112 



GENERAL PRINCIPLES OF ZOOLOGY, 



the firmer arteries (a) leading to the capillary region, and the 

 thinner- walled veins (v) leading back to the heart. 



Correlation of Respiratory Organs and Blood System, It is a 

 law that in all animals the blood-vascular system has been influ- 

 enced in its arrangement and structure more by respiration than 



FIG. 65. Scheme of circulation in a fish, a', ascending (ventral) aorta; a 2 , descend- 

 ing (dorsal) aorta ; c, carotid , da, intestinal arteries ; dc, intestinal capillaries , 

 dv, intestinal veins; 7i, auricle; k, ventricle; Tea, afferent gill-a.rteries; kv, efferent 

 gill-arteries ; Zc, liver-capillaries ; sc, body-capillaries ; i>c, cardinal veins ; vTi, 

 hepatic vein ; ty', jugular vein. 



by nutrition in the narrower sense; there exists a correlation 

 between the organs of respiration 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 



