INTRODUCTORY 25 



Birds differ from Reptiles and agree with the Mammalia in 

 having a completely four-chambered heart, whereby the ad- 

 mixture of arterial and venous blood is prevented. The heart 

 of birds, however, differs in important respects from that of 

 mammals. 



Owing to their excessive activity the heart-beats of the bird 

 are quicker than in any other animals, numbering 120 to the 

 minute during rest, and during flight reaching a far higher 

 figure. In a bird which has just alighted the pulsations are 

 beyond the count of the ear. 



Birds, like mammals, have but a single aortic arch, but while 

 in the former this is the left, in the latter it is the right of the 

 originally double arch which persists. The corresponding arch 

 of the opposite side, in both cases, gives rise to part of the 

 subclavian artery. The carotid arteries exhibit some interest- 

 ing modifications, but these appear, like so many other char- 

 acters, to have no bearing, no traceable bearing, on the struggle 

 for existence. And the same remarks apply to the femoral 

 arteries which may be supplanted by the sciatic. Though 

 attempts have been made to use these vessels for taxonomic 

 purposes, they have been only partially successful. The Pen- 

 guins alone among birds develop a rete mirable, which is, of 

 course, connected with their diving habits, though other diving 

 birds, it is to be remarked, have not developed a similar ar- 

 rangement of the blood-vessels, which is met with again in the 

 Cetacea among the mammals. An ingenious use of the veins 

 supplying the intestinal mesentry has been made by Dr. 

 Chalmers Mitchell, but the discussion of this, as of other peculi- 

 arities of the vascular system, does not come within the scope 

 of this work. 



During the work of brooding it is to be remarked the 

 blood-vessels of the abdomen become greatly distended, and 

 form large " inflamed " areas known as brood spots, which, 

 applied to the surface of the egg during incubation, generate 

 the heat necessary for the development of the growing chick. 



The Muscular System 



The muscular system of birds presents no characters which 

 have any really important bearing on the problems with which 

 this book is concerned. Nevertheless, interesting illustrations 



