. Elements of Comparative Anatomy. 370 COMPARATIVE Fig. 196. Vertical section throuo;li an Anodonta. v Ventricle, a Auricles. pp' Pericardial cavity, i Hind-gut. w Mantle, hr hr' Branchiaj. / Foot. length. In most of the LameUibranchiata the heart is divided into two limbs, whicli embrace the hind-gat {i), and which give ofJ the aorta after they unite. In Area this passage of the hind-gut through the heart leads to the formation of two ventricles, which are represented by two completely separate cham- bers, each of which is provided with an aui"icle. Each ventricle gives off an
. Elements of Comparative Anatomy. 370 COMPARATIVE Fig. 196. Vertical section throuo;li an Anodonta. v Ventricle, a Auricles. pp' Pericardial cavity, i Hind-gut. w Mantle, hr hr' Branchiaj. / Foot. length. In most of the LameUibranchiata the heart is divided into two limbs, whicli embrace the hind-gat {i), and which give ofJ the aorta after they unite. In Area this passage of the hind-gut through the heart leads to the formation of two ventricles, which are represented by two completely separate cham- bers, each of which is provided with an aui"icle. Each ventricle gives off an aorta, which unites with its fellow of the opposite side before it gives off any branches, so that there is a single aorta all the same. The same holds for the posterior arterial trunk. The anterior arterial trunk passes as far as the region of the mouth, where it gives off branches and opens into wide hsemal spaces. The pos- terior arterial trunk, the length of which is dependent on the develop- ment of the hinder portions of the mantle, which represent the siphons, also passes into htemal spaces or lacunae. Spaces of this kind, which are essentially marked off by connec- tive tissue, ramify not only in the mantle, but also between the viscera. They may be distinguished into lai'ger or smaller recep- tacles for blood, according to their width; while, further, they take the place of a capillary and of a venous system. Larger sinuses are regularly present at the base of the gills, and a median azygos one—which collects together the venous spaces in the foot —extends between the two adductor muscles. All of these haemal spaces are connected together, and form in different parts a more or less wide mesh-work. The two lateral spaces also communicate with the organs of Bojanus (§ 290). If we follow out the course which is taken by the blood sent from the arteries to the peripheiy, although, indeed, many points about it have not been definitely made out, we find that some of it
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Keywords: ., bo, bookcentury1800, bookdecade1870, bookpublisherlondonmacmillan
