TEGUMENTARY ORGANS. 



497 



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 long, pale, some- 

 times striated fibres, which may or may not 

 present remains of elongated endoplasts. 

 Besides the latter and the pigment granules, a 

 multitude of stria3 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 (Jig. 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 pig- 

 ment granules arise in the periplast ; it ac- 

 quires 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 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 

 integument, 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 



* Griffith, Loiul. Mod. Gazette, ISIS. 



elongated transversely, and which form the 

 superficial layer of the endcron. Within this 

 is a structureless layer, the commencement 

 of the ecderon, enclosed by which are the re- 

 presentatives of the cellular ecderon, the so- 



Fig, 316. 



Diagram illustrative of the posit'wn of the different 

 layers of the hair sac in a young hair. 



a, b, outer rootsheath ; c, fenestrated rootsheath ; 

 d, imperforate rootsheath. 



called rootsheaths. These are commonly de- 

 scribed as two, the outer (a) and the inner (e, 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 1845, 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 therete mucosum, 

 and needs no particular description. 



Thefencstratcd, inner roots/tenth 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 (./%. 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 () 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- 

 K K 



