GENERAL ORGANOLOGY 101 



free in the body cavity or enclosed in a special pouch (p), the pericardium. 

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

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

 (k), is of less functional importance; hence is not carried out in all cases. 

 There are also valves {kl), which, by closing, prevent the blood from 

 flowing back 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 out again to the tissues. The 

 part of the course of circulation concerned in this must have easily 

 permeable walls, must be widely distributed in the body, and have a large 

 superficial area. These demands are met by the capillaries (c), extremely 

 fine, thin-walled and permeable epithelial tubes, which surround and 

 penetrate all organs. Between the heart and the capillaries (here exists, 

 corresponding to their diiierent functions, great differences in structure; 

 they must therefore be united Ijy special transitional vessels (i) vessels 

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

 thinning of their walls, pass gradually into the capillaries, the arteries (o) 

 and (2) vessels (veins) which start from the capillaries and lead back to 

 the heart, uniting to form larger and stronger vessels (v). 



Correlation of Respiratory Organs and Blood System. — It is a 

 law that in all animals the blood-vascular system has fjeen influenced in 

 its arrangement and structure more by respiration than by nutrition in the 

 narrower sense; there exists a correlation between the organs of respira- 

 tion and of circulation. A double capillary region must be distinguished; 

 besides the body capillary system already mentioned there is the respiratory 

 capillary region, whose exclusive office is to remove the carbon dioxide 

 from the blood and to furnish oxygen to it (gill and lung capillaries). A 

 twofold capillary region makes necessary also a twofold system of arteries 

 and veins (systemic arteries and systemic veins, respiratory arteries and 

 respiratory veins). The accompanying diagram (fig. 66) of the blood 

 circulation of fishes illustrates this. Veins lead from the capillary region 

 of the tissues of the body to the auricle of the heart. The contraction or 

 systole of the auricle drives the blood into the ventricle. While the auricle 

 enlarges (diastole) and refills with blood from the veins, the systole of the 

 ventricle forces the blood through the gill arteries to the gill capillaries. 

 Since systole and diastole of a heart chamber alternate, the heart acts as a 

 suction and force pump, and the systole of auricle and ventricle must 

 alternate in time. From the gill capillaries the blood goes to the 'gill- 

 veins' (eiferent gill arteries), which unite into a single large trunk; this 

 again gives off lateral branches passing into the capillary region of the 



