43. The last division of the body comprehends the upper and lower extremities, The upper consist of the shoulder, arm, fore-arm, and hand. The bones composing the shoulder are two, the clavicle, or collar bone, and the scapula, or the blade bone The clavicle, named from its resemblance to a key, resembles the italic s, and extends across the upper part of the chest, from the shoulder to the breast bone, and it serves not only to sustain the upper extremity and connect it with the chest, but also to prevent its falling forward upon the thorax, and to afford a fixed point for steadying the arm in the performance of its various actions.

44.The scapula, or shoulder blade, is a large, flat, triangular bone placed upon the upper and back part of the chest, and extends from the second to the seventh ribs. It lies embedded in muscles, and has no connection with any other bone except the clavicle at a single point. It is separated from the thorax by a double layer of muscles, on which it is placed as on a cushion. It serves for the attachment of sixteen muscles which go to the ribs, the bone of the tongue, the arm, the head, and the spine. It thus serves not only as a support, but a fulcrum for every action of the superior extremity.

Scapula. a, superior angle; d, the glenoid cavity, or socket for the round head of the arm bone; m, the aeromion process: n, the coracoid process, which serve to protect the joint; f, the base; g, the costa, or inferior border, and h, the superior border of the triangle; l, the spine; o, the semi-lunar notch, for the passage of an artery, vein, and nerve.

45. As a general rule, in the joints, strength and security are sacrificed, in some degree, to obtain great freedom and latitude of motion. Accordingly, we find that the shoulder joint, which allows of more extensive motion than any other, is also more frequently dislocated. In the hinge joints, such as the knee and ankle, such an accident is comparatively Where a bone is dislocated, a regularly educated surgeon should, if possible, always be employed to reduce it, as there is great danger in trusting to a natural bone-setter, so called, who is entirely ignorant of the anatomy of the parts.

rare.

46. The os humeri, or arm bone, is of a cylindrical shape, and forms at the elbow a perfect hinge-joint with the two

bones of the fore-arm, called radius and ulna.\{This bone is

susceptible of all kinds of motion, elevation, depression, advancing, retreating, circumlocution, and rotation Its scapular extremity is lodged in a strong membraneous bag, called the capsular ligament, and when the arm is raised up, the bone slides downward in the glenoid cavity, and thus distends the lower part of the capsular ligament. In every motion of the arm, except in carrying it backwards, the scapular readily moves or follows it; it is therefore during motions of the latter kind, that dislocations of the joint are most apt to occur. If, therefore, the scapula could always follow the motions of the arm, it would rarely be forced out of its socket, and then only by extreme violence.

47. In the fore-arm we find two kinds of motion, one at the elbow, backward and forward, and also a rotary motion, by which the palm is turned upward or downward, as occasion requires. These motions are called supination and pronation. Flexion and extension of the arm are performed by means of the ulna, which being articulated, with the os humeri, by a hinge joint, carries the radius along with it in all its movements. Now while the larger part of the ulna is above, the larger part of the radius is below, so that while the former presents a large surface for articulation at the elbow, the latter does the same at the wrist, and this inverse arrangement also contributes to the uniform diameter of the fore-arm. While the fore-arm is thus attached to the os humeri, the radius is attached to the wrist; so that when we turn the palm of the hand, the radius rolls on the ulna carrying the hand with it. Indeed so admirable is this contrivance, that both motions may be performed at the same time, for while we are bending the arm, we may also be rotating or turning it upon its axis. To facilitate these motions, it will be observed that near the elbow, a tubercle of the radius plays into a socket of the ulna, whilst near the wrist, the radius finds the socket, and the ulna the tubercle.

g, the ulna; a, the radius.

48. It has been remarked that the ulna has a hooked process, the olecranon, e, which catches round the lower end of the humerus, forming with it a hinge joint. The radius also, has a neat, small, round head, b, bound to the ulna by ligaments, which, as it turns, carries the hand, which is attached to its lower extremity along with it. Now in animals that have solid hoofs, such a motion would be useless and a source of weakness, accordingly we find these bones united together and consolidated in such animals. (By an examination of these bones alone, the anatomist is able to determine whether the animal to which they belonged, perhaps thousands of years ago, was carniverous or graminiverous, that is, whether he was an animal or a vegetable feeder; whether he had claws or hoofs. If he finds merely the end of the radius, and notices in it a smooth depression where it bears against the humerus, and the smooth surface that turns on the cavity of the ulna, he concludes at once, that the animal had a paw, and a motion of the wrist which implies claws. It was in this way that Cuvier and Buckland made those singular and interesting discoveries in relation to antediluvian fossil bones, which have given such importance to geological researches.

49. But let us examine this point a little further. If the

examination of a single bone, or even the end of a bone, like that of the radius, shows that the animal to which it belonged was carniverous, like the tiger, lion or leopard, it also shows the form of all the other bones; not only that the animal had teeth to rend his prey, and claws to hold it, but a spinal column to admit of such motion, such writhing and turning as is necessary to secure it, and such a stomach and intestines as are adapted to digest it, in short, such as belong to the carniverous class. How beautiful is that process of reasoning, and how interesting that science which enables us from a small portion of a skeleton, to determine the existence of a carniverous animal, of a fowl, or a bat, a lizard, or a fish; which teach us the wisdom and the extent of that plan, which adapts the members of every creature to its proper office; which exhibits a system extending through the whole range of animated beings, whose motions are conducted by the operation of muscles and bones.

50. Twenty-nine bones enter into the composition of the human hand, of which eight constitute the wrist. The metacarpal bones support the fingers, and are four in number, the thumb being directly articulated with the wrist. From this arrangement there results great strength, mobility, and elasticity. Indeed, it may be said, that on the length, strength, free lateral motion, and perfect mobility of the thumb, depends the power of the human hand. In strength it is said to be equal to that of all the fingers, hence it is called pollex, from "pollere," to have much strength. If we examine the thumb of the monkey, we find that it extends no farther than to the root of the fingers. The fingers would be comparatively of little use, were it not for the fleshy bed of the thumb.