362 ZOOLOGY. 



Can yon conceive any use it might be in steering, for the purpose of rising 

 or sinking in the water? What would be the effect of compressing the 

 air-bladder at one end more than at the other? 



378. The Circulation. Little needs be said here in addition 

 to what has been said in the general discussion of the verte- 

 brate circulation (see Fig. 164-169). The heart is two-cham- 

 bered. The auricle receives the venous blood from the system ; 

 it is passed to the ventricle through a valve which forbids its 

 passage in the reverse direction. From the ventricle the blood 

 passes through a valvular region into the ventral aorta, which 

 carries it, by a series of right and left branches, to the gills. 

 Here aeration takes place, the pure blood being gathered from 

 the gills by a series of efferent branches which combine (ex- 

 cept some anterior branches which go to the head) to form a 

 dorsal aorta. The dorsal aorta gives off branches to the body 

 wall, to the paired appendages, to the liver, digestive tract and 

 kidneys, continuing into the tail where it breaks up in the 

 muscles. The impure blood from the capillaries of the tail is 

 brought back to the kidneys by the renal portal vein, where it 

 again passes through capillaries ; here the blood is purified of 

 its urea and similar impurities. The blood supplied direct to 

 the kidneys from the aorta and that of the renal-portal circu- 

 lation is returned to the heart by way of right and left (cardi- 

 nal) veins which join corresponding right and left veins 

 (jugular) from the head to form the veins (ductus Cuvieri) 

 which empty into the auricle. The blood which was distributed 

 to the stomach and intestines is gathered into a vessel (hepatic 

 portal vein) which carries it to the liver, together with much 

 of the food absorbed from the intestines. The hepatic portal 

 vein here breaks up into capillaries. The blood from the liver 

 and from the appendages unites with that carried by the 

 ductus Cuvieri before it reaches the heart. The student should 

 carefully follow out the course of the circulation in the accom- 

 panying diagrams (Figs. 178, 179). Variations from this 

 typical condition are numerous, accounts of which must be 

 sought in more extended texts. 



