18 



ZOOLOGY. 



between auricle and ventricle, one valve is not enough 

 to make it impossible for the blood to pass back into 

 the auricle during the contraction of the ventricular 

 walls. There are two or three 

 valves there (Fig. 8, KL), fixed 

 by fibres to the wall of the 

 ventricle. In order that the 

 blood which is forced into the 

 artery {8.A.) may not pass into 

 the ventricle during its relaxa- 

 tion, there is another valve (not 

 indicated in Fig. 8), at the base 

 of the artery. 



An arrangement like that so 

 far described is found in fishes. 

 The heart consists in them of 

 an auricle, into which is returned 

 the blood that has traversed the 

 body, and of a ventricle which 

 moves it on again. But the 

 blood that has traversed the 

 body is on that account poor in 

 oxygen, and consequently unfit 

 to be circulated again when it is 

 returned to the heart. It is 

 necessary for it to take up fresh 

 oxygen before being circulated again. In fishes this 

 difficulty is met by the blood, poor in oxygen, which 

 flows out of the ventricle, first going to the gills and 

 streaming through them. The gills consist of a very 

 large number of small, thin-walled outgrowths arranged 

 in regular rows on the firm gill-arches. The blood, 

 poor in oxygen, passing out of the ventricle and 

 through various arterial branches to the gill-filaments, 

 takes up fresh oxygen as it streams through these 

 from the oxygen dissolved in the water which con- 

 stantly surrounds them. For this purpose a stream 

 of pure water is regularly taken in by the mouth and 



Fig, 8. — Diagram of the Heart 

 in a Fish. 



