It is refreshing in these Darwinian days to open a book on anatomy which became popular in England rather more than a century ago. The book was written by Archdeacon Paley, and entitled " Natural Theology," the mechanism of the human body being one of the chief subjects of discourse. The Archdeacon had studied anatomy in London to excellent purpose under William Hunter—John Hunter's eldest brother—one of the best anatomists at the latter half of the eighteenth century. The Archdeacon expressed in clear and simple language the views which the anatomists of his time held of the human body. To them, it was a machine of superlative construction, the final work of a power which had created all things. How far the anatomists of to-day have departed from the Archdeacon's point of view may be seen from the preceding chapters. We have in later days neglected the study of the mechanism of the human body. Modern anatomists look on it as an anonymous missive which has come to them through the post of Time, stamped with certain marks from which they try to interpret —and with some success—something of the whence and how it has come. With the best of intentions the writer has been unable to break away from the detective's outlook; with each new chapter he hoped to leave evolution behind him and study the cunning mechanism of the human body without a mention of creation or evolution. How far that can be done the present chapter will show.
The manner in which the head is jointed to the body is an example of an effective and most delicate mechanism worked out under circumstances of peculiar difficulty. The head, like the telescope, is so jointed that the eyes can sweep the horizon from east to west by a turning movement, and scan the heavens from base to zenith by a nodding or extensory one. In the very centre of the movement is placed the most vital structure in the body— the lower end of the stem of the brain, which issues from the skull by the foramen magnum to pass into the canal of the backbone as the spinal cord. We can flex and extend the head, rotate it from side to side, and yet the " silver cord " is never in the slightest degree injured. When we examine the anatomy of the parts we see that the nodding movements occur between the skull and the first vertebra or atlas, and that the rotating movement takes place between the atlas and the second vertebra or axis. This elaborate mechanism, which can be employed at all times with the utmost surety and safety, has been accomplished by a simple modification of the first vertebra. Like all other vertebrae the atlas was originally made up of three parts —a body, which forms the solid front wall of the canal for the spinal cord, an arch which encloses the canal at the sides and at the back, and a third minor element—the front bar. The body of the atlas or first vertebra of the neck has been transferred from its proper position to become fixed as a pivotlike process on the upper surface of the secend vertebra or axis. The arch of the atlas having no longer a body to make up its anterior deficiency unites with the third or minor element—the front bar. The atlas thus appears ring-shaped; the pivot-like process of the axis lies within the front part of the ring. When the head is rotated so that the face is turned from side to side, the atlas, which supports the head, moves round the pivot which has been shaped from its own body. When, however, the head is rotated thirty degrees to one side or to the other, the movement is suddenly checked by special ligaments. Were the ligaments or bands of fibrous tissue not present, then the rotation might be continued until the arch of the atlas was carried against the spinal cord; indeed, in the dead body when the ligaments and muscles are divided, it is possible to rotate the head until the cord is completely crushed by the rotating arch of the atlas. Yet the writer knows of no case where death has been caused by a forcible movement of the kind just described.
While the joint between the atlas and axis is a perfect example of a " pivot-joint" that between the atlas and the skull, where nodding movements occur, represents a modified form of another type of joint— the " ball and socket." The hip-joint, where the semispherical head of the thigh-bone fits within the cup formed on the pelvis by the " acetabulum," is the most perfect example of a ball and socket joint in the human body. In order to allow the stem of the brain to emerge at the foramen magnum, the central parts of both " ball " and " cup " have been cut away. If the ball were perfect it would be placed over the foramen magnum and the cup within the arch of the atlas. Only a minor part of the ball has been left on each side of the anterior end of the foramen magnum, and all that is left of the cup are two corresponding sockets placed on the upper surface of the atlas. The centre of the joint is the position of least movement, and is occupied by the commencement of the spinal cord. The movements are limited and checked by ligaments and muscles so that in the most extreme movements of the head the vital nerve centres are not endangered. One marvels at the simplicity of the elements out of which the most complex and delicate mechanisms are formed. By transferring the body of the first to the second vertebra and by a simple modification of the joint between the first vertebra and the skull a mechanism has been secured which gives mobility to the head and safety to the nerve cord.
The various orders of levers might be studied at the joint between the head and body. The head is a lever of the first order; the atlas is the fulcrum which supports the lever. When the head is bent backwards, the power is represented by the muscles of the nape of the neck; the weight lifted is the front part of the head. The various kinds of levers, however, are better exemplified in the limbs, as well perhaps at the elbow joint as anywhere. We shall consider the forearm as a lever. If a weight be placed on the upturned palm and the forearm bent on the upper arm or brachium we see a lever of the third order. The elbow joint is the fulcrum; the power, represented by the biceps and anterior brachial muscle, is applied to the lever between the fulcrum (elbow joint) and the weight (hand), but much nearer the fulcrum than the weight. Power is thus lost, but speed is gained. It is apparent that the shorter the forearm or lever then the greater will be the purchase. Now it is a remarkable fact that man differs from the anthropoid apes in having a comparatively short and therefore powerful forearm. It is shorter in European than in African races. The gibbon is noted for the length of its forearm. If we watch the anthropoids using their arms in progression on the trees we at once realize the significance of their long and of our short forearm. The anthropoid seizes the branch with its hand and then draws upwards the weight of the body by flexing the arm at the elbow joint. The moving lever is not the forearm ; it is the brachium or upper arm. We are still dealing with a lever of the third order ; the fulcrum is still placed at the elbow joint; the power is furnished by the biceps and anterior brachial muscle, but is applied to the brachium, which is the moving lever, at some distance from the fulcrum, thus increasing power obtained. The weight is represented not by an object placed in the hand but by the whole body of the animal which is attached to the arm or lever at the shoulder joint. In the climbing anthropoid the upper arm is short and forms a powerful movable lever; in working man, the forearm is the movable lever, and therefore, compared to the brachium, is short and powerful. We see that the short forearm of man and the long one of anthropoids are functional modifications.