606 ORGANS OF THE SENSES. 



a flattened form, lose their nuclei, and finally become closely compacted together 

 into a firm, dense, horny texture. In chemical composition, the nails resemble the 

 epidermis. According to Mulder, they contain a somewhat larger proportion of 

 carbon and sulphur. 



Hairs are peculiar modifications of the epidermis, and consist essentially of the 

 same structure as that membrane. They are found on nearly every part of the 

 surface of the body, excepting the pa*lms of the hands and soles of the feet, and 

 vary much in length, thickness, and color in different parts of the body, and in 

 the different races of mankind. In some parts they are so short as not to project 

 beyond the follicle containing them ; in other parts, as upon the scalp, they are of 

 considerable Jength ; along the margin of the eyelids and upon the face, they are 

 remarkable for their thickness. A hair consists of a root, the part implanted in 

 the skin ; the shaft, the portion projecting from its surface ; and the point. They 

 generally present a cylindrical or more or less flattened form, and a reniform out- 

 line upon a transverse section. 



The root of the hair presents a bulbous enlargement at its extremity ; it is 

 whiter in color, and softer in texture, than the stem, and is lodged in a follicular 

 involution of the epidermis, called the hair-follicle. When the hair is of con- 

 siderable length, the follicle extends into the subcutaneous cellular tissue. The 

 hair-follicle is bulbous at its deep extremity, like the hair which it contains, and 

 has opening into it, near its free extremity, the orifices of the ducts of one or more 

 sebaceous glands. In structure, the hair-follicle consists of two coats : an outer 

 or dermic, and an inner or cuticular. The outer coat is formed mainly of areolar 

 tissue; it is continuous with the corium, is highly vascular, and supplied by 

 numerous minute nervous filaments. The inner or cuticular lining is continuous 

 with the epidermis, and, at the bottom of the hair-follicle, with the root of the hair ; 

 this cuticular lining resembles the epidermis in the peculiar rounded form and soft 

 character of those cells which lie in contact with the outer coat of the hair-follicle, 

 and the thin, dry, and scaly character of those which lie near the surface of the 

 hair, to which they are closely adherent. "When the hair is plucked from its 

 follicle, this cuticular lining most commonly adheres to it, and forms what is called 

 the root-sheath. At the bottom of each hair-follicle is a small conical-shaped 

 vascular eminence or papilla, similar in every respect to those found upon the 

 surface of the skin; it is continuous with the dermic layer of the follicle, is highly 

 vascular, and probably supplied with nervous fibrils : this is the part through 

 which material is supplied for the production and constant growth of the hair. 

 The root of the hair rests upon this conical-shaped eminence, and is continuous 

 with the cuticular lining of the follicle at this part. It consists of nucleated cells, 

 similar in every respect to those which in other situations form the epidermis. 

 These cells gradually enlarge as they are pushed upwards into the soft bulb, and 

 some of them, contain pigment granules, which exist either in separate cells, or 

 the granules are separate, but aggregated round the nucleus ; it is these granules 

 which give rise to the color of the hair. It occasionally happens that these 

 pigment granules completely fill the cells in the centre of the bulb, which gives 

 rise to the dark tract of pigment often found, of greater or less length, in the axis 

 of the hair. 



The shaft of the hair consists of a central part or medulla ; surrounding this is 

 the fibrous part of the hair, covered externally by the cortex. The medulla 

 occupies the centre of the shaft, and ceases towards the point of the hair. It is 

 usually wanting in the fine hairs covering the surface of the body, and commonly 

 in those of the head. It is more opaque and deeper colored than the fibrous part, 

 and consists of cells containing pigment or fat-granules. The fibrous portion of 

 the hair constitutes the chief part of the stem ; its cells are elongated, and unite 

 to form flattened fusiform fibres. These also contain pigment granules, which 

 assume a linear arrangement. The cells which form the cortex of the hair consist 

 of a single layer which surrounds those about to form the fibrous layer ; they aro 

 converted into thin flat scales, having an imbricated arrangement. 



