FISH IN GENERAL. 85 



ment they are erected, and their base grows speedily hard and 

 resisting. 



Deglutition. Most osseous fishes having no muscles in 

 the lips, or if they be fleshy, none proper to retain the ali- 

 ment in their mouths, have on the inside of each jaw, behind 

 the anterior teeth, a kind of membranous valve, formed by 

 means of a fold of the internal skin, and turned towards the 

 gullet, preventing in particular the water taken into the mouth 

 for respiration from escaping back again. Aliment also, held 

 by the teeth of the jaws, retained by the valve, carried further 

 back by the teeth of the palate and tongue, when they are 

 found in the species, is prevented passing between and injur- 

 ing the respiratory apparatus, by the denticulations upon the 

 branchiae. The action of the jaw and of the tongue operates 

 only in the direction of the pharynx, where the food is further 

 acted upon by the pharyngeal teeth, which both triturate 

 and force it onwards into the oesophagus. This viscus is 

 covered with a layer of strong muscular fibres, closely joined, 

 and forming sometimes divers fasciculi, whose contractions 

 force the alimentary bolus towards the stomach, and thus 

 complete deglutition, for fishes being destitute of neck, the 

 oesophagus is necessarily very short. 



Intestinal canal. The digestive organs are inclosed in the 

 abdominal cavity, which is separated forwards from that con- 

 taining the heart, by means of a kind of diaphragm of small 

 dimensions, composed of extensions of the pericardium and 

 peritoneum, and containing within its texture the great 

 venous sinus. Another cavity extends along the spine, con- 

 taining the kidneys and the swim-bladder. It is divided from 

 the abdomen, properly so called, by the peritoneum, which 

 as in other animals forms also a fold to suspend the viscera 

 it contains, namely, the intestinal canal, the liver, spleen, and 

 pancreas, when there is one. The organs of generation and 



