IV. VERTEBRATA: MAMMALIA 



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Class III. Mammalia. 



The mammals occupy the highest place among the vertebrates, and 

 consequently in the animal kingdom; they also possess a special interest 

 for us, for man, in structure and development, belongs to the group, 

 although separated in intelligence from the most highly organized of the 

 members by a wide gap. 



The most striking characteristics of the mammals are furnished by the 

 skin. In fact one may, vv'ith Oken, call them hair-animals, since hair 

 is as diagnostic as feathers are for birds. The hairs (fig. 596, H) are 

 cuticular structures which are seated on papilla; of the corium, and 

 are nourished by blood-vessels in these. The lower end, the root of the 

 hair, lies in a pit in the epidermis, the hair follicle, and is surrounded by a 



Fig. 596. — Section of skin of man (from Wiedersheim). Co, corium (derma); 

 D, oil gland; F, fat; G, blood-vessels; GP, vascular papilla; H, hair; A'', nerves; Np, 

 nerve papilla; Sc, stratum comeum; SD, SD', sweat gland and duct; SM, stratum 

 Malpighii. 



double envelope, the epithelial root sheath, formed by an inpushing of 

 the epidermis and an outer connective-tissue follicular sheath. Small 

 muscles attached to the base of the larger hairs serve for their erection. 

 Since side branches are lacking, the structure of the hair is more simple 

 than that of feathers, and the forms fewer. Wool is characterized by its 

 spiral turns; then there is straight hair which, by increase in size, forms the 

 'whiskers' {vibrissa:) on the upper lip of many mammals, bristles (swine), 

 and lastly the spines of hedgehogs and porcupines. In the pelts of many 

 animals two kinds of hair may occur, wool below and straight hair outside. 

 Histologically hair consists of cornified cells, often arranged in medullary 

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