346. Through this extended surface the whole blood of the system must circulate, and every point of it must be unceasingly bathed with fresh streams of water. To generate the force necessary for the accomplishment of these objects, an increase of power must be communicated both to the circulating and to the respiratory apparatus. Neither the contractile power of the vessels by which in some of the simpler animals the nutritive fluid is put in motion, nor the contraction of the rudimentary heart by which in creatures somewhat higher in the scale a more decided impulse is given to the blood, are sufficient. A muscular heart, capable of acting with great power, is now constructed, which is placed in such a position as to enable it to propel with velocity the whole blood of the body through the myriads of capillary vessels that crowd every point of the surface of the branchial leaflets. To bring the water with the requisite degree of force into contact with this flowing stream, the apparatus of cilia is wholly inadequate. The water entering by the mouth, is driven with force, by the powerful muscles of the thorax, through apertures that lead to the branchial cavities. At the instant that the branchial leaves receive the currents of water through the appropriate apertures, the cartilaginous or bony arches which sustain the leaves, separate to some distance from each other, and to that extent expand the leaves and proportionally increase the surface exposed to the water: at the same time, the rush of water through the leaves unfolds and separates each of the thousand minute filaments of which they are composed, so that they all receive the full action of the fluid as it flows over them. 347. After the venous blood of the system has been thus exposed to the action of the respiratory medium, it is taken up by the vessels called the branchial veins (fig. CXXIX. 6), which for the reason assigned (372) are functionally arteries, as the branchial artery (fig. CXXIX. 4) is functionally a vein. The branchial veins uniting together form the great arterial trunk of the system, (fig. CXXIX. 7) by which the aërated blood is carried out to every part of the body.