THE MUSCLES. 89 



but witlKAit energy. This is the nature of the masculai 

 fibre ; and being so, it is evident that the reciprocal eyicr- 

 gctic motion of" the limbs, by which we mean motion vith 

 force in opposite directions, can only be produced by the in- 

 strumentality of opposite or antagonist muscles — of flexors 

 and extensors answering to each other. For instance, the 

 biceps and brachialis internum muscles,^ placed in the lionl 

 part of the upper arm, by their contraction, bend the elbow, 

 and with such degree of force as the case requires or the 

 strength admits of. The relaxation of these muscles after 

 the eflbrt would merely let the fore-arm drop down. For 

 the hack stroke, therefore, and that the arm may not only 

 bend at the elbow, but also extend and straighten itself with 

 force, other muscles, the longus et brevis brachialis exter- 

 niis,'\ and the anconscus, placed on the hinder part of the 

 arms, by their contractile twitch, fetch back the fore-arm 

 into a straight line with the cubit, with no less force than 

 that with which it was bent out of it. The same thing ob- 

 tains in all the limbs, and in every movable part of the body. 

 A finger is not bent and straightened without the coiitrac- 

 tion of two muscles taking place. It is evident, therefore, 

 that the animal functions require that particular disposition 

 of the muscles which we describe by the name of antagonist 

 muscles. And they are accordingly so disposed. Every 

 muscle is provided with an adversary. They act hke tv/o 

 sawy^ers in a pit, by an opposite pull ; and nothing, surely, 



* Plate IIL, Fig. 2. The biceps, ce, arises by two portions from 

 the scapula ; these form a thick mass of flesh in the middle of tlie 

 arm, which is finally indented into the upper end of the radius.. The 

 brachicEus internus, 6, arises from the middle of the himaerus, and is 

 mscrted mto the ulna. Both these muscles bend the fore-arm. 



t Plate IIL, !FiG. 2. The long and the short brachiceus intcrnus 

 :r the triceps extensor cubiti^ c, is attached to the inferior edge of the 

 scapula and to the humerus by three distinct heads, which unite and 

 invest the whole back part of the bone ; it then becomes a strong ten- 

 don, and is implanted into the elbow. It is a powerful extensor of the 

 fore-arm. The anconcEus, d, is a small, triangular muscle, situ ited at 

 the outer side of the elbow ; it assists the muscle c. 



