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 (Cholcepus) 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." 



