The chest is supplied with numerous muscles which move the ribs up and down in the act of breathing. A great, fan-shaped muscle, called the pectoralis major, lies on the chest. It extends from the chest to the arm and helps draw the arm inward and forward. The arm is raised from the side by a large triangular muscle on the shoulder, the deltoid, so called from its resemblance to the Greek letter delta, Δ. The biceps, or two-headed muscle, forms a large part of the fleshy mass in front of the arm. Its use is to bend the forearm on the arm, an act familiarly known as “trying your muscle.” Its direct antagonist is the three-headed muscle called the triceps. It forms the fleshy mass on the back of the arm, its use being to draw the flexed forearm into a right line. On the back and outside of the forearm are the extensors, which straighten the wrist, the hand, and the fingers. On the front and inside of the forearm are the flexors, which bend the hand, the wrist, and the fingers. If these muscles are worked vigorously, their tendons can be readily seen and felt under the skin. At the back of the shoulder a large, spread-out muscle passes upward from the back to the humerus. From its wide expanse on the back it is known as the latissimus dorsi (broadest of the back). When in action it draws the arm downward and backward, or, if one hangs by the hands, it helps to raise the body. It is familiarly known as the “climbing muscle.” Illustration: Fig. 37.—A Few of the Important Muscles of the Back. Passing to the lower extremity, the thigh muscles are the largest and the most powerful in the body. In front a great, four-headed muscle, quadriceps extensor, unites into a single tendon in which the knee-cap is set, and serves to straighten the knee, or when rising from a sitting posture helps elevate the body. On the back of the thigh are several large muscles which bend the knee, and whose tendons, known as the “hamstrings,” are readily felt just behind the knee. On the back of the leg the most important muscles, forming what is known as the calf, are the gastrocnemius and the soleus. The first forms the largest part of the calf. The soleus, so named from resembling a sole-fish, is a muscle of broad, flattened shape, lying beneath the gastrocnemius. The tendons of these two muscles unite to form the tendon of Achilles, as that hero is said to have been invulnerable except at this point. The muscles of the calf have great power, and are constantly called into use in walking, cycling, dancing, and leaping. 77. The Effect of Alcoholic Drinks upon the Muscles. It is found that a man can do more work without alcohol than with it. After taking it there may be a momentary increase of activity, but this lasts only ten or fifteen minutes at the most. It is followed by a rapid reduction of power that more than outweighs the momentary gain, while the quality of the work is decidedly impaired from the time the alcohol is taken.