The structure of the heart in the various Vertebrates must also be noticed. In no Vertebrate is this organ so simple in structure as in the scheme given in Fig. 6; such an arrangement, moreover, would involve great difficulties. One great difficulty would be that while the blood was leaving the heart at _a_ (Fig. 6), no fresh blood could enter, so that the blood in the veins would stand still. Even in the lowest Vertebrates (the Fishes) this difficulty is obviated, for where the main vein (or veins) opens into the heart an enlargement of this vein is found, where the blood can collect as long as the heart continues to contract. This expansion is also reckoned as part of the heart, and named the _auricle_ (Fig. 8, _V.K._), while the heart proper is termed the _ventricle_ (_K._). It is also easy to see that there must be a tolerably wide opening between the two chambers, so that as soon as the ventricle becomes flaccid the auricle can force blood into it. But there being such a wide aperture between auricle and ventricle, one valve is not enough to make it impossible for the blood to pass back into the auricle during the contraction of the ventricular walls. There are two or three valves there (Fig. 8, _Kl._) fixed by fibres to the wall of the ventricle. In order that the blood which is forced into the artery (_S.A._) may not pass into the ventricle during its relaxation, there is another valve (not indicated in Fig. 8), at the base of the artery. [Illustration: FIG. 8.—Diagram of the Heart in a Fish. ]