356 RESPIRATION. 



no special respiratory organ, the interchange of gases being 

 effected through the general surface. Higher in the animal 

 scale, special organs are found, which are called gills, when 

 the animals live under water and respire the air which is in 

 solution in the water, and lungs, when the air is introduced 

 in its gaseous form. 1 Animals possessed of lungs have a tol- 

 erably perfect circulatory apparatus, so that the blood is 

 made to pass continually through the respiratory organs. In 

 the human subject and warm-blooded animals generally, the 

 lungs are very complex, and present an immense surface 

 by which the blood is exposed to the air, only separated from 

 it by a delicate permeable membrane. These animals are 

 likewise provided with a special heart, which has the duty of 

 carrying on the pulmonary circulation. Though respiration 

 is carried on to some extent by the general surface, the lungs 

 are the important and essential organs in which the inter- 

 change of gases takes place. 



The essential conditions for respiration in animals which 

 have a circulating nutritive fluid are : air and Nood, sepa- 

 rated l}y a membrane which will allow the passage of gases. 

 The effete products of respiration in the. blood pass out and 

 vitiate the air. The air is deprived of a certain portion of its 

 oxygen, which passes into the blood, to be conveyed to 

 the tissues. Thus the air must be changed to supply fresh 

 oxygen and get rid of the carbonic acid. The rapidity of 

 this change is in proportion to the nutritive activity of the 

 animal and the rapidity of the circulation of the blood. 2 



1 Insects have no lungs ; but the air is disseminated throughout the organism 

 by a system of air-bearing tubes (true arteries), or trachea?, and is probably ap- 

 propriated directly by the tissues, without the intervention of the blood. 



2 The manner in which this change of air is effected in the different classes of 

 animals constitutes one of the most interesting subjects in comparative physiology. 

 Its study has shown how, as we pass from the lower to the higher orders of ani- 

 mals, and the functions become more active, a division of labor takes place. 

 Functions which in the lowest animals have no special organs, one part, as the 

 integument or alimentary track, performing many offices, in the higher classes 

 are assigned to special organs, which are brought to a high condition of develop- 



