SECT. XII. i. OF STIMULUS AND EXERTION. 69 



mufcle, according to the fecond law of animal cau- 

 fation in Sect. IV. In thofe conftitutions, which are 

 termed weak, the fpirit of animation becomes 

 fooner exhaufted, and tremulous motions are pro- 

 duced, as in the hands of infirm people, when they 

 lift a cup to their mouths. The quicker cxhauft- 

 ion 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, whe- 

 ther it acts in the mode of irritation, fenfation, 

 volition, or aflbciation, 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 

 fhaking of the hands of weak people, when they 

 attempt to write. In a manuscript epiftle of one of 

 rny correfpondents, which is written in a fmall 

 hand, I obferved from four to fix zigzags in the 

 perpendicular ftroke of every letter, which mews 

 that both the contractions of the fingers, and in- 

 tervals between them, muft have been performed in 

 very (hort periods of time. 



The times of contraction of the mufcles of en- 

 feebled 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 there- 

 fore there is a lefs quantity of it to be received at 

 each interv/* of the activity of the fibres. Hence 

 in repeated motions, as of the fingers in perform- 

 ing on the harpfichord, it would at firfi fight ap- 

 pear, that fwiftnefs and ftrength were incompati- 

 ble ; neverthelefs the fmgle contraction of a mufcle 



b 



