Cassell's concise natural history : being a complete series of descriptions of animal life / [E. Perceval Wright].

  • Wright, E. Perceval (Edward Perceval), 1834-1910.
Date:
1885
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Licence: Public Domain Mark

Credit: Cassell's concise natural history : being a complete series of descriptions of animal life / [E. Perceval Wright]. Source: Wellcome Collection.

    18/682 (page 4)
    (frogs and toads); and 5, Pisces (fishes). Each of these classes of vertebrate animals must in due course engage our attention. CLASS I.—MAMMALIA. The first class that will engage our attention is that of the Mammalia. In this class we find the following peculiarities, which will serve to distinguish it from all the others :—The skull is fastened to the spinal column by a double articulation; each half of the lower jaw is composed of but a single bone; the solid particles (corpuscles) of the blood are, for the most part, red, disc-shaped, and circular in form, and without a nucleus ; there is a complete diaphragm, separating the contents of the thoracic cavity (the lungs, heart, &c.) from those of the abdominal cavity (stomach, liver, &c.); the air-tubes of the lungs do not end in air- sacs ; the tegumentary system somewhere throughout its extent exhibits horny modifications of the epidermis, known as hair, scales, &c.; and, most essential feature of all, the young, after their birth, are nourished by a fluid called milk, which is secreted by glands called mammary glands, from whence the name of the class is derived. Very generally the little ducts which form these glands open upon one common elevation, known as the teat, or nipple. To this the young mammal applies its mouth when moved by the sensation of hunger. Although in each order of Mammalia many apparent differences in the degree and form of development of the various systems so often alluded to will be met with, yet it seems most convenient that we should give a general sketch of the peculiarities of each system as introductory to our special study of the orders. The Tegumentary System.—This system will come more particularly under our notice when considering the different natural orders. As a rule, the skin or integument of mammals is covered with hair, but in the Cetacea or whales hair is absent. In some mammals it is covered with horny scales (Manis), or bony plates (Dasypus), or spines (Erinaceus). In some, as in the rat and beaver, there is a mixture of hair and scales. The texture and quantity of the hair varies much, and so also does the thickness of the integument to which the hair is an appendage, being of great thickness and immensely rigid in the rhinoceros, and being wonderfully thin and fine in some rodents. In connection with hair and scales must be mentioned nails, claws, horns, and hoofs, the various forms of which will be mentioned presently. There are two classes of tegumentary organs, differing in structure, chemical composition, and mode of development. Eirst, the horny and glandular organs produced by the conversion of the outer layer of the skin (ecderon), which latter we may regard as the free area of metamorphosis of the external covering of any animal. Second, certain structures, which appear very frequently to be developed by a process of excretion. Of the former, we have nails, hoofs, the horny sheath of the horns of some ruminants, as well as hairs and spines. These latter, in addition to their own proper structure, are at first wholly, and always partially, enclosed in peculiar sac-like forms. These sacs, as in the case of the hair-sac, are often formed by a turning-in of the whole of the skin, so that they are composed of both an inner (enderonic) and an outer (ecderonic) portion. Among the conversionary productions of the outer layer are the so-called glands, which organs are engaged in the secretion of a fatty substance (sebaceous glands), or in that of a clear fluid (sweat glands). Between these two it is very difficult to draw any clear line of demarcation. The Osseous System.—The skeleton of mammals varies in the different groups. It consists of a back-bone composed of a series of individual bones, called vertebrae, hence the name vertebral column, to which are articulated the skull, a series of ribs, and the anterior and posterior extremities, or legs and arms. The vertebral column is divided into five regions—the cervical, the dorsal, the lumbar, the sacral, and the caudal. In some animals, as in the whales, there is but little distinction between the lumbar and the sacral regions. Although the neck or cervical region varies much in length, being in the gorilla about one- seventh of the whole vertebral column, while in the giraffe (Fig. 1) it is about three- sevenths, yet it is found to consist, with perhaps three exceptions, of seven vertebrae. In animals having hoofs (Fig. 1) it has been remarked that the length of the neck corre- sponds in great measure to that of the fore legs; but the elephant (Fig. 2) forms a noteworthy exception to this rule. There are generally thirteen dorsal vertebrae, as in most of the ruminating mammals,