Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/957

Rh SSIN.j tongue contains numerous smaii tubular or branched glands, more especially on the dorsum near its root, which secrete mucus. Depressions also occur in this part of the mucous membrane, around the walls of which groups of lymphoid cells are collected in the sub-epithelial connective tissue, which have an arrangement closely resembling the structure of the adjacent tonsils, and form an example of adenoid tissue. The SKIN, or Integument, invests the entire outer surface of 1;he body, and contains structures by the excitation of which the properties of things are determined by the sense of touch. The skin also contains accessory structures, as the nails, hairs, sebaceous glands, and sweat glands. The skin consists of a non-vascular, cuticle or epidermis, and of a vascular and sensitive corium, or cutis vera. The Cuticle, Epidermis, or scarf skin, forms the outer covering of the skin, and protects the cutis. It is a laminated structure, and consists of numerous layers of cells superimposed on each other. As these cells cover a free surface exposed to the air, they belong to the epithe lium group. The thickness of the cuticle varies in diffe rent localities from y^h * 5&quot;$&quot;o~t ;n ^ ncn &amp;gt; wner e the skin is frequently exposed to pressure, as in the soles of the feet, the cuticle is the thickest and hardest ; and the hands of those accustomed to manual labour have a hard and horny cuticle. The increase in thickness in these localities is for the purpose of protecting the highly sensi tive cutis from injury. The outer surface of the cuticle in many parts of the body, especially the palm of the hand and the fingers, is marked by ridges and furrows ; the ridges indicate the position and arrangement of the papillae of the cutis, whilst the furrows are due to the sinking of the cuticle into the spaces between the rows of papilla?. The mouths of the sweat glands open on the surface of these ridges. The cuticle is divided into two strata. The super ficial horny stratum consists of layers of flat, polygonal scales like a tessellated epithelium ; the cells in the superimposed layers firmly adhere to each other by their surfaces, and in vertical sections this stratum presents a fibrous appearance ; but the cells may be readily isolated by digestion in a caustic alkali. The deeper or mucous stratum, or rete Malpiyhii, lies next the cutis, and closely follows the undulations of its papillary surface. The cells forming the layer next the cutis are columnar in shape, those in the layers immediately succeeding are rounded or cubical, whilst those next in order are polygonal, and not unfrequently possess pointed processes or prickles projecting from them, hence the name, prickle cells, employed by Schultze. The cells &quot;which lie next the horny stratum assume the scale-like form. It is in the cells of the mucous stratum that the colouring matter of the skin is found, which in the fair races of men forms the isolated coloured spots called freckles and moles, but in the dark races the pigment granules are uniformly distributed through the cells of this stratum. The super ficial cells of the horny stratum of the cuticle are continually being shed, so that the cells of the deeper layers gradually approach the surface, and new cells are continually being formed in the deeper part of the rete &quot;Malpighii. The cuticle is closely adherent to the cutis in the healthy living skin, but on the application of a blister, or when putrefac tion sets in after death, it separates from it. The Cutis vera. When the cuticle is removed the surface of the cutis is seen to be studded with multitudes of minute elevations, the papillse of the skin. These papillae are either simple conical structures, or compound with two or three branches. They are largest in the palm and sole, being from T -g-th to yj^th of an inch high, and are arranged in ridges, but more usually they are much shorter and irregularly distributed. The cutis is formed of counec- h Fio. 87. Vertical section through the skin and sutcuta- neons tissue, hs, horny stratum, and rm, rete Malpighii of cuticle; pp, papillae of cutis; t, a touch corpusclei with n, a nerve fibre ; J&amp;gt;c, a Mood and k, a lynij h capillary ; ct, connective subcutaneous tissue ; /, fat lobule; *, a sweat gland with its duct. 897 tive tissue, in which stellate connective tissue corpuscles and elastic fibres are abundant. The deeper surface of tho connective tis sue of the cutis is reticulated, and is continu ous with the bundles of con nective tissue that form the areolar subcu- ^MH2^iffi!niHKi^ taneous tissue. In the papillse themselves the fibres of the connective tis sue are not so well marked, and the surf ace of the papillse possesses more of a homoge neous aspect, which gives rise to the ap pearance de scribed as a basement membra n e. The cutis is highly vascu lar ; the small arteries which go to the skin give off branches to the lobules of fat in the subcutaneous tissue, then pene trate the cutis, and form a plexus from which capillaiies arise, which enter the papilla*, and form vascular loops within them. The lymphatic vessels of the skin are numerous; they form a plexus in the cutis, which lies beneath the vascular plexus, forms, as Neumann s injections show, a network around both the sebaceous and sweat glands, and gives off capillary loops into the papilla?. The nerves of the skin are the cutaneous branches both of the spinal and of certain of the cranial nerves, the origin and distribution of which have already been described. They run through the subcutaneous tissue, and enter the deep surface of the cutis, where they divide into branches. As these pass Nerves towards the papilla? they unite to form a nerve plexus, touch - from which smaller branches arise to enter the papilla?, and terminate, more especially in the skin of the palm of the hand, fingers, and sole, which are the surfaces most sensitive to touch impressions, in the tactile or touch corpuscles. The touch corpuscles discovered by Wagner and Meissner are the peripheral end-organs of the nerves of touch. They may be single or compound ; are usually ovoid inform, not unlike a minute fir cone; and are trans versely marked, from the transverse direction of the nuclei of fusiform cells which form an investing capsule. Each single corpuscle and each division of a compound corpuscle is penetrated by one, and, according to Thin, by never more than one, medullated nerve fibre, but the exact mode of termination of the axial cylinder of the fibre has not been ascertained. Virchow and other German observers have stated that the papillce which contain capillaries do not contain nerves or touch corpuscles, and vice versa ; but Dalzell and Thin have shown that certainly the majority of papilla? that contain nerve fibres and touch corpuscles are also vascular papillae. Non-medullated nerve fibres ascend to the surface of the cutis, and, according to Lan- gerhans, pass into the rete Malpighii between the cells of the mucous layer. Nails. On the back of the last phalanx of each thumb 3 Nails.