But as this kind of sensibility is remarkably subject to vary, its relation to substances foreign to the organ changes also; thus the part of the capillary system which rejected blood, admits it at the moment when its sensibility has been increased. Irritate a part of the skin, it reddens in an instant; the blood flows there; while the irritation continues, it remains; when it ceases, it disappears. Whatever be the external means which raise the cutaneous or mucous sensibility, we observe the same phenomenon. We can in this way bring more or less blood into some parts of the capillary system. Bring the hand to the fire, the heat exalts the sensibility of its system, more blood enters it; take it away, this property resumes its natural type, and the blood is brought back to its ordinary quantity. The internal organs which are subjected less to the causes of excitement, have less varieties in their capillary system; yet, however, there are many, and all arise from the same principle. A series of organized tubes are unlike an assemblage of inert ones. These last require mechanical obstacles to prevent the communication of fluids with each other; where there is a communication between the tubes, there is a communication in the fluids. On the contrary, in the living economy, it is the peculiar vitality with which each tube is animated, which serves for an obstacle and a limit to the different fluids; this vitality performs the part of different machines that we place in the communicating tubes, to separate them from each other. Every organized vessel is then truly active; it admits or rejects fluids which enter there, according as it is able or not to support their presence. Disproportion of capacity has nothing to do with this phenomenon; a vessel may have mere than four times the capacity of the particles of a fluid, and yet refuse to admit them, if this fluid is repugnant to its sensibility. It is in this point of view that the theory of Boerhaave has a great defect. At the period in which this physician wrote, the vital forces had not been analyzed. It was necessary to employ physical forces to explain vital phenomena; hence it is not astonishing that all his theories are so incoherent. In fact, theories in the vital phenomena borrowed from physical forces, exhibit the same inadequacy, as those would in the physical phenomena borrowed from vital laws. What would you say, if in explaining the motion of the planets, rivers, &c. they should talk of irritability and sensibility? you would think it absurd; it is equally absurd, in explaining the animal functions, to talk of gravity, impulse, inequality of the capacity of the tubes, &c.