processes; but in the more precise system of nomenclature introduced by Sir Richard Owen, the one which is situated highest on the arch (see Fig. 4, m), projects more or less forwards as well as outwards, is usually thick and rounded, and is nearly always in relation with the anterior zygapophysis, is termed metapophysis ;1 the one placed rather lower (Fig. 4, a), and which projects more or less backwards, and is generally rather slender or styliform, is called anapophysis.

These, with the zygapophyses before mentioned, sometimes called oblique processes, but which are rather articular surfaces than true processes, are all the processes commonly met with on any Mammalian vertebra.

Development of the Vertebra.-The first indication of the formation of a vertebral column in the embryo is the appearance of a longitudinal primitive dorsal groove in the germinal membrane, the edges of which (lamine dorsales) rise up and meet above, so as to convert the groove into a canal. From the tissue lining this canal (uppermost layer of the germinal membrane) the brain and spinal cord are developed, and in its walls are formed anteriorly the cranium, and posteriorly the vertebral column; the canal itself becoming the cerebral cavity and the neural canal of the spine.

In the floor of this canal, formed by a horizontal lamina which separates it from another and larger, ventral or hæmal canal (formed by the approximation in the middle line below. of the lamina ventrales), a slender rod of peculiar structure is developed. This is the notochord or chorda dorsalis, around which the bodies of the future vertebræ are developed. In the Mammalia it almost completely disappears at 1 It is also called "mammillary process in some works on Human Anatomy.

The various forms of joints may be arranged under two principal heads. In one, the contiguous surfaces of the bones are connected by interposed fibrous tissue, passing directly from one to the other, filling up the space between them, and allowing of only a limited amount of motion, as is the case with the bodies of the vertebræ.

The other and more frequent and more perfect form of joint is that in which the contiguous extremities of the bones are covered by a thin layer of very smooth cartilage, and surrounded by a capsular ligament, attached only round the edges of the articular surfaces, and which is lined by a synovial membrane, so called from its secreting a viscid lubricating fluid termed synovia. The amount of motion permitted in these "synovial joints" varies according to the form of the opposed articular surfaces and the arrangement of the ligaments which hold them together. When the two surfaces are nearly flat, and the bones firmly bound by strong short ligaments, as in those which compose the carpus and tarsus, the motion is reduced to an extremely slight gliding of one on the other. Joints in the form of a hinge, as at the elbow, allow of a free motion in one plane only. Ball and socket joints, as at the shoulder and hip, allow of the greatest variety of movements.

The Endoskeleton is divided into an axial portion, belonging to the head and trunk, and an appendicular portion, belonging to the limbs. There are also certain bones called splanchnic, being developed within the substance of some of the viscera. Such are the os cordis and os penis found in some Mammals. These, however, are more appropriately treated of with the anatomy of the organs of which they form a part.

The Axial Skeleton consists of the vertebral column, the skull, the sternum, and the ribs.

CHAPTER III.

THE VERTEBRAL COLUMN.

General Characters.-The Vertebral Column consists of a series of distinct bones called Vertebra, arranged in close connection with each other along the dorsal side of the neck and trunk, and in the median line. It is generally prolonged posteriorly beyond the trunk to form the axial support of the appendage called the tail. Anteriorly it is articulated with the occipital region of the skull.1

The number of distinct bones of which the vertebral column is composed varies greatly among the Mammalia, the main variation being due to the elongation or otherwise of the tail. Apart from this, in most Mammals, the number is not far from thirty, though it may fall as low as twentysix (as in some Bats) or rise as high as forty (Hyrax and Cholapus).2

The different vertebræ, with some exceptions, remain through life quite distinct from each other, though closely connected by means of fibrous structures which allow of a certain, but limited, amount of motion between them.

1 For the sake of uniformity, in all the following descriptions of the vertebral column, the long axis of the body is supposed to be in the horizontal position.

2 These numbers are not exact, owing to the uncertainty in the mode of reckoning the sacral vertebræ.

The exceptions are: Near the posterior part of the trunk, in nearly all Mammals which possess completely developed hinder limbs, two or more vertebræ become ankylosed together to form the "sacrum," the portion of the vertebral column to which the pelvic girdle is attached. As a rule, none of the other vertebræ are normally united by bone, but in some species there are constant ossific unions of certain vertebræ, more particularly in the region of the neck. These will be specially noticed presently.

FIG. 2.-Anterior surface of human thoracic vertebra (fourth), . c body or centrum ; nc neural canal; pedicle and 7 lamina of the arch; t transverse process; az anterior zygapophysis.

Although the vertebræ of different regions of the column of the same animal, or of different animals, present great diversities of form, there is a certain general resemblance among them, or a common plan on which they are constructed, which is more or less modified by alteration of form or proportions, or by the superaddition or suppression of parts to fit them to fulfil their special purpose in the economy.

An ordinary vertebra (see Fig. 2) consists in the first place of a solid piece of bone, the body or centrum (c), of the

form of a disk or short cylinder. The bodies of contiguous vertebræ are connected together by a very dense, tough, and elastic fibrous material, called the intervertebral substance, of peculiar and complex arrangement. This substance forms the main, and in some cases the only, union between the vertebræ. Its elasticity provides for the vertebræ always returning to their normal relation to each other and to the column generally, when they have been disturbed therefrom by muscular action.

A process (p) rises on each side from the dorsal surface of the body. These meeting in the middle line above form together an arch, surrounding a space or short canal (nc). As in this space lies the posterior prolongation of the great cerebro-spinal nervous axis, or spinal cord, it is called the neural canal, and the arch is called the neural arch, in contradistinction to another arch on the ventral surface of the body of the vertebra, called the hamal arch.1 The last is, however, never formed in Mammals by any part of the vertebra itself, but only by certain bones, placed more or less in apposition with it, and which will not here be considered as parts of the vertebral column, strictly speaking.

The lower portions of each side of the arch (p), usually thick and more or less vertical in direction, constitute its pedicles. The upper more compressed and more horizontal portions () are the lamina. The pedicles are usually notched in front and behind, but most deeply behind, to form the sides of the intervertebral foramina for the transmission of the nerves issuing from the spinal cord. Occasionally the foramina for these nerves perforate the pedicles, instead of being truly intervertebral.

The laminæ meet in the median line above, at a more or

1 So called because it incloses the heart and the great central bloodvessels

C