ligaments and periosteum, which they cover: but they scarcely admit of being detached from the surface of cartilages. I have one preparation, however, in which I accidentally succeeded in raising an uniform membrane from the cartilage covering the head of the humerus. Membranes of this description take their name from the viscid fluid called sinovia, which they secrete, and which lubricates the internal surfaces of joints.

From an analysis by M. Margueron it appears that sinovia is composed of the following ingredients:

Joints which combine these various elements are distinguished into different classes according to the form which they affect, and the kind of motion of which they allow.

A ball and socket joint, or enarthrosis, like the hip, gives great security, and at the same time permits very

extensive motion.

A joint, in which surfaces nearly plane are opposed to each other, is termed an arthrodia; the motion allowed in such a case is very limited, but takes place in a degree in every sense.

A joint which allows of motion in one plane only is termed a ginglymus or hinge joint. Of this joint there are

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Analogical Design in the Skeleton.

two kinds; in one the motion is angular, as in the knee, or rotatory as between the atlas and dentata.

Such are the materials of the skeleton, and the different modes in which they are joined together to form one frame.

When we seek in the skeleton for illustrations of that analogical design, which is evident not merely in entire classes but in single objects of nature's workmanship, we remark that the head is not a part which corresponds with any subdivision of the frame, but rather seems an epitome of all the rest. Thus the embryo when first seen consists of two coherent nodules (not differing materially in volume even, one of which becomes the head, while the other spreads into the trunk and limbs.

When we compare together the several regions of the trunk, we observe that it is laid out in corresponding organs on either side of a centre, which consists of the five lumbar vertebræ. Above the lumbar vertebræ are the dorsal vertebræ, above these the cervical; below the lumbar vertebræ are the sacral bones, below these the coccygeal. To the dorsal vertebræ and to the sacrum bones are articulated, which have the double office of forming a visceral cavity, and of throwing to a convenient distance from the median plane the bones of the extremities. The ribs and sternum, the clavicle and scapula form with the dorsal vertebræ an organ strictly analogous to that formed by the ossa innominata and the saBut the chest for the function of respiration requires to be continually altering its dimensions, and the upper extremity is characterized by the extent and velocity rather than by the strength of its motions: to suit

crum.

Analogical Design in the Skeleton.

349

both these objects, the chest and shoulder are formed of many bones, that are moveable in various senses; the ribs are capable of rotating upon their sternal and vertebral joints, and of being raised or depressed upon their vertebral joints carrying with them the sternum; the clavicle again revolves upon the sternum, and the scapula rolls upon the convexity formed by the angles and shafts of the ribs. On the other hand the pelvis, as regards the viscera, is intended merely for their support, or if during labour a temporary enlargement of its lower aperture be requisite, the flexibility of the joints of the os coccygis in the female skeleton seems a sufficient provision for this object: the inferior extremities again require to be articulated to a solid unyielding platform, upon which they may poise the incumbent weight of the trunk and head. The bones of the pelvis are for these reasons, few, weighty, massive, and knit together immoveably. Thus accurately do the points, in which a resemblance is wanting between the chest and pelvis, preserve the analogy between these parts.

As

It is needless to dilate upon the correspondence of the femur with the humerus, of the tibia, patella and fibula with the radius and ulna, of the tarsus with the carpus, of the bones of the foot with those of the hand. mobility is the prevailing character of the upper extremity, the radius plays upon the ulna, the bones of the wrist are so disposed as to form three ball and socket joints, and the metacarpal bone of the thumb moves freely on a hinge joint. As stability is the leading character in the lower extremity, the knee moves in one plane only, the fibula has no motion upon the tibia, the joints of the tarsus do no more than yield sufficiently to

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The Skeleton fitted for the erect Posture.

break the force with which the frame alights upon the ground, and neither of the metacarpal bones move on their carpal joints.

It would appear fanciful to enlarge upon the points of correspondence between the head and trunk. As the vertebral canal contains the spinal chord, the cranial cavity contains the cerebrum and cerebellum; as the main parts of the respiratory organs are contained in the upper cavity of the trunk, and the digestive viscera are supported by the lower, the nostrils are the cavities attached to the fore and upper part of the cranium, and the fauces are attached to the lower part. If the chest supports the organs of prehension, the pelvis those of pursuit, the orbits, the nostrils, the cavities of the temporal bone have points in common with the former, and the fauces, which contain the tongue, have a trivial analogy with the latter.

If we look to the physical strength of the skeleton, we may begin by enumerating the globular form of the skull, and the security it derives from the sphenoïd and temporal bones; but to pursue this theme would lead me to repeat remarks which have already been made, or would anticipate what belongs to the next head. Let me rather observe, that we are not to seek for indefinite strength and powers of resistance in the frame, but to admire the degree of security given to such frail materials, the equal strength of the whole, and the exact proportion of the means of defence in each part to the risks to which it is exposed.

When we look at the skeleton as designed for beings distinguished by the erect posture and erect progression, we notice the head so placed as to be poised upon the vertebral column,-the lower cervical vertebræ deficient in

351

Of the Muscles, which move the Frame. those processes, which in quadrupeds give attachment to a ligamentum nuchæ adequate to support a pendent head,—the vertebral column becoming broader towards its base, the spinal column resting upon an elastic hoop disposed in a plane nearly vertical-the centre of gravity falling in the right line which joins the two acetabula, so that the body may be swayed with security in any direction upon the ossa femoris,—the margin of either acetabulum thickest and deepest at its upper and back part,-and the foot forming an elastic arch upon which the frame securely rests.

The structure of the muscular substance employed in supporting and moving the frame has been already described. The muscles of the trunk and limbs have at least two attachments to bone, one of which is called their origin, the other their insertion. The former term is usually applied to that attachment which is nearest the centre of the body, or which under ordinary circumstances is the fixed point during the action of the muscle. By its origin and insertion a muscle adheres to two separate bones, which either are articulated together or have a third bone or even several interposed. In the latter case a single muscle is adapted to bend or extend several joints.

Muscular fibres in some instances adhere directly to the periosteum of a bone, in others are united to it by an intermediate chord of the same texture with a ligament, in this case termed a tendon or sinew. Every muscle of the class under consideration has a tendon at one extremity; and commonly at both, tendinous fibres are wrought up in its texture.

Some have illustrated the connexion between a muscle and a tendon in the following manner. Each fasci