Practicum II. Muscle Mechanics. 15 



36. Ligamentous Action of the Biceps. Hold the arm by the elbow, 

 the hand at the left, and the brachium at right angles with both the scap- 

 ula and the antibrachium. Depress the scapula, i. e., flex it upon the 

 brachium ; the antibrachium will be flexed. Then extend the antibrach- 

 ium and the scapula will rise to its former position. In all this the biceps 

 acts just as if it were a ligament or a cord passing over a pulley at the 

 shoulder. 



a. During life the conditions are not quite identical because the 

 muscle is slightly extensible and may contract considerably ; but one of 

 the reasons for the difficulty we have in keeping the legs perfectly 

 straight at the knees when we bend forward and try to touch the floor 

 with the finger-tips, is that certain muscles of the thigh, attached to the 

 leg below the knee, arise from the pelvis at such a place that when the 

 hips are tilted as in stooping they are put upon the stretch ; their tendons 

 (ham-strings) may be felt to become tense if the fingers are placed at the 

 at the "inner" side of the knee. 



37. Transect the biceps at about the middle of its length and note 

 the cut area ; the power of a muscle, other things being equal, depends 

 upon the thickness, while its length, if the fascicles are parallel, deter- 

 mines the distance through which it can contract. 



a. This general statement is more intelligible if we compare the 

 contracting muscle to a strip of rubber that has been stretched ; the longer 

 it is the greater will be its shortening ; the thicker it is the greater the 

 force exerted. 



b. The triceps is much larger in the cat and most quadrupeds than 

 in man ; this is not so much for the forcible extension of the arm as such, 

 but because this muscle also serves for the support and propulsion of the 

 body. 



38. The Ligaments at the Elbow. Trim off the triceps and other 

 muscles that may remain about the elbow. Note that the arthral ends of 

 the bones are enclosed in a CAPSULE as was the hip (Pract. I, 5). 

 If the humerus is turned from side to side there will be seen, at the ulnar 

 side a thicker strip of the capsule extending from a prominence of the 

 humerus obliquely distad to the ulna ; a similar one on the other side 

 crosses the head of the radius. Cut away the rest of the capsule and note 

 that these two LATERAL LIGAMENTS permit the usual flexion and exten- 

 sion, but prevent any considerable lateral movement ; if either is cut the 

 bones separate readily at that side. 



a. Divide both ligaments ; disconnect the humerus ; note the form 

 of the apposed ends of the bones. 



39. Relations of the Ulna and Radius. Separate the proximal 

 ends of the two bones, and note the cylindrical form of the latter, turning 

 as a pivot at the side of the former. 



a. Disconnect the distal ends and note the reversal of the relative 

 sizes of the two bones. The elbow joint is formed mainly by the ulna, 

 the wrist by the radius. 



40. Flexors and Extensors of the Hand and Fingers. Grasp the 

 mass of muscles at the "inner" side of the antibrachium and pull toward 

 the elbow ; the hand will be flexed at the wrist ; pull those on the "outer" 

 side and it will be extended. 



a. Observe the actions of the same groups of muscles upon your 

 own arm ; in the cat as in man the flexors are more powerful ; indeed in 



