176 STRUCTURE OF FISH. 



purpose, the heart of a fish is situated very near the 

 gills ; and sends off from its ventricle one artery, which 

 is ultimately ramified over the whole fibrous mass of 

 the gills in a very minute manner, and forming a tissue 

 which is very tender and sensible, and bleeds profusely 

 when lacerated. The surface which the gills present 

 to the water is very great ; for Dr. Monro, whose re- 

 searches threw much light upon this curious branch of 

 Natural History, calculates that those of a large skate 

 at 2250 inches, about equal to the whole surface of a 

 man's body. 



In the cartilaginous bodies, which have their skele- 

 tons comparatively soft and pliable, and are therefore 

 without distinct joints, the gills are fixed ; while in 

 bony fishes they are free ; each gill, or mass of fringe, 

 being attached to a separate curved and moveable bone. 

 The gills are, with at least few if any exceptions, open 

 to receive the water from the mouth only. The fila- 

 ments float backwards from the bones, and the action 

 is produced from the motion of the gills themselves 

 and the gill-covers and the gill-flaps in which these termi- 

 nate in some species. If the water enters the gills from 

 behind, the filaments appear to get entangled, the cir- 

 culation of the blood is stopped, and the fish is stran- 

 gled, or as it is usually called, drowned. The very 

 same takes place when, by wounding the muscles that 

 move the breathing apparatus, the motion of the gills is 

 prevented, and also, when the application of any caustic 

 substance, such as quick-lime, destroys the surface. 

 The breathing apparatus of fishes is thus liable to be 

 deranged both by mechanical and by chemical injuries. 



It is impossible to contrast this complicated respi- 



