Page:Popular Science Monthly Volume 42.djvu/783

Rh straight above him. If you go through these movements you will find that your first impulse is to take a slight breath, which is chiefly effected by the diaphragm, and on looking at the brachial plexus you find that the first branch which is given off from the fifth nerve is a filament to the phrenic (Fig. 13). Then, just as you proceed to raise your shoulder, you take a still deeper breath, and this, too, is represented in the plexus by the posterior thoracic nerve, starting from the fifth and sixth nerves and going to the serratus magnus, which has little power to raise the ribs and act as an inspiratory muscle while the shoulder is depressed, but will

do so when the arm is raised. If your hand is hanging by your side, you will find the shoulder slightly drawn backward by the rhomboid and the arm rotated a little outward by the infraspinatus, which is supplied hy the suprascapular nerve. Next, you raise your arm, and as you do so you will find that unconsciously you bend it and turn it out, you extend the wrist and flex the fingers. The raising of the arm and turning it out is effected by the deltoid and teres minor, which are innervated by the circumflex nerve, while the shoulder is still further raised by the trapezius, which gets its nervous supply from a higher source—the spinal accessory. The biceps and other flexors of the arm receive their supply through