8 OF STIMULUS AND EXERTION. Sscr.XIL U 



2. The immediate effect of the action of the fpi- 

 rit of animation or fenforial power on the fibrous* 

 parts of the body, whether it acts in the mode of 

 irritation, ienfation, volition, or afTociation, is a 

 contraction of the animal fibre, according to the 

 fecond law of animal caufation. Seel. IV. Thus 

 the ftinnulus of the blood induces the contraction 

 of the heart ; the agreeable tafte of a ftrawberry 

 produces the contraction of the mufcles of deglu- 

 tition ; the effort of the will contracts the mufcles, 

 which move the limbs in walking; and by affocia- 

 tion other mufcles of the trunk are brought into 

 contraction to preferve the balance of the body. 

 The fibrous extremities of the organs of fenfe have 

 been fliewn, by the ocular fpectra in Seel. III. to 

 fuffer iimilar 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 re- 

 fpecl to the irritative motions this is exemplified in 

 the periftaltic contractions of the bowels ; which 

 ceafe and are renewed alternately, though the fti- 

 mulus of the aliment continues to be uniformly ap- 

 plied ; in the fenfitive motions, as in ftrangury, 

 tenefmus, and parturition, the alternate contracti- 

 ons and relaxations of the mufcles exift, though 

 the ftimulus is perpetual. In our voluntary exer- 

 tions it is experienced, as no one can hang long by 

 the hands, however vehemently he wills io to do; 

 and in the aflbciate motions the conftant change of 

 our attitudes evinces .the neceflity of relaxation to 

 thofe mufcles, which have been long^n 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 

 pf the fpirit of animation previoufly refidem in the 



