The main substance of the rest of the shaft of all hairs, and its entirety in some, is com posed of the cortical tissue. This is a horny hard substance, clear And homogeneous in white hairs, but filled with pigment granules, and moreover having its own special colora tion in coloured hairs, which may be broken up mechanically, or by the action of strong alkalies and acids, into lung, pale, some times striated fibres, which may or may not present remains of elongated cndoplasts. Besides the latter and the pigment granules, a multitude of strim and dots are visible in the cortical substance, which are produced by canals and cavities containing air.
The cortical substance results from the metamorphosis of the corresponding portion of the hair bulb. The primarily rounded vesicular endoplasts (fig. 315. A), become greatly elongated and spindle-shaped, without ever, so far as I have been able to observe, becoming surrounded by a distinct cell cavity or wall (fig. 315, n). At the same time pig ment granules arise in the periplast ; it ac quires a fibrous appearance, becomes horny, and splits up m ore and more readily into plates and fibres in the direction of its length. As it attains its perfect structure, rounded and elongated vacuolm, which there is no reason whatever to suppose result from confluent cell cavities, arise in it and become filled with air. In fact,the perfect cortical substance is a sort of rudimentary horny dentine.
Lastly, the medullary substance — which at tains a considerable development in the short thick hairs of man, and in those of the body of many mammals, but is frequently absent, as in the hair of the head of man, and according to Briicke (Reichert's " Bericht," 1849) in the bristles of the pig, the whiskers of the dog, seal, walrus and the long hairs of Myrmeco phaga jubata —consists of a horny matter like that of the cortex and continuous with it, excavated into polygonal cavities, which fre quently contain air bubbles and pigment granules. The cavities communicate, and the air may be driven from one into the other.* In the fully formed hair, they contain no remains of endoplasts. The medullary substance, like the cortical, proceeds from the metamorphosis of the indifferent tissue of the pulp, but the process, instead of being one of vacuolation and fibrillation, is essentially one of cellulation. The endoplasts, instead of
elongating, remain rounded. Cavities are de veloped round them, whose partition walls become thick and granular. The cavities then gradually enlarging eventually open into one another, and the endoplasts disappear. The whole structure and mode of develop ment of this tissue, in fact, show its complete identity with the " pith " of feathers, as we shall see more fully below.
The hair sac is an involution of the whole integutnent, and as such is composed of an enderonic and of an ecderonic portion. The former, which is continuous with the subcu taneous tissues, when well developed, consists externally of a network of fine elastic fibres, within which is a layer of homogeneous tissue containing endoplasts which are more or less elong,ated transversely, and which form the superficial layer of the enderon. Within this is a structureless layer, the commencement of the ecderon, enclosed by which are the re presentatives of the cellular ecderon, the so called rootsheaths. These are commonly de scribed as two, th e outer (a) and the inner (c, d); the latter again being composed of two struc tures, an external, the fenestrated inner root sheath of Henle, and an internal, which I de scribed in 1815, and which may be called the imperforate inner rootsheath. The outer root sheath, like the others, is thicker above than below, thinning out where it joins the bulb at the bottom of the sac. It consists entirely of tissue resembling that of the rete mucosum, and needs no particular description.
The fenestrated, inner rootsheath lies in im mediate contact with the outer rootsheath. It is composed of more or less rounded or polygonal flat plates, with faintly marked boundaries, united by their narrow ends, and leaving spaces between their sides (fig. 315. F). It is very tough and resistant, both to mechanical and chemical action, and no endo plasts can be seen in its elements. The im perforate rootsheath (a) is composed of flat thin flexible plates not unlike those of the preced ing layer ; but they present no intervals, their boundaries are strongly marked, and in the centre of each there is a peculiar, elongated, often more or less dumb-bell-shaped endo.
plast. In the human hair sac there are usually only one or two larninw in this layer, but in Rodents there are said to be many.