CHAPTER IX 



THE CIRCULATION 



THE blood circulates in a closed system of tubes, continuous 

 from the heart back to the heart. The walls of these vessels 

 separate the blood from the tissues. Nowhere, except in the 

 spleen, does it come into contact with any cells other than the 

 lining cells of the vessels in which it flows, and the exception 

 made by the spleen is more apparent than real. The spleen 

 (p. 79) is a kind of sponge invested with a firm capsule. Small 

 arteries discharge their blood into its spaces ; small veins collect 

 it. But the organ is essentially a part of the vascular system. 

 Its spaces take the place of the capillary vessels which connect 

 arteries with veins in other situations. 



The blood makes a double circuit. From the right heart it 

 passes through the vessels of the lungs. Returning to the left 

 heart, it is driven through the body. Although the heart con- 

 sists of two separate pumps, it makes but a single organ. Its 

 division into right auricle and ventricle and left auricle and 

 ventricle is but slightly indicated on the surface. In most 

 invertebrate animals the two pumps are distinct. In some the 

 lung-heart and the body-heart are on opposite aspects of the 

 body. But one must not, when thinking of the morphology of 

 the vertebrate heart, picture it as formed by the juxtaposition 

 of two, originally separate, pumps. Truly, in its very earliest 

 stage of growth, it is represented by two tubes which lie, in the 

 embryo, far apart. But these, before we can speak of the 

 existence of a heart, fuse into a single tube, with four con- 

 tractile bulbs in series. As the heart develops, the dilatation at 

 its hinder or venous end and the dilatation at its anterior or 

 arterial end disappear. A partition is formed which divides the 

 two middle bulbs into right and left auricle and right and left 

 ventricle respectively. Immediately after birth the lungs are, 



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