66 HEART OF MONOTREMATA CHAP. 
stance in the heart of the Rabbit. The heart of the Monotre- 
mata presents differences of some importance from those of other 
Mammalia; the modern knowledge of the Monotrematous heart 
is mainly due to Gegenbaur’ and Lankester,” in whose memoirs 
references to the older literature will be found. The principal 
features of interest in which the heart of the Monotremata differs 
from that of the higher Mammalia are these. When the two 
ventricles are cut across transversely, the cavity of the right is 
seen to be wrapped round that of the left in a fashion precisely 
like that of the bird’s heart; on the other hand in the higher 
mammal the two cavities lie side by side. The main difference 
petween Monotremes and other Mammals concerns the right 
auriculo-ventricular valve. The differences which it presents 
from the corresponding structure of the rest of the Mammalia 
are two: in the first place, the valve itself does not com- 
pletely surround the ostium; it is only developed on one side; 
the septal half (ze. that turned towards the interventricular 
septum) is either entirely absent or more generally represented 
by a small bit of membrane; nevertheless I found’ recently in 
an Ornithorhynchus heart a complete septal half to the right 
auriculo-ventricular valve. The second point of interest in 
connexion with this valve is, that the muscul papillares instead 
of ending in chordae tendineae attached to the free edge of the 
valve are directly attached to the valve, and in some cases pass 
through its membranous flap, to be attached to its origin at the 
boundary of the auricle and of the ventricle. The invading of 
the valve-flap by muscle in this way is highly interesting, as it 
recalls the heart of the bird and of the crocodile. The im- 
perfect condition of the valve (from which, as has already been 
stated, the septal half is as a rule nearly absent) is a point of 
resemblance to the heart of the bird; the corresponding valve 
of the crocodile’s heart being complete. 
There are also features in the system of arteries and veins 
which are eminently distinctive of mammals. In the first place, 
the aorta leaving the heart and conveying blood to the body 
is only a half arch, and bends to the left side as seen in 
Fig. 43. The right and left halves are present in reptiles, 
and meet behind the heart. In the bird the right half alone 
1 Jen. Zeitschr. ii. 1866, p. 365. 
2 Proc. Zool. Soc. 1883, p. 8. 3 Proc, Zool. Soc. 1894, p. 715. 
