Sect. XII. i. 3. AND EXERTION. 47 



fimilar contraction by each of the above modes of excitation ; 

 and by their configurations to conftitute our ideas. 



3. After animal fibres have for fome time been excited into 

 contraction, a relaxation fucceeds, even though the exciting 

 caufe continues to act. In refpect to the irritative motions this 

 is exemplified in the periftaltic contractions of the bowels ; 

 which ceafe and are renewed alternately, though the ftimulus of 

 the aliment continues to be uniformly applied ; in the fenfitive 

 motions, as in ftrangury, tenefmus, and parturition, the alter- 

 nate contractions and relaxations of the mufcles exift, though 

 the ftimulus is perpetual. In our voluntary exertions it is expe- 

 rienced, as no one can hang long by the hands, however vehe- 

 mently he wills fo to do j and in the afibciate motions the con- 

 stant change of our attitudes evinces the neceffity of relaxation 

 to thofe mufcles, which have been long in action. 



This relaxation of a mufcle after its contraction, even though 

 the ftimulus continues to be applied, appears to arife from the 

 expenditure or diminution of the fpirit of animation previoufly 

 refident in the mufcle, according to the fecond law of animal 

 caufation in Sect. IY. In thofe conftitutions, which are termed 

 weak, the fpirit of animation becomes fooner exhaufted, and 

 tremulous motions are produced, as in the hands of infirm peo- 

 ple, when they lift a cup to their mouths. This quicker ex- 

 hauftion of the fpirit of animation is probably owing to a lefs 

 quantity of it refiding in the acting fibres, which therefore more 

 frequently require a fupply from the nerves, which belong to 

 them. 



4. If the fenforial power continues to act, whether it acts in 

 the mode of irritation, fenfation, volition, or afFociation, a new 

 contraction of the animal fibre fucceeds after a certain interval ; 

 which interval is of fhorter continuance in weak people than in 

 ftrong ones. This is exemplified in the making of the hands of 

 weak people, when they attempt to write. In a manufcript epif- 

 tle of one of my correfpondents, which is written in a fmall 

 hand, I obferved from four to fix zigzags in the perpendicular 

 ftroke of every letter, which fhews that both the contractions of 

 the fingers, and intervals between them^ muft have been per- 

 formed in very fhort periods of time. 



The times of contraction of the mufcles of enfeebled people 

 being lefs, and the intervals between thcfe contractions being 

 lefs aifo, accounts for the quick pulfe in fevers with debility, 

 and in dying animals. The ihartnefs of the intervals between 

 one contraction and another in weak conftitutions, is probably 

 owing to the general deficiency of the quantity of the fpirit of 

 animation, and that therefore there is a lefs quantity of it to be 



received 



