterior iliac spines. They are best seen when the sub- 

 cutaneous adipose tissue is well developed. 



The cutis is made up of two layers which are structurally and developmentally 

 markedly different. The superficial ectodermic portion, epidermis, is made up 

 almost entirely of closely packed epithelial cells, the deeper mesodermic part, 

 corium, is formed largely of connective-tissue fibres. 



The epidermis (cuticle, scarf-skin) is a cellular non-vascular membrane which 

 forms the whole of the superficial layer of the skin and at the great openings 

 through the skin, as the mouth and anus, blends gradually with the mucous mem- 

 brane. It represents from one-tenth to over half the thickness of the skin, in 

 different parts of the body, the usual thickness being .05 to .2 mm., ranging from 

 .03 mm. to nearly 3 mm. The thickness varies also in different individuals. 

 Its deep surface is molded exactlj^ to the underlying corium but its superficial 

 surface fails to reproduce all of the irregularities of the latter. In spite of this 

 close association, blood-vessels never enter the epidermis. 



Structure of the epidermis. — ^The cells of the epidermis are packed together in many irregu- 

 lar layers. The deepest cells are soft protoplasmic, somewhat elongated, perpendicular to 



