248 OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



dissolve the latter, though they are themselves dissolved by a stronger solution, 

 which does not act upon the horny layer of the cuticle ;* and it seems not improba- 

 ble that these permanently retain their place, and are not successively carried to 

 the surface by the formation of new layers beneath, as are the spheroidal cells 

 (Fig. 38, c, d) which lie upon them. In what way these spheroidal cells originate, 

 has not yet been ascertained. It has been generally supposed that they are 

 formed upon free nuclei in the midst of a blastema that intervenes beneath the 

 cutis and the " stratum Malpighii;" but the researches of Prof. Kolliker tend 

 to negative this idea, and to render it probable that they multiply by endogen- 

 ous production. 3 In whatever mode they are generated, the nutriment which 

 they require for their growth and development must be drawn from the vessels 

 of the Cutis, through the medium of the basement membrane; since, however 

 thick may be the substance of the Epidermis, it is never penetrated by vessels. 

 The Epidermis is pierced by the excretory ducts of the sebaceous and sweat- 

 glands, those of the latter passing through it with a somewhat corkscrew-like 

 turn (Fig. 37, g y i) } and both being lined with an epithelium (/) which is con- 

 tinuous with that of the mucous layer of the cuticle. It is also pierced by the 

 Hairs, with whose substance (as we shall presently see) it has a like relation of 

 continuity through their follicles. The horny layer has the same chemical com- 

 position with Nails, Hoofs, Horns, Hair, and Wool; the formula of all of them, 

 being 480, 39H, 7N, 180. 



241. The Epidermis covers the whole exterior of the body, not excepting 

 the Cornea and the Conjunctival membrane, where, however, it has more the 

 character of an Epithelium ; this -continuity is well seen in the cast skin or 

 slough of the Snake, in which the covering of the front of the eye is found to be 

 as perfectly exuviated as that of any part of the surface. The Epidermis 

 appears solely destined for the protection of the true Skin, from the mechanical 

 injury and the pain occasioned by the slightest abrasion, and from the irritating 

 influence of exposure to air and of changes of temperature : we perceive the 

 value of this protection, when it has been accidentally destroyed. The cuticle is 

 very speedily and completely replaced, however ; the increased determination 

 of blood to the Cutis, which is the consequence of the irritation, being favorable 

 to the accelerated production of Epidermic cells from its surface. It is proba- 

 ble that pressure and friction may act in the same manner ; for although the 

 peculiar thickness of the Epidermis on the palms and soles is well marked even 

 in the foetus (in obvious preparation for the future requirements of these parts), 

 yet, when parts of the surface on which the Cuticle was originally thin, are 

 habitually exposed to pressure or friction, its substance undergoes a great aug- 

 mentation. The Cuticle is completely exuviated at the close of some Exanthe- 

 matous disease, especially Scarlatina ; and we are probably to regard this as 

 one of the modes in which morbific matter is eliminated from the system 

 ( 215). It usually " desquamates" in minute shreds, or peels off in larger 

 patches; but sometimes the entire cuticle of the hand or foot, even with the 

 nails attached, comes off at once, like a glove drawn from the hand. A new 

 Epidermis is always, preformed beneath that which is thus shed; as in the 

 normal exuviation of the lower animals. 



242. Mingled with the ordinary Epidermic cells, we find some which secrete 

 Colouring-matter ; these are termed Pigment-cells. They are not readily dis- 

 tinguishable in the epidermis of the fair races of mankind, except in certain 

 parts, such as the areola around the nipple, and in freckles, naevi, &c. But 

 they are very obvious, on account of their dark hue, in the newer layers of the 

 Epidermis of the Negro and other colored races; and, like true Epidermic cells, 



1 See Messrs. Todd and Bowman's "Physiological Anatomy," p. 862, Am. Ed. 



2 See his " Mikroskopische Anatomic," band ii. $\ 14-22. 



