78 PHYSIOLOGY. 



able for them. For the most part, accordingly, we find that cer- 

 tain muscles, not only act against their antagonists, but likewise 

 (2) against the weight of the member they sustain ; and many 

 muscles seem manifestly to be more numerous, and more large and 

 powerful, because they have to support the weight of the part ; 

 as the muscles of the lower jaw, the muscles of the temple, 

 the Pterygoid muscles, which are more powerful than their an- 

 tagonists that pull down the jaw ; and whenever these are con- 

 siderably relaxed or weakened, as by the power of sleep, or by 

 narcotic powers, as in the case of drunkenness, the jaw falls 

 down of itself. Many more instances may be given of this: thus, 

 the biceps muscle, in the present position of my arm, not only acts 

 against the triceps extensor, but against the weight of my arm ; 

 so that if an external power takes off this, the biceps is contract- 

 ed, or at least considerably relaxed. The weight of the parts, 

 therefore, has a share. But (3) their extension is kept up by 

 fluids distending the cavities which they surround. This, I say, 

 takes place in the whole arterial system. I am not to prove at 

 present that the arteries have muscular fibres ; but they are kept 

 extended, I say, both in the systole and in the diastole of the 

 heart, beyond their natural condition, by the quantity of blood 

 in the arteries ; and this is the foundation of the tension which 

 is given particularly to every portion of muscular fibres in every 

 part of the system, the distention of the arteries by the fluids 

 they contain, and particularly as these are impelled by the force 

 of the heart. So far we have no difficulty with respect to a 

 great number of muscular fibres being kept in a state of disten- 

 tion, but we add also, (4) by their connexion with such distended 

 cavities. Perhaps there is no muscular fibre that is not con- 

 nected with parts surrounding cavities, which are upon occasion 

 inflated or distended, but many cavities are alternately so 

 distended and again contracted to a considerable degree, and 

 then the muscular fibres are hardly in an extended state ; thus 

 in the stomach, the fibres there are in a state of extension, but 

 there are occasions when the stomach is contracted to a small 

 capacity, and we might expect that they were in a particular 

 state of contraction. But there is ground to believe that mus- 

 cular fibres every where have another particular cause of disten- 

 tion, viz. in consequence of arteries every where accompanying 



