(1614). The fifth division of the animal kingdom is composed of four great classes of animals, closely allied to each other in the grand features of their organization, and possessing in common a general type of structure clearly recognizable in every member of the extensive series, although, of course, modified in accordance with the endless diversity of circumstances under which particular races are destined to exist. The immeasurable realms of the ocean, the rivers, lakes, and streams, the fens and marshy places of the earth, the frozen precincts of the poles, and the torrid regions of the equator, have all appropriate occupants, more favoured as regards their capacities for enjoyment, and more largely endowed with strength and intelligence, than any which have hitherto occupied our attention, and gradually rising higher and higher in their attributes, until they conduct us at last to Man himself. Pishes, restricted by their organization to an aquatic life, are connected by amphibious beings, that present almost imperceptible gradations of development, with terrestrial and air-breathing Reptiles: these, progressively attaining greater perfection of structure and increased powers, slowly conduct us to the active, hot-blooded Birds, fitted by their strength, and by the vigour of their movements, to an aerial existence.

From the feathered tribes of Vertebrata, the transition to the still more intelligent and highly-endowed Mammalia is effected with equal facility; so that the anatomist finds, to his astonishment, that throughout this division of animated nature, composed of creatures widely differing among themselves in form and habits, an unbroken series of beings is distinctly traceable.

(1615). The first grand character that distinguishes the vertebrate classes is the possession of an internal jointed skeleton, which is not, as in the preceding classes, extravascular and incapable of increase except by the successive deposition of calcareous laminae applied to its external surface, but endowed with vitality, nourished by blood-vessels and supplied with nerves, capable of growth, and undergoing a perpetual renovation by the removal and replacement of the substances that enter into its composition.

(1616). In the lowest tribes of aquatic Vertebrata the texture of the internal framework of the body is permanently cartilaginous, and it continues through life in a flexible and consequently feeble condition; but as greater strength becomes needful, in order to sustain more active and forcible movements, calcareous particles are found to be deposited in the interstices of the cartilaginous substance, and, in proportion as these accumulate, additional firmness is bestowed upon the skeleton, until at length it assumes hardness and solidity proportioned to the quantity of the contained earthy matter, and becomes converted into perfect bone.

(1617). Phenomena precisely similar are observable in tracing the formation and development of the osseous system, even in those genera possessed, when arrived at maturity, of the most completely organized skeletons.

(1618). In the very young animal the bones consist exclusively of cartilage; but as growth proceeds, earth becomes deposited by the blood-vessels in the as yet soft and flexible pieces of the skeleton, until by degrees they acquire density and strength as the animal advances towards its adult condition.

(1619). The complete skeleton of a vertebrate animal may be considered as being composed of several sets of bones employed for very different purposes, - consisting of a central portion, the basis and support of the rest, and of various appendages derived from or connected with the central part. The centre of the whole osseous fabric is generally made up of a series of distinct pieces arranged along the axis of the body; and this part of the skeleton is invariably present; but the superadded appendages, being employed in different animals for various and distinct purposes, present the greatest possible diversity of form, and are many of them wanting in any given genus; so that a really complete skeleton, that is, a skeleton made up of all the pieces or elements which might, philosophically speaking, enter into its composition, does not exist in nature, inasmuch as it is owing to the deficiency of some portions and the development of others in particular races that we must ascribe all the endless diversity of form and mechanism so conspicuously met with in this division of the animal world.

(1620). Nevertheless, although there is no such a thing in creation as a fully-developed skeleton, it will be necessary, in order to prepare the student for the contemplation of the numerous modifications met with in this portion of the animal economy, hereafter to be described, briefly to enumerate the component parts which might theoretically be supposed to enter into the construction of the framework of an animal; and thus by comparison he will be enabled, as we proceed, to appreciate more readily the variations from a general type apparent throughout the vertebrate classes. It may likewise be as well thus early to caution the anatomist who has confined his studies to the contemplation of the human body, against taking the skeleton of Man as a standard whereby to direct his judgment; for Man, so highly raised by his intelligence and mental powers above all other beings, is, so to speak, a monstrosity in the creation; and, so far from finding in the human frame the means of elucidating the laws of animal organization, it is found to have been constructed upon principles the most aberrant and remote from those which an extensive investigation of the lower animals has revealed to the physiologist.

(1621). A skeleton, described generally, is made up of the following portions: first, of a chain of bones, placed in a longitudinal series along the mesial line of the back, and more or less firmly articulated with each other, so as to permit certain degrees of flexure. These bones, examined individually, present various additional parts destined to very different ends: some defend the central axis of the nervous system from external violence; others, when present, guard and enclose the main blood-vessels; and the rest, acting as prominent levers, either serve to give insertion to the muscles which move the spine, or afford additional security to the articulations between the vertebral pieces. Those vertebrae which defend the posterior portions of the nervous axis, usually called the spinal cord, constitute the spine; while those enclosing the anterior extremity of the nervous axis, which, for reasons hereafter to be explained, becomes dilated into large masses forming collectively the brain, are by the human anatomist distinguished as the cranium or skull.