122 PBINCIPLES OP VETERINARY SCIENCE 



In other words, when fats are broken down their end-products 

 are carbon dioxid and water. The same thing is true of the car- 

 bohydrates to which the equation below applies: 



C 6 H 12 6 + = 6C0 2 + 6H 2 



Normally a perfect balance is maintained in the blood between 

 the production of carbon dioxid and the consumption of oxygen 

 by breathing either faster or slower. The rapidity of breathing 

 is regulated by a collection of nerve-cells in the medulla oblongata 

 known as the respiratory center. Special nerve fibers run from 

 this center, by way of the vagus nerve trunks, to the lungs and 

 muscles concerned with respiration. Like other important 

 nerve centers, it is automatic, that is, when its vagus nerve con- 

 nections are severed it continues to discharge impulses. In this 

 event, however, the respiratory movements are not sufficient 

 properly to aerate the blood and the animal sooner or later suc- 

 cumbs to carbon dioxid poisoning. 



Carbon dioxid, when raised above a certain level in the blood 

 by exercise or any other cause, stimulates the respiratory center 

 to discharge rapidly. There results from this, by reflex action, 

 a series of contractions in the muscles of respiration, an increase 

 in the number of respirations per minute, and rapid ventilation 

 of the lungs. The action is due to the direct influence of the 

 carbon dioxid on the cells of the center, rather than to the accu- 

 mulation of this gas in the blood, for it is readily eliminated from 

 the blood as the latter flows through the lungs. These cells may 

 accordingly be looked upon as agents to protect all other body 

 cells from carbon dioxid poisoning. When this gas is in excess 

 it produces, besides a marked increase in the number of respira- 

 tory movements, great depression, and finally tranquil death 

 from paralysis. 



On the other hand, when a decrease occurs in the amount of 

 oxygen in the blood, the respiratory center is but slightly stimu- 

 lated to action. The effects are best observed at high altitudes 

 where the atmosphere is rarefied. This is because the hemoglo- 

 bin of the blood has the power of taking oxygen from air that 

 contains less than that normally breathed, so the effects of dimin- 

 ished oxygen in the air are not at once noticeable. If oxygen 

 is withheld, animals exhibit great excitement, cyanosis, dizziness, 

 albuminuria, and convulsive death, but little or no increase in 



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