514 VETERINARY PHYSIOLOGY 



from the surface covered by a very thin layer of integument, 

 just below which is a tuft of capillary blood-vessels. The 

 oxvgen passes from the water to the blood ; the carbon 

 dioxide from the blood to the water. 



A lung is simply a gill or mass of gills, turned outside 

 in, with air, instead of water, outside the integument. 

 While in aquatic gill-bearing animals there is constantly a 

 fresh supply of water passing over the gills, in lung-bearing 

 animals the air in the lung sacs must be exchanged by some 

 mechanical contrivance. 



{Tlie structure of the various parts of the respiratory 

 troM must be studied practically.) 



The lungs consist of myriads of small thin-walled air sacs 

 attached round the funnel-like expansions (infundibular 



passages) in which the air 

 passages terminate. These 

 infundibula are the most ex- 

 pansile structures in the 

 lung, and they are largest 

 where the expansion of the 

 lung is greatest (fig. 207). 

 Each air sac is lined by a 

 Fig. 207. — Scheme of the Distribu- layer of simple squamoUS 



ptr;:r„7AttcA°utLutg' epithelium, with smaller, more 



granular cells between them. 

 The cells are readily stimulated to proliferate by the action of 

 irritant substances, and the cells so produced take upon them- 

 selves a phagocytic action. The epithelium is placed upon a 

 framework of elastic fibrous tissue richly supplied with blood- 

 vessels. It has been calculated that, if all the air vesicles 

 in the lungs of a man were spread out in one continuous 

 sheet, a surface of about 100 square metres would be pro- 

 duced and that the blood capillaries would occupy about 75 

 square metres of this. Through these vessels about 5000 

 litres of blood pass in twenty-four hours, and during 

 muscular exercise the flow may be increased some sevenfold 

 (p. 413). 



The larger air passages are supported by pieces of hyaline 

 cartilage in their walls, but the smaller terminal passages, 



