ccxviii SKIN. 



Development of the sebaceous glands. The rudiments of the sebaceous glands 

 sprout like little buds from the sides of the hair-follicles; they are at first, in fact, 

 excrescences of the external or mucous layer of the root-sheath (fig. cxix., n, n), and 

 are composed entirely of nucleated cells. Each little process soon assumes a flask 

 shape and is at first solid ; but in due time a group of cells containing fat particles 

 appears in its centre, and gradually extends itself along the 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 estab- 

 lished in its office. Additional saccules and recesses, by which the originally simple 

 cavity of the gland is complicated, are formed by budding out of its epithelium, as 

 the first was produced from the epithelial root-sheath, and are excavated in a similar 

 manner. 



It would thus appear that the rudiments of the hair-follicles, sweat-glands, and 

 sebaceous glands, are all derived from the same source. 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 which, of course, 

 yields the material required both for the production of new cells for their further 

 growth and for the maintenance of their secreting function. 



Functions and vital properties of the skin. The skin forms a general external 

 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 resembling certain 

 parts of the mucous membrane Under certain conditions, moreover, it performs the 

 office of an absorbing surface, but this function is greatly restricted by the epidermis. 

 Throughout its whole extent the skin is endowed with tactile sensibility, but in very 

 different degrees in different parts. On the skin of the palm and fingers, which is 

 largely supplied with nerves and furnished with numerous prominent papillae, the 

 sense attains a high degree of acuteness ; and this endowment, together with other 

 conformable arrangements and adaptations, invests the human hand with the cha- 

 racter of a special organ of touch. A certain 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 skin caused by naked exposure to 

 cold and by certain mental emotions, and producing the state of the surface named 

 " cutis anseriua," in which the muscular bundles protrude the hair-follicles with which 

 they are connected, 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 scrotum, as is well known, becomes 

 shrunk and corrugated by the application of cold or mechanical irritation to its sur- 

 face ; but in this case the contraction takes place in the subcutaneous tissue and the 

 skin is puckered. 



Reproduction of skin. When a considerable portion of the skin is lost, the breach 

 is repaired partly by a drawing inwards of the adjoining skin, and partly by the for- 

 mation of a dense tissue, less vascular than the natural corium, and in which, so far 

 as I know, hairs and glands are not reproduced, so that some deny that the cutaneous 

 tissue is regenerated. Still the new part becomes covered with epidermis, and its 

 substance sufficiently resembles that of the corium to warrant its being considered as 

 cutaneous tissue regenerated in a simple form. I may add that, in small breaches of 

 continuity from cuts inflicted in early life, the uniting part sometimes acquires fur- 

 rows similar to those of the adjoining surface. 



SECRETING GLANDS. 

 The term gland has been applied to various objects, differing widely from 



hair follicles were infested by a worm, which he has described and delineated in Mullet's 

 Archiv for 1842. Since then, further interesting details respecting this curious parasite, 

 with observations on its development, have been contributed by Mr. E. Wilson. Phil. 

 Trans. 1344. 



