The left ventricle has very thick parietes, which form the apex of the heart ; the mitral valve is membranous ; the larger flap is at tached to two strong columnw carnem ; the smaller flap to three smaller columnw.
The small left auricle (r) receives two pulmo nary veins.
In the Echidna the free appendix of the right auricle is slightly indented. The ter minal orifice of the superior cava is protected by a membranous semiltmar valve, extending from its left side. The musculi pectinati di verge from a strong fasciculus, which extends from the appendix to the orifice of the in ferior cava; this fasciculus bounds the left side of a wide fossa ovalis, which is imper forate. The inferior cava is protected by a large membranous Eustachian valve ; the left vena innominata terminates by a distinct aperture to the left of the preceding, and is also defended by a process of the Eustachian valve. The inner surface of the right ventricle is more irre gular than in the Ornithorhynchus ; the free wall is attached to the fixed one by several col umnm carnere and short chordm tend inex ; the tricuspid valve is membranous and consists of one principal portion attached to the exterior circumference, and a smaller portion closing the outer angle ; the free margin of the valve is attached to the extremity of a large fleshy column, arising by different roots from both the fixed and the free walls of the ventricle ; a short fleshy column is attached to the left ex tremity of the valve; some chords: tendinex are fixed to the right angle of the valve. The rest
of the structure of the heart corresponds with that in the Ornithorhynchus.
The aorta (fig. 187, d) bends, as in the Mammalia, over the left bronchus. The pri mary branches come off from the arch, in both Monotremes, as in Man, viz. arteria in nominata, left carotid, and !eft subclavian. The innominata divides, after a course of three lines, into the right subclavian and carotid (fig. 187, i), the latter being the smallest branch. Both subclavians emerge from the thorax above the first rib, and pass between it and the coracoid.
The phrenic, cceliac, and mesenteric arteries are given off from the abdominal aorta ; the renal artery is short, wide, and single; there is no inferior mesenteric artery, but the abdominal aorta terminates by dividing into the two com mon iliac and the caudal arteries, the arterial system agreeing in this and the other essential characters with the Mammalian type. The crural artery is shown at y, fig. 180.
Each of the superior venm caves receives the azygos vein of its respective side. The inferior cava has a long course in the thorax ; it is greatly dilated in the liver in the Ornithorhyn chus, as it is in the Placental divers, the Otter and Seal for instance.* The veins of the kidney are continued from the renal artery, and communicate solely with the inferior cava. The vena ports' is consti tuted as in other Mammalia.