68 



PHYSIOLOGY OF FARM ANIMALS 



[CH. 



contact with innumerable capillaries which form a close raeshwork. 

 Thus these capillaries are separated from the air in the air sacs b}' 

 the thinnest possible layer of protoplasm through which a gaseous 

 exchange is effected. The result of the repeated branching and 

 subdivision of the air tubes is to bring a very large surface of 

 blood inider the influence of the air in the air sacs of the lungs. 

 In the human subject it is estimated that no less than 2000 

 square feet of blood surface is exposed to the air in this way. 



In the act of expiration the air passes through the same 

 passages as in inspiration but in the reverse direction. 



Each lung is surrounded externally bj^ a layer of fibrous tissue , 



it '■' %^^^ J ^ 



*» '' ^^ 



I L 



Fig. 38. Section through lung tissue (alter Klein, etc., 

 from Halliburton). A.lJ alveolar duct, .V septa, 

 N alveoli or air-sacs, M unstriated muscle. 



and the wall of the thoracic cavity is lined by a similar layer. 

 NormaUy these two layers or pleurae are separated by only a very 

 small quantity of serous fluid which however increases in amount 

 in the inflammatory condition known as pleurisy. 



Inspiration is brought about by the expansion of the thorax, 

 the surfaces of the lungs continuing in contact with the pleurae 

 and the pleurae with the thoracic wall. In this way the lungs 

 are caused to expand, and air is drawn into them through the 

 trachea in order to fill uj:) the spaces Avhich would otherwise 

 occur. The thoracic movement involved in inspiration is pro- 

 duced by the contraction and consequent recession of the 



