partly by the renewal of vivid impressions; and partly, as it seems, by vibrations excited in the pleura and peritonæum, and thence communicated to the diaphragm, and to the muscles of the breast and belly.
That the last cause has a real efficacy, may appear from the following instance:—Let respiration be supposed to be at a stand for a small time, on account of the person’s running, or exerting an act of great strength. It is evident, that the blood will both be accumulated in the lungs, and heated there, during this interruption of respiration, since respiration both ventilates the blood, and promotes its motion through the lungs. The external membrane of the lungs will therefore be both distended and heated, i.e. will have an increase of vibrations communicated to it. But this membrane is continuous to the pleura, and, indeed, is the same membrane with it. An increase of vibrations will therefore be communicated to the pleura, and consequently to the diaphragm, and muscles of the breast, which it invests.
The peristaltic motion of the intestines is, in part, to be deduced from the second and third classes of motory vibrations, in the same manner as the motion of the heart, since that motion, like this, returns at intervals incessantly. And there is reason to believe, that vigorous vibrations, either of the sensory or ideal kind, impart an extraordinary degree of activity to the stomach and bowels. However, they derive also a great part of their motions, probably the major part, from the impressions which the aliment, bile, and fæces, make upon the villous coat; the vibrations excited by these impressions both running directly into the muscular coat, for the purpose of contracting that part which adjoins to the seat of impression, and also running upwards and downwards along the villous coat, so as to exert some efficacy at a distance from this seat.
It is very remarkable, that the pale fibres of the intestines, in men, and many other animals, preserve their power of alternate contraction and relaxation for a considerable time after death, whereas the red fleshy muscles of the same animals lose theirs soon after the effusion of their blood. It is a phænomenon of a like kind with this, that the whole muscular system of some animals that are exanguious, or nearly so, retain their activity for a considerable time after these animals are cut into pieces. And both may serve to intimate that the electricity, or other attractive virtue, of pale fibres and fluids, at the same time that it is feebler than that of red ones, is however, of a more durable kind, and, as was observed above of the hearts of frogs and vipers, capable of being put into action by a less degree of heat.
The actions of sneezing, swallowing, coughing, hiccoughing, vomiting, and expelling the fæces and urine, with others of a like nature, are to be deduced from the first and fourth classes of motory vibrations; i.e. either from those vibrations which first ascend up the sensory nerves, and then are detached down the
