“_Ligaments._--The office of the ligaments with respect to locomotion, is to restrict the degree of flexion, extension, and other motions of the limbs within definite limits.
“_Effect of Atmospheric pressure on Limbs._--The influence of atmospheric pressure in supporting the limbs was first noticed by Dr. Arnott, though it has been erroneously ascribed by Professor Müller to Weber. Subsequent experiments made by Dr. Todd, Mr. Wormald, and others, have fully established the mechanical influence of the air in keeping the mechanism of the joints together. The amount of atmospheric pressure on any joint depends upon the area or surface presented to its influence, and the height of the barometer. According to Weber, the atmospheric pressure on the hip-joint of a man is about 26 lbs. The pressure on the knee-joint is estimated by Dr. Arnott at 60 lbs.”[13]
FIG. 5. Shows the muscular cycle formed by the biceps (_a_) or flexor muscle, and the triceps (_b_) or extensor muscle of the human arm. At _i_ the centripetal or shortening action of the biceps is seen, and at _j_ the centrifugal or elongating action of the triceps (_vide_ arrows). The present figure represents the forearm as flexed upon the arm. As a consequence, the long axes of the sarcous elements or ultimate particles of the biceps (_i_) are arranged in a more or less horizontal direction; the long axes of the sarcous elements of the triceps (_j_) being arranged in a nearly vertical direction. When the forearm is extended, the long axes of the sarcous elements of the biceps and triceps are reversed. The present figure shows how the bones of the extremities form levers, and how they are moved by muscular action. If, _e.g._, the biceps (_a_) shortens and the triceps (_b_) elongates, they cause the forearm and hand (_h_) to move towards the shoulder (_d_). If, on the other hand, the triceps (_b_) shortens and the biceps (_a_) elongates, they cause the forearm and hand (_h_) to move away from the shoulder. In these actions the biceps (_a_) and triceps (_b_) are the power; the elbow-joint (_g_) the fulcrum, and the forearm and hand (_h_) the weight to be elevated or depressed. If the hand represented a travelling surface which operated on the earth, the water, or the air, it is not difficult to understand how, when it was made to move by the action of the muscles of the arm, it would in turn move the body to which it belonged, _d_ Coracoid process of the scapula, from which the internal or short head of the biceps (_a_) arises, _e_ Insertion of the biceps into the radius. _f_ Long head of the triceps (_b_). _g_ Insertion of the triceps into the olecranon process of the ulna.--_Original._]