1st. Experimental excitation, by means of galvanism or mechanical irritation, causes different results when applied to spinal nerves, to different parts of the spinal cord, or to different parts of the brain. Galvanism applied to a spinal nerve, determines, it has been said, dilatation of blood-vessels, and increased secretion in glands. But galvanism applied to the spinal cord in the neck, causes contraction of blood-vessels. Mechanical irritation of other parts of the spinal cord, on the other hand, causes vaso-motor paralysis and dilatation of blood-vessels. This is especially true of that part lying in the loins, and which contains a peculiar nervous centre, that stands in special relation to the uterus and ovaries, and is involved in many of their diseases, either as a cause or effect. Systematic galvanic irritation of the brain has been little attempted, until in some very recent experiments; but its effects are already known to be most various, according to the part to which it is applied. The brain is not a single organ, but rather a collection of organs, differing from one another in function even more than in situation, and among them only some are really concerned in the production of thought. 2d. In the medulla oblongata exists a nervous centre called the vaso-motor centre, because of its close relations with the vaso-motor nerves. Stimulation of this centre causes contraction of the blood-vessels. Severing the same part causes paralysis of the vaso-motor nerves and dilatation of the blood-vessels. The conditions of the brain that have been most clearly shown to influence the circulation, are those that can be proved to take an effect on this vaso-motor centre. If, as is probable, different forms of cerebral action induce or depend on different cerebral conditions, or involve different sections of the cranial masses, this effect would necessarily be different, and the influence on the circulation vary accordingly.