833 



SKIN. 



SLATE. 



831 



organ of tbe body. Experiments have in fact proved that animals 

 prevented from perspiring die of suffocation as certainly, though not 

 BO rapidly, as when their respiration is obstructed. The quantity of 

 perspiration secreted amounts to about two pounds in 24 hours ; but 

 it is liable to considerable variations, according to tbe habits of the 

 individual, the state of the atmosphere, the activity of other glands, 

 such as the lungs and kidneys, and other circumstances. 



Another secretion from the skin is that of the oily sebaceous 

 matter by which its surface is always kept in a slight degree greasy, 

 so that water adheres to it only in drops, and does not easily soak into 

 the substance of the epidermis. The sebaceous glands by which this 

 secretion is produced, as well as the hair-follicles on which they are 

 almost always attendant, are described in the article HAIR. 



The loss of fluid by these secretions from the skin is in some 

 measure compensated by the absorption which it also exercises. It is 

 uncertain how much, if any, of the vapour of the atmosphere around 

 us is thus imbibed; but it is certain that the skin absorbs fluids 

 placed for a short time in contact with it, and this so rapidly, that 

 (especially after long fasting) a perceptible increase of weight is 

 observed after a person has been immersed in a bath. The obstacle 

 to a more constant and considerable absorption of fluid is the nearly 

 impenetrable layer of epidermis ; and hence the substances most 

 rapidly absorbed are those which most easily pass through it, such as 

 water, after having been imbibed into ita deepest layers, vapours of 

 sulphureted hydrogen, hydrocyanic acid, &c., oils rubbed upon it, or 

 corrosives which destroy its texture. 



Besides its secretions, there are produced from the vessels of the 

 skin materials of which are formed certain appendages for its protec- 

 tion and other purposes, such as the cuticle, the hair, and the nails. 



The cuticle, or epidermis, is an insensible and non-vascular mem- 

 brane, which is laid over the whole of the external surface of the body 

 in a layer, the thickness of which is varied according to the protection 

 required for the wellbeing of the subjacent cutis. The under surface, 

 which lies next to the cutis, is accurately fitted into all its irregu- 

 larities, and sends prolongations down into the interior of all its 

 glands and follicles ; the outer surface, which is exposed to friction, is 

 comparatively smooth. The epidermis is composed of several layers 

 of cells : of the two layers into which it may commonly in an ordinary 

 dissection be split, the lower is called Rete Mucosum, or Rete Malpighii; 

 the upper and outer, more particularly, Epidermis. In the deeper 

 layers the epidermis is composed entirely of minute polygonal cells, 

 adhering by their edges, and containing nuclei and a thin fluid ; in 

 the layers nearer the surface are cells of the same kind, but larger and 

 flatter; and those on the very outer surface are dry and scale-like; 

 they have lost almost all trace of form, and becoming loose, are 

 removed by friction at exactly the same rate ns, under ordinary 

 circumstances, new cells are produced at tbe surface next the cutis. 

 Thus the epidermis is subject to constant and rapid change : its cells, 

 as fast as they dry and are removed in the form of scurf from its 

 exterior, being replaced by new ones at its interior ; and thus, what- 

 ever waate (within certain limits) it is subject to, its thickness is not 

 diminished, but rather, as the waste is increased, so is its thickness, 

 till it attains that degree, which is competent to the protection of the 

 subjacent cutis ; as any one may see in the palms of his hands, soon 

 after he has begun to occupy himself in a more than usually laborious 

 handicraft. 



The epidermis is the seat of the characteristic national colours of 

 the skin, as well as of the colours of freckles and other superficial 

 marks. In dark-complexioned races, especially in negroes, it is very 

 thick, and ita cells are filled with minute black or otherwise coloured 

 pigment-granules, many of which also lie loose among them. [ALBINO; 

 PIGMENT.] The thickness of the epidermis in these tribes renders it 

 less penetrable by the rays of heat; and it is hence (and not on account 

 of its colour, which would have an opposite effect) that a negro can 

 bear the exposure of his skin to a degree of solar heat which blisters 

 that of a European. 



The nails are thin lamin;c of horny tissue, produced by the cutis on 

 the back of the ends of the fingers and toes. Under each of the more 

 perfect of the nails, such as those of the fingers and the great toe, the 

 cutis has a peculiar structure, called the matrix of the nail, composed 

 of large sharply-pointed and very vascular papilla;, which at the root 

 are arranged irregularly, but at the body of the nail are placed in 

 close set rows or longitudinal ridges. By all this vascular surface the 

 substance of the nail is produced in minute cells, which subsequently 

 coalesce and form the dense, obscurely fibrous, and transparent mass 

 of the body of the nail. The crescentio opaque part at the root of the 

 nail owes its whiteness in part to ita own substance, which in the 

 deeper layers is softer and more opaque than in those of the body, and 

 in part to the surface beneath it being less vascular than the rest. 



The under surface of the nail is grooved or otherwise marked in 

 correspondence with the matrix, to which it closely fits; the outer 

 surface, exposed to friction, is comparatively smooth, though still it 

 presents traces of the ridgea in which, when it was at the under 

 surface, it was formed ; for the nails are produced in the same method 

 as the cuticle ; as fast as their exposed surfaces or their ends are 

 worn away, they are replaced by layers growing from the matrix ; and 

 the whole mass of the nail, growing at once from below its body and 

 from its root, is constantly pushed forwards and thickened, at the 



MAT. HIST. DJV. VOL. IV. 



very same rate as its free extremity is cut or worn down, and its bjdy 

 thinned by friction. 

 SKIN-MOTHS. [DERMESTID.E.] 



SKINK. [SCINOIDJE.] 



SKIPPER. [HESPEUIID*.] 

 SKIPPER, a Fish. [SUOMBERESOX.] 

 SK1RRET. [SIDM.] 

 SKORODITE, a Mineral. [IRON.] 

 SKULL. [SKELETON.] 



SKULL-CAP. [SCUTELLARIA.] 



SKULPIN. [CALLIO.VYMCS.] 



SKUNK. [MDSTELIDJS.] 



SKYLARK. [ALACDA.] 



SLATE. By some geological writers tlie laminar structures which 

 prevail in many stratified and in some metamorphic rocks are called 

 slaty or schistose ; but, in consequence of the progress of investigation, 

 one of these structures, locally superinduced in deposited strata, 

 which is characterised by planes of cleavage generally meeting those 

 of deposition at considerable angles, is specially called the slaty struc- 

 ture. If, in the diagram below, c, s, I, represent iu section a scries of 



deposited beds of clay (c), sandstone (s), and nodules of limestone (I), 

 all dipping, as the arrow S (south) indicates, at 20 : the lines which 

 cro3s these beds at oblique angles, and are more highly inclined, as 

 in the arrow K = 60, are the edges of innumerable parallel planes 

 of cleavage, which are continuous through tho finely argillaceous 

 beds c ; more or less twisted in and about the limestone nodules I ; 

 more or less interrupted by the arenaceous beds s, or represented 

 therein by lines more nearly rectaugled to the plane of deposition. 

 The law here indicated of the want of coincidence in the planes of 

 cleavage and deposition 'u almost universally observed in nature. 

 Nearly horizontal strata are crossed by inclined cleavage; highly 

 inclined strata are traversed by nearly veriical cleavage. In strata 

 which dip different ways from an axis or to an axis, the cleavago 

 planes are sometimes found to be parallel throughout the mass on both 

 sides of the axis ; and even where strata are variously contorted, they 

 are frequently dissected through a great part or the whole of their 

 mass by cleavage planes passing in one direction. Hence the conclu- 

 sion is obvious that this slaty structure, this monohedral symmetry 

 (if we may not call it crystallisation), is the fruit of a general cause 

 acting subsequently to the deposition and disturbance of the strata, 

 capable of pervading and re-arranging the particles so as to polarisa 

 and systematise their mutual attractions, but not to fuse them together, 

 destroy their original distinctness, or obliterate the evidence of their 

 original condition. This force was so general, that along many mil'js 

 of country, as, for example, in the whole Snowdonian chain, ona 

 particular direction (north-north-east), in North Devon and Pembroke- 

 shire another (nearly east and west), is found to prevail more or less 

 distinctly in all the rocks; though, as before observed, arenaceous and 

 pebbly beds are least influenced by it, and limestones are unequally 

 and variously affected. 



This dependence of the slaty structure on the nature of the rock is 

 sometimes very positively pronounced, as in some classes of rock the 

 cleavage does change and even reverse its inclination where contortions 

 prevail. (This is very observable in some cases of cleavage in the old 

 red-sandstone of Pembrokeshire.) On a first view it appears to bo 

 equally dependent on geological time, since it is principally among 

 the older strata that it is well exhibited on a large scale ; but on this 

 head doubt arises, when we find the Silurian rocks, which are not 

 slaty at Ludlow, become so near Llandovery ; the old red-sandatone 

 slaty in Pembrokeshire and not so in Monmouthshire; the mountain 

 limestone shales slaty near Tenby and not so in Yorkshire ; the lias 

 shales slaty on the northern slopes of the Alps, but not so in England. 



There are then local conditions which influence the development of 

 slaty cleavage, and it is essential to a general solution of the problem 

 which this structure involves, that these conditions should be deter- 

 mined. Proximity to rocks of igneous origin has been freely appealed to 

 for this purpose ; but this appears an insufficient and not often applicable 

 cause. The most general condition which has occurred to our obser- 

 vation is the fact of remarkable displacement of the strata on one or 

 more anticlinal or synclinal axes; and it is of consequence to this 

 inference to remark that very often, approximately or even exactly, 

 the horizontal edge ('strike') of tho inclined cleavage planes coincides 

 with the axis of movement (and therefore with the strike) of tho 

 stratification. Pressure in some peculiar application appears to us to 



3 ii 



