TEGUMENTARY ORGANS 375 



sac there was a bulging process, the centre of which was occupied by 

 a mass of fatty-looking granules, the future sebaceous glands of the 

 hair. 



Hairs are not normally susceptible of indefinite growth, but have, 

 like the teeth, a fixed form to attain. This form is always that of a 

 more or less elongated spindle, inasmuch as the hairs are sharp at 

 their points, becoming broader and thicker in the middle, and dimin- 

 ishing again at their proximal ends. When fully formed, and ready 

 to fall out, in fact, this end of the hair is either pointed, or more or 

 less ragged and brush-like. 



As soon as the finishing process of any hair begins the foundation 

 of a new one is laid by the development of a diverticulum of the outer 

 rootsheath towards its base, in which a young hair is developed, in the 

 manner already described, and gradually pushes out the old one. 



The varieties of form and appearance presented by the hairs of 

 animals (for which see the works of Heusinger, Eble, Busk, and Quekett, 

 cited at the end of this article) are produced ; ist, by the relative 

 proportions of the medullary and cortical substances, and the 

 arrangement of the former with respect to the latter. Thus the 

 peculiar appearance of Rodent hairs is due to the disposition of the 

 medullary substance. 2nd, by the development of the cuticular layer, 

 whence arise the whorled scales of bat's hair — the imbricated plates of 

 seal's hair, &c. ; 3rd, by the shape of the shaft, which may be cylin- 

 drical, as in ordinary hair of the head in man ; or evenly flattened, 

 as in the short curly hairs ; or narrow and cylindrical below, and wide 

 and flattened above, as in the hairs of the deer tribe. The spines of 

 certain mammals, such as Hystrix and Erinaceus, present some 

 interesting peculiarities of form ; offering, as they do, a sort of 

 transition between hairs and feathers.^ 



The porcupine's " quill," as it is called, is a cylindrical tube which 

 gradually diminishes to a point above and below. At its apex , the 

 cavity of the quill is simply conical, but lower down its section 

 becomes polygonal, and, the angles of the polygon, being prolonged, 

 resembles a four-rayed star. Still further towards the root of the 

 quill, each ray of the star divides into two secondary rays, and then 

 the secondary rays subdivide into two tertiary rays ; so that eventually 

 the cavity of the spine is a complicated star with four and twenty 

 branches. Below its middle, the quill diminishes in diameter, and at 

 the same time the complexity of its internal cavity likewise dis- 

 appears, the tertiary rays disappearing first, and then the secondary 



1 See Briicker (Reichert's Bericht, 1849), from whom the account in the text is taken, 

 though the main points have been independently verified. 



