motory nerves, which communicate with these by some common trunk, plexus, or ganglion, or else from those vibrations that run along the surfaces of uniform membranes, and so affect all the muscles which lie contiguous to any part of these membranes. It is a strong argument in favour of the hypothesis here delivered, that all the above-mentioned motions arise in the neighbourhood of vivid sensations, increase when they increase, and languish when they languish.
In examining this hypothesis by the actions of sneezing, swallowing, and coughing, regard must be had to the nose, uvula, and epiglottis, respectively, as being extreme and pointed parts, and consequently liable to be affected with extraordinary vibrations, agreeably to the ninth phænomenon of the sixth proposition.
In like manner, the numerous plexuses and ganglions of the eight pair of nerves and intercostal nerve must have great influence in the motions and functions of the parts contained in the thorax and belly.
As the motory vibrations of the second and third classes are of a gentle kind, for the most part, and descend constantly into the whole system of the muscles, it may be expected, that young children should move all their limbs at times, with some irregular kind of succession, from this cause. And this seems to be the fact. Strong contractions of the limbs are often excited by frictions, gripes, and other vivid sensations; but then the motory vibrations here are those of the first and fourth classes. General convulsions, from acidities, and other irritations in the bowels, seem to be excited in the same way, the intercostal nerve serving to communicate the vibrations with more readiness to the muscles of the trunk and limbs.
It appears to me also, that the intercostal nerve, which makes those of each side a separate system, as it were, has some share in determining hemiplegias to one side. In like manner, the great brachial and crural ganglions make all the nerves of the same limb sympathize with one another.
Whether the nerves of the same names throughout the body have not some sympathetic influences over each other, may be doubted. If those of the right side arise from the left part of the brain, and vice versâ, which seems to be the opinion of the best anatomists, then one would imagine, that the homonymous nerves of the right and left sides must, in crossing over, lie somewhere contiguous to each other, and so impart vibrations to each other. And there seem to be some facts from whence this may be inferred; but we cannot expect to be able to distinguish, with certainty, so feeble an influence, amidst so many others that are far stronger.
Yawning and stretching may, perhaps, when considered in all their circumstances, take in all the five classes of motory vibrations. When they happen in the attacks of fever-fits, and other
