A COMPARISON OF THE JOINTS OF THE ARM AND LEG AND THE SIGNIFICANCE OF THE STRUCTURAL DIFFERENCES BETWEEN THEM By C. P. MARTIN Trinity College, Dublin THE work embodied in this paper commenced in an attempt to find an explanation of the many differences in structure between the elbow- and knee- joints in Man. The arm and leg are built on a common plan and in most respects closely resemble each other. Consequently the difference in the structure of the joints is the more remarkable. The difference can be explained in part by the fact that the bones of the fore-arm have retained the movements of pronation and supination, while in the leg the bones are almost immovably fixed together. A further part of the difference can be explained by the fact that the ulna, or post-axial bone, preponderates at the elbow, but the tibia, or pre-axial bone, preponderates at the knee. Parsons has shown (1) that in the Reptiles the great extensor muscle of the fore-limb is inserted into the ulna, but in the leg the corresponding muscle is inserted into the tibia. This apparently led to the difference in the bones which predominate at these two joints. In the Mammals the limbs have departed from the reptilian position and have been turned in under the body, the two limbs rotating in opposite directions during this process. The limbs have then become supporting as well as propelling organs. In the leg the tibia carries the foot and is placed on the medial side of the limb, that is the side best fitted for supporting the weight of the body; in the fore-arm the radius carries the "hand," and by pronation of the "hand" is also placed on the medial side of the limb. Thus in many quadrupeds the tibia and radius come to preponderate in the leg and fore-arm respectively; the fibula often disappears completely, but the upper end of the ulna into which the extensor muscle is inserted always persists, its persistence has been determined by the insertion of this muscle. This prominence of the ulna at the elbow-joint leads to con- siderable difference between that joint and the knee. There is, however, one matter of contrast between the joints which cannot be wholly explained by these facts; namely that the ulna at the elbow projects as the olecranon above the level of the joint and interlocks with the trochlea of the humerus, whereas the tibia at the knee has no upward projection and in consequence there is no interlocking ofthe bones. The ulna cannot be compared morphologically with the tibia, but, as stated above, the great extensor muscles of the two limbs are inserted into these bones and therefore they are com- parable functionally. 512 C. P. Martin The presence of the olecranon is associated to some extent with the freedom of movement between the bones of the fore-arm, for it fixes the ulna as regards rotation and therefore assists it to act as a base for the movements of pronation and supination of the hand. As, however, a well-developed olecranon is found in animals which have lost the power of supinating the hand, it must serve some other function. It is this other function of the olecranon which has to be defined. The greatest development of the olecranon is found in the highly specialised quadrupedal Mammals, the Carnivora and the Ungulata. In them the ole- cranon is one of several similar features in the limbs. The skeleton of each limb consists of three segments, the adjacent segments meeting one another at an angle. In the fore-limb the humerus passes downwards and backwards; the radio-ulna passes downwards and slightly forwards, or sometimes, especially in the standing posture of the animal, straight downwards; and the carpus is Fig. 1. Diagram of limbs of a typical quadruped. Arrow points in direction of animal's head. Note that segments meet each other at an angle and that the upper end of segments which slope downwards and forwards is prolonged up above articulations in which they take part. usually aligned with the radio-ulna or inclines slightly backwards in digitigrade animals and straight forwards in plantigrade animals. In the hind-limb the femur passes downwards and forwards, the tibia downwards and backwards, and the tarsus downwards and forwards (see fig. 1). In each limb, therefore, the bones pass downwards and forwards or downwards and backwards, and the corresponding segments in the two limbs pass in different directions. In both limbs, with the exception of the carpus which is very short, the upper ends of the bones which pass downwards and forwards are prolonged above the level of the articulations in which they take part. Thus in the femur the great trochanter is prolonged above the level of the hip; in the tarsus the calcaneus is prolonged above the ankle, and in the radio-ulna the olecranon projects above the level of the elbow. In these quadrupeds therefore the great trochanter at the hip-joint and the posterior projection of the calcaneus at the ankle-joint are features similar to the projection of the olecranon at the elbow; and if any functional significance can be ascribed to the great development of the Joints of the Arm and Leg and the Structural Differences 513 olecranon in these animals then the same significance will apply to the upward projection of the great trochanter and the posterior end of the calcaneus. When such an animal moves forwards it does so by pushing backwards on the ground with its hind-feet, and first pulling and then pushing backwards with its fore-feet. These actions are, by the resistance of the ground, converted into a forward motion of the animal's body. The various segments of the limbs are levers by which the animal brings about these stresses and, as the feet are fixed on the ground, the lower ends of the segments are the fulcra of the levers. In the case of the upper segments of each limb the lower end of the segment is not absolutely fixed, for it is not on the ground, but it is fixed relatively to the upper end. The weight bears on these levers at the centre of the articulations at their upper ends. In the case of those segments which pass downwards and backwards, fixation of their lower ends and movement of their upper ends around this fixed point will not produce any forward movement of the animal, or at most can only produce a movement downwards and slightly forwards. But at each step the animal requires a forward and upward impulse, the upward element being necessary to balance the force of gravity while the foot is off the ground and the limb is being brought forwards preparatory to another step. But in the case of those segments of the limbs which pass downwards and forwards, movement of their upper ends around their fixed lower ends will obviously produce a forward and upward impulse to the animal's body. Accordingly, Mivart(2) recognised that the tarsus is the lever mainly concerned in forward movement in the cat, and Huxley (3) recognised the same fact as regards Man. It appears therefore that the bones in the limbs which are aligned in a down- wards and forwards direction are the levers by which an animal moves forwards. As already stated all of these bones are prolonged up above the articulations at their upper ends, and the extensor muscle by which the power is applied to the-lever is inserted into the extreme upper end of the prolongation. The bones are therefore levers of the second order in which the weight is between the power and the fulcrum. The mechanical advantage of such a lever is obtained by dividing the total length of the lever from power to fulcrum by the distance from the weight to the fulcrum. It is evident, therefore, that the upward prolongation of these bones confers a mechanical advantage on the extensor muscles when an animal is propelling itself forwards. Figs. 2 and 3 show the actual conditions found at the ankle- and elbow- joints in quadrupeds. Figs. 4 and 5 show the conditions as they would be if the tarsus and radio-ulna did not project up above these joints. In this last case if the feet were on the ground and the animal were propelling itself forwards the tarsus and radio-ulna would still be levers of the second order, but the points of application of the power and of the weight would approximately coincide and no mechanical advantage would accrue. For it should be noted that the lower end of the extensor muscles is composed of non-contractile tendon, and the point of application of the power is not at the insertion of the Anatomy Lxvmn 33 514 C. P. Martin muscle but at the place where the tendon passes round the upper end of either the tarsus or the radio-ulna. The mechanism of the lower limb in Man when he is moving forwards is very similar to that of the hind-limb in quadrupeds during the same action. Keith(4), Hooton(5) and Huxley(3) all recognise that the foot is a lever of the second order, and that the backward projection of the calcaneus adds to the F Fig. 2. Fig. 3. Fig. 2. Plan of condition which is actually found at elbow- and ankle-joints of a quadruped. Fig. 3. Plan to show that radio-ulna and tarsus are levers of the second order when the animal's feet are on the ground and these bones are used to propel the animal forwards.