GENERAL SCHEME OF CIRCULATION 



301 



lieart. From it we learn that tlie heart is not a simple structure. 

 In the diagram lour distinct cavities can be seen, viz. : right antl 

 left ventricles, left atrium and aortic space — the right atrium is 

 not shown. The heart is really a double pump consisting of a 

 main pump or systemic heart (left atrium and ventricle) and 

 a subsidiary pump or j)ulmonary heart. In Fig. 80 is given 

 a scheme of the circulation. By contraction of the left ventricle 

 the blood is forced along a series of conducting tubes or arteries 

 {Art.) which lead to every part of the body and end in the substance 

 of the tissues in a network of innumerable hair-like canals, the 

 capillaries {Cap.). These capillary vessels are the wharves of the 

 tissues. Through their walls takes place the exchange of imports 



CAP 



Art. 



Fig. 86. — Scheme of tlie Circulation. S.H., systemic lieart .sending blood to the capillaries 

 in the tissues, Cap. The blood brought back by veins, and the exuded lymph by lymphatics, 

 Ly., passing through glands ; blood sent to the alimentary canal, Al.C, and from that to the 

 liver, Lir. ; blood also sent to the kidneys. Kid. ; the blood before again being sent to the 

 body is passed through the lung.s by the pulmonic heart, P.H. 



(Noel Paton's Essentials of Human Physiology.) 



and exports by which we measure the metabolism of the tissues. 

 Consequently it is found, when the capillaries join together to 

 form the wider conducting canals, venules and veins, that the 

 blood has lost some of its cargo of oxygen and nutrient matter, 

 and has gained a certain amount of waste matter. This with- 

 drawal of nutrient material is made good by the diversion of 

 some of the blood from an arterial canal to the capillaries in the 

 walls of the intestine {Al.C). Some waste matter is eliminated, 

 as we have seen, by a capillary mechanism in the kidney. Chemical 

 changes occur on the passage of the blood through the capillaries 

 of certain factories, e.g. liver and spleen. The loss of oxygen is 

 not made good until the blood has been carried by the veins into 

 the right atrium, passed from this reception house into the body 

 of the pump, the right ventricle {P.H.) and forced by the action 

 of this subsidiary pump into the lung capillaries. There, as we 

 saw in the last chapter, it gets rid of the excess of carbon-dioxide 



