DIGESTIVE SYSTEM OF FISHES. 



233 



the return of such fishes or mollusks as may have been SAvallowed 

 alive and uninjured by the small obtuse teeth of this great Shark. 

 In many Osseous Fishes Ave may, finally, notice the communication 

 of the ' ductus pneumaticus ' Avith the oesophagus, usually by a small 

 simple foramen ; but provided with special muscles in the Lepidosteus, 

 where it opens upon the dorsal aspect of the oesophagus, and Avith a 

 sphincter and cartilage in the Polypterus, and Lepidosiren, Avhere 

 it communicates like a true glottis Avith the ventral surface of 

 the beginning of the oesophagus. In the Globe-fishes {^Diodon, 

 Tetrodoii) the great air-sac seems to be a more dii'ect development, 

 as a cul de sac, of the oesophagus (prep. 2095.). These singvdar fishes 

 blow themselves up by sAvallowing the air, Avhich escapes through a 

 large anterior oblique orifice into the sac : and this again communi- 

 cates Avith the fore-part of the oesophagus by a second orifice much 

 smaller than the first, and having a tumid A^alvular margin. * 



The cardiac orifice of the stomach is occasionally defined by a con- 

 striction, as in the Planirostra (yfig. 61. e), and Mormyrus ^fig. 63. e) : 

 but an increased expansion with increased vascularity and a more 

 delicate epithelial lining of the mucous membrane more usually in- 

 dicate, in Fishes, the beginning of the digestive cavity. The stomach 

 is a simple and commonly an ample cavity, with a great disproportion 

 in the diameters of the cardiac and pyloric orifices ; in the Cornish 

 Porbeagle- Shark, for example, the cardiac entry will readily admit a 

 child's head, Avhilst the pyloric outlet will barely allow of the passage 

 of a crow-quill. 



There are two predominant forms of the stomach in Fishes, viz. the 

 ' siphonal' and the ' caecal ;' in the first it presents the form of a bent 

 tube or canal, as in the specimens f from the Turbot, Flounder, Sole, 

 Cod, Haddock, Salmon, Carp, Tench, Ide, Lump-fish, Lepidosteus, 

 Sturgeon, Paddle-fish {Jxfj. 61. ^, /), and most Plagiostomes ; in the 



Digestive Organs in situ. Planirostra. 



second form the cardiac division of the stomach terminates in a blind 



* Lxvi. t. iii. p. 271. pi. 47. 



■f- Reference was made to prcparntions or recent dissections on tlie lectnre- 

 1al)lc. 



