RESPIRATION 195 



possessing both gills and a kind of lungs, the swim-bladder being 

 surrounded with a plexus of bloodvessels and taking on a respiratory 

 function. 



In all the higher vertebrates the respiration is carried on by lungs ; 

 the trifling amount of gaseous interchange which can possibly take 

 place through the skin is not worth taking into account. The lungs 

 are to be regarded as developed from outgrowths of the alimentary 

 canal, beginning near the mouth. 



The object of all special respiratory arrangements being, in the 

 first instance, to facilitate the gaseous exchange between the sur- 

 rounding medium (air or water) and the blood, a prime necessity of 

 a respiratory organ, be it skin, gill, trachea, or lung, is a free supply 

 of blood, in vessels so fine and thin that diffusion readily takes place 

 into them and out of them. But a free supply of blood would be of 

 no avail if the medium to which the blood gave up its carbon dioxide 

 and from which it drew its oxygen was not being constantly and 

 sufficiently renewed. 



Sometimes the natural currents of the water or the air are of 

 themselves sufficient to secure this renewal ; in other cases, artificial 

 currents are set up by cilia, or special bailing organs, like the scapho- 

 gnathites of the lobster. In all the higher animals active move- 

 ments, by which air or water is brought into contact with the respira- 

 tory surfaces, are necessary ; and it is possible that such movements 

 take place even in the tracheae of insects and other air-breathing 

 arthropoda. Fishes, by rhythmical swallowing movements, take in 

 water through the mouth and pass it over the gills and out by the 

 gill-slits, while the frog distends its lungs by swallowing air. 



Physiological Anatomy of the Respiratory Apparatus. In man 

 the respiratory apparatus consists of a tube (the trachea) widened at 

 its upper part into the larynx, which contains the special mechanism 

 of voice, and communicates through the nose or mouth with the 

 external air. Below, the trachea divides dendritically into innumer- 

 able branches, the ultimate divisions of which are called bronchioles. 

 Each bronchiole breaks up into several wider passages, or infundibula, 

 the walls of which are everywhere pitted with recesses or alcoves, 

 called alveoli. The trachea and larger bronchi are strengthened by 

 hyaline cartilage in the form of incomplete rings, connected behind 

 by non-striped muscular fibres, which also exist in the intervals 

 between the rings. The middle-sized bronchi within the lungs have 

 the cartilage in the form of detached pieces in the outer portion of 

 the wall, while nearer the lumen lies a complete ring of non-striped 

 muscle. 



In the bronchioles, no cartilage is present, but the circularly- 

 arranged muscular fibres still persist, and also form a thin layer in 

 the infundibula. In the air-cells, or alveoli, however, there are no 

 muscular fibres. Their walls consist essentially of a network of 

 elastic fibres, continuous with a similar layer in the infundibula and 

 bronchioles, and covered on the side next the lumen by a single 

 layer of large, clear epithelial scales, with here and there a few 

 smaller and more granular polyhedral cells. 



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