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.

Even in the case of hard work that must be speedily done, alcohol does not help, but hinders its execution.  The tired man who does not understand the effects of alcohol often supposes that it increases his strength, when in fact it only deadens his sense of fatigue by paralyzing his nerves.  When put to the test he is surprised at his self-deception.

Full intoxication produces, by its peculiar depression of the brain and nervous system, an artificial and temporary paralysis of the muscles, as is obvious in the pitifully helpless condition of a man fully intoxicated.  But even partial approach to intoxication involves its proportionate impairment of nervous integrity, and therefore just so much diminution of muscular force.  All athletes recognize this fact, as while training for a contest, rigid abstinence is the rule, both from liquors and tobacco.  This muscular weakness is shown also in the unsteady hand, the trembling limbs of the inebriate, his thick speech, wandering eye, and lolling head.