546 ORGANS OF RESPIRATION. 



symmetrical as in ophidia, are here always placed in the dorsal 

 region of the trunk, as in the highest pulmonated mollusca. 

 Some vertebrata, as the lowest fishes, are exclusively 

 branchiated; others, as reptiles, birds, and mammalia, are 

 exclusively pulmonated ; others pass, by development, from 

 the one mode of respiration to the other; others remain 

 pulmo-branchiated, or truly amphibious, through life. But 

 the temperature of the blood, and the energy of the muscular 

 organs, directly correspond, in all, with the extent and acti- 

 vity of the respiratory system. 



Although thus elevated in the scale, the respiration of 

 fishes, like that of the lowest worms, is only aquatic and 

 branchial ; but in order to render this mode of respiration as 

 effective as possible, the surface of these aquatic organs is 

 here greatly extended, the surrounding element is rapidly 

 renewed on their exterior, the whole blood of the system is 

 sent through them for aeration, and its velocity is accelerated 

 by the entire force of the heart. The respiratory organs of 

 fishes consist of variously formed gills, attached to osseous 

 or cartilaginous branchial arches, extending from the sides of 

 the os hyoides upwards to the sides of the cranium. In the 

 osseous fishes, there are four of these branchial arches sus- 

 pended, free and moveable, under a wide opercular covering 

 on each side of the neck. The branchiae consist of numerous 

 small, flat, tapering, cartilaginous laminae, approximated closely 

 to each other, bifurcated at their free ends, or cleft to their 

 base, and arranged in a regular series, like the teeth of a comb, 

 over the whole exterior convex margin of each branchial arch. 



Over the very extensive surface presented by these 

 innumerable, free, pendent laminae, the whole venous 

 blood of the system, sent from the heart and bulbus 

 arteriosus, is conveyed by the minute ramifications of the 

 branchial artery. The water inhaled by the mouth, is con- 

 veyed backward by an act like deglutition, and escapes on 

 each side, between the separated branchial arches, and over 

 all the vascular laminae of the gills ; and thus the venous 

 blood of fishes is oxygenated, and decarbonized, and fur- 

 nished with nitrogen for the gaseous secretion of their air-sac. 

 Vibratile cilia have been detected both on the external and 

 internal mucous surfaces of fishes. Some fishes are observed 

 to swallow atmospheric air, to be conveyed through the 



