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Show 202 PROF. T. H. HUXLEY ON MENOBRANCHUS. [Mar. 17, of muscular fibres, coated, on each side, by a layer of the cardiac epithelium. This plate represents the auricular septum ; but it extends for but a very short distance forwards, and then, as it were, frays out into separate branched muscular bands, each of which is invested by its own epithelial cells. On the dorsal side, these bands proceed to be attached to the wall of the auricle about midway between its anterior and its posterior ends ; but the ventralmost band makes an arch across the auriculo-ventricular aperture, and passes into a single muscular pillar, which is attached on the ventral side of that aperture. Anteriorly, this column branches out, and its divisions attach themselves to the left wall of the auricle and unite with the ramifications of the muscular bands proceeding from the dorsal side of the auriculo-ventricular opening (figs. 4 & 5). The "plate," the "pillar," and the branched muscular bands which proceed from them, which have just been described, are all that represent the septum of the auricles, which therefore can have but little efficacy as a partition between the pulmonary and the systemic venous blood. These two kinds of blood must mix freely through the wide meshes of the network of fibres invested by epithelium ; and it is only above and behind, where the meshes become closer and the network gradually passes into the impervious "plate," that the pulmonary blood can be guided to the auriculo-ventricular aperture by a special channel *. The auriculo-ventricular valves are mere narrow folds of the endocardium, bounding the margins of the auriculo-ventricular aperture, which is triradiate, in consequence of a notch in its ventral lip. The ventricle is oval in form ; its cavity is small, directed transversely, and bounded by thick, spongy, muscular walls. The left end of the cavity communicates with the auricle ; the right end opens into the elongated truncus arteriosus. The moiety of this truncus which lies nearest the heart is a tube, divided by a slight transverse constriction into two but little-marked dilatations. As Van der Hoeven has stated, there are three semilunar valves set in a transverse row in the first dilatation, just above the aperture of communication with the ventricle, while three other such valves are disposed across the middle of the second dilatation. The division of the truncus arteriosus which contains these valves may be * Stannius ('Handbuch,' p. 216, note) states that, according to his own and Hyrtl's observations, the separation of the auricles is apparently incomplete (anscheinend unvollkommen) in Proteus, Menobranchus, and Siren. As regards Proteus, in the only specimen I have dissected I have failed to find any trace of a septum; but in Siren I have found it complete, and extending between the auriculo-ventricular valves to terminate with a free edge, just as in the Frogs. According to Fritsch ("Zur vergleichende Anatomie der Amphibienherzen," Archiv fiir Anatomie, 1869), the septum is sometimes very imperfectly developed in adult (massig kraftige) specimens of Bana temporaria and B. esculenta. A series of Frogs examined in the spring of 1860 exhibited this condition. But in these cases the septum was reduced to a fold of the auricular wall, while in the specimen of Menobranchus here described the septum extends nearly to the auriculo-ventricular aperture, but is perforated. I have not met with the condition of the septum described by Fritsch in adult Frogs. |