80 



says Dr Monro, there are four double gills, or gills 

 with two sides each, and one single gill ; or there 

 are in all eighteen sides or surfaces on which the 

 branchial artery is spread out. On each of these 

 sides, there are about fifty divisions or doublings of 

 the membrane of the gills ; and each division has 

 on each side of it one hundred and sixty subdivi- 

 sions or folds of its membrane, the length of which, 

 in a very large skate, is about one-eighth part of an 

 inch, and its breadth about one-sixteenth part : so 

 that in the whole gills there are about 1 44,OOO sub- 

 divisions or folds, the two sides of each of which are 

 equal to the sixty-fourth part of a square inch j or the 

 surface of the whole gills in a large skate is equal to 

 2.250 square inches, that is, to more than fifteen 

 square feet, which have been supposed equal to the 

 whole external surface of the human body. When, 

 after a good injection of the branchial artery, a mi- 

 croscope is applied, the whole extent of the mem- 

 brane of the gills is seen covered with a beautiful 

 network of exceedingly minute vessels ; and if dis- 

 tilled oil of turpentine, coloured with vermilion, has 

 been injected with moderate force in a living or re- 

 cently dead skate, some of the colourless parts of 

 the oil exude upon the surface of the gills *. From 

 all these facts, concerning the respiration of fishes, 

 we learn, that the oxygen gas of the air contained 

 in water, is changed (63.) into carbonic acid by the 

 medium of their gills ; and that their blood, like 

 that in the lungs of breathing animals, loses at 



* Monro on the Structure, &c. of Fishes, p. 



