120 A MANUAL OF VETERINARY PHYSIOLOGY 



the nostrils to the infundibular cavities. The air existing in 

 the alveoli is spoken of as the alveolar ; it is the important air 

 in respiration, and carries out the effective respiratory changes. 

 From the air-sacs to the nostrils is the ' dead space ' — that is 

 to say, it contains air which is for the time being functionless 

 from a respiratory point of view. The amount of the ' dead 

 space ' has not been determined for all animals ; in man one- 

 third of an inspiration fills it, while the remaining two-thirds 

 goes to the alveoli. This proportion must necessarily vary in 

 different animals, depending on the length of the head and 

 neck. 



The composition of the air in the respiratory passages is 

 obviously not the same throughout ; it grows progressively 

 poorer in oxygen and richer in carbon dioxide from nostrils to 

 alveoli. During expiration the air from the dead space is 

 the first to leave, and some of the alveolar follows. During 

 inspiration the incoming air is diluted with that already in the 

 lungs, and its chemical composition altered. If it takes a 

 minute to ventilate the lungs — viz., to replace entirely the air 

 they originally contained — it is obvious that though some of the 

 inspired air may reach favourably placed infundibular cavities 

 at once, the bulk can only reach it gradually. In man it is 

 estimated that about one-eighth of the alveolar air is changed 

 at each respiration. How far an ordinary inspiration travels 

 it is difficult to determine. Under the most favourable circum- 

 stances some of the axial stream of the current might reach 

 the alveoli, especially of the anterior lobes, but the bulk of it 

 will get no further than the bronchi. Some of this may effect 

 deeper penetration at the next inspiration, while some of it will 

 be expelled unchanged. In the infundibula the air is changed 

 by the process of diffusion. 



The composition of the air in the alveoli of the lungs has for 

 years been a difficult matter to determine, yet in order to explain 

 pulmonary respiration this knowledge is essential. The air in 

 the alveoli possesses less oxygen and more carbon dioxide than 

 expired air, for, as we have previously seen, expired air is a mixture 

 of alveolar and fresh air, the latter passing away at the beginning. 

 The alveolar air at the end of the expiration, which lies between 

 these two extremes, is the mixed air. The air at the end of the 

 expiration represents that in the alveolar spaces, as it has come 

 from where the actual passage of the oxygen into the blood 

 and the carbon dioxide out of it has taken place, and in 

 consequence a knowledge of its composition is of the utmost 

 moment 



Haldane, to whom we are indebted for a method of obtaining 

 the alveolar air, shows that in man the percentage of carbon 



