bone. It keeps the shoulders apart from the chest, and throws the action of the muscles proceeding from the ribs, upon the arm-bone; which would otherwise be drawn inwards, and contract the upper part of the trunk.

If we examine the motions of the anterior extremity in different animals, it will guide us to see why in some this bone is perfect, and in others, entirely wanting. Animals which fly, or dig, or climb, as bats, moles, porcupines, squirrels, ant-eaters, armadilloes, and sloths, possess the collar-bone; for having a lateral or outward motion of the extremity, that bone is required to keep the shoulders apart. There is also a degree of freedom of motion in the anterior extremity of the lion, cat, dog, martin, and bear; they strike with the paw, and rotate the wrist more or less extensively; and they have therefore a clavicle, though an imperfect one. In some of these, as the lion, the bone occupying the place of the collar-bone is very imperfect indeed; although attached to the shoulder, it does not extend to the breast-bone (A), but lies concealed in the flesh, and is like a mere rudiment of the bone. Yet, however imperfect, it marks a correspondence in the bones of the shoulder to those of the arm and paw, and the extent of motion enjoyed.

When the bear stands up, we perceive by his ungainly attitude and the movements of his paws, that there must be a wide difference in the bones of his upper extremity from those of the hoofed or cleftfooted animal. can take the keeper's

He

hat from his head, and hold it; or can hug an animal to death. The ant-bear especially, as he is deficient in teeth, possesses extraordinary powers of hug

ging with his great paws; and, although harmless in disposition, he can on occasion squeeze his enemy, the jaguar, to death.

C

These actions, and the power of climbing, result from the structure of the shoulder, from there being a collar-bone, however imperfect.

Although in man the clavicle is perfect, thereby corresponding with the extent and freedom of motion of his hand, yet in some animals which dig or fly, as the mole and bat, the bone is comparatively stronger and longer.

Preposterous as appears the form of the kangaroo, yet, even in this animal, a relation is preserved between the extremities. He sits upon his strong hind-legs and tail, tripod-like, with perfect security; and has his fore-paws free. He has a clavicle, and it is from possessing that bone and the corresponding motions, that he can employ his paws as a means of defence; for with the anterior extremities he will seize the most powerful dog, and then drawing up his hinder feet, dig his sharppointed hoofs into his enemy, striking out, and tearing him to pieces. Though possessed, therefore, of no great speed, and unprovided with horns, teeth, or claws, but, as we should suppose, totally defenceless, nature has not been negligent of his protection.*

It cannot be better shown, how the function or use of a part determines its structure, than by looking to the clavicle and scapula of the bird.

The chief peculiarity of birds is, that in flying they do not strike out their wings with an alternate motion, but their extremities, as we may continue to call them, move together. Now, three bones converge to constitute the shoulder-joint of the bird: the scapula, clavicle, and coracoid bone.† But neither the scapula nor clavicle has the resemblance which their names would imply. The scapula is the long thin bone, like the blade

able manner, to the enormous hinder extremities. The subject is taken up in the "Additional Illustrations" at the latter part of the volume, on the "General Form of the Skeleton."

*In the form of the kangaroo, and especially in its skeleton, there is something incongruous, and in contrast with the usual shape of quadrupeds. The head, trunk, and forepaws appear to be a portion of a In man, the coracoid bone is a smaller animal unnaturally joined process of the scapula; but late comto the legs of another of greater parative researches into the "type" dimensions and strength. It is not of the vertebral skeleton, make it easy to say what are, or what were, appear that, although joined in man the exterior relations corresponding to the shoulder-blade, the coracoid with the very peculiar form of this is a distinct elementary bone of the animal; but the interior anatomy is scapular arch," or basis of the accommodated, in a most remark-upper extremity.-(S.)

66

of a knife; and the clavicles are united at the centre, near the breast-bone, to form the furculum, or fork-bone, which, in carving, we detach, after removing the wings of a fowl. This leaves that stronger portion of bone which is articulated with the breast-bone, as a new

part; and although it corresponds with the place of the clavicle, yet, from its bearing an analogy to a process of the irregularly-formed scapula in mammalia, it is called coracoid bone. However this may be, what we have to admire, is the mode in which the bones are fashioned to strengthen the articulation of the shoulder, and to give extent of surface for the attachment of

the muscles which move the wings, as long levers, in flight.

OF THE SCAPULA.

By attending to the scapula, or shoulder-blade, we shall better understand the influence of the bones of the shoulder on the motions and speed of animals. The scapula is that flat triangular bone which lies on the ribs, and is cushioned with muscles. On its anterior angle there is a depressed surface, the glenoid cavity or socket for the arm-bone. The scapula shifts and revolves on the ribs with each movement of the arm. To produce these movements, the muscles converge towards it from all sides, from the head, spine, ribs, and breast-bone, and, by acting in succession, they roll the scapula and toss the arm in every direction. When the muscles combine in action, they fix the bone, and either raise the ribs in drawing breath, or give firmness to the whole frame of the trunk.

Before remarking further on the influence of the scapula on the motions of the arms, I shall give an instance to prove their importance to the function just referred to, that of assisting in

drawing in the breath. Hearing that there was a poor lad of fourteen years of age born without arms, and whose unhappy condition had excited the benevolence of some ladies, I sent for him. I found that indeed he had no arms, but he had clavicles and scapulæ. When I made this boy draw his breath, the shoulders were elevated; that is to say, the scapula, being drawn up, became the fixed points from which the broad muscles diverging from it towards the ribs, acted in raising and expanding the chest in respiration. We would do well to remember this double office of the scapula and its muscles; that whilst it is the foundation of the bones of the upper extremity, and never wanting in an animal that has the most remote resemblance to an arm, yet it is the centre also and point d'appui of the muscles of respiration, and acts in that capacity even when there are no extremities at all.*

We have seen that it is only in certain classes of animals, that the scapula is articulated to the trunk by bone through the medium of a clavicle. A slight depression, therefore, on that process of the scapula (acromion process, E. fig. p. 32) to which the clavicle is attached, when discovered in a fossil bone, will declare to the geologist the class to which the animal belonged. For example, there are brought over to this country the bones of the Megatherium, an animal which must have been larger than the elephant; of the anterior extremity, the scapula only has been found; but on the end of the process, called acromion, of this bone, the mark of the attachment of a clavicle is discovered. Now that alone points out the whole constitution of the extremity; that it enjoyed perfect freedom of motion. Other circumstances will declare whether that extensive motion was bestowed to enable the animal to dig with its huge claws, like some of the edentata, or to strike out in defence, like the feline tribe.

Some interest is attached to the position of the scapula, in the horse. In him, as well as in other quadrupeds, with the exceptions already pointed out, the clavicle is absent; the connexion between the anterior extremity and the trunk exists solely through muscles: and the muscle called serratus magnus, which is large in man, is particularly powerful in the horse; for

* Some curious facts, illustrative | the author's paper on the "Voice," of this office of the muscles of the arm in the Philosophical Transactions, situated on the chest, are stated in 1832.-(S.)

the weight of the trunk hangs almost exclusively upon this muscle. But the speed of the horse, as in most quadrupeds, results from the strength of his loins and hinder extremities; it is the action of the muscles situated there, which propels him forwards, in the gallop or at the leap. We accordingly perceive that if the anterior extremities had been joined to the trunk firmly, as by a clavicle, that bone could not have withstood the shock from the descent of the whole weight of the animal when thrown forwards. Even though the fore-legs had been formed as powerful as the posterior extremities, they would have suffered fracture or dislocation. We cannot but admire, therefore, this provision, in all quadrupeds whose speed is great and spring extensive, for diminishing the shock of descending, and giving an elasticity to the anterior extremities.

In observing the relative position of the bones of the anterior extremity in the horse, we shall perceive that the scapula is

oblique to the chest; the humerus oblique to the scapula; and the bones of the fore-arm at an angle with the humerus. Were these bones arranged in a straight line, end to end, the shock of alighting would be conveyed as through a solid column; and the bones of the foot, or the joints, would suffer from the concussion. When the rider is thrown forwards on his hands, and

*The serratus magnus, attached | the upper border of the scapula, near extensively to the ribs near the the withers.

breast-bone, ascends convergingly to