OF THE EXTERIOR MUSCLES. 453 



In this case and in the preceding, which are of extremely 

 frequent occurrence in the animal mechanism,* the part which 

 is to serve as a fixed point is retained by the contraction of 

 other muscles which render it motionless. The motions ap- 

 parently the most simple almost always require the simultane- 

 ous action of a greater number of other muscles than those 

 which are destined to produce them immediately. 



729. It is in efforts especially that we observe these mus- 

 cular synergies. 



An effort, t nisus, is any muscular action of extraordinary 

 intensity, destined to surmount an external resistance, or to 

 perform a laborious function, whether accidentally or natural- 

 ly. Thus, the action of raising or carrying a heavy body, 

 parturition, difficulty of passing the urine, &c. require efforts 

 before they can be executed. 



In every effort, there is an extraordinary nervous influx 

 upon the muscles. Sometimes this influx is voluntary, some- 

 times involuntary. In the latter case, it is irresistibly deter- 

 mined by the connexion already remarked between the invo- 

 luntary internal muscles, and their external congenerous 

 muscles. In every effort, also, a great number of muscles, 

 sometimes the whole apparatus of motion is in action. Lastly, 

 in every effort, the lungs are first filled with air by an inspira- 

 tion, the glottis is closed or simply narrowed, the muscles of 

 expiration are contracted, and the walls of the thorax are thus 

 rendered immoveable, in order to present fixed points of at- 

 tachment to the muscles of the abdomen and limbs. 



The effects of efforts are to retard or prevent the entrance 

 of the venous blood into the thoracic trunks, whence its re- 

 flux and its stasis in the veins of the neck, the head, the abdo- 

 men, and even the limbs; to compress the thoracic and abdo- 



* Winslcnv, Mem. de PAcad. des Sc., 1719-23-26-29-30-39-40, &c. 



f Js. Bourdon, Recherches sur le mt-canisme de la respiration et de la circu- 

 lation du sang. Paris, 1820. J. Cloquet, De V influence, des efforts sur les 

 organ cs renfermcs dans la cavite thorachique. Paris, 1820. Magendie, 7> 

 ^influence des mouvemens de la poitrine, et des efforts, sur la circulation du 

 sung. Journal de Physiologic, vol. i. 



