Sect. XII. i. 5. AND EXERTION. 49 



health, which I believe is near the truth, the refult will be, that 

 the velocity of the contractile fides of the arteries will be in a 

 pleurify as two and a half to one, compared to the velocity c£ 

 their contraction in a date of health; for if the circumference 

 of the fyflole of the artery be three lines, and the diaftole in 

 health be twelve lines in circumference, and in a pleurify 

 eighteen lines ; and fecondly, if the artery pulfates thrice in the 

 difeafed ftate for twice in the healthy one, it follows, that the 

 velocity of contraction in the difeafed date to that in the healthy 

 ftate will be forty-five to eighteen, or as two and a half to one. 

 From hence it would appear, that if we had a criterion to de- 

 termine the velocity of the arterial contractions, it would at the 

 fame time give us their ftrength, and thus be of more fervice in 

 diftinguifhing difeafes, than the knowledge of their frequency., 

 As fuch a criterion cannot be had, the frequency of puliation, 

 the age of the patient being allowed for, will in fome meafure 

 afiift us to diftinguifh arterial ftrength from arterial debility, 

 fince in inflammatory difeafes with ftrength the frequency fel- 

 dom exceeds one hundred and eighteen or one hundred and. 

 twenty pulfations in a minute ; unlefs under fome peculiar cir- 

 cumstance, as the great additional ftimuli of wine or of exter- 

 nal heat. 



5. After a mufcle or organ of fenfe has been excited intb 

 contraction, and the fenforial power ceafes to act, the laft filia- 

 tion or configuration of it continues ; unlefs it be diflurbed by 

 the action of fome antagonift fibres, or other extraneous power. 

 Thus in weak or languid people, wherever they throw their 

 limbs on their bed or fofa, there they lie, till another exertion 

 changes their attitude ; hence one kind of ocular fpectra feems. 

 to be produced after looking at bright objects •, thus when a 

 fire-flick is whirled round in the night, there appears in the eye 

 a complete circle of fire ; the action or configuration of one 

 part of the retina not ceafing before the return of the whirling 

 fire. 



Thus if any one looks at the fetting fun for a fhort time, and 

 then covers his clofed eves with his hand, he will for manv ^qc- 

 ondsoftime perceive the image of the fun on his retina. A 

 fimilar image of all other bodies would remain fome time in the 

 eye, but is effaced by the eternal change of the motions of the 

 extremity of this nerve in our attention to other objects. See 

 Sect. XVII. 1.3. on Sleep. Hence the dark fpots, and other 

 ocular fpectra, are more frequently attended to, and remain 

 longer in the eyes of weak people, as after violent exercife, in- 

 toxication, or want of fleep. 



6. A contraction of the fibres (omewhat greater than, ufual 

 VQ L> L H intrc d ... •, 



