SECT. XIL 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 refpecl 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 ; and in the aflbciate motions the con- 

 firm change of our attitudes evinces the neceflity of relaxation 

 to thole mu Teles, 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 

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

 caufation in Seel:. IV. In thole conftitutions, which are termed 

 weak, the fpirit of animation becomes looner 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 aft, whether it arts in 

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

 contraction of the animal fibre fucceeds after a certain interval ; 

 which interval is of (horter continuance in weak people than in 

 ftrong ones. '1 his is exemplified in the (baking 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 obferve from four to fix zigzags in the perpendicular 

 ftroke of every letter, which (hews 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 thofe contractions being 

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

 and in dying animals. The fhortnefs 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 



