SAXGUIFEROUS SYSTEM. 503 



cavities of the heart are always separated, at maturity, by 

 impervious septa; the descending aorta is formed by the 

 right arch, and not by the left as in birds ; the thick mus- 

 cular fold of the right ventricle of inferior vertebrata is here, 

 with few exceptions, as the ornithorhyncus, supplied by a 

 more delicate and complicated membranous tricuspid valve ; 

 the fibrous and serous coats of the vessels are more distinct ; 

 the valvular apparatus throughout the sanguiferous system 

 is more perfect and effective, and the valves of the veins 

 more numerous than in the inferior classes; the whole 

 heart is proportionately larger, and situate more posteriorly 

 in the trunk. The depressed and flattened form of the heart, 

 common in the chondropterygious fishes and chelonian rep- 

 tiles, is seen in the lowest mammalia, as in many of the 

 edentulous, the pachydermatous, and the cetaceous animals, 

 where we observe also in the adult condition of the herbi- 

 vorous lamantins, the rytinse, and the dugongs, the primi- 

 tive cleft or detached form of the two ventricles, throughout 

 half their extent from the apex towards the base, as in the 

 embryo state of this organ in higher animals and in man. 

 This retention of the earlier bifid condition of the apex of 

 the heart, resulting from the drawing up of the septum in 

 dividing the original single ventricular cavity into two, which 

 is seen also in the porpoise and slightly in the seal, is in 

 accordance with the general inferiority marked in the other 

 systems of these lower aquatic mammalia. 



The first portions of the two great arterial trunks, near 

 their origins from the heart, are sometimes found enlarged 

 in cetacea, like the bulbi arteriosi of inferior classes ; and in 

 the diving amphibious mammalia, as the seals, besides the 

 usual dilated inferior vena cava or sinus venosus common in 

 most aquatic vertebrata, to allow of their prolonged submer- 

 sion and suspended respiration, we often find the foramen 

 ovale, the ductus arteriosus, and the ductus venosus, remain- 

 ing to the adult state, quite pervious as in the earlier foetus 

 of higher tribes. The parietes of the ventricles are thicker 

 in mammalia than in lower classes, the right ventricle is less 

 extended around the left. than in birds, and the interior 

 surface of these cavities is more even, and presents fewer 

 detached muscular chords, than in reptiles. In the lower 

 orders of mammalia the heart is generally more median in 



