The brain is also larger in man, quadrupeds, and birds, because in them the quantity of nerves is greater than in fishes and insects, which on this very account have very little sentiment; they have but a small brain, in proportion to the small number of nerves which it nourishes. And here I cannot help remarking, that man has not, as has been said, a proportionably larger brain than any other animal. There are species of apes, and of cetaceous animals, which, proportioned to the size of their bodies, have more brains than man; another fact which proves that the brain is neither the seat of sensation, nor the principle of sentiment, since were it so those animals would have more sensations, and more sentiment, than man. By observing the nutrition of plants we shall perceive that they do not absorb the gross parts of earth or water, and that these must first be reduced by heat into tenuous vapours. In like manner the nerves are nourished by the subtle moisture of the brain, which is received by their extremities or roots, and thence carried into all the branches of the sensitive system. This system, as we have already remarked, forms an individual whole, of which the parts have so close a connection that we cannot wound one without injuring all the rest. The slightest irritation of the smallest nerve is sufficient to throw the whole body into a convulsion, nor is it possible to cure the pain, or remove the convulsion, but by cutting away the nerve above the injured part, and then all the parts to which this nerve joined become at once motionless and insensible. The brain ought not to be considered as an organic part of the nervous system, because it differs both in properties and substance, and is neither solid, elastic, nor sensible. I own that, when compressed, a stop is put to sensation; but this proves it a body foreign to the system, which, from acting with a weight on the nerves, benumbs them in the same manner, as a heavy weight applied to the arm or leg, deadens the feeling; and this is evident, because the moment the compression is removed sentiment revives, and the motion is re-established. I own likewise that, by injuring the brain, convulsions, and even death, will ensue, but these effects are produced from the nerves being injured in their very source. To these reasons I might add particular facts, which would also prove that the brain is neither the centre of sentiment nor the seat of sensation. There have been animals, and even children, born without either head or brain, yet endowed with sentiment, motion, and life. In insects and worms the brain is not perceptible, having only a part which corresponds with the spinal marrow, and therefore the spinal marrow might more reasonably be supposed the seat of sensation, being common to all animals, which the brain is not.