RESPIRATION. 



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general plan, being essentially membranous prolongations of the 

 external surface, adapted by its permeability and vascularity to 

 bring the blood into close relation with the surrounding medium. 



As this medium may be either air or water, we find two principal 

 forms of respiratory organs, one of which is adapted for each. In 

 aquatic animals the membrane is usually prolonged externally into 

 tutls or fringes, each one of which is supplied with arteries and veins, 

 during the circulation through which the aeration is accomplished. 

 These organs are called gills. 



In air-breathing animals the aerating surface is reflected inwardly, 

 forming passages or chambers in which the air is received, and on 

 which the capillary vessels are distributed. 



In insects we find a series of tubes called trachecs, ramifying 

 through the body and carrying air to every part of it. 



In regard to the human lung, according to the observations of 

 Mr. Rainey, a bronchus, when traced from its commencement to its 

 termination, is seen in the first part of its course, to be more or less 

 cartilaginous ; it then becomes destitute of cartilage, retaining, how- 

 ever, a perfectly circular form and having no air-cells opening into 

 it ; farther on, being still circular, numerous air-cells open into it ; 

 lastly, the air-cells increase so much in number, and open into the 

 -bronchus so closely to one another, that the tube can no longer retain 

 its circular form, but becomes reduced to an irregular passage 

 running between the cells, and ultimately reaching the surface of the 

 lobule, ends by forming a terminal air-cell. 



The air-cells are small irregularly shaped cavities, having usually 

 four or five unequal sides ; those which are situated close to the small 

 bronchial passages open into them by well-defined circular aper- 

 tures, while those at a distance from these passages open one into 

 the other, as in the lung of the frog and serpent ; in fact each lobule 

 of the lung of the mammal and man, with its bronchial passages and 

 appended cells, may be regarded as a repetition of the whole lung 

 of the frog. 



The sides or walls of the air-cells are formed of a thin transparent 

 membrane, and the capillary vessels are so placed between the walls 

 of two adjacent cells as to be exposed on their two sides to the action 

 of the air. The number of capillary plexuses is not the same as that 

 of the air-cells, one network passing between and supplying several 

 cells ; or in other words, one terminal branch of the pulmonary artery 

 supplies the plexuses of several cells. 



It has been calculated by M. Rochoux that as many as 17,990 

 air-cells are clustered around each terminal bronchus, and that their 

 total number amounts to 600 millions. 



The foetal lungs, according to Mr. Rainey, prior to respiration are 

 distinctly seen, when injected, to possess air-cells fully formed, and 



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