72 PHYSIOLOGY OF FARM ANIMALS [CH. 



breathing become deeper and quicker. This is what happens 

 during physical exertion, but the condition can be produced by 

 allowing an animal to breathe air containing an abnormal amount 

 of carbon dioxide (2 to 3 per cent. Rumination also causes 

 a slight increase in the respiratory movements in cattle and 

 sheep). Laboured breathing due to excess of carbon dioxide 

 is known as dyspnoea, and if continued maj'^ pass into as- 

 phj'^xia. On the other hand if the carbon dioxide tension falls 

 below a certain level (as by forced breathing which removes an 

 abnormal amount of the gas from the lungs and therefore from 

 the blood) the respirator}^ movements are slowed or stopped for 

 a time. This condition is known as ajDnoea. 



The rate of the respiratory movements is influenced further 

 by other acids than carbon dioxide, and in particular by lactic 

 acid which is produced during muscular exercise; moreover a 

 condition of acidosis which quickens the rate of movement 

 occurs in certain diseases (e.g. diabetes, when oxybutyric or 

 other organic acids may be present in the blood). 



The respiratory movements are also affected by nervous 

 influences, as by stimulating the vagus when the movements 

 become quickened. If on the other hand the vagus is cut the 

 movements become slowed. The same result is produced by 

 stimulating the superior larjaigeal nerve which therefore acts 

 antagonisticall}^ to the main branch of the vagvis. It is a familiar 

 experience that the respiratory movements are affected by 

 emotion or fright, and also that unlike the movements of the 

 heart they are under the control of the will. The complete 

 nervous mechanism is very complicated and only imperfectly 

 understood. The efferent nerves concerned are the intercostal 

 and the phrenic nerves, the former passing to the intercostal 

 muscles and the latter to the diaphragm. 



The Respiratory Quotient. Carbon dioxide contains its own 

 volume of oxygen. Thus if carbon is burnt in air the volume 

 of gas remains unchanged. Carbohydrates (e.g. CgH^QOg) con- 

 tain sufficient oxygen to oxidise their Itydrogen. Proteins on the 

 other hand do not, and still less do fats. Consequentlj' when 

 proteins or fats are consumed in metabolism the carbon dioxide 

 produced is less than the oxygen used ; that is to say, oxj^gen 

 has been employed to oxidise the hydrogen as well as the carbon. 

 The following are the exact fiaures : 



