259 



are an infinity of very narrow passages, which receive 

 and divide the water taken in by the mouth, into mi- 

 nute particles. Then the air, its prison-doors being in 

 some measure opened, escapes and joins the blood of 

 all ihe little arteries. 



The gills have an alternate motion of dilatation 

 and compression. When they dilate, the water is 

 taken in ; when they contract, it is driven out. It 

 seems that in the instant of contraction, the air ex- 

 pressed from the water is forced in to the blood-vessels. 

 It is the same, as to our lungs. The air enters them 

 at the time of inspiration, but is received into the 

 blood at the tirae of expiration only. So that the 

 water which is taken hi f by the mouth of fishes, when 

 stripped of its air, is carried off by the gills. Meantime 

 the air which is thus gained is distributed first to 

 those fine ramifications of the arteries, wh'ich are ex- 

 panded upon the gill throughout, and then to the 

 Teins inosculated therewith. And fishes can no more 

 live without a constant supply of this, than land ani- 

 mals can. 



The gills in all fishes are eight, four on each side. 

 The lower gill is always smaller than the rest. The 

 other three on each side are gradually larger to the 

 top one, which is always the largest. Each of .these is 

 formed of a bony substance, bent into the shape either 

 of a semicircle or a bow. On the convex side of this, 

 there is a sort of plumes or leaves, each of which con- 

 sists of a double row of bony lamella, formed like so 

 many sickles, and fixed to the convex side of the bow 

 by means of the membrane wherewith it h covered. 



These lamella have one part convex, and the other 

 concave. The concave part of each lamella is applied 

 to the convex part of the next opposite lamella. 

 Every lamella is invested with a fine membrane^which 

 receives the ramifications of the blood-vessels. Every 

 gill has an artery, a vein and a nerve. The gills re* 

 ceive the blood which is thrown from the heart into 

 the aorta, and derive it to the utmost parts of the 



