The hard structures of nails, hoofs, and horns (i. e. horny sheath of the horns of Ruminants) are developed in exactly the same manner ; nor am I aware that any tissue enters into these organs, which is not entirely produced by the horny conversion of a cel lular ecderon. The hoof of a fcetal lamb was entirely composed of such horny cells.
Structure of hairs, spines, and feathers.— In these tegumentary organs, we have to con sider, first, their own proper structure, and, secondly, that of the sacs in which they are at first wholly, and always partially, enclosed.
The shaft of a hair is composed of three distinct structures, an external, the czttic e ; a middle, the cortex ; and an internal, the medulla.
The cuticle ( fig. 315. c, D, E) on that por tion of the shaft which lies within the hair sac, consists of two layers, while only the inner of them remains in the protruded por tion. Viewed in section, as when a hair is observed in its totality, the cuticular layers form a thin double margin to the shaft, the outer (b) having the appearance of minute rhomboidal cells, joined end to end ; the inner (a) seeming to be composed of close set fibres arranged parallel to one another, and obliquely to the axis of the hair. If, however, the focus of the microscope be adjusted to the surface of the hair, or if the cuticular layer be detached from the shaft, these rhomboidal cells and parallel strim are found to be the expression or irregular transparent structure less plates, overlapping one another, and closely united into tough membranes, to which their projecting edges give a striated ap pearance. No trace of endoplasts is visible in the older of these plates, and the matter of which they are composed is singularly un changeable, remaining untouched on the ad dition of strong sulphuric acid, or of caustic potash, which completely dissolve the inner substance of the base of the shaft, and leave the cuticle in the form of a transparent, colour less, double membrane. In man, the outer layer of the cuticle ceases at the level of the sebaceous glands ; and the edges of the plates of the inner layer lie very closely appressed to the shaft ; in many of the lower animals, however, the plates are at a greater angle to the axis of the hair, and their projecting edges give rise to the most elegant sculpturings of its surface.
The cuticle proceeds from the horny meta morphosis of the two outermost layers of the pulp of the hair. The lowest portion of the bulb of a hair, if viewed in section, presents a sharply defined edge ( fig. 315., c), which may occasionally be raised up by reagents as a distinct structureless membrane ; but is normally perfectly continuous with the sub jacent transparent homogeneous periplast of the pulp, in which lie the ordinary rounded or oval vesicular endoplasts of young indif ferent tissue. Tracing the margin of the hair upwards, we find, next, that the two most superficial series of these endoplasts (D, a, b) are distinguished from the rest, by being free from that deposit of pigment granules which surrounds the endoplasts of the proper shaft substance ; and these two series are more or less distinctly contained in cavities or cells. The outer series is disposed more parallel, the inner more perpendicular to the surface. Still higher, (E) the cavities of the outer series are larger, and their party walls straight and sharply defined, while the endo plasts, which were at first plainly visible, dis appear. In the inner series, both cavities and endoplasts disappear, and the periplast seems to split up into thin parallel horny plates (E), whose edges become more and more strongly marked. Such are the steps in the development of the cuticular layers which may be observed in short thick human hairs, such as those of the nostril. In those of the head, however, and in the hairs of the body of the calf, I have been unable to trace the cuticle into anything but a structureless layer, wrinkled externally, which passed into the superficial structurcless layer of the deepest part of the bulb (c). I formerly thought that this indicated an important difference, but it is readily accounted for, if we suppose the process of development to be the same in each case, the endoplasts only disappearing very early in the latter.