GENERAL OKGANOLOGY. 133 



cavity (Fig. 61). Hence we see that the heart always lies, 

 either free in the body-cavity, or enclosed in a special 

 pouch (/>), the pericardium (in all cases a portion of the 

 general body-cavity which has become independent). 

 The division of the heart, into a part which receives the 

 blood, the auricle (//), and a part which drives the blood 

 onward, the ventricle (k), is of less functional importance; 

 hence this division is not carried out in all cases. There 

 are also special mechanisms within the heart, the valves 

 (/), which, by closing, prevent the blood from flowing 

 back into any other chamber of the heart when the walls 

 relax at the end of the contraction. 



Blood-vessels. In order that the blood system may 

 properly perform its function, in addition to circulation, it 

 is necessary that the nutritive substances be readily taken 

 up and given over again to the tissues. The part of the 

 course of circulation concerned in this must be general, 

 i.e., widely distributed in the body, and having a lumen 

 of large superficial area. These demands are met by the 

 liairlike vessels or capillaries (r), extremely fine and thin- 

 walled tubes, which surround and permeate all organs. 

 Through their walls, usually formed by a thin epithelial 

 layer, the proteid substances for nourishing the tissues can 

 pass, and the oxygen can be exchanged for carbonic acid. 

 Between the heart and the capillaries there exists, corre- 

 sponding to their different functions, the greatest con- 

 ceivable difference in structure; they must therefore be 

 united by special transitional vessels vessels which begin 

 large and thick- walled at the heart, and by branching, 

 and thinning of their walls pass gradually into the capil- 

 laries; of such vessels there are two kinds, the firmer 

 arteries (a) leading to the capillary region, and the thin- 

 walled veins (?') leading back to the heart. 



Correlation of Respiratory Organs and Blood Sys- 

 tem. It has been determined, as a law, that in all animals 

 the blood-vascular system has been influenced in its arrange- 

 ment and structure more by respiration than by the taking 

 up of food in the narrower sense; there exists a correlation 



