256 



ANIMAL MECHANICS. 



for a force is transmitted without diminution by a perfectly flexible cord 

 passing over a perfectly smooth surface, and the tendon may, for all 

 practical purposes, be considered as perfectly flexible, and the surface 

 over which it passes as perfectly smooth. 



A muscle like the brachialis, which passes from the anterior surface 

 of the humerus, and is inserted into the coronoid process of the ulna, 

 not only exerts a force upon the ulna, but exerts an ecpual and 

 opposite force upon the humerus from which it springs. If the humerus 

 be free to move at the shoulder-joint, this force may cause a rotation 

 of the humerus as well as a rotation of the ulna at the elbow-joint. 

 Fischer, 1 who has investigated this subject with great care, studied 

 chiefly the actions of the brachialis muscle and the triceps. He made a 

 mechanical model of the arm, corresponding to it not only in gross 



weight, but in the position 

 of the centres of gravity 

 of both the upper and 

 lower arm, and likewise 



arm, 



in the general distribution 

 of mass in the direction 

 of its length. The muscle 

 was represented by an 

 elastic band, fastened in 

 the position of the muscle, 

 and balanced by the 

 tension of a string. On 

 burning this string the 

 elastic band shortened, 

 and when it represented 

 the brachialis muscle it 

 bent the model at the 

 elbow, and also extended 

 the shoulder backwards. 

 The model was free to 

 rotate on a vertical pivot 

 at the shoulder, the whole 

 moving on a glass plate 

 in the horizontal plane, to 



Fig 



A B 



144. — A represents the action of the triceps upon 

 the humerus and ulna. As the ulna is extended 

 the humerus flexes. B represents the action of 

 the brachialis which flexes the ulna, and at the 

 same time extends the humerus. In both A and B 

 the black figures represent the bones before the 

 muscles have contracted. — After Fischer. 



avoid the effect of gravity. 

 When the band represented the triceps it extended the elbow, and at 

 the same time flexed forward the humerus (Fig. 144). 



Stimulation of these muscles in the living body by faradic currents 

 gave similar results. 



It appears, therefore, that a muscle which only passes over one joint does yet 

 effect movements on neighbouring joints, and this as a rule in an opposite 

 direction ; thus a flexion at the elbow causes extension at the shoulder, and 

 extension at the elbow causes flexion at the shoulder. 



Muscles which pass over two joints. — While certain muscles, 

 such as those which move the jaws — the deltoid, the brachialis anticus, 

 etc. — pass over but one joint in their course from origin to insertion, 

 many muscles, especially those of the limbs, pass over two joints, or even 

 more. Thus the long head of the biceps passes over both the shoulder- 



1 Fischer, " Beitr. zu einer Muskeldynamik," Leipzig, 1895, S. 56-197. 



