In the furthest advanced vibrissae the tissue of the axis of the sac was converted into horny cells, the rudiment of the " fenes trated " or of the inner, horny rootsheath. Over the papilla the rudiment of the hair shaft was indicated by a conical process, horny at its apex and marked by radiating lines. Finally, on each side of the neck of the 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 inde finite 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 dimi nishing again to 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 man ner already described, and gradually pushes out the old one.
The varieties of form and appearance pre sented by the hairs of animals (for which see the works of Hensinger, Eble, Busk, and Quekett, cited at the end of this article) are produced ; 1st, 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 deve lopment of the cuticular layer, whence arise the whorled scales of bat's hair — the imbri cated plates of seal's hair, &c. ; 3rd, by the shape of the shaft, which may be cylindrical, 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 inter esting peculiarities of form ; off'ering, 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 conica4, but lower down its section becomes polygonal, and, the angles of the polygon being prolonged, resem bles 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 fonr and twenty branches. Below its middle, the quill dimi nishes 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, &c., until at last the cavity is circular as at the apex. The boun dary of the quill cavity is immediately formed by medullary substance ; bat the cortical sub stance follows to a certain extent the con tour of the inner cavity, so that in a transverse section of the middle of the quill the cortical substance presents the same general outline as the medullary, though its processes and insec tions are less marked.
In the adult condition, the central cavity is filled by an irregular horny, mass, which Reichert and Bri3cker regard as the dried-up pulp, but which is probably, as in the feather (vide infra), simply the last horny product of the pulp, filling up the space vvhich the latter once occupied; for it is certain that every por tion of the porcupine quill has, like every por tion of a feather, at one time constituted a cap over the corresponding portion of its pulp. The pulp, in fact, commences like that of a feather, as a smooth conical process upon which the apex of the quill is moulded. As it grows, however, the pulp assumes an angular form, and then, as that of a feather would do, becomes produced into larnellze. By the constant production of new elements at the surface of these lamellm and their cornification, the "quill" is produced, and retains internally the impression of the mould on which it was formed. Apart from the arrangement of the lamellm, the principal difference from a feather which the " quill" presents, is simply that it does not, as it is formed, split up along the lines of the lamellm of the pulp.