THE RESPIRATORY SYSTEM 123 



the number of respiratory movements. This shows that a defi- 

 ciency of oxygen does not cause stimulation of the respiratory 

 center to such an extent as results from an increase in carbon 

 dioxid. 



The amount of oxygen in the blood leaving the lungs cannot 

 be materially increased even by causing the animal to breathe 

 deeply or by offering it pure oxygen gas to breathe, as the blood 

 normally is near the point of saturation. This means that 

 increased ventilation of the lungs without an accompanying 

 increase in the rate and force of the heart beat will not supply 

 more oxygen to the tissues. 



Asphyxia is the condition of suffocation that results from an 

 increase of carbon dioxid or a decrease of oxygen in the blood. 



Dyspnea is the condition of distressed or labored breathing 

 caused by lack of air. The affected animal is said to be "air 

 hungry." It is seen in wind broken horses and those affected 

 with heaves after violent exercise. 



The amount of oxygen absorbed and other gases excreted 

 varies with the size of the animal, the activity of the tissues of 

 the body, the rate and depth of respirations, and the body tem- 

 perature. In a state of rest a horse of 1100 pounds weight has 

 been found to absorb 5260 liters of oxygen, and give out 5060 

 liters of carbon dioxid in twenty-four hours. During exercise 

 these amounts are greatly increased due to more rapid combustion 

 of the carbon-containing materials in the tissues. Exposure to 

 cold causes increased oxidation and a corresponding increase in 

 the respiratory exchange. In a fasting animal respiratory activity 

 is slow; on a non-nitrogenous ration -it is somewhat more rapid, 

 still more rapid on a mixed ration, and most rapid on a 

 nitrogenous ration. 



The amount of air breathed by a horse was found by King to 

 be 142 cubic feet per hour and about 3400 cubic feet per day. 

 This demand quickly uses up the air in the immediate vicinity 

 of the nose for an equal amount of air that has been breathed is 

 discharged. It is obvious that a constant stream of fresh air is 

 needed to remove the vitiated air and to supply oxygen. This 

 calls for some means of keeping the air in motion. The electric 

 fan and window ventilators are used in our homes for this pur- 

 pose. In the stable air shafts are employed. For most effective 



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