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STOMACH AND INTESTINE. 



intestinal cavity. These have been supposed 

 to constitute a rudimentary pancreas.* 



The alimentary canal of Fishes is simple, 

 wide, and short, compared with that of other 

 Vertebrata. Its chief subdivisions are an 

 oesophagus, a stomach, and an intestine. 



The oeso'phagus is large, dilatable, and mus- 

 cular. Its mucous membrane is generally 

 simple, sometimes involuted or glandular ; 

 and offers a remarkable contrast to the redder 

 and more vascular membrane of the stomach 

 at the point of their junction. As the dia- 

 meter of the tube rarely undergoes any great 

 and sudden increase in this situation the above 

 contrast of structure is often the only distinc- 

 tion between the two cavities. 



The stomach varies greatly in size and 

 shape. Usually, however, it forms a curved 

 tube like a siphon. The obliteration of the 

 concave side of this tube converts it, by many 

 gradations in different genera, into the shape 

 of a two-necked flask, or of a blind tube with a 

 double orifice at one end. In other instances 

 it is dilated, or almost globular. Where tubu- 

 lar, it generally tapers away towards the p\ lo- 

 rus. And this end of the stomach, which is 

 usually more muscular than the cardiac, some- 

 times approaches the structure of a gizzard, 

 having constricted extremities, a thick muscu- 

 lar coat, and a scaly epithelium on its mucous 

 membrane. The valve itself is almost always 

 present, as a circular ridge of muscular fibre, 

 covered by a fold of mucous membrane. 



The intestine of the fish is short and wide : 

 and generally consists of two portions, which 

 are separated from each other by a slight con- 

 striction into a small and large intestine. The 

 first receives the bile-duct, and the follicles 

 which form the rudimentary pancreas. The 

 latter also occasionally receives a cascal tube. 

 The intestine has the usual three coats 

 serous, muscular, and mucous. The serous 

 membrane is often pigmentary, and its cavity 

 communicates by apertures with the exterior. 

 It rarely forms a continuous mesentery : the 

 attachment of the intestine being generally 

 ligamentous or filamentous, or even, as in 

 one instance f, by means of a mass of areolar 

 tissue that involves the whole tube. The 

 muscular coat is of unstriped fibres J, which 

 form two layers, the circular generally ex- 

 ternal.^ The mucous membrane is variously 

 folded : it sometimes contains ductless glands : 

 rarely it is ciliated. || The chief deviations from 

 these the ordinary characters of the intestine 



* Many anatomists, however, consider the office 

 of this gland to be fulfilled bv the cavity previously 

 mentioned. But strong arguments against this view 

 might be derived from the development of glands in 

 general, and of the pancreas in particular ; both in 

 the phases of individual life, and in the advance of 

 the animal series. In addition to this, the gastric 

 character of this cavity, and the unfitness of a giz- 

 zard for solvent or digestive functions, further justify 

 us in preferring the above interpretation. 



\ The Tetrodon mola. 



+ In the Tench (Cyprinus tincii), thej'are striped. 



Reversing their ordinary arrangement in the 

 Mammalia. 



|| As in the Branchiostoma. 



are, either an increase of length, which is some- 

 times accompanied by a diminution in width ; 

 or an equally real increase of active surface, 

 which is due to the development of folds, such 

 as the spiral valve of the Shark. 



The appendices pyloriccs, or pancreatic 

 follicles, are absent in many fishes. They 

 vary in number from one to two hundred. 

 In structure they range from simple, short 

 canals, to elaborate branches, which are united 

 by areolar tissue and vessels, and are enclosed 

 in a muscular tunic. 



The alimentary canal of Reptiles preserves 

 much of the simplicity, width, and shortness, 

 seen in that of Fishes. But it offers important 

 differences in many respects. The thick, 

 semi-transparent, gelatinous-looking intes- 

 tinal parietes generally possessed by the Fish, 

 are strongly contrasted with the thinner and 

 more condensed and opaque tube present 

 in the Reptile. Such a comparison seems 

 to indicate a great advance in the develop- 

 ment of the Reptilian digestive canal. This 

 advance, though no doubt correlative with that 

 of the tissues generally, probably depends 

 chiefly on the increased efficiency of the respi- 

 ratory function. 



The oesophagus varies greatly in size and 

 appearance. It is usually large and dilatable. 

 In the Ophidians this width and laxity 

 are so greatly increased, that it forms a tube 

 which can be distended to almost any extent ; 

 and the parietes of which are so thin, that 

 they may be regarded as supplanted by the 

 muscular parietes of the belly itself. 



The stomach rarely possesses any well-marked 

 cardiac constriction. Hence the characters of 

 its mucous membrane are the chief means by 

 which it can be distinguished from the oeso- 

 phagus. Its form, in the Chelonian and Ba- 

 trachian divisions, somewhat resembles that 

 seen in many fishes. Beginning by a dilated 

 cardiac pouch, it tapers away towards the py- 

 lorus, taking a curve like a siphon. In the 

 Crocodiles, the stomach may be regarded as 

 consisting of two portions. Of these, the first 

 is a gizzard : which resembles the form and 

 appearance of that of the Cuttle-fish ; and con- 

 sists, like it, of a plane of muscular fibres, that 

 radiate from a central tendon on each side of 

 the organ. The second is a small pyloric 

 pouch or diverticulum, which passes out of 

 the gizzard at its upper part, close to where 

 it receives the dilated oesophagus. In many 

 Serpents the pyloric extremity is so narrow 

 and muscular, that the organ has been distin- 

 guished into two parts : an upper, or cardiac, 

 which is thin and saccular ; and a lower, or 

 pyloric, which is narrow, strong, and tubular. 

 The pyloric valve varies in development. But 

 even where best marked, it never approaches 

 the distinctness seen in man and the higher 

 Mammalia. It consists, as usual, of a pro- 

 jection, which is formed by the circular 

 muscular fibres, and is covered by a fold of 

 mucous membrane. 



The intestine is short, and rather wide. It 

 is usually divided into small and large by a 

 circular constriction or valve. 



