16 RESPIRATORY APPARATUS OF FISHES. 



Finally, the venous blood is not all directly returned into the 

 sinus, which we have before mentioned ; for that of the intes- 

 tines and of some other parts, before being sent back into the 

 heart, is distributed by the portal vein through the liver. We 

 see, then, that the blood, in passing through the circle of the 

 circulation, entirely traverses the respiratory apparatus, as 

 amongst the Mammalia and Birds ; but in this course it only 

 passes once into the heart, which consequently renders its pro- 

 gress much slower. The heart itself corresponds in its functions 

 to the right half of the same organ amongst the higher Yertebrata. 



536. The Respiration of Fishes is effected by means of the 

 air dissolved in the water ; and takes place at the surface of a 

 multitude of projecting and very vascular membranous plates, 

 fixed on the external edge of the branchial arches, the position of 

 which has been already pointed out. In general there are on either 

 side four gills, each composed of two rows of lengthened plates. 

 In several of the Cartilaginous fishes, there are five, and in the 

 Lamprey we find seven. Amongst nearly all the Osseous fishes 



these plates are simple, and 

 fixed at the bottom only; in 

 a small number, such as the 

 Hippocampus^ commonly call- 

 ed Sea Horse, they are, on 



FIG. 259. HIPPOCAMPUS. .- , , 



the contrary, rammed, and 



somewhat resemble bunches of feathers. Lastly, amongst the 

 greater part of the Cartilaginous fishes, such as the Rays and 

 Sharks, they are fixed to the skin by their external edges, as 

 well as to the branchial arches by their internal. 



537. The water necessary for respiration enters into the 

 mouth, and by a movement of deglutition is forced through the 

 openings which exist between the branchial arches, and thus 

 arrives at the gills ; after bathing the surface of these, it escapes 

 outwards by the opening of the gills. "We see, in fact, the 

 animal alternately opening the mouth, and raising its operculum. 

 Amongst those fish whose gills are free at their external edge, it 

 is sufficient to have one of these openings on each side ; but when 

 the gills are fixed, it is necessary for the exit of the water to 



