Respiration. 83 



appears to be to ensure the volume of air being raised to 

 the proper temperature. 



The air having been warmed by passing over the septum 

 and turbinated bones enters the glottis, the arytenoid car- 

 tilages of which are separated to a greater or less extent to 

 enlarge the opening ; in quiet respiration this enlargement 

 of the glottis is not very remarkable, but during work the 

 cartilages are powerfully drawn upwards and backwards, 

 and the Y-shaped glottis fully opened. It is a remarkable 

 fact that the glottal opening should be so comparatively 

 small, considering the diameter of the trachea and the size 

 of the nasal openings. 



During inspiration the lar} r nx and trachea slightly descend 

 to ascend during expiration. This is particularly well seen 

 in horses during the hurried respirations of disease, pro- 

 ducing a well-marked rhythmical movement of the laryngeal 

 region and base of the tongue. 



Kespiratory Changes in the Air and Blood. — We must now 

 consider the changes which the air undergoes on passing 

 into the lungs. 



Atmospheric Am contains in 100 Parts : 



By Volume. By Weight. 



Oxygen - - 20-96 23016 



Nitrogen - - 7901 76-985 



Carbonic acid - -03 



The proportion of carbonic acid is small, it is a natural 

 impurity in the air. The atmosphere also contains moisture 

 the amount of which depends upon the temperature ; the 

 higher the temperature the greater the amount of water 

 which the air can contain as vapour, and the lower the 

 temperature the less the amount. 



Air may be dry or saturated, the latter term implying 

 that it contains as much vapour as it can hold at the 

 observed temperature ; it generally contains about 1 per 

 cent, of moisture, and is spoken of as dry if it contains 

 { per cent. The air which passes from the lungs is always 

 saturated with moisture. 



6—2 



