foreign to nutrition, can change so rapidly. M. Lepelletier1 ascribes it to two very different causes. First, to defective secretion of the colouring fluid, without any privation of nutrition. In this case, the hairs may live and retain their hold, as we observe in young individuals: -and secondly, to the canals, which convey the fluid into the hair, being obliterated, as in old age. The same cause, acting on the nutritious vessels of the bulb, produces, successively, privation of colour, death, and loss of those epidermoid productions. According to other physiologists, the seat of colour is in the horny covering of the hair; and, in the largest hairs or spines of the porcupine, this seems to be the case, the pith being white, and the horny covering coloured. There is often an intimate relationship observed between the colour of the hair and that of the rete mucosum. complexion is accompanied with light hair; a swarthy with dark ;— and we see the connexion still more signally displayed in those animals that are spotted-the colour of the hair being variegated like that of the skin. A fair Hairs differ materially according to the part of the body on which they grow. In some parts they are short, as in the armpits; whilst on the head it is not easy to say what would be the precise limit to the growth, were they left entirely to nature. In the Malay, it is by no means uncommon to see them touch the ground. The hair has various names assigned to it, according to the part on which it appears,-beard, whiskers, mustachios, eyebrows, eyelashes, &c. In many animals it is long and straight; in others crisped, when it is called wool. If stiff, it is termed a bristle; if inflexible, a spine. It is entirely insensible, and, excepting in the bulbous portion, is not liable to disease. Dr. Bostock affirms, that under certain circumstances hairs are subject to a species of inflammation, when vessels may be detected, at least in some of them, and they become acutely sensitive. Their sensibility under any known circumstances may be doubted. They appear to be anorganic, except at the root; and, like the cuticle, resist putrefaction for a length of time. The parts that do not receive vessels are nourished by transudation from those that do. Bichat and Gaultier were of the opinion of Dr. Bostock ;-misled, apparently, by erroneous reports concerning plica polonica; but Baron Larrey has satisfactorily shown that plica is confined to the bulbs: the hairs themselves continue devoid of sensibility. It is difficult to assign a plausible use for the hair. That of the head has already engaged attention; but the hair, which appears on certain parts at the age of puberty and not till then, and that on the chin and upper lip of the male sex only, set our ingenuity at defiance. In this respect, the hair is not unique. Many physiologists regard certain parts, which exist in one animal, apparently without function, but which answer useful purposes in another, to be vestiges indicating the harmony that reigns through nature's works. The generally useless nipple and mamma of one sex might be looked upon in this light; but 1 Traité de Physiologie Médicale et Philosophique, tom. iii. p. 42, Paris, 1832. the tufts of hair on various parts cannot, in any way, be assimilated to the hairy coating that envelopes the bodies of animals; and is, in them, manifestly intended as a protection against cold. TIL Fig. 46. n m There is another class of bodies connected with the skin, and analogous in nature to the last described,―the nails. These serve a useful purpose in touch, and consequently require notice here. In the system of De Blainville, they constitute a subdivision of the hairs, which he distinguishes into simple and compound,simple, when each bulb is separated, and has a distinct hair;-compound, when several pileous bulbs are agglomerated, so that the different hairs, as they are formed, are cemented together to constitute a solid body of greater or less size,-nail, scale, horn, &c. In man, the nail alone exists; the chief and obvious use of which is to support the The cuticle and nail, n, detached pulp of the finger, whilst it is exercising touch. Animals are provided with horns, beaks, hoofs, nails, spurs, scales, &c. All these, like the hair, grow from roots; and are considered to be analogous in their physical and vital properties. Meckel, and De Blainville, Bonn, Walther, Lavagna, and others, are of opinion, that the teeth are of the same class; and that they belong, originally, to the skin of the mouth. Section of the Skin on the end of the Finger. from the cutis and matrix, m. The nails, near their origin, are seen, under the microscope, to consist of primary cells, almost identical with those of the epidermis; these gradually dry into scales; and the growth of the nail appears to be effected by the constant generation of cells at its root and under surface; and as successive layers are pushed forward, each cell becomes larger, flatter, and drier, and more firmly fixed than those around it.1 The chemical composition of the epidermis and the nails is similar to that of the hair: yet according to Mulder, there are material differences in their properties;-the latter, being almost insoluble in strong acetic acid, in which the other two are readily soluble: hence-he infersthe composition of hair and of horn and whalebone must differ materially; and, that, accordingly, Scherer's conclusion, that they are all identical is incorrect. The following are the results of the analysis of each of these bodies. 2 For physiological purposes, the above description is sufficient. A few Henle, edit. cit., i. 289, Paris, 1843. 2 The Chemistry of Vegetable and Animal Physiology, translated by Fromberg, p. 527. Edinb. and London, 1849. words will be necessary regarding the mucous membranes, which resemble the skin so much in their properties, as to be, with propriety, termed dermoid. If we trace the skin into the various outlets, we find, that a continuous, soft, velvety membrane-epithelium-exists through their whole extent; and, if the channel has two outlets, as in the alimentary canal, this membrane, at each outlet, commingles with the skin; and appears to differ but slightly from it. So much, indeed, do they seem to form part of the same organ, that physiologists have described the absorption, that takes place from the intestinal mucous membrane, as external. They cannot, however, in the higher order of animals, be considered completely identical; nor is the same membrane alike in its whole extent. They have all been referred to two great surfaces;-the gastro-pulmonary-comprising the membranes of the outer surface of the eye, ductus ad nasum, nose, mouth, and respiratory and digestive passages; and the genito-urinary-which line the whole of the genital and urinary apparatuses. In addition to these, a membrane of similar character lines the meatus auditorius externus, and the excretory ducts of the mammæ. The analogy between the skin and mucous membranes is farther shown by the fact, that if we invert the polypus, the mucous membrane gradually assumes the characters of skin; and the same circumstance is observed in habitual descents of the rectum and uterus. In the mucous membranes-especially at their extremities, which appear to be alone concerned in the sense of touch-the same superposition of strata is generally considered to exist as in the skin-viz., epidermis or epithelium, rete mucosum, corpus papillare, and cutis vera. They have, likewise, similar follicles, called mucous; but nothing analogous to the hairs; unless we regard the teeth to be so, in corre⚫spondence with the views of Meckel, De Blainville, and others. The attention of anatomists has been closely directed to the ultimate structure of the mucous system. In the mucous tissues two structures have been separately described, especially by Mr. Bowman,' who has thrown much light on the subject. These are the basement membraneas he terms it—and the epithelium. The former is a simple, homogeneous expansion, transparent, colourless, and of extreme tenuity, situate 1 Cyclopædia of Anat. and Physiology, pt. xxiii. p. 486, April, 1842. on its parenchymal surface, and giving it shape and strength. This serves as a foundation on which the epithelium rests. It may frequently be demonstrated with very little trouble in the tubuli of the glands, especially of the kidney, which are but very slightly adherent, by their external surface, to the surrounding tissue. M. Flourens' considers that every mucous membrane is composed of three laminæ or layers,-the derma, epidermis, and corpus mucosum situate between the derma and epidermis. The corpus mucosum of mucous membranes is continuous at all the outlets of the body, and is identical with the second epidermis; differing, therefore, from the corpus mucosum of the skin, a term which-as elsewhere remarked-he thinks ought to be abolished. Histological examination exhibits the epithelium to consist of cells, which are termed epithelial, and have various shapes. The two chief are tesselated or pavement epithelium, and cylinder or conical epithelium. Epithelium is not, however, confined to mucous membranes, but, of late years, has been found to exist elsewhere; it is always Fig. 48. in contact with fluids, and of a soft, pliant character. Tesselated epithelium covers the serous and synovial membranes, the lining membrane of the blood-vessels, and the mucous membranes, except where cylinder epithelium exists. It is spread over the mouth, pharynx and œsophagus, conjunctiva, vagina, and entrance of the female urethra. The cells composing it are usually polygonal; and are well seen in the marginal figure. Cylinder epithelium is found in the intestinal canal, beyond the cardiac orifice, in the larger ducts of the salivary glands, in the ductus communis choledochus, prostate, Cowper's glands, vesiculæ seminales, vas deferens, tubuli uriniferi, and urethra of the male; and lines the urinary passages of the female from the orifice of the urethra to the beginning of the tubuli uriniferi of the kidneys. In all these situations, it is continuous with tesselated Tesselated Epithelium. Extremity of one of the tuof an adult; showing its tes buli uriniferi, from the kidney selated epithelium. - Magni fied 250 diameters. (Wagner.) Scales of Tesselated Epithelium. (After Henle.) A. Section of epithelium of conjunctiva with some scales loosened. B. Scales from surface of cheek. c. The more deeply seated scales from the human conjunctiva. epithelium, which lines the more delicate ducts of the various glands. The cells have the form of long cylinders or truncated cones, arranged side by side, the apices attached to the mucous membrane or to flat 'Op. cit., p. 80. Diagram of the Structure of an Involuted Mucous Membrane, showing the continuation of its elements in the follicles and villi. F, F. Two follicles. b. Basement membrane. c. Submucous tissue. e. Epithelium. v. Vascular layer. n. Nerve. v. Villus, covered with epithelium. v'. Villus, whose epithelium has been shed. epithelial cells lying upon it; the base being free. Each cell, nearly midway between the base and apex, encloses a flat nucleus with nucleoli. Cylinders of Intestinal Epithelium. (After Henle.) A. From the cardiac region of the human stomach. B. From jejunum. c. Cylinders seen when looking on their free extremities. D. Ditto, as seen in a transverse section of a villus. Epithelium is sometimes furnished with cilia, or is said to be ciliated. The nature and uses of these cilia, as well as the different varieties of mucous membrane, will be described hereafter. 2. PHYSIOLOGY OF TACT AND TOUCH. In describing the physiology of the sense of touch, it will be convenient to revert to the distinction already made between the sense when passively and actively exerted; or between tact, and touch. The mode, |