BLOOD SYSTEM. 



385 



COMPARATIVE MORPHOLOGY OF THE BLOOD SYSTEM IN 

 VERTEBRATES. 



Mammals. — In all essential features the blood sys- 

 tem of mammals is exactly the same as that of man, 

 already described. All mammals have a four-chambered 

 heart — i. e., a double heart with no communication be- 

 tween except through the capillaries. They all, there- 

 fore, have a complete double circulation, with the whole 

 blood passing through both. 



Birds. — Birds have also a double heart and complete 

 double circulation. There is only one thing worthy of 

 note, and that only on account of its significance in the 

 evolution of birds. It is that the aortic arch turns to 

 the right instead of to the left, as in mammals. 



Reptiles and Amphibians. — The first important 

 variation from the model already given is found in these. 

 The heart of the reptile and the amphibian is three- 

 chambered instead of four-chambered. If the heart of 

 mammals and birds may be compared to two tenements, 

 each with two chambers, joined together, but with dead 

 wall between, then the heart of reptiles and amphibians 

 may be compared to one tenement with a suite of three 

 rooms. With such a structure it is impossible to have a 

 complete double circulation. The pulmonic circulation 

 is only a branch of the systemic, and therefore only a 

 part of the blood passes through the lungs to be 

 aerated, and therefore also not pure oxygenated, but 

 more or less mixed blood goes to the tissues. 



Course of Circulation. — Fig. 265 is a schematic 

 diagram showing the course of circulation in reptiles. 

 The heart, as is seen, consists of two auricles and one 

 ventricle. The blood is thrown out of the ventricle into 

 the aorta, which makes an arch, but there divides and 

 sends a large branch to the lungs, while the main branch 



