RESPIRATION. 471 



effected by the rotation of the arytenoid cartilages outward, 

 and the rendering tense the vocal cords. The dilatation of 

 the lungs takes place simply to obviate the formation of a 

 vacuum, which would otherwise result from the increased 

 capacity of the thorax.* In becoming distended, the lungs 

 must necessarily alter their position relatively to the walls of 

 the mediastinum and thorax. Their gliding over these parts 

 is, however, greatly facilitated by the smoothness of the layer 

 of epithelium covering the adjacent surfaces of the plurse, and 

 by the moisture with which these are bedewed. 



The lateral dilatation of the thorax, which is greatest 

 towards the lower extremity of the false ribs, has been found, 

 by measurement, to be about two inches and seven lines 

 during quiet ordinary respiration in the horse. The ordinary 

 increase on the antero-posterior diameter of the thorax, as 

 ascertained by opening the abdominal cavity, and employing 

 a graduated measure, is about four and a-half inches in the 

 horse. The dilatation, in either case, is necessarily much 

 increased when the respiration is excited. The quantity 

 of air taken into the lungs of the horse, during quiet respira- 

 tion, is found to vary from a quart to a quart and a hal 

 (COLIN.) This does not indicate, however, the amount of air 

 present in the lungs after an inspiration; the trachea, bronchial 

 tubes, and even the air cells, always retain a considerable 

 quantity of air which, by mixing with the cold air inhaled, 

 raises its temperature and moistens it, so as to render it less 

 liable to irritate the delicate walls of the pulmonary cells. 



Expiration is effected by a series of actions exactly the 

 converse of those just described. 



The lateral diameter of the thorax is diminished by a 

 rotation of the ribs backwards and inwards. When the various 

 agencies leading to inspiration are thrown out of use, this 

 movement is induced by the arrangement and elasticity of 



