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 l and Lankester, 2 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 

 between 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 (i.e. that turned towards the interventricular 

 septum) is either entirely absent or more generally represented 

 by a small bit of membrane ; nevertheless I found 3 recently in 

 an Ornithorliynchus heart a complete septal half to the right 

 auriculo -ventricular valve. The second point of interest in 

 connexion with this valve is, that the musculi 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. 



