148 STUDIES IN ADVANCED PHYSIOLOGY. 



power to propel it. In every water- works system there 

 must be at least one central pumping 1 station. The central 

 station in the circulation of the blood is, of course, the 

 heart. From this arise vessels which lead the blood away 

 from it, called arteries. These arteries are of two sets, one 

 carrying the impure blood to the lung, the pulmonary arter- 

 ies, the other carrying the pure blood over the entire body, 

 the systemic arteries. The arteries divide and sub-divide 

 as they proceed, and finally entering their respective tissues 

 lead into the plexuses, or net-works of tiny capillaries. In 

 these capillaries the walls are thin enough to permit the 

 nourishment held in solution in the plasma to dialize through 

 and soak in among the tissues. The oxygen, too, in the 

 capillaries becomes disassociated from the corpuscles which 

 have carried it to this point, and along with some of the 

 plasma which soaks through it enters the lymph of the tis- 

 sues. It will be seen, therefore, that in the capillaries the 

 arterial stream, so to speak, divides, the larger portion of 

 it, together with all of the corpuscles, remaining in the 

 capillaries, and, being returned to the heart through the 

 veins, while the other part soaks through the capillary 

 walls and slowly and leisurely circulates between, and per- 

 meates, the tissues. This latter part is designated as lymph. 

 Lymph, therefore, is but that portion of the blood which has 

 dialized out through the thin capillary walls. 



The returning streams, then, are two. First, the veins 

 which carry the returning blood that has not left the capil- 

 laries, and which, because they contain all the red corpuscles, 

 look red, and are therefore spoken of as blood veins; and 

 second, a system of so-called veins, the lymphatics, quite 

 similar to the ordinary blood veins which gather up the 

 lymph of the tissues, and finally through the thoracic duct 

 return this stream to the original blood circulation, where 

 the thoracic duct empties itself into the left subclavian vein. 



THE ROUTE OF ONE COMPLETE CIRCULATION. 



Starting with a drop of blood say in the right ventricle, 

 it is sent by the contraction of this chamber through the 



