a bed, they often fall into a great uneasiness from the stoppage of respiration, and recover from it by crying; and that rocking, or any other gentle motion, will prevent this. The last circumstance is particularly favourable to the notion of vibrations.
The night-mare in adults appears to be a disorder of the same kind. A full supper, and lying upon the back, dispose to it, by hindering the free motion of the diaphragm. Motion, and especially when attended with quick respiration, laughter, &c. prevent it, by giving activity to the diaphragm. Shaking the person affected, calling to him, &c. rouse from it, by exciting vibrations. The uneasiness from accumulated blood does the same thing at last.
It is easy to see, that if a like disposition to inaction should arise in the hearts of new-born children, or adults, the motion and distending force of the venal blood must renew their action for similar reasons. It is remarkably coincident with this, that forcing the venal blood upon the heart has in some cases been found to renew its action even after death.
Seventhly, The actions of sighing, crying, coughing, sneezing, &c. by recurring frequently, and making all the muscles, which can in any degree contribute to respiration, unite therein, enlarge the range of respiration by degrees, and make this action be performed with greater strength perpetually in the young child. The increase of strength in the whole muscular system must however be considered likewise.
Eighthly, The same actions have also a great share in begetting voluntary respiration. For more muscles are put into action in voluntary respiration, than in that which is ordinary.
Ninthly, It is agreeable to the foregoing theory, and to parallel observations on the heart, that respiration should be stronger in fevers, in sleep, and in general wherever the heat of the blood is increased, than in ordinary cases. For respiration depends chiefly on the second and third sources of motory vibrations. By this means the force of the respirations answers in a general way, as it ought to do, to the force with which the blood is moved, or to the force of the heart.
I now come to the particular consideration of the actions of sighing, coughing, laughter, and sobbing, and shedding tears from grief.
Sighing is an extraordinary contraction of the diaphragm. It arises not only from all causes, which accumulate the blood in the lungs, and thus excite the diaphragm to an extraordinary contraction; but also from such compressions of the abdomen as hinder the free motion of the diaphragm for a time. For thus a disposition to vibrate is accumulated in the fibres of the diaphragm. It is however to be observed, that this compression of the abdomen does also accumulate blood in the lungs. The abdomen seems to be compressed in a state of sorrow, attention, &c. The frequent recurrency of sighing makes it voluntary early
