THE RESPIRATORY SYSTEM 



119 



Colin poured seven quarts of water per hour into the trachea of a 

 horse and upon autopsy at the end of three and a half hours did 

 not find any water in the air sacs. 



The process of respiration consists of two separate and distinct 

 exchanges of gases: external respiration and internal respiration. 

 The former both precedes and fol- 

 lows the latter. 



External respiration, or breathing, 

 is the exchange of gases between 

 the external medium, air, and the 

 blood. It consists of inspiration, 

 a movement in which the lungs 

 are filled with air r and expiration, 

 by which the inspired air is expelled 

 from the lungs and these organs 

 are ventilated. Normally a short 

 interval occurs between these 

 movements. 



Inspiration is brought about by a 

 contraction of the diaphragm, 

 which causes this partition to 

 flatten, particularly at the sides, so 

 that the length of the thorax is 

 increased (Fig. 36). The ribs are 

 rotated outward and forward by 

 the external intercostal muscles 

 attached to them. As a result of 

 these changes both the girth and 

 transverse diameter of the chest 

 are increased, and the thorax is 

 greatly expanded, so as to permit 

 the maximum intake of air. In- 

 spiration requires exertion. 



Expiration is made possible by 

 the relaxation of the muscles employed in inspiration. Nor- 

 mally it is mainly a passive act; no muscular effort being required. 

 The diaphragm springs forward rapidly, due to pressure from 

 the abdominal viscera. This causes the lungs to be compressed, 

 and forces most of the air out of them. The balance of the 

 air normally exhaled depends for its expulsion on the contraction 



Fig. 36. — Horizontal section of 

 chest of horse to show the move- 

 ments of diaphragm: a, Right 

 lung; b, left lung; 1, position of 

 the diaphragm during deep ex- 

 piration; c, liver; d, stomach during 

 deep expiration; e, spleen during 

 deep expiration; 2, position of the 

 diaphragm during deep inspiration ; 

 e', liver; d', stomach; e', spleen; 

 /, posterior vena cava as it passes 

 through its foramen in the dia- 

 phragm. (Sussdorf.) 



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