ZOOLOGY 



SECT. 



s^; 



The blood- vascular system attains a far higher degree 



o f complexity 

 than in any of 

 the groups pre- 

 viously studied : 

 its essential fea- 

 tures will be best 

 understood by a 

 general descrip- 

 tion of the circu- 

 latory organs of 

 Fishes. 



The heart (Figs. 

 769 and 781) is 



IfcUai 



S*w*i" 



SSI^I 



a muscular organ 

 contained in the 

 pericardial cavity 

 and composed of 

 three chambers, 

 the sinus venosus 

 (s. v.), the auricle 

 (au.), and the ven- 

 tricle (v.), which 

 form a single 

 longitudinal 

 series, the hind- 

 most, the sinus 

 venosus, opening 

 into the auricle, 

 and the auricle 

 into the ventricle. 

 They do not, 

 however, lie in a 

 straight line, but 

 in a zigzag fash- 

 ion, so that the 

 sinus and auricle 

 are dorsal in posi- 

 tion, the ventricle 

 ventral. Usually 

 a fourth chamber, 

 the conus arterio- 

 sus (c. art.), is 

 added in front of 

 the ventricle. The 

 various chambers 

 are separated 



