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 Vertebræ, 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.

less open angle. At the point of their junction there is usually a single median process projecting dorsally, called the spinous process or neural spine.

In most cases upon the anterior and posterior edges of the laminæ of the arch are flattened, slightly projecting, more or less oval, smooth surfaces or facets, which in the natural state are covered by a thin layer of cartilage, and come into contact and articulate (by synovial joints) with the corresponding surfaces of the immediately antecedent and succeeding vertebræ. These have been called by Sir Richard Owen zygapophyses; that placed on the front edge of the arch being the anterior zygapophysis, that on the hinder edge the posterior zygapophysis. As a general rule the latter have their faces directed downwards, overlying the upward directed anterior zygapophyses of the vertebra next behind. This is a useful rule to remember in ascertaining which is the front and which the posterior surface of a vertebra. Sometimes, especially in the lumbar region, the posterior zygapophyses have their faces directed outwards, in which case the corresponding anterior zygapophyses look inwards (Fig. 3, az).

These articular surfaces on the arch constitute a second mode by which the vertebræ are united, and their size and conformation aid to regulate the amount of motion allowed between the component parts of the column. They are often entirely wanting when flexibility is more needed than strength, as in the greater part of the caudal region of long-tailed animals.

In addition to the body and the arch, there are certain projecting parts called processes, more or less developed in different vertebræ. Many difficulties exist about the signification, homologies, and terminology of these processes. Probably, when more is known of the development of the

vertebræ in a large series of animals, some further light will be thrown on the subject; but at present it does not appear that there is that uniformity in the plan of construction of all vertebræ which has often been supposed, and definitions of the different parts applicable in every case have not yet been arrived at, and it may even be doubted whether this will ever be possible.

8

m

az

t

FIG. 3.-Anterior surface of the lumbar vertebra of Hare (Lepus timidus). s spinou process; m metapophysis; az anterior zygapophysis; transverse process; hypapophysis.

The principal processes commonly met with are as follows:

I. From the middle of the upper part of the arch, a process (Fig. 3, s) generally single, but sometimes bifid at the end, grows out vertically. This is the spinous process, or neural spine already mentioned; about its homology in different vertebræ there never can be any question. It may, however, be completely absent when the arch is round or smooth above, as in the cervical region in some animals; on the other hand, it may grow out into a very long conspicuous rod of bone, as in the anterior dorsal region of others.

2. Occasionally a process grows in the median line from the under-surface of the body. This may be single and long and slender, as in the anterior lumbar vertebræ of the Hare

, or a sharp median ridge, as in the cervical verteAo many Ungulata and the cervical and caudal vertebræ Ornithorhynchus, or double, as in the atlas vertebra of kext named animal and the caudal vertebræ of many others. termed a hypapophysis. Most commonly there is not elew a trace of

any

such

process.

From the sides of the lower part of the arch, or from body, lateral processes project more or less directly outwants. These are called transverse processes. There may be bat one, or there may be two, superior and inferior, on each

de of a vertebra. In the latter case the superior is sometimes called a diapophysis, and the inferior a parapophysis; though it is questionable whether the processes to which these terms have been applied can always be regarded as strictly homologous.

FIG. 4. Side view of first lumbar vertebra of Dog (Canis familiaris), 4. s spinous process; az anterior zygapophysis; pz posterior zygapophysis; m metapophysis; a anapophysis; t transverse process.

4. Besides these principal laterally projecting processes, there are often others arising from the side of the arch, more especially developed in the lumbar region, though by no means constant even there.

Of these there may be one or

two on each side. They have often been called accessory