GENERAL ORGANOLOGY. 113 



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, respira- 

 tory arteries and respiratory veins). The accompanying diagram 

 (fig. G5) 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; from the auricle the blood flows into the 

 ventricle, and through the afferent gill-arteries into the gill-capil- 

 laries. Thence it is conducted through the 'gill-veins 7 (eiferent 

 arteries), 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 its course through the 

 body the blood twice changes its chemical character and corre- 

 spondingly its color. The blood which flows from the body 

 capillary region has given up its oxygen to the tissues, receiving 

 in exchange carbon dioxide, and has become dark red. This 

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

 becomes oxygenated, giving up the carbon dioxide and becoming 

 bright red. The different character of the blood can be recognized 

 in the arteries and veins of the systemic circulatory system; the 

 dark blood containing carbon dioxide is called venous, and the 

 bright red, containing 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. 65), because they easily lead to the false assumption that veins 

 must always conduct blood containing carbon dioxide, and arteries 

 always oxygenated blood. In opposition to this, the diagram 

 shows that, in the respiratory circulation (the shorter course), the 

 conditions must be the reverse of those in the systemic circula- 

 tion, since here the arteries contain ( venous/ while the veins 

 contain ' arterial/ blood. 



Closed and Lacunar Blood-vascular Systems. Such a blood- 

 vascular system as has here been described is called a closed one, 

 because the blood always flows in special tubes provided with their 

 own walls. Opposed to the closed stands the lacunar blood-vascular 

 system; here the blood-vessels lose, after a time, the character of 

 tubes and become wide cavities, or sinuses, which, without special 

 walls, are enclosed between the intestines and other organs 

 (hsemocoale, supra). 



