This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(2143). The highest boon conferred upon the lower animals, "Heaven's last, best gift," is parental affection. The cold-blooded Ovipara, unable in any manner to assist in the maturation of their offspring, were necessarily compelled to leave their eggs to be hatched by the agency of external circumstances; and their progeny, even from the moment of their birth, were abandoned to chance and to their own resources for a supply of nourishment. In Birds, the duties and the pleasures inseparable from the necessity of incubating their ova, and of providing nutriment for their callow brood, are indeed manifested to an extent unparalleled in the preceding orders of Vertebrata; but it is to the Mammalia alone, the most sagacious and intelligent of all the inhabitants of this world, that the Creator has permitted the full enjoyment of paternal and maternal love, has thrown the offspring absolutely helpless and dependent upon a mother's care and solicitude, and thus confers upon the parent the joys and comforts that a mother only knows - the dearest, purest, sweetest bestowed upon the animal creation.
(2144). The grand circumstance whereby the entire class of beings generally designated under the name of Quadrupeds may be distinguished from all other members of the animal kingdom is, that the females of every species are furnished with mammary glands - secerning organs appointed to supply a secretion called milk, whereby the young are nourished from the moment of their birth until they have reached a sufficient age to enable them to live upon such animal or vegetable substances as are adapted to their maturer condition. The possession of these lactiferous glands would indeed be in itself a sufficiently decisive characteristic of the whole group; and if to this we add that their visceral cavity is separated into a thorax and abdomen by a muscular diaphragm, and that they breathe by means of lungs precisely similar to our own, we need not in this place dwell upon any more minute definition of the Mammiferous Vertebrata.
(2145). The Mammalia, as we might be prepared to anticipate from their importance, are extensively distributed. The generality of them are terrestrial in their habits, either browsing the herbage from the ground, or, if of carnivorous propensities, leading a life of rapine by carrying on a bloodthirsty warfare against animals inferior to themselves in strength or ferocity. Many inhabit the trees; some burrow beneath the surface of the soil; a few can raise themselves into the air and flit about in search of insect prey; the Otter and the Seal persecute the fishes even in their own element; and the gigantic Whales, wallowing upon the surface of the sea,"tempest the ocean" in their fury.
(2146). With habits so diverse, we may well expect corresponding diversity in their forms, or in the structure of their limbs; and, in fact, did we not compress our description of these particulars into reasonable limits, we might easily test the perseverance of the most patient reader in following us through the mass of details connected with this part of our subject. We shall therefore, commencing as we have hitherto done, with the osteology of the class, first describe, in general terms, the characters of a Mammiferous skeleton, and then, as we arrange the Mammalia under the various orders into which they have been distributed, speak of the most important aberrations from the given type.
(2147). The vertebral column of all Mammals, with the remarkable exception of the Cetacea, is divisible into the same regions as in the human skeleton, viz. the cervical, dorsal, lumbar, sacral, and coccygeal or caudal portions.
(2148,) The cervical vertebrae are invariably seven in number. The Sloth (Bradypus tridactylus) was, until recently, regarded as forming a solitary exception, it having been supposed to possess nine cervical vertebrae; the researches of Professor Bell, however, show that even this animal conforms to the general law. The distinguished naturalist referred to has demonstrated *, "that the posterior two of these vertebrae have attached to them the rudiments of two pairs of ribs, in the form of small elongated bones articulated to their transverse processes; they must therefore be considered as truly dorsal vetebrae, modified into a cervical form and function suited to the peculiar wants of the animal." Professor Bell further observes that "the object of the increased number of vertebrae in the neck of the Sloth is evidently to allow of a more extensive rotation of the head; for, as each of the bones turns to a small extent upon the succeeding one, it is clear that the degree of rotation of the extreme point will be in proportion to the number of pieces in the whole series.
When the habits of this extraordinary animal are considered, hanging as it does from the under surface of boughs, with the back downwards, it is obvious that the only means by which it could look towards the ground must be by rotation of the neck; and as it was necessary, to effect this without diminishing the firmness of the cervical portion of the vertebral column, to add certain moveable points to the number possessed by the rest of the class, the additional motion was acquired by modifying the two superior dorsal vertebrae, and giving them the office of cervical, rather than by infringing on a rule which is thus preserved entire without a single known exception".
(2149). The occipital bone articulates with the atlas by two lateral condyles, instead of by a single central articulating surface, - a circumstance which depends upon the greatly-increased development of the encephalon, and the consequent expansion of the cranium.
* Cyclop, of Anat. and Phys. art. Edentata.
(2150). The number of dorsal vertebrae depends upon that of the ribs: thus in the Bat tribe there are only eleven; while in some of the Pachydermata (as, for example, in the Elephant and Tapir) as many as twenty dorsal vertebrae may be counted. The lumbar and sacral vertebras will likewise be more or less numerous in different genera; and in the number of pieces composing the coccyx, or tail, there is every variety, from four to five-and-forty.