THE RABBIT. 31 



precaval veins, nor anything in the venous system answer- 

 ing to the remarkable arches of the aortic and pulmonary 

 arteries. Why does not the simple rule, that what is the 

 best course for artery is best for vein, hold here also ? 

 Partly it may be explained by particular necessities, as in 

 the case of the portal system ( 7). But no such explan- 

 ation will hold for the neck and head vessels, nor, above all, 

 for the remarkable arching forwards and to the left of the 

 trunk arteries. Here in fact we find for the first time, but 

 by no means for the last, details of structure which cannot 

 be explained by any reference to the individual needs of the 

 rabbit itself. How they may possibly be explained we may 

 see later on, when we have studied other animals besides 

 the rabbit. Meanwhile, we may just carry farther an 

 analogy already suggested. When, in travelling on a 

 railway, we find that the up and down lines do not run 

 side by side, but differ in line or level, and no obvious 

 reason exists for this, we naturally conclude that the two 

 were not engineered at the same date, that the railway has 

 had a past history and does not represent the carrying out 

 of a single plan, but of several successive plans superim- 

 posed on one another. May it not be possible to explain 

 in some similar way these anomalies in the rabbit ? 



15. Summary of Circulation. Summarizing the course 

 of the circulation, starting from the right ventricle, we 

 have pulmonary artery, pulmonary capillaries, pulmonary 

 vein, left auricle, left ventricle, aorta, arteries, and 

 systemic capillaries. These smallest and ultimate ramifica- 

 tions of the circulation penetrate every living part of the 

 animal, so that if we could isolate the vascular system we 

 should have the complete form of the rabbit in a closely 

 meshed network. It isjn Jbhe capillaries that the exchange 

 of material between the blood and the tissues occurs, 

 nutritive material and oxygen passing out to the tissues 

 and katastases in from them ; they are the essential factor 

 in the circulatory system of the mammal veins, arteries, 

 and heart simply exist to remove and replace their contents. 



From the systemic capillaries the blood, with less oxygen 

 and less plasma than before, passes into the veins. From 



