138 NUTRITION OF FARM ANIMALS 



must at least equal the amount required by the tissues. It is 

 a familiar observation that the rate of ventilation of the lungs 

 varies with the varying activity of the body cells. This is 

 true of all these activities, but is most familiar in the case of 

 muscular work which, as everyone knows, promptly increases 

 the rate and depth of breathing, so that severe exercise, such as 

 rapid running, for example, brings into play all the reserve re- 

 sources of the breathing mechanism. As already stated 

 (190), the muscles which are used in breathing are ordinarily 

 controlled from the so-called " respiratory center " and it is 

 through this center that the regulation is effected. If, for 

 example, an animal be supplied with air largely diluted with 

 some indifferent gas, such as nitrogen or hydrogen, the partial 

 pressure of the oxygen in the alveoli is so reduced that the hae- 

 moglobin of the blood is only partially saturated with oxygen. 

 Such a deficiency of oxygen stimulates the respiratory center 

 and produces more active breathing and a corresponding in- 

 crease in the rapidity with which the air in the alveoli (residual 

 air) is renewed. 



Under ordinary conditions, however, the stimulus to the 

 respiratory center is not a lack of oxygen in the blood but an 

 excess of carbon dioxid. As has already been implied, the 

 lungs serve not only for the absorption of oxygen but for the 

 elimination of the carbon dioxid produced by respiration, 

 which passes by way of the lymph to the blood and thence to 

 the air in the alveoli of the lungs. Any increase in the ac- 

 tivity of the tissues by which more carbon dioxid is produced 

 tends to increase the content of this substance in the blood. 

 Even a very slight increase, however, promptly stimulates the 

 respiratory center and so causes greater activity of the muscles 

 concerned, resulting especially in deeper and to some extent more 

 rapid breathing. By this means the ventilation of the lungs 

 is augmented and so provision is made for the removal of a 

 greater amount of carbon dioxid. 



It is plain, however, that a simple increase in the lung ven- 

 tilation alone is not sufficient, except in a limited degree, to carry 

 away more carbon dioxid from the tissues. Along with the 

 increased ventilation there must be an increase in the rapidity 

 of the blood current which is the medium by which the transfer of 

 gases between the lungs and the lymph takes place. Accordingly, 



