704 STRUCTURE OF FEATHERS AND HAIRS. 



of aggregations of cells filled with horny matter, and frequently 

 much altered in form. This structure may generally be made-out 

 in Horns, Nails, &c, 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 modifications of this 

 structure are presented to us in Hairs and in Feathers ; which 

 forms of clothing are very similar to each other in their essential 

 nature, 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 distri- 

 bution of vessels to its walls. When a Hair is pulled-out ' by its 

 root,' its base exhibits a bulbous enlargement, of which the exte- 

 rior is tolerably firm, whilst its interior is occupied by a softer 

 substance, which is known as the ' pulp ; ' and it is to the con- 

 tinual augmentation of this pulp in the deeper part of the follicle, 

 and to its conversion into the peculiar substance 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. 



551. Although the Hairs of different Mammals differ greatly in 

 the appearances they present, we may generally distinguish in 

 them two elementary parts ; namely, a cortical or investing sub- 

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

 substance, usually of a much softer texture, occupying the interior. 

 The former can sometimes be distinctly made-out to consist of 

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

 the Hairs of the Sable (Fig. 364) ; whilst, in the same hairs, the 

 Medullary substance is composed of large spheroidal cells. In the 

 Mush-deer, on the other hand, the Cortical] substance is nearly 

 undistinguishable ; and almost the entire hair seems made-up of 

 thin-walled polygonal cells (Fig. 365). The Hair of the Rein-deer, 

 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 Rodents (Fig. 366, a, b), the Cortical substance forms 

 a tube, which we see crossed at intervals by partitions that are 

 sometimes complete, sometimes only partial ; these are the walls of 

 the single or double line of cells, of which the Medullary substance 

 is made-up. The Hairs of the Bat tribe are commonly distrn-. 



