14 Practicum II. Counteracting Muscles. 



30. Disarthral Muscles. But the biceps arises from the scapula 

 and is inserted upon the antibrachium ; hence between its attachments 

 intervene an entire segment (brachium) and two joints (shoulder and 

 elbow) ; it and similar muscles may therefore be called DISARTHRAL. The 

 monarthral were formerly called "short" and the disarthral "long." 



31. Actions of the Biceps. Pull upon it when (a) the brachium is 

 fixed and show that it is then a flexor of the antibrachium ; (&) when the 

 antibrachium is fixed and show that it then a flexor of the brachium. 

 So far it acts like the brachialis. 



But (c} grasp the arm at the elbow so that movement there is pre- 

 vented and pull the biceps ; the scapula will be extended, i. e., brought 

 more nearly into line with the brachium. Ordinarily the scapula is fixed 

 and the whole arm extended at the elbow. This may be illustrated by 

 resting one's arm upon the table, placing the fingers of the other hand 

 upon the biceps, and then lifting the arm as a whole ; the biceps may be 

 felt to contract although no flexion occurs at the elbow. 



32. The Biceps as a Supinator. Pronate the hand as completely as 

 possible (PI. 5, last paragraph of description) grasp the elbow and pull 

 the biceps strongly proximad ; the hand will be supinated partly so that 

 the pollex is above instead of at one side. 



a. Cut the brachialis entirely away ; pronate the hand again, noting 

 that the tendon of the biceps is drawn down between the ulna and the 

 radius ; when the muscle is pulled the tendon is drawn out again, and the 

 radius revolves upon its long axis. 



33. Repeat the experiment, cutting away the remnants of other 

 muscles so as to ascertain that the tendon of the biceps is attached to a 

 tubercle (the bicipital tuberosity) of the radius which is visible only when 

 the hand is forcibly and completely supinated ; when the hand is pronated 

 the radius is rotated upon its own axis and the tendon is wound upon it 

 as is the rope upon the cylinder at an old fashioned well. Now as pull- 

 ing upon the rope would turn the cylinder in the opposite direction, so 

 pulling upon the tendon turns the radius and thus supinates the hand. 



34. Observe the supinating action of the biceps upon your own 

 arm by placing the fingers over the muscle during vigorous supination ; 

 it will be felt to harden. 



a. Supination is employed not only in the actions named in the de- 

 scription of PI. 5 but in turning the handle of a door, in using a sword, 

 etc. 



35. Counteracting Muscles. The single muscle, biceps, may by its 

 contractions accomplish either of three different things, viz., (i) flex the 

 arm at the elbow ; (2) extend the whole limb at the shoulder ; (3) supi- 

 nate the hand. 



a. Either two, or even all three, of these may be performed at once, 

 but more commonly only one at a time. The other two actions are then 

 prevented by the simultaneous contraction of the antagonizing muscles. 



b. For example, in supination, flexion at the elbow is prevented by 

 the triceps. This may be observed upon the dissected arm of the cat, and 

 upon one's own as follows : 



c. Grasp the muscles of the brachium between the thumb and fingers 

 so that the latter are upon the triceps ; when the hand is supinated and yet 

 not bent at the elbow there will be felt a hardening of not only the 

 biceps but also of the triceps. 



