received, or of the chyme upon the intestines, or of the in-flowing blood upon the walls of the heart, in accordance with which the act is digestion, or motus peristalticus, or beating of the heart, &c.
But if now, in this process, we go one step further, we find that the muscles are the product of the blood, the result of its work of condensation, nay, to a certain extent they are merely solidified, or, as it were, clotted or crystallised blood; for they have taken up into themselves, almost unaltered, its fibrin (cruor} and its colouring matter (Burdach's Physiologie, Bd. v. § 686). But the force which forms the muscle out of the blood must not be assumed to be different from that which afterwards moves it as irritability, upon nerve stimulus, which the brain supplies; in which case it then presents itself in self-consciousness as that which we call will. The close connection between the blood and irritability is also shown by this, that where, on account of imperfection of the lesser circulation, part of the blood returns to the heart unoxidised, the irritability is also uncommonly weak, as in the batrachia. Moreover, the movement of the blood, like that of the muscle, is independent and original; it does not, like irritation, require the influence of the nerve, and is even independent of the heart, as is shown most clearly by the return of the blood through the veins to the heart; for here it is not propelled by a vis a tergo, as in the case of the arterial circulation; and all other mechanical explanations, such as a power of suction of the right ventricle of the heart, are quite inadequate. (See Burdach's Physiologie, Bd. 4, § 763, and Rösch, Ueber die Bedeutung des Blutes, § 11, seq.) It is remarkable to see how the French, who recognise nothing but mechanical forces, controvert each other with insufficient grounds upon both sides; and Bichat ascribes the flowing back of the blood through the veins to the pressure of the walls of the capillary tubes, and Magendie, on the other hand, to the continued action of the impulse of the heart (Précis de Physiologie par Magendie, vol. ii. p.
