CHAPTER II 
GENERAL ANATOMICAL CHARACTERS 
I. TEGUMENTARY STRUCTURES 
Hair.—The external surface of the greater number of members of 
the class is thickly clothed with a peculiarly modified form of 
epidermis, commonly called hair. This consists of hard, elongated, 
slender, cylindrical or tapering, filiform, unbranched masses of 
epidermic material, growing from a short papilla sunk at the 
bottom of a follicle in the derm or true skin. Such hairs upon 
different parts of the same animal, or upon different animals, assume 
various forms, and are of various sizes and degrees of rigidity,—as 
seen in the delicate soft velvety fur of the Mole, the stiff bristles 
of the Pig, and the spines of the Hedgehog and Porcupine, 
all modifications of the same structures. Each hair is composed 
usually of a cellular pithy internal portion, containing much air, 
and a denser or more horny cortical part. In some animals, as 
Deer, the substance of the hair is almost entirely composed of the 
medullary or cellular substance, and it is consequently very easily 
broken ; in others the horny part prevails almost exclusively, as in 
the bristles of the Wild Boar. In the Three-toed Sloth (Bradypus) 
the hairs have a central horny axis and a pithy exterior. Though 
generally nearly smooth, or but slightly scaly, the surface of some 
hairs is strongly imbricated, notably so in some Bats; while in the 
Two-toed Sloth (Cholepus) the hairs are longitudinally grooved or 
fluted. Though usually more or less cylindrical or circular in 
section, hairs are often elliptical or flattened, as in the curly-haired 
races of men, the terminal portion of the hair of Moles and Shrews, 
and conspicuously in the spines of the Rodents Xerus and Platacantho- 
mys. Hair having a property of mutual cohesion or “felting,” 
which depends upon a roughened scaly surface and a tendency to 
curl, as in domestic Sheep (in which animal this property has been 
especially cultivated by selective breeding), is called “wool.” 
