THE COD. 115 



XXVI. Open the air-bladder by a longitudinal incision 

 along its left side, and dissect away the muscles, 

 &c., which still obscure its anterior end, taking 

 care not to injure the precaval vein (§ 141) ; note 



138. The characters of the air-bladder: its lateral and 

 ventral walls are very thick and tough, its dorsal wall so 

 thin that the vertebral column, aorta (§ 142), and kidneys 

 (§ 143) can be readily seen through it ; its ventro-lateral 

 portions are deeply sacculated, and its antero-lateral angles 

 produced each into a long coiled tube, blind at its free end ; 

 of these the left only is seen at present, imbedded among 

 the muscles at the anterior end of the air-bladder. 



139. The rete mirabile of the air-bladder, a large, soft, 

 cake-like mass of rounded outline and red colour, situated 

 within the air-bladder in contact with its ventral wall ; as 

 already seen, it receives a branch from the mesenteric artery 

 and pours its blood into the portal vein. 



140. The anterior portion of the kidney — usually called 

 the head-kidney — a dark-brownish red mass, lying above 

 and in front of the anterior end of the air-bladder. 



141. The (left) precaval vein (ductus Cuvierii) (Fig. 32, 

 pc), a large vein passing from the sinus venosus upwards 

 to the anterior part of the kidney, crossing the gullet as it 

 goes, and lying immediately behind the last branchial arch ; 

 it receives the corresponding spermatic vein, and at its 

 dorsal end is formed by the confluence of two veins, the 

 left jugular bringing the blood from the head, and the 

 left cardinal returning that from the trunk generally. 



142. The dorsal aorta (Fig. 32, d.ao\ a large median 

 artery, seen through the thin dorsal wall of the air-bladder; 

 to show it plainly, the latter should be dissected away. 



The dorsal aorta is continued posteriorly as the caudal artery [c.a), 

 which passes through the hjemal arches of the tail ; before doing so it 



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