OTHER INVOLUNTARY MOTIONS. 155 time ; for then it is very perceptible, being accompanied with great uneafinefs and anxiety. The adlion of the air, aliments, and bile, upon the inteftines, which is the caufe of their periftaltic motion, is commonly unperceived by us ; but let thefiimulus adling on the in¬ teftines be increafed, as is the cafe when any ftrong purgative is taken, or when any acrid humours are lodged in the prhnœ vite, and it will be felt very fenfibly. The fiimulus of light upon the retina, which makes the pupil contrail, is feldom perceived or re¬ garded, unlefs when the degree of light is much ftronger than what the eye, immediately before, had been expofed to. The aélion of the returning blood upon the heart, though it be ufually impercep¬ tible, in fome cafes is plainly to be felt: for people, efpecially fuch as have weak nerves, after a fudden fright, (which makes the blood return more haftily, and in greater quantity, than ufual to the heart), are fenfible of a particular feeling from this organ’s being more than ordinarily affeefted by a furcharge of that fluid. In various parts of the body, pulfations, or fmall alternate convulfions, are fometimes perceived, which, as they keep not time with the beating of the heart, cannot be arterial vibrations, but muft be the alternate contrariions of mufcles, or, rather, of a fmall parcel of their fibres. There is no fenfation of aftimnlus in the part before thefe motions begin, or while they continue; and yet, as they frequently happen to peo¬ ple in health, whofe brain and nervous fyflem are found, it is pro¬ bable that they are owing to fome obftru&ing matter, which over- firetches the fibres of the fubtiler veffels, or to acrid particles in the fluids touching the tender nerves of the convulfed part. ß The fiimulus occasioning the vital motions is unperceived byus, not only on account of its gentlenefs, but alfo becaufewe have been accuftomed to it from our birth. The force of cullom is great and unaccountable ; what we have been long ufed to, we become fcarce- ly fenfible of, while things wdiich are new, though more trifling, and of weaker impredion, affetf: us remarkably. Thus he who is accuftomed to the country, is much affe&ed with the noife and bridle of a populous city : but by ufe, he daily becomes lefs and'lefs U 2 fenfible