Whatever may be the substance which serves as the vehicle of sentiment, and produces muscular motion, it is certainly propagated by the nerves, and is communicated in an indivisible instant from one extremity to the other. In whatever manner this motion may be effected, (whether by vibrations, as in elastic fibres, or by a subtile fire, similar to that of electricity, which not only resides in animated, and in all other bodies, but is constantly regenerated in the former by the motion of the heart and lungs, by the action of the blood in the arteries, and also by that of exterior causes on the organs of sense) certain it is that the nerves and membranes are the only sensible part of the animal body. The blood, the lymph, the fat, the bones, the flesh, and all other solids and fluids, are of themselves insensible; the brain is a soft and unelastic substance, and on that account incapable of producing or propagating the vibrations of sentiment. What may have given rise to the opinion that the brain was the seat of sensation, and the centre of sensibility, is the circumstance that the nerves, which are the organs of sensation, terminate in the brain; for which reason it was considered as the only part that could receive every agitation or impression. This supposition appeared so simple, and so natural, that no attention was paid to the physical impossibility that attends it, though abundantly evident; for how is it possible that a soft and insensible substance should not only receive impressions, but retain them for a length of time, and propagate all their agitations over the solid and sensible parts? Perhaps it will be answered after Descartes and Peyronie, that it is not in the brain, but in the pineal gland that this principle of sensation resides; but it is very easily distinguished that the pineal gland, the callous substance in which they would enclose the seat of the sensations, have no connection with the nerves, but are surrounded with the insensible substance of the brain, and so separated from the nerves that they cannot receive the motions of them, and therefore these suppositions, like the former, must fall to the ground. But what, in this case, is the use and functions of this very noble and principal part of the body? Is not the brain to be found in every animal? Do we not find it larger in man, quadrupeds, and birds, which have all much sentiment, than in fishes, insects, and other animals which have but little? When compressed, is not all motion suspended? Does not every action cease? If this part is not the principal of motion, why is it so essentially necessary to it? Why is it proportioned, in every species of animals, to the quantity of sentiment with which they are endowed?