Practicum II. Flexors and Extensors. 13 



radius ; if the hand be supinated vigorously and as far as possible, the 

 attachment may be seen. 



a. Carry the scalpel along the humerus between it and the brachialis 

 so as to detach the muscle except at its origin. 



23. Actions of Certain Muscles upon the Antibrachium. The anti- 

 brachium is a LEVER divided at the elbow joint into a LONG ARM, and a 



SHORT ARM, the OLECRANON, PI. I. 



a With either of the muscles to be experimented with, biceps, tri- 

 ceps and brachialis, the natural contraction may be imitated sufficiently 

 by pulling proximad in the line of its length ; the effect is the same as 

 if the muscle shortened itself like a piece of rubber that has been 

 stretched. 



24. Flexion. Grasp the humerus, the elbow down. Pull upon the 

 brachialis. The hand rises and the arm is flexed just as when, in life, 

 the muscle contracts. 



The atitibrachium constitutes a FLEXION LEVER, the power (brach- 

 ialis) being applied between the weight (hand) and the fulcrum (elbow 

 joint). 



25. Extension. Grasp the humerus, the elbow up. Pull upon the 

 triceps. The arm is extended just as when, in life, the triceps contracts. 

 The antibrachium now constitutes an EXTENSION LEVER, the fulcrum 

 (elbow joint) being between the weight (hand) and the power (triceps). 



26. Rising. Grasp the humerus, the elbow down, it and the hand 

 resting upon the table. Pull upon the triceps so as to straighten the arm 

 at the elbow. 



a. The condition is the same as when, upon all-fours, we rest the 

 hand and antibrachium upon the floor and then raise the body by straight- 

 ening the arm at the elbow. The weight (brachium, etc.,) is between the 

 fulcrum (contact of the hand with the floor) and the power (triceps). 



b. A more familiar example of this action is in rising upon the ball 

 of the foot ; the weight (of the leg and body) is between the fulcrum 

 (contact of the foot with the floor) and the power (muscles of the "calf") 

 inserted at the heel. Illustrate this by means of the cat's leg ; rest the 

 foot on the table and pull upon the tendo Achillis so as to raise the heel. 



27. Antagonism of Flexors and Extensors. The biceps and brach- 

 ialis and the triceps act upon the antibrachium in opposite directions ; 

 they are OPPONENTS or ANTAGONISTS. If they should contract at once 

 and with equal power there would be no movement ; the arm would sim- 

 ply be "fixed" at the elbow ; illustrate this upon your own arm. 



28. Reversal of the Fixed Point. In the previous observations the 

 brachium is supposed to be fixed and the antibrachium movable ; this is 

 the more frequent condition ; but in climbing the hand and antibrachium 

 are fixed and the brachium is flexed upon the latter. 



a. Illustrate this by tying a string tightly about the humerus and 

 the proximal end of the brachialis ; then grasp the antibrachium and 

 pull upon the muscle ; the brachium is flexed at the elbow. So far as 

 the muscle is concerned the action is the same as in 24, but the lo- 

 cation of the fixed point is reversed. 



29. Monarthal Muscles. The brachialis crosses but one joint 

 (arthron) ; its attachments are upon two adjacent segments of the limb, 

 and a single joint intervenes between the origin and the insertion ; hence 

 it and similar muscles may be called MONARTHRAL. 



