594 VBRTEBRATBD ANIMALS. 



all Epidermic appendages ; that is, they are produced upon the 

 surface, not within the suhstance, of the true skin, and are allied 

 in structure to the Epidermis (§ 419); being essentially com- 

 posed of aggregations of cells, filled with horny matter; and 

 frequently much altered in form. This structure may generally 

 be made out in horns, nails, &e., with little difficulty, by treating 

 thin sections of them with a dilute solution of soda; which after 

 a short time causes the cells that had been flattened into scales, 

 to resume their globular form. The most interesting modifica- 

 tions of this structure are presented to us in Hairs and in Fea- 

 thers ; which forms of clothing are very similar to each other in 

 their essential structure, and are developed in the same manner, 

 — namely, by an increased production of epidermic cells at the 

 bottom of a flask-shaped follicle, which is formed in the substance 

 of the true skin, and which is supplied with abundance of blood 

 by a special distribution of vessels to its walls. When a hair is 

 pulled out "by its root," its base exhibits a bulbous enlarge- 

 ment, of whioh the exterior is tolerably firm, whilst its interior 

 is occupied by a softer substance, which is known as the "pulp ;" 

 and it is to the continual augmentation of this pulp in the deeper 

 part of the follicle, and to its conversion into the peculiar sub- 

 stance of the hair when it has been pushed upwards to its narrow 

 neck, that the growth of the hair is due. The same is true of 

 feathers, the stems of which are but hairs on a larger scale ; for 

 the "quill" is the part contained within the follicle, answering 

 to the "bulb" of the hair; and whilst the outer part of this is 

 converted into the peculiarly solid horny substance forming the 

 " barrel" of the quill, its interior is occupied, during the whole 

 period of the growth of the feather, with the soft pulp, only the 

 shrivelled remains of which, however, are found within it after 

 the quill has ceased to grow. 



411. Although the Hairs of difiierent Mammals differ greatly 

 in the appearances they present, we may generally distinguish 

 in them two elementary parts ; namely, a cortical or investing 

 substance, of a dense horny texture, and a medullary or pith-like 

 substance, usually of a much softer texture, occupying the inte- 

 rior. The former can sometimes be distinctly made out to con- 

 sist of flattened scales arranged in an imbricated manner, as in 

 some of the hairs of the Sahle (Fig. 308) ; whilst in the same 

 hairs, the medullary substance is composed of large spheroidal 

 cells. In the Musk-deer, on the other hand, the cortical sub- 

 stance is nearly un distinguishable ; and almost the entire hair 

 seems made up of thin-walled polygonal cells (Fig. 309). The 

 hair of the Reindeer, though much larger, has a very similar 

 structure ; and its cells, except near the root, are occupied with 

 air alone, so as to seem black by transmitted light, except when 

 penetrated by the fluid in which they are mounted. In the hair 

 of the Mouse, Squirrel, and other small Eodents (Fig. 310, a, b), 

 the cortical substance forms a tube, which we see crossed at 



