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 pul- 

 monary 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 aper- 

 ture, which is trhadiate, in consequence of a notch in its ventral lip. 



The ventricle is oval in form ; its cavity is small, directed trans- 

 versely, 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 con- 

 striction 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 divi- 

 sion 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 Fritscb (" Zur vergleichende Anatomie der Amphibienherzen," 

 Archiv fur Anatomie, 1809), the septum is sometimes very imperfectly developed 

 in adidt (massig kraftige) specimens of Rana (emporaria and S. esculcnta. 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 con- 

 dition of the septum described by Fritsch in adult Frogs. 



