ORGANS OF SECRETION. 597 



beyond the pyloric valve, and close to the opening of the 

 pancreatic duct. Besides the hepatic artery, common to the 

 invertebrated classes, in which it alone furnishes the materials 

 of the bile as well as the nourishment of the liver, this organ in 

 fishes is supplied with several portal veins, derived chiefly from 

 the chyloposetic viscera, which ramify with the hepatic artery 

 along the pinnated tubuli of the liver, affording the bile from 

 the mixed blood of their capillaries. The portal veins are 

 less extensively ramified through the liver in fishes and the 

 lower vertebrata than in the higher warm-blooded classes, as 

 in the earlier conditions of the human embryo. There are 

 frequently hepato-cystic ducts in fishes which convey the 

 bile directly into the fundus of the gall-bladder, as well as 

 the usual hepatic ducts leading to the cystic ; and sometimes 

 as in the orthagoriscus, the bile is still poured directly into the 

 cavity of the stomach, as in most of the invertebrated classes. 

 From the aqueous nature of the pancreatic secretion, that 

 gland, like the salivary, is little required in fishes, and is 

 there found, as in the mollusca, chiefly in the simplest folli- 

 cular form, occurring as a single pyloric follicle in the ammo- 

 dites, and arriving at the most conglomerate form in the highest 

 cartilaginous fishes. In some fishes where the free pancreatic 

 tubuli are still wanting, as the cyprinus, esox, murrena and 

 ophisurus, their place is supplied by a simple cellated or 

 follicular structure of the internal lining of the duodenum. 

 The pancreatic follicles of this class open into the duodenum, 

 close to the biliary duct, beyond the pyloric valve, by a most 

 variable number of orifices, according to the degree of de- 

 velopment or concentration of the organ. The follicles, when 

 simple, are commonly cylindrical, aggregated into groups, and 

 open into the duodenum by several apertures sometimes 

 nearly as wide as the intestine itself; but in the most con- 

 glomerate form of this organ, in the plagiostome fishes, the 

 common duct of all the minute tubuli composing the lobules 

 of the pancreas, terminates by a single and narrow orifice. 

 The spleen first makes its appearance in the class of fishes ; 

 it is sometimes wanting as in the lamprey, the myxene, and 

 the gastrobranchus, generally it is small and single, but in 

 some cartilaginous species it is divided into detached lobes, 

 as in some of the cetacea ; it is appended to the left side of 

 the stomach, and is highly vascular and largely supplied with 

 lymphatics. 



