TEGUMENTARV ORGANS 



413 



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. B). 

 At the same time pigment granules arise in the periplast ; it acquires a 

 fibrous appearance, becomes horny, and splits up more and more readily 

 into plates and fibres in the direction of its length. As it attains its 

 perfect structure, rounded and elongated vacuolae, which there is no 

 reason whatever to suppose result from con- 

 fluentcell 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 

 contains a considerable development in 

 the short thick hairs of man, and in those 

 ■of the body of many mammals, but is fre- 

 quently absent, as in the hair of the head 

 of man, and according to Briicke (Reichert's 

 " Bericht," 1 849) in the bristles of the pig, 

 the whiskers of the dog, seal, walrus and 

 the long hairs of Myrmecophaga jubata — 

 consists of a horny matter like that of 

 the cortex and continuous with it, ex- 

 cavated 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 con- 

 tain no remains of endoplasts. The me- 

 dullary 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 developed 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 development of this tissue, in fact, 

 show its complete identity with the " pith " of feathers, as we shall see 

 more fully below. 



Fig. 316. — Diagram illustrative 

 of the jDosition of the differ- 

 ent layers of the hair sac 

 in a young hair, a, b, outer 

 rootsheath ; c, fenestrated 

 rootsheath ; d, imperforate 

 rootsheath. 



The hair sac is an involution of the whole integument, and as such 



1 Griffith, Lond. Med. Gazette, 1848. 



