506 ANATOMY OP VERTEBRATES. 



conceal the ventricle. The two precavals and the postcaval 

 terminate in a sinus, which the pulmonary venal trunk seems to 

 enter, but to the inner surface of which it adheres in its course to 

 its proper auricular chamber. The ventricle is obtuse and some- 

 times sub-bifid at the apex : it is connected to the pericardium by 

 the usual reflection of the serous layer upon the bulbus-arteriosus, 

 and also by a fold reflected from the apex upon the coronary vein, 

 which is thence continued to the venous sinus. The muscular 

 parietes of the ventricle are about a line in thickness, and loosely 

 fasciculate. The cavity is partially divided by an incomplete 

 septum, terminating by a concave border opposite the orifice of 

 the artery, on each side of which are the valves closing the two 

 auriculo-ventricular orifices. The aorta, narrow, and with thin 

 walls at its commencement, after a short subspiral course, thickens 

 into an elongate ' bulbus arteriosus,' which includes a longitudinal 

 valvular prominence, grooved at its fore-part in correspondence 

 with the origins of the branchial arteries. There is a pair of 

 valves at the origin of the aorta, and a second pair near the 

 beginning of the bulb. The distinction of the pulmonary from 

 the systemic auricle, first observed in Siren, has been since deter- 

 mined in Mcnobranchus^ , Axolotes"^, Amphiuma, and Menopoma.^ 

 In Proteus, in which some of the blood of the puny lungs is con- 

 veyed to systemic veins, the auricular septum is not complete, 

 according to Hyrtl.'' In Amphiuma the auricle is smaller and 

 less fimbriated than in Siren. The ventricle is similarly connected 

 to the pericardium by the apex, as well as by the artery. This 

 forms a half spiral turn at its origin, and dilates into a broader 

 and shorter bulb than in Siren. 



In Mcnopoma the auricles are still more reduced in size, and lie, 

 as in Salamandra, fig. 333, a, when undistended, to the left of the 

 ventricle: their outer sui'fiice, as in Menohranchus, is entire. The 

 ventricle is of a flattened triangular form : its cavity is occupied 

 by the loose fasciculate muscular structure through Avhich the 

 blood filters, as through a sponge, from the small contiguoiis 

 auricular apertures, each of which has a simple valve, to the 

 ' ostium artcriosum.' The artery inclines, with a slight twist, to 

 the left, and swells into a subspherical bulb. The valves are 

 confined to the narrower part, and are in two transverse rows, 

 four in each row, each valve of a conical shape, pointing forward.'^ 

 The first row is just aliove the ostium : the second is haliVay 

 between this and the bull). 



' CCLXXVIII. p. 73. = coLxxix, p. 45. ^ cci-xix. p. 215. 



* ccuoix. p. 258. » XX. ii. p. 45, pi. xxiii. fig. 2. 



