Outlines of the chief developments of the dermo-skeleton in different vertebrates which are usually more or less ossified are added to the endoskeletal archetype: as, e.g. the median horn supported by the nasal spine (15) in the Rhinoceros; the pair of lateral horns developed from the frontal spine (11) in most Ruminants; the median folds (D 1, D 11) above the neural spines, one or more in number, constituting the "dorsal" fin or fins in Fishes and Cetaceans. Similar folds are sometimes developed at the end of the tail, forming a "caudal" fin, C, and beneath the haemal spines constituting the " anal" fin or fins, A, of Fishes, and other analogous developments.

(1652). From an extended survey of the organization of the skeleton throughout the vertebrate series, it is easy to perceive that, however diversified in adaptation to external circumstances, there is a general agreement between the various parts of the osseous framework, sufficient to convince us that all have been constructed in accordance with an ideal plan or archetype, from which ίδεa, as Plato expresses it, the Creator has not deviated since the earliest appearance of the palaeozoic races of Vertebrata up to the present period; and this "archetype" Professor Owen has illustrated by the diagram in the preceding page.

(1653). The nervous system of the Vertebrata is by far more complex and elaborately organized than that of any of the four preceding divisions of the animal world, and consists, in fact, of several distinct systems, differently disposed, and appropriated to different offices. Certain largely-developed ganglia, situated in the cavity of the cranium, generally considered by themselves on account of their disproportionate size when compared with the other nervous centres, are commonly grouped together under one common designation, and form what is called the brain, or encephalon: these masses, however, as we shall hereafter see, preside over various and widely-different functions, and with them perception, volition, and intelligence are essentially connected.

(1654). Continued from the brain, and lodged in a canal formed by the superior arches of the vertebral column, is a long chain of ganglionic centres, so intimately united that they appear confused into a long medullary cord, usually denominated the spinal marrow (medulla spinalis).

(1655). The spinal medulla in reality consists of two double series or columns, composed of symmetrical and parallel ganglia, - one pair of columns, the anterior, presiding over those muscular movements which are under the control of the will, while the posterior are destined to receive impressions derived from the exterior of the body; these columns, therefore, are denominated respectively the motor and sensitive tracts of the spinal cord.

(1656). From the lateral aspects of the medulla spinalis are derived, at intervals, symmetrical pairs of nerves, which escape from the spinal canal by appropriate orifices situated between the different bones of the vertebral column, and are distributed to the voluntary muscles and integument of the two sides of the body.

(1657). The spinal nerves, however, are not so simple in their composition as they were considered to be by the older anatomists: each of them has, in fact, been found to arise from the spinal cord by two distinct roots, one derived from the anterior, the other from the posterior column of the corresponding side; so that each nerve is evidently made up of two distinct sets of filaments, one set communicating with the motor, the other with the sensitive tracts; and thus every nerve derived from the spinal cord is a compound structure, being composed of filaments distinct in office, although enclosed in the same sheath, some being connected with the muscular movements, the others with sensation. But in addition to the cerebro-spinal ganglia and the symmetrically-arranged nerves emanating therefrom, that are distributed to the organs of sensation and movement, there exists in the Vertebrata a distinct system of nervous centres lodged among the viscera, appropriated to the performance of the automatic functions, and presiding over those involuntary movements of the body upon which depend the operations connected with nutrition.

These ganglia are variously distributed, being situated in the head, the neck, the thorax, and the abdomen; and from them arise large plexuses of nerves, destined to supply the organs belonging to digestion, circulation, and secretion, - thus forming extensive ramifications, formerly distinguished by the name of the sympathetic nerve, but now more properly considered as a distinct system presiding over organic life, as the former is connected with the phenomena of animal life.

(1658). With the increased development of the nervous system in the vertebrate classes we find the organs of the senses assume a proportionate perfection of structure and regularity of arrangement. The auditory apparatus, of which we have seen only rudiments in the lower animals, gradually becomes more and more elaborately organized. The eyes, now invariably two in number, are lodged in cavities formed for their reception by the osseous framework of the face, and exhibit, in the simplicity of their structure, a higher type of organization than any we have hitherto examined. Organs of smell, also double, but of very variable construction, are likewise constantly present. The tongue becomes slowly adapted to appreciate and discriminate savours; and the sense of touch, the most generally diffused of all, is especially conferred upon organs of different kinds peculiarly adapted to exercise this faculty. Thus with increased intelligence higher capabilities of enjoyment are allotted, and sagacity developes itself in proportion as the nervous centres expand. But there are minor points, characteristic of the vertebrate division of the animal world, which must not be omitted in this preparatory survey of their organization.