At birth the animal nervous system experiences a remarkable revolution, in consequence of the red blood that penetrates it. Heretofore black blood only circulated in its vessels. The sudden difference that the circulation experiences, has a manifest influence upon its functions. In fact the least foreign substance, differing from red blood, which during life is forced towards the brain by the carotid, is sufficient to produce there a remarkable derangement, and oftentimes even death, as I have frequently convinced myself. Why? because it is not only as a vehicle of nutritive matter, that the fluid sent by the arteries acts upon the brain, but also as an excitant, a stimulant. The change of excitement which the brain suddenly experiences at birth, inevitably increases its vital activity, gives it that which is new and renders it fit for the functions it has never before performed, those of receiving sensations. Asphyxia is real always when the lungs are not developed after birth, when they do not receive air and consequently do not send red blood to the brain. Some muscular motions may undoubtedly be made; but animal life never begins in its perfection, until the organs that execute it are influenced by red blood. This blood is a general cause of internal excitement. This direct acts simultaneously with the sympathetic excitement that the brain experiences from the skin and mucous surfaces, which the external agents act upon immediately after the exit of the fœtus from the womb. The lungs and the brain influence each other reciprocally at this period, the first by sending red blood to the second, and this by putting in action the diaphragm and the intercostals, which make the air, that is necessary for the production of this red blood penetrate the lungs; hence we see that other excitants act before that of this blood, since before its formation, the brain has already in it a principle of motion. Besides, the brain and the whole nervous system are the more powerfully excited by the new principles that the blood has derived from the air, as, 1st. their vessels are in proportion larger and more numerous than afterwards; and, 2d. as all the cerebral arteries enter at that part of the base of the brain, where is found the origin of the nerves, and which is without doubt the most sensitive part of the whole organ.