230 



THE SKIN. 



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Fig. 159. — Development of a Se- 

 baceous Gland in a six months' 

 F(ETus, 2i30 Diameters (Kolliker). 



a, hair ; 6, inner root-slieath 

 outer root-sheath of hair-follicle 

 rudiment of sebaceous gland. 



axis of the pedicle until it penetrates through the root-sheath, and the fat-cells 



thus escape into the cavity of the hair- 

 follicle, and constitute the first secretion 

 of the sebaceous gland. They are soon 

 succeeded by others of the same kind, 

 and the little gland is established in its 

 office. Additional saccules and recesses, by 

 which the originally simple ca"\aty of the 

 gland is complicated, are formed by budding 

 out of its epithelium, as the fii'st was pro- 

 duced from the epithelial root-sheath, and 

 are excavated in a similar manner. 



It would thus appear that the rudiments 

 of the haii'-foUicles, sweat-glands, and 

 sebaceous glands, are all derived from the 

 same soui-ce. They all originally appear as 

 solid bud-like excrescences of the soft 

 Malpighian or mucous layer of the epidermis, 

 (for the outer stratum of the root-sheath 

 must be regarded as such) ; these grow 

 down into the corium. in which recesses 

 are formed to receive them, and whiiih, of 

 course, yields the material required both for 

 the production of new cells, for their fm-ther 

 growth, and for the maintenance of their 

 secreting function. 



Functions and vital properties of the skin.— The skin fonns a general ex- 

 ternal tegument to the body, defining the surface, and coming into relation with 

 foreign matters externally, as the mucous membrane, with which it is continuous 

 and in many respects analogous, does internally. It is also a vast emunctory, by 

 which a large amount of fluid is eliminated from the system, in this also resem- 

 bling certain jDarts of the mucous membrane. Under certain conditions, moreover, 

 it performs the office of an absorbing sui-face, but this function is gi-eatly re- 

 stricted by the epidemiis. Throughout its whole extent the skin is endowed with 

 tactilv scmihilifi/. but in very different degi-ees in different jDarts. On the skin of 

 the palm and fingers, which is largely supplied ^^•itll nerves and fimiished ^vith 

 numerous prominent papillte, the sense attains a high degree of acuteness ; and 

 this endowment, together with other confonnable arrangements and adaptations, 

 invests the human hand with the character of a special organ of touch. A ceitain 

 though low degree of vital contractility, depending doubtless on the muscular 

 fibres in its tissue, also belongs to the skin. This shows itself in the general 

 shrinking of the sldn caused by naked exposui-e to cold and by certain mental 

 emotions, and producing the state of the sui-face named '• cutis anserina," in 

 which the muscular bundles protrude the hair-follicles with which they are con- 

 nected, whilst they retract or depress the intermediate cutaneous tissue ; and this 

 condition of the skin may be produced locally by the electric stimulus applied by 

 means of the magneto-electric apparatus. The scrotmn, as is well known, becomes 

 shmnk and coiTUgated by the application of cold or mechanical irritation to its 

 surface ; but in this case the contraction takes place in the subcutaneous tissue 

 and the skin is puckered. 



Reproduction of skin. — "\Mien a considerable portion of the skin is lost, the 

 breach is repaired partly by a di-awing inwards of the adjoining skin, and partly 

 by the foi-mation of a dense tissue, less vascular than the natural corium, and in 

 which, so far as we know, hairs and glands are not reproduced, so that some deny 

 that the cutaneous tissue is regenerated. Still the new part becomes covered with 

 epidennis, and its substance sufficiently resembles that of the corium to warrant 

 its being considered as cutaneous tissue regenerated in a simple form. In small 

 breaches of continuity from cuts inflicted in early life, the uniting part sometimes 

 acquii'es fuiTOws similar to those of the adjoining siu'face. 



