This sub-kingdom has received its name in consequence of all its members possessing a vertebral column; a segment of this column is designated a vertebra, which Professor Owen defines as " one of those segments of the endo-skeleton which constitute the axis of the body and the protecting canals of the nervous and vascular trunks."

Unity of type pervades the whole sub-kingdom. All its members are fashioned after the same model-hence, a "typical" or "ideal" vertebra has been constructed, from which, however, there are many and various deviations; the nearest approach to it exists in the caudal vertebra of the crocodile, and in the skeleton of the fish. The accompanying diagram explains the "ideal vertebra."

1. Neural spine, sometimes bifid. 2. Zygapophyses, articular processes of neural arch. 3. Neurapophyses, forming sides of neural arch. 4. Diapophyses, upper transverse processes. 5. Centrum, or body. 6. Pleurapophyses, ribs. 7. Parapophyses, lower transverse processes. 8. Hæmapo

Cranial Vertebra. Divisions of Vertebrata.

27

physes, forming sides of hæmal arch. 9. Zygapophyses, articular processes of hæmal arch. 10. Hæmal spine, sometimes bifid. * Denotes the exogenous elements. All the others are the autogenous elements.

The accessory process situated behind the base of the transverse process of each lumbar vertebra, and the mammillary process situated on the posterior aspect of each superior articulating process of the same vertebræ in the human subject, have been respectively called the Anapophyses and Metapophyses.

Professor Owen has, moreover, constructed a 66

or "Archetype Skeleton.”

Typical "

The bones of the Cranium are now believed to be composed of four modified vertebræ, and are designated:

The Vertebrata are divided by Professor Huxley into three divisions, and by Professor Owen into two, which comprise five classes (sic.) :

CHAPTER II.

MAMMALIA: GENERAL CHARACTERS.

MAMMALIA are vertebrate animals that possess mammary glands; all but two (the echidna and ornithorhyncus paradoxus) have teats; the young are always born in a helpless condition, and the skin is invariably covered, more or less, with hair.

OSTEOLOGY.-The skull is united to the spinal column by means of two condyles; the lower jaw consists of two halves, each composed of a single piece and united in front, there is no quadrate bone; the vertebral column is, with few exceptions, divided into cervical, dorsal, lumbar, sacral and caudal regions. The cervical vertebræ are always seven in number;* the dorsal vertebræ generally thirteen, often more (in man there are only twelve); the lumbar vertebræ are usually six or seven, rarely less than four (in man only five); the sacral vertebræ are generally amalgamated to form a single bone, the sacrum (absent in whales); while the caudal vertebræ vary in number from four to forty-five. The thoracic cavity is enclosed by a series of ribs, which correspond in number to the dorsal vertebræ; the ribs, as a rule, are united to the sternum by cartilages, and not by bony pieces, as in birds; the front ribs, which reach the sternum, are termed "true," the hinder ones, which do not, are called "false." The sternum consists of several pieces placed one behind the other, which are usually amalgamated to form a single bone.

*For a long time it was believed that the sloth had nine, and the dugong six cervical vertebræ.

The normal number of limbs is four, hence Mammals have sometimes been incorrectly denominated Quadrupeds, for many of the Amphibia and Reptilia have also four legs. Whales and dolphins have only the anterior limbs. The anterior and posterior limbs respectively are connected with the body by the scapular and pelvic arches, and are variously modified in the different orders. Professor Haughton states that the best example of the Scapular arch exists in the bird, where the humerus works in a socket formed by four bones, viz., acromion, scapula, clavicle, coracoid; while the best example of the pelvic arch is seen in the alligator, consisting also of four bones, viz., ilium, ischium, marsupial, pubis.

Of the bones which typically form the scapular and pelvic arches, one or more may be absent, but it is important to remember that both the anterior and posterior limbs are formed after a simple and symmetrical plan, and are followed in succession by a series of bones. This is thus exemplified:

The bones enumerated above present many changes and modifications in the different classes of animals, the most remarkable of which will be mentioned. In Mammals the anterior limbs are always present, but the clavicle is frequently absent, and the acromion is often a mere process of the scapula, the coracoid, which is a distinct bone in birds and reptiles, is attached to the scapular arch. In the Monotremata alone of Mammals the coracoid remains separate.

The humerus and femur, as well as the bones of the forearm and leg, resemble each other; the bones of the wrist and ankle are subject to many modifications, while the fingers vary in number from one to five-thus, the horse has only one finger, which is believed to be the middle finger. The toes undergo similar variations, and do not always correspond in number with the fingers-e. g., in the dog tribe (Canidæ) the fore feet have five toes, while the hind feet have only four; and in the Hyænidæ there are only four toes to each foot.

The bones of the different orders of Mammals do not differ much from those of man; in general, however, they are coarser, and in some, as in those of the head of the elephant, there are extensive air cells; while those of the Cetacea have no medullary canals, are coarse and fibrous externally, and spongy or cellular internally, the cells assuming a tubular arrangement. On examining a thin transverse section of a long bone belonging to man with a microscope of moderate power, there will be observed the openings of the vascular canals, called Haversian, surrounded with concentric laminæ, and "plasmatic canals,” called “canaliculi” piercing the laminæ at right angles; the dilatations on these canals are termed "lacunæ " or "bone cells," and are arranged concentrically between the lamina. It will be remarked further on, that the microscopic appearance of bone varies in the different classes of vertebrate animals.

DIGESTION. The great majority of Mammals possess two sets of teeth, deciduous and permanent, hence they have been termed by Professor Owen, Diphyodonts (dís, twice; púw, I produce; odoús, a tooth). Some have only one set, and are called by the same author Monophyodonts (uóvos, single; pów, I produce; ỏdoús, a tooth); they are the Monotremata, Edentata, and Cetacea. In the scaly and great ant-eaters there are no teeth whatever, and they are present only in the