79 



of the fish : others, as the lamprey, receive the wa- 

 ter into their gills by the mouth, and expel it through 

 several holes in their sides, while those fishes that 

 move rapidly, and make long migrations, take in the 

 water largely by the mouth, and reject it very often by 

 the gills. The gills were by the ancients considered 

 to perform an office for fishes similar to that which 

 the lungs perform for land-animals ; and Mayhow 

 conceived them to be especially constituted for se- 

 parating the air from water, whereby some vital 

 aerial property was conveyed into the mass of blood. 

 Hence it is, he adds, that fishes alternately draw in 

 and expel water, as land-animals receive and expire 

 air *. 



65. But a somewhat more particular description 

 of the structure of the gills of fishes will much as- 

 sist our conception of the manner in which the re- 

 spiratory function is performed by them. In fishes, 

 the heart consists only of one auricle and one ven- 

 tricle ; and from the latter, one artery is sent off, 

 which is spent entirely on the gills. This artery 

 conveys venal blood from the heart, which, in its 

 passage through the gills, assumes a florid hue, and 

 being afterwards collected by the branchial veins 

 into one large trunk, is distributed, without the inter- 

 vention of a second heart, to all parts of the body ; 

 from which it is again brought back to the heart in 

 a venal state, to undergo the same circulation. The 

 gills, upon which the branchial artery ramifies, are 

 of great extent. In each side of the body of a skate, 



* Tractat. Quinq. p. 259. 



