cess of consolidation of the skeleton certain bones originally distinct become fused together. In Plates XIL, XIII., and XIV., front, back, and side views of the entire skeleton are given, together with figures of the skull and several of its constituent bones.

ANATOMY OF THE ORGANS OF LOCOMOTION.

The organs of locomotion consist of the muscles or active organs, and the bones and joints or passive organs. The anatomy of the bones will first attract our attention. OSSEOUS SYSTEM-OSTEOLOGY-SKELETON.-The word Skeleton (from σkéλw, to dry) signifies literally the dry or hard parts of the body. When used in a limited sense it is applied merely to the boncs, but when used in a wider and more philosophic sense it comprises not only the bones or osseous skeleton, but the cartilages and fibrous membranes which complete the framework of the body. The first evidence of a skeleton in the embryo is the appearance of membranes in many parts of which cartilage is developed, and in course of time this cartilage is converted into bone. In some animals, however, as in the cartila. ginous fish, the osseous conversion does not take place, and the skeleton remains permanently cartilaginous; and in the very remarkable fish called Lancelet, or Amphioxus, the skeleton consists almost entirely of fibrous membrane.

The skeleton serves as a basis of support for the soft parts, as affording surfaces of attachment for muscles and as a protection for many delicate organs. In the vertebrata the osseous skeleton is clothed by the muscles and skin, and is technically called an endo-skeleton. In invertebrata the skeleton is not unfrequently on the surface of the body, and is termed an exo- or dermo-skeleton. In some vertebrates (e.g., the armadillo, tortoise, and sturgeon), in aldition to the proper endo-skeleton, skeletal plates are developed in connection with the integument, so that they possess a dermo-skeleton likewise. In some vertebrates, also, a partial skeleton is formed within the substance of some of the viscera-e.g., in ruminant animals a bone is situated in the heart; in the walrus and other carnivora, in rodents, bats, and some monkeys, a bone lies in the penis; and in the leopard, jackal, and other carnivora, a cartilaginous style lies in the middle of the tongue. These parts form a splanchno- or visceral skeleton. By some anatomists the teeth, which are unquestionably hard parts of the body, are also referred to the splanchno-skeleton, though they are special modifications of the papillæ of the mucous membrane of the gum. In man, the teeth being excluded, there is neither exo- nor splanchno-skeleton, but only an endo-skeleton.

In each of the great subdivisions of the body an endoskeleton exists, so that we may speak of an Axial Skeleton and an Appendicular Skeleton. The Axial Skeleton consists of the bones of the spine and head, the ribs, and the breastbone; the Appendicular Skeleton, of the bones of the limbs. The number of bones in the skeleton varies at different periods of life. In the adult there are about 200,

D 12:

PS

Ms

Vertebra

We shall commence the description of the AXIAL SKELE TON by giving an account of the bones of the spine. The SPINE, SPINAL or VERTEBRAL COLUMN, chine, or back- Spine. bone, consists of a number of superimposed bones which are named Vertebræ, because they can move or turn somewhat on each other. It lies in the middle of the back of the neck and trunk; has the cranium at its summit; the ribs at its sides, which in their turn support the upper limbs; whilst the pelvis, with the lower limbs, is jointed to its lower end. The spine consists in an adult of twentysix bones, in a young child of thirty-three, certain of the bones in the spine of the child becoming ankylosed or blended with each other in the adult. These blended bones lose their mobility, and are called false vertebræ ; whilst those which retain their mobility are the true vertebræ. In the vertebrata the bones of the spine are arranged in groups, which may be named from their position - vertebræ of the neck or cervical; of the chest, dorsal or thoracic; of the loins, lumbar; of the pelvis, FIG. 5.-The Axial Skeleton. C, the sacral; and of the tail, coccygeal or caudal; and the number of vertebræ in each group may be expressed in a formula. In man the formula is as follows:-C7D12L5S5C0c4 = 33 bones, as seen in the child; but the five sacral vertebræ fuse together into a single bone— the sacrum-and the four coccygeal into the single coccyx. Hence the sacrum and coccyx of the adult are the false, whilst the lumbar, dorsal, and cervical are the true vertebræ.

Coc 4

cervical vertebræ; D12, the dorsal; Ls, the lumbar; S. the sacral; Coc4 the coccygeal; CC, the series of twelve ribs on one side; Ps, the præ-sternum; Ms, the meso-sternum; Xs, the xiphisternum. The dotted line VV represents the vertical axis of the spine.

The vertebræ are irregularly-shaped bones, but as a rule have certain characters in common. Each possesses a body and an arch, which enclose a ring, with certain processes and notches. The Body, or Centrum, is a short cylinder, which by its upper and lower surfaces is connected by means of fibro-cartilage with the bodies of the vertebræ immediately above and below. The collective series of vertebral bodies forms the great column of the spine. The Arch, also called Neural Arch, because it encloses the spinal marrow or nervous axis, springs from the back of the body, and consists of two symmetrical halves united behind in the middle line. Each half consists of an anterior part or pedicle, and a posterior part or lamina. The Rings collectively form the spinal canal. The Processes usually spring from the arch. The spinous process projects backwards from the junction of the two laminæ, and the collective series of these processes gives to the entire column the spiny character from which has arisen the