PISCES. 



983 



SalmonidtE, they are extremely numerous, wliile 

 in other races, the PleuronectideB for example, 

 there are only two coeca attached to the pylorus. 

 That the pyloric appendages are strictly analo- 

 gous to the pancreas, exhibiting that gland in 

 its simplest condition, such as it presents in 

 the embryonic state of the higher animals, is 

 proved by the gradual transition that may be 

 traced from the condition in which they exist 

 as above described, and the undoubted pan- 

 creatic gland met with in the more highly 

 organized cartilaginous Fishes. Thus in the 

 Cod-fish (Gadus morrhua) the pancreatic 

 cueca unite together as they approach the duo- 

 denum, so as to communicate with that intes- 

 tine by comparatively few orifices situated in 

 the vicinity of the pylorus ; and in the Stur- 

 geons this coalescence of the ccecal tubes is 

 still more conspicuous, for in that race of 

 Fishes the pyloric appendages are very short, 

 and so connected together by vessels and cel- 

 lular membrane into a single mass, that they 

 here present a. precisely intermediate condition 

 between the free cceca of the osseous Fishes 

 and the conglomerate pancreas possessed by 

 the Sharks and Itays. 



In the plagiostome cartilaginous Fishes last 

 named, the pancreas is in fact a true conglo- 

 merate gland, resembling that of Quadrupeds, 

 and in the same manner pouring its secretion 

 into the intestine through a single excretory 

 duct, which runs obliquely for a considerable 

 distance between the coats of the duodenum, 

 and terminates in the vicinity of the dttctus 

 c/ioledochus. 



Liver. The liver of Fishes is generally of 

 very great relative size, and frequently contains 

 in its tissue such an enormous quantity of oil, 

 that this alone forms an important article of 

 commerce. Its texture is exceedingly soft, and 

 liom the arrangement of the vessels in its inte- 

 rior sometimes exhibits a fibrous appearance. 

 The lobes of which it consists are generally 

 very numerous, but there is always a gall-blad- 

 der, from which the bile is poured into the 

 intestines through a single duct, which termi- 

 nates in the duodenum near the pylorus. 



Spleen. This viscus, which is peculiar to 

 the Vertebrata, first makes its appearance as 

 we trace the animal series upwards in the class 

 under consideration. In Fishes it is of very 

 variable size, but is always present. Its usual 

 position is among the folds of the intestines, 

 and its relations with the stomach are so diffe- 

 rent to what obtains in the mammiferous races, 

 that its functions cannot be in any way influ- 

 enced by pressure caused by the distension of 

 that organ. In its supply of arterial blood, 

 which is subsequently transmitted to the portal 

 system, it differs in no essential particular from 

 what occurs in the other vertebrate classes. 



All the above viscera are lodged in the cavity 

 of the abdomen, in which they are suspended 

 by numerous irregular folds of peritoneum ; 

 the generative and urinary organs, as well as 

 the swimming bladder hereafter to be described, 

 being situated beneath the spine external to the 

 peritoneal sac, the peritoneum only covering 

 their anterior surfaces. In many Fishes the 



peritoneum does not form a closed bag, as is 

 the case with the serous membranes in general, 

 but on the contrary is penetrated by two large 

 orifices situated in the vicinity of the anus, 

 through which the peritoneal membrane be- 

 comes continuous with the external integument, 

 and in this respect assimilates a mucous sur- 

 face. Such is the case in the Sturgeons, Lam- 

 preys, Salmons, Sharks, and Rays : in the two 

 last genera, indeed, the connection is still fur- 

 ther extended by two orifices, through which 

 the peritoneal bag communicates with the ca- 

 vity of the pericardium. The mesentery is 

 in Fishes very incomplete, consisting merely 

 of irregular bands, which enclose the principal 

 bloodvessels and unite the viscera to each other. 

 Sometimes there are processes filled with oily 

 fat representing the omentum. 



In the Brtmc/iiostouia the oesophagus com- 

 mences at the bottom of the buccal cavity be- 

 hind the opening of the branchial sac, between 

 the latter and the spinal axis, and after a short 

 course terminates in a simple dilatation, which 

 forms the stomach. About the middle of the 

 branchial sac the alimentary tube becomes im- 

 bedded in the liver, and terminates in a delicate 

 intestine, which extends to the anal opening. 



Lymphatic system. The principal lacteal 

 vessels are situated near the large branches 

 of the cceliac and mesenteric arteries and veins, 

 and the large lymphatic trunks derived from 

 the spleen, liver, and pancreas, accompany the 

 chief bloodvessels of those parts.* 



The lacteals and lymphatics of the assistant 

 chylopoietic viscera are much larger in propor- 

 tion to the bloodvessels than in Quadrupeds, 

 Birds, or even in Reptiles. Their branches 

 communicate with each other freely and repeat- 

 edly, and instead of uniting into one or two 

 trunks, they form a right and left plexus, which 

 are continued undiminished in size till they are 

 about to join with the lymphatic system of the 

 rest of the body. Neither the lacteal nor 

 lymphatic vessels are quite cylindrical, but by 

 being contracted a little in many places seem 

 to be jointed, so that the anatomist would ex- 

 pect to find numerous valves in their course, 

 yet these are entirely wanting, except at the 

 termination of the whole system. 



The lacteals terminate in a very remarkable 

 structure, which is situated along the great 

 curvature of the stomach. This consists of an 

 elongated viscus, the interior of which is en- 

 tirely cellular, so that, when prepared by infla- 

 tion and drying, its internal texture resembles 

 the cancellated structure of bones, being com- 

 posed of a great many cells of very irregular 

 shapes, and all communicating with each other, 

 so that probably this cellular viscus may per- 

 form the office of the absorbent glands of other 

 animals, which in Fishes are totally wanting. 

 Pursuing the right and left plexuses formed by 

 the lacteals and lymphatics of the chylopoietic 

 organs, we are led upwards along the sides 

 and back part of the oesophagus to the sides of 

 the spine and of the inferior venae cavae, and 



* Monro, Structure and Physiology of Fishes, 

 fo), 1785. 



