TEGUMENTARY ORGANS 



411 



nor am I aware that any tissue enters into these organs, which is not 

 entirely produced by the horny conversion of a cellular 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 consider, 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. 



The cuticle {fig. 315. C, D, E) on that portion of the shaft which 

 lies within the hair sac, consists of two 

 layers, while only the inner of them 

 remains in the protruded portion. 

 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 micro- 

 scope be adjusted to the surface of 

 the hair, or if the cuticular layer be 

 detached from the shaft, these rhom- 

 boidal cells and parallel stride are 

 found to be the expression of irregular 

 transparent structureless plates, over- 

 lapping one another, and closely united into tough membranes, 

 to which their projecting edges give a striated appearance. No 

 trace of endoplasts is visible in the older of these plates, and the 

 matter of which they are composed is singularl}' unchangeable, re- 

 maining untouched on the addition 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, 

 colourless, 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 



Fig. 315. — Hair, 

 from the nose 



Man. A, E, D, E, F, 

 C, from the head. 



