DIGESTIVE APPARATUS. 651 



siderable distention ; but occasionally, as for example in the Mullets, its 

 muscular walls are so thick that it might almost deserve the name of 

 gizzard, and in such fishes its power of crushing the food is no doubt 

 considerable. 



(1779.) The intestinal canal in the osseous fishes is a simple tube 

 (fig. 317, i), folded in sundry gyrations proportioned to its length ; but 

 in the cartilaginous families, such as the Sharks, the Kays, and the 

 Sturgeons, it presents internally a very remarkable arrangement, evi- 

 dently intended to increase the extent of surface over which the digested 

 aliment may be spread, for the purpose of absorbing its nutritive por- 

 tions. In these tribes a spiral valve (fig. 327, 7i) winds in close turns 

 from the pyloric to the anal extremity of the capacious intestine ; so 

 that, although externally the intestine appears short in proportion to 

 the size of the animal, its mucous lining is exceedingly extensive. 



(1780.) In addition to the biliary secretion which we have met with 

 in the lower animals, another system of chylopoietic glands for the first 

 time makes its appearance in the class before us, from which a fluid 

 termed the pancreatic is poured into the intestine. In the osseous fishes 

 this viscus presents the simplest condition of a gland, consisting of 

 simple caeca (fig. 317, n ri); sometimes, as in the Perch, only three in 

 number ; at others, as for instance in the Salmonida?, extremely nume- 

 rous. Prom these appendages a glairy fluid, resembling saliva in com- 

 position, is abundantly secreted, and becomes mixed with the bile im- 

 mediately upon its entrance into the intestine. 



(1781.) In the cartilaginous fishes, such as Sharks and Rays, the 

 pancreas exhibits a more perfect development, and already presents the 

 appearance of a conglomerate gland (fig. 327, /), from which the pan- 

 creatic fluid is conveyed into the intestine through a common duct. 



(1782.) The liver of fishes is proportionately very large, and generally 

 contains abundance of oil. The bile derived from it is received into a 

 gall-bladder (fig. 317, c), from which a duct of variable length in different 

 species conveys it into the intestine, in the immediate vicinity of the 

 pylorus. 



(1783.) It is in these animals that we for the first time find the 

 biliary secretion separated from venous blood ; and consequently they are 

 provided with a new arrangement of the blood-vessels of the abdomen, 

 which they possess in common with the other Vertebrata, forming what 

 is termed by anatomists the system of the vena portce. The veins de- 

 rived from the stomach, the intestines, and the spleen, which last viscus 

 now makes its appearance, instead of conveying their contents to the 

 heart, plunge into the substance of the liver and there again subdivide 

 into capillary tubes, thus furnishing to the liver abundance of venous 

 blood, from which the hepatic secretion is elaborated. 



(1784.) The spleen, now for the first time met with in the animal 

 creation, is a highly vascular organ, generally enclosed in the mesentery 



