In man, and in the animals which resemble him, the diaphragm appears to be the centre of sentiment; it is on this nervous part that the impressions of pain and pleasure are directed; it is on this that all the movements of the sensitive system are exercised. The diaphragm, in a transverse form, divides the body into two equal parts, of which the superior contains the heart and lungs, and the inferior the stomach and the intestines. This membrane is possessed of the utmost sensibility; it is also so necessary for the propagation and communication of feeling, that the slightest injury of it is always accompanied with convulsions, and often with death. The brain, which is considered as the seat of sensation, is not, therefore, the centre of sentiment, since it may be wounded, and even parts of it removed without causing the death of the animal. Let us then distinguish sensation from sentiment. Sensation is nothing more than an agitation or impression on the sense, whereas sentiment is this very sensation rendered agreeable or disagreeable by the propagation of the agitation through the sensitive system, for the essence of sentiment, its sole characteristic is pleasure or pain, and all other movements, notwithstanding they pass within us, are totally indifferent, nor do they affect us. It is on sentiment that the whole exterior movements, and the exercise of animal force depend; it acts only in proportion as it feels, and the very part which we consider as the centre of sentiment is also the centre of force. A slight examination will shew us that all lively emotions, whether of pain or pleasure, in a word, all sensations, whether agreeable or disagreeable, are felt internally in the region of the diaphragm. On the contrary, there is no token of sentiment in the brain; in the head there are none but pure sensations; we only recollect that this or that sensation has been agreeable or disagreeable; and if this operation in the head is followed by a lively and real sentiment, then we feel the impression of it within the region of the diaphragm. Thus the foetus, where this membrane is without exercise, is without sentiment, and the little motions of the foetus may therefore rather be considered as mechanical, than dependent either on sensation or on the will.