GENERAL ORGANOLOGY. 135 



makes necessary also a twofold system of arteries and veins 

 (systemic arteries and systemic veins, respiratory arteries 

 and respiratory veins). The accompanying diagram (Fig. 

 62) of the course of the blood-circulation of the fish ex- 

 plains this. From the capillary region of the functional 

 tissues of the body, veins lead to the auricle of the heart ; 

 from the auricle, the blood flows into the ventricle and 

 through the gill-arteries on into the gill-capillaries. 

 Thence it is conducted through the gill-veins, which 

 unite into a single large trunk; this again gives off lateral 

 branches passing into the capillary region of the body. 

 Since the branches of the main trunk formed by the gill 

 veins lead again into a capillary region, they must, like 

 the main stem, be called arteries. 



Arterial and Venous Blood. During the course of 

 the blood through the body, it twice changes its chemi- 

 cal character and correspondingly its color. The blood 

 which flows out from the body capillary region has given 

 up its oxygen to the tissues, receiving in exchange car- 

 bonic acid, and has taken on a dark red color. This char- 

 acter is maintained until, in the gill-capillaries, it again 

 becomes oxygenated, giving up the carbonic acid and 

 becoming bright red in color. The different character of 

 the blood can be recognized in the arteries and veins of the 

 systemic circulatory system: the dark blood containing 

 carbonic acid is called venous, and the bright red, con- 

 taining oxygen, arterial blood, since the former flows in 

 the veins, the latter in the arteries. These terms are 

 entirely unsuitable, as can readily be seen from the above 

 diagram (Fig. 62), because they easily lead to the false 

 assumption that veins must always conduct blood con- 

 taining carbonic acid and arteries always oxygenated blood. 

 In opposition to this, the diagram shows that, in the re- 

 spiratory circulation (the shorter course), the conditions 

 must be the reverse of those in the systemic circulation, 

 since here the arteries contain " venous," while the veins 

 contain "arterial," blood. 



Closed and Lacunar Blood-vascular Systems. Such 



