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 of 

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

 of the fyftole 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 (late for twice in the healthy one, it follows, that the 

 velocity of contraction in the difeafed (late 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 pulfation, 

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

 afliit us to diitinguifh 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 puiiations in a minute ; unlefs under fome peculiar cir- 

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

 nal heat. 



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

 contraction, and the fenforial power ceafes to aCt, the laft fitua- 

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

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

 Thus in weak or languid people, wherever they throw their 

 limbs on rheir 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-ftick is whirled round in the night, there appears in the eye 

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

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

 lire. 



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

 then covers his clofed eyes with his hancl, he will for many fec- 

 onds of time perceive the image of the fun on his retina. A 

 fimiiar 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. i. 3. on Sleep. Hence the dark fpots and other 

 ocular ipeCtra, are more frequently attended to, and remain 

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

 toxication, or want of ileep. 



6. A contraction of the fibres fornewhat greater than ufual 

 VOL. I. H introduces, 



