828 



THE POPULAR EDUCATOR. 



native of Australia, has certain features which are essentially 

 bird-like in character; e.g., it has a bill like that of a duck, 

 webbed feet, etc. It spends much of its time in water, though 

 it has burrows excavated in the adjacent banks of the stream. 

 A little time ago naturalists had some difficulty in determining 

 whether this animal was a mammal or not. Later researches 

 have, however, satisfactorily settled the question in the affirma- 

 tive. Closely allied to it is a peculiar hedgehog-like animal, 

 furnished likewise with a bill, and prickly spines on its back, 

 the porcupine ant-eater (Echidna). 



In the next stage towards mammalian perfection, we find an 

 extensive order of animals, principally found in Australia, Van 

 Diemen's Land, and the islands of the Asiatic Archipelago, as 

 far as Java. A few species are found also in America. These 



formidable and invincible barrier to the would-be antagonist. 

 Every hair is divided into a free part, or shaft (Fig. I., 1 ), with 

 its tapering point, and a root (Fig. I., 2) inclosed within a sac. 

 In straight hairs, the former is generally straight and rounded ; 

 in the curly and woolly hairs it is twisted spirally, and quite flat, 

 or slightly ribbed. The root is always straight and cylindrical, 

 and softer and thicker than the shaft ; at least, at its lower part. 

 In living hairs it ends in a still softer knob-like enlargement, 

 two to three times thicker than the shaft, the bulb of the hair 

 (Fig. I., 3), which is placed, cap-like, upon a little process of the 

 sac named the hair papilla.* (Fig. I., 9.) 



The nails and claws are, like the hairs, modified epidermic 

 processes, and, like it, consist of a soft and horny layer. 



Some animals, as the elephant, hippopotamus, rhinoceros, hog 



JV. 



MAMMALIA. I. HAIR AND HAIR FOLLICLE (MAGNIFIED 50 TIMES). II. HUMAN TEETH. III. VERTICAL SECTION OF A HUMAN MOLAR TOOTH. 

 IV. TRANSVERSE SECTION OF A HUMAN MOLAR TOOTH. V. SURFACE OF THE ENAMEL, WITH EXTREMITIES OF THE ENAMEL FJBSES 

 (MAGNIFIED 350 TIMES, AFTER KOLLIKER). VI. OSSEOUS HEAD OF THE GREENLAND WHALE, WITH THE WHALEBONE PRESENT. VII. THE 

 WHALEBONE. VIII. TEETH OF A CARNIVOROUS ANIMAL. IX. TEETH OF AN INSECTIVOROUS ANIMAL. X. TEETH OF AN HERBIVOROUS 

 ANIMAL. XI. TEETH OF A FRUGIVOROUS ANIMAL. 



Eefs. to Nos. in Figs. I. 1, shaft ; 2, root; 3, bulb ; 4, epidermis of the hair ; 5, inner root sheath ; 6, outer root sheath ; 7, structureless 

 membrane of the hair follicle; 8, transverse and longitudinal fibrous layers of the same; 9, papilla of the hair follicle; 10, excretory ducts 

 of two sebaceous glands ; 11, cwtis ; 12, mucous and, 13, horny layer of the epidermis the latter entering a certain way into the follicle ; 

 14, end of the inner root sheath. II. 1, incisive teeth ; 2, canine or eye-tooth ; 3, small or premolars ; 4, large molars. III. and IV. 

 1, 1, enamel ; 2, 2, puip cavities ; 3, 3, cement ; 4, 4, dentine. 



are the marsupials, or pouched quadrupeds (kangaroos, opossums, 

 etc.), so named from the presence of a bag, developed from the 

 skin of the belly, in which they carry their prematurely brought- 

 forth young during the helpless condition of infancy. Safe 

 from danger in the pouch, the young are enabled to reach the 

 maternal teats, by which they hang and are fed.* 



With few exceptions, the mammalia have their skin protected 

 with hair. In colour, shape, and strength, the hairs vary consi- 

 derably, from the curly wool which keeps the sheep warm, to 

 the protective spines of the hedgehog. The hair fulfils the follow- 

 ing conditions : Provides warmth to the body, adds to the beauty 

 of the animal, forms a protective covering to the skin, and like- 

 wise, as in the timid hedgehog, a spiked coat of mail, a most 



* Plutarch, in his treatise on the love of parents for their children, 

 mentions these animals as an illustration of affection for their offspring. 



horse, ass, etc., have remarkably thick skins, and on thig 

 account were formerly classed by Cuvier as a distinct order, 

 under the name Pachydermata (iraxvs, thick; 5tp/j.a, skin).f 



The Teeth. For variety and beauty, the teeth excel every 

 other part of the mammalian body. They are confined to the 

 jaws, and arranged in an uninterrupted series. Each jaw is 

 hollowed out into a number of pits, or alveoli, in which the 

 teeth are lodged, connected to the bone through the intervention 

 of a membrane called the periosteum, which lines the tooth- 

 socket. Each tooth is composed of dentine, or ivory (Figs. III. 

 and IV.. 4, 4), which forms the greater part of its substance. 

 The projecting part, or crown, is covered with a very hard 



* Kolliker. 



t The elephants now form the order Proboscidea, the hyrax 

 Hyracoidea, and the remainder may be called Ungulata. (Huxley.) 



