96 PHYSIOLOGY. 



of here, is, that the tension operates in exciting contraction, 

 even when the muscle is entirely separated from the body ; and 

 therefore we say it operates by increasing the inherent power, 

 not merely by acting upon the whole system, and thereby upon 

 the muscular fibre. There is this further circumstance to 

 be observed, that a certain condition of the inherent power 

 seems to be necessary to the action of the animal power upon it, 

 and therefore do we find it so necessary to give a tension to 

 these fibres which, by the power of the will, we would excite to 

 contraction. I have had occasion to point out this, and to 

 mention the use of a full inspiration ; and the effect which the 

 frown, the contracting the eyelid, &c. has upon the muscles of 

 the diaphragm, is, I think, no otherwise to be explained, than 

 by supposing that the tension of the whole is necessary to the 

 tension of every single muscle. But we find that this is deter- 

 mined to be in a certain degree according to the power of habit 

 and custom, which regulates the degree of tension that is neces- 

 sary to the actions of every muscle. Thus, when any external 

 means of tension, as the designed or accidental appendage of 

 weights, which have been employed, and have become habitual, 

 are taken away, the action is weakened. In various exercises, 

 and in the practice of several arts, we find that it is necessary 

 to the due and steady performance of the action, that the vari- 

 ous tools or instruments should be of a certain weight. In 

 another respect we observe the power of custom in regulating 

 the degree of tension. The most part of our muscles are stretched 

 upon bones, and by means of this contrivance the antagonists 

 are made to balance one another. But this tension will be 

 varied in every difference of the flexure of the joints to which 

 the muscles belong, and, therefore, in every different posture of 

 the body ; but if we are habituated to perform any particular 

 action in a given posture, that becomes afterwards necessary to 

 the performance of the same ; and this will explain the difference 

 which takes place among different men in performing the same 

 action, and how the most awkward posture for performing an ac- 

 tion may be as necessary as the most proper. The principal means 

 of tension in our system, is that which is given to the arteries 

 by the blood propelled into them (see page 79) ; and this can be 

 communicated to every fibre of the system. Hence the dif- 



