598 SCIENCE PROGRESS 



diminution of the contraction or discharge of its antagonist, 

 even to complete suppression in the latter. 



While thus laying stress on the reflex co-ordination of 

 antagonistic muscles by reciprocal innervation, it must be re- 

 membered that not all muscles and pairs having antagonistic 

 mechanical effects are dealt with in this way. The mechanical 

 antagonism of muscles exhibits two main types. Muscles, 

 according as they act directly on one joint or on more than 

 one, are distinguishable into a single-joint class and a double- 

 joint class, and so on. Single-joint muscles exert their pull 

 from origin to insertion across one joint only ; they directly 

 affect that one joint alone. The most direct and simplest case 

 of antagonism is between single-joint flexors and extensors 

 acting on one and the same hinge, such as brachialis flexing 

 elbow and humeral head of triceps extending elbow, or the 

 lateral and medial muscles of the eyeball rotating the ball in 

 opposite directions about the same vertical axis. In all cases 

 of this direct antagonism the co-ordination of the antagonistic 

 muscle pair is by reciprocal innervation. 



A less simple kind of antagonism occurs where double-joint 

 muscles are involved — muscles which act directly on two joints. 

 At the knee the vasto-crureiis (fig. 5) muscle which passes across 

 that joint only is the great extensor of the joint, while a double- 

 joint muscle, semitendinosHS^ is the main flexor of the joint ; 

 but this latter, besides flexing the knee, extends the hip. If the 

 hip be prevented from extending and, still more, if it be actually 

 flexed, the action of semitendinosus as a knee-flexor is enhanced. 

 Experimental examination of the act of reflex flexion of the 

 knee shows that when semitendinosus contracts to produce 

 flexion, its antagonist at knee is relaxed by inhibition ; but 

 that at the hip, where semitendinosus is an extensor, it and the 

 hip flexors are contracted together. The analysis of the reflex 

 shows that semitendinosus, although potentially trom its position 

 an extensor of hip as well as a flexor ot knee, is emplo3^ed 

 in fact by the nerve centres as a knee-flexor and not as a 

 hip-extensor; nor is there evidence that it is ever used in any 

 other than the former way. In using it the nervous system 

 favours its effect at knee by simultaneously inhibiting its direct 

 antagonist there ; but it throws into contraction its antagonists 

 at hip, thus immobilising the point from which the muscle as 

 a knee-flexor takes its pull. A muscle which by fixating a 



