35 rangements of its auricle, differing as they do strikingly from those of the same com- partment in the mammalian heart, help us by that contrast to get a true idea of the working of this latter. ' Firstly, the walls of the Bird's right auricle are relatively thicker, not only as compared with the walls of its own ventricle, but also as compared with the walls of the corresponding auricle in the Mammal, the musculi pectinati standing out in as sharp relief as the similarly working muscular ridges in a hypertrophied bladder, and inclosing anfractuosites and recesses almost as deep. But, secondly, and what is of more importance, the Bird's auricle is furnished with a large and functionally ac- tive valve, protecting the entrance of the great veins, and preventing regurgitation into those vessels just as the auriculo- ven- tricular valves prevent regurgitation from the ventricles. It is fair to argue a priori that if the Mammalian auricle had counted for as much in the action of the heart as the Bird's, its force would have been economised by the placing of a large and functionally useful valve in the site of the rudimentary D 2
