morbid cases, the first seems to be owing to pretty sudden and strong contractions in the membrane of the mouth, fauces, aspera arteria, and œsophagus; the last to contractions in the whole skin.
As the bowels derive their peristaltic motion, in part, from the second and third classes; so it seems that the secretory and excretory vessels of the glands must be constantly agitated with a like motion, from the same causes, performing their ordinary secretions and excretions thereby. Their extraordinary ones are generally owing to irritations in the membranes, in which the mouths of their excretory vessels lie. And this agrees remarkably with the doctrine of vibrations. For the vivid vibrations excited in the membranes by the irritating cause must diffuse themselves every way; and when they come to the mouths of the excretory vessels, penetrate them, and, by passing up into the vessels, both excretory and secretory, greatly increase their peristaltic motion, and, by consequence, their secretions and excretions. All this seems equally to hold, in respect of the exhaling and absorbing vessels dispersed throughout the body.
The external motions of the eyes in young children are probably owing, in part at least, to the immediate action of light upon the tendinous expansions of the four strait muscles, and particularly upon those of the adducens and abducens. But the light which passes through the pupil seems also to have some share, as will be shewn hereafter. As to the internal motions, it appears, that the light which falls upon the cornea and uvea must excite the greater and lesser rings to contraction, in proportion to its strength; and, consequently, prepare the eye to see distinctly, at different distances, in the manner explained by Dr. Jurin. The hypothesis of this proposition does therefore give and receive light from his ingenious theory of this matter.
The two muscles which relax the membrana tympani, are much more exposed to the air than the musculus internus, or the musculus stapedis. When therefore the air is agitated with strong vibrations, as in loud sounds, it will excite the first-named muscles to action, and consequently relax the membrana tympani, as it ought to do. For what reasons the last-named muscles are contracted in feeble sounds, is a question of a more difficult nature, as is the parallel one in the eye, viz. why the radiated fibres of the uvea are contracted in small degrees of light, so as then to dilate the pupil.
The reader is desired to take notice, that, in all the instances of this proposition, I consider the motions as merely automatic. Their voluntary and semi-voluntary state will be accounted for in the two next propositions.
