DEVELOPMENT OF THE SWEAT-GLANDS. 



429 



capsule round the body of the gland. Each little sweat-gland is supplied with a 

 dense cluster of capillary blood-vessels. 



Distribution. Sweat-glands exist most numerously in regions unprovided with 

 hairs, but they occur in all parts of the skin, and may in some cases open into hair- 

 follicles. According to Krause, nearly 2,800 open on a square inch of the palm of 

 the hand, and somewhat fewer on an equal extent of the sole of the foot. He assigns 

 rather more than half this number to a square inch on the back of the hand, and 

 not quite so many to an equal portion of surface on the forehead, and the front and 

 sides of the neck. On the breast, abdomen, and fore-arm, he reckons about 1100 to 

 the inch, while on the lower limbs and the back part of the neck and trunk, the 

 number in the same space is not more than from 400 to GOO. 



The size of the sweat-glands also varies. According to the observer last named, 

 the average diameter of the coil is about -fa inch ; but in some parts they are larger 



Fig. 491. DEVELOPING SWEAT-GLANDS FROM A SEVEN 



MONTHS FOETUS. MAGNIFIED 50 DIAMETERS. (Kolliker. ) 



a, horny layer of the epidermis ; b, Malpighian layer ; 

 d, rudimentary gland ; e, lumen of the duct, opening at 

 / upon the surface of the skin. 



than this as, for example, in the groin, but 

 especially in the axilla. In this last situation 

 Krause found the greater number to measure 

 from ^ to ^ inch, and some nearly -J- inch 

 in diameter. 



The development of the sweat-glands 

 has been carefully studied by Kolliker. Their ' 

 rudiments, when first discoverable in the 

 embryo, have much the same appearance 

 as those of the hairs, and, in like manner, consist of processes of the mucous 

 layer of the epidermis, which pass down and are received into corresponding 

 recesses of the corium (fig. 491). They are formed throughout of cells collected 

 into a solid mass of a club shape, continuous by its small end with the .Malpighian 

 layer of the epidermis, and elsewhere surrounded by homogeneous limiting membrane 

 which is prolonged above between the corium and cuticle. The subsequent changes 

 consist in the elongation of the rudimentary gland, the formation of a cavity along 

 its axis at first without an outlet the prolongation of its canal through the 

 epidermis to open on the surface, and, in the meantime, the coiling up of the 

 gradually lengthening gland-tube into a compact ball, and the twisting of the excre- 

 tory duct as it proceeds to the orifice. The plain muscular tissue of the sweat- 

 glands is said to be developed from some of the epithelium-cells of the rudimentary 

 gland (Ranvier). If this be so, it is the only known instance, in the higher animals, 

 of muscular tissue being developed from the epiblast. 



The ceruminous glands in the auditoiy passage consist of a tube coiled into a 

 ball, like the sweat-glands ; and there is such a further correspondence between 

 the two, in structure and mode of development, that the ceruminous glands may be 

 regarded as a variety of the sudoriferous. 



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 Malpighian layer of the epidermis, (for the 

 outer stratum of the root-sheath must be regarded as such) ; these grow 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. 



