496 



TEGUMENTARY ORGANS. 



distended and rendered transparent, by the 

 action of acids or alkalies. The horny 

 stratum of the epidermis is therefore the result 

 of the conversion of the walls or periplast of a 

 whole layer of the cells of the ecderon into 

 horn. 



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 



1>roduced by the horny conversion of a cel- 

 ular ecderon. The hoof of a foetal 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 cuticle; 

 a middle, the cortex; and an internal, the 

 medulla. 



Fig. 315, 



Hair, Man. 

 A, B, D, E, F, from the nose ; c, from the head. 



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 (6) 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 striae are found to be the 



expression of 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 (j%. 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 structureless 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. 



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 



