42 HIGH SCHOOL ZOOLOGY. 



last-mentioned duct, is the duct of the pancreas, a gland of 

 different structure and function, which is independent of the 

 liver in higher animals, but in many fishes, is either wholly 

 or partly concealed within it, entering the frame work of the 

 liver beside the portal vein, the chief blood vessel of that organ, 

 through which, as we shall hereafter see, a considerable amount 

 of the blood of the body passes on its way to the heart. 



54. The liver like the intestine, is within the peritoneum and 

 covered with a serous coat like the intestine, but the air-bladder 

 which we have now to examine is only covei'ed by the pe- 

 i-itoneum on its ventral surface. It is therefore an extra- 

 peritoneal structure ; it communicates with the hinder end 

 of the oesophagus by a narrow tortuous duct which lies between 

 the folds of the mesentery, and enters the air-bladder a little 

 in front of its middle. As the air-bladder is a recess or diverti- 

 culum of the intestine, we should expect to find a similar arrange- 

 ment of its coats. As a fact however, the muscular coat is 

 merely represented by a stout white opaque layer, in which only 

 connective and no muscular tissue is present, while this sepa- 

 rates with great readiness from the inner mucous coat on 

 account of the scantiness of the submucous tissue. When the 

 air-bladder is first exposed it appears to be undivided, but when 

 the outer coat is removed there is seen to be a partition sub- 

 dividing the hinder part into two cavities, and narrowing the 

 apertures by which theso communicate with the single anterior 

 cavity. The inner coat can readily be removed without ruptur- 

 ing it, and then the three communicating compartments are 

 readily seen : it is at the junction of these that the duct 

 entei"s. As for the structure of the mucous coat, it is 

 very unlike that of the intestine, for its connective-tissue layer 

 is very delicate, very poorly supplied with blood-vessels, and its 

 epithelial layer, is formed of thin pavement-like hexagonal 

 cells. Only the outer coat is connected with the altered verte- 

 brae and their processes as described in § 47. 



