PHYSIOLOGY. 



viz., oxygen. This is, as it were, a second repeated killing of the 

 nutriment, or, in its true sense, a real consuming of it. Where this 

 consuming attains its culmination the blood and consequently the whole 

 body becomes warm, and thence arises, at least chiefly, the uniform heat 

 of birds and mammalia. 



A distinct organ of respiration is entirely wanting in the lowest 

 animals, viz., in the Infusoria, Polypi, Acaleplice, and many of the 

 intestinal worms ; and if they really breathe it can only be by means 

 of the exterior integument, in the same way as the internal skin 

 imbibes the nutrimental juices from the food. The first instance of a 

 true respiratory apparatus speaks in favour of this opinion, for where 

 found it is a continuation of the exterior integument, a sort of tufted 

 or ramose fold of the skin, which projects into the medium, loaded with 

 oxygen. Such respiratory organs, which are called branchiae, we find 

 in the muscles, the majority of snails, and in all the Crustacea, and 

 even among fishes and the naked amphibia, either throughout their 

 whole lives or during the time they remain in the water. The respiratory 

 organ being merely at one part of the body, a motion of the juices to 

 this spot is requisite, and thus originate the vessels as new organs con- 

 necting the functions of the intestinal canal and branchiae. Vessels 

 must consequently be found in all animals with a partial respiratory 

 apparatus, and they may therefore be deficient in such as have this 

 apparatus universally distributed. 



If the fold of skin which becomes developed to the respiratory organ 

 pass inwardly, it is then called not gill, but lung (pulmo}. The 

 medium, which is generally the air that contains the oxygen, is received 

 into the lung, wherein the oxygen becomes incorporated with the nutri- 

 tive fluid. This also is in general merely partial, and then consists of 

 membranous bags, which in its highest grade of organisation consists 

 of a web of small cells, that by degrees unite into common ducts, the 

 last and largest of which, the trachea, opens outwardly. Vessels convey 

 the nutritive fluid (the blood) to the surface of these cells and bags, 

 and by means of other vessels it is conducted hence to all the parts of 

 the body. These organs of respiration are common to the majority of 

 amphibia, all the birds, and mammalia ; their first indication is found 

 in the pulmonary Mollusca and in the Arachnida, A universally dis- 

 tributed lung, the analogue of the similar branchia, would require no 

 vessels, as the oxydisation of the nutritive fluid Avould take place 

 everywhere. We also absolutely find that animals whose body is 



