142 A MANUAL OF VETERINARY PHYSIOLOGY 



The next table is given by Munk : 



The Influence of Work on Respirations. — It is not clearly 

 known why an increase in the number of respirations results 

 from muscular work. During moderate work the increase of 

 carbon dioxide in the alveoli of the lungs is sufficient, as we 

 have seen, to increase the pulmonary ventilation ; but with 

 prolonged and especially severe work it is probable that there is 

 formed in the muscle fibre some products of its metabolism, 

 for example, lactic acid, which may, by circulating through the 

 blood, either stimulate the respiratory centre, or render it still 

 more sensitive to the percentage of carbon dioxide circulating 

 through it. 



Evidence that the panting respirations of work may be due to 

 the presence of a chemical substance circulating in the blood 

 is afforded by the experiment of dividing the spinal cord in the 

 dog, and stimulating the muscles of the hind limbs. The animal, 

 of course, is unconscious of any movement, but the respirations 

 are increased as if it had been running for some distance. What 

 the substance is that gives rise to this is not known. Lactic acid 

 has been suggested, and dilute acids injected into the blood give 

 rise to much the same condition. Hurried respirations may 

 also be produced through the circulatory system. In an animal 

 in training the breathlessness which it is one of the objects of 

 training to get rid of, is due to the fact that more blood is brought 

 to the lungs than can be disposed of. If the right heart pumps 

 into the lungs more blood than the lungs can return to the left 

 heart, breathlessness follows. The gallop by which an animal 

 gets its ' wind ' and ' staying ' power, operates through the cir- 

 culatory system. Fortunately, the vessels of the lungs are 



stant communication with the respiratory apparatus, from which samples 

 of the expired air were taken for analysis. It has been shown that this 

 latter is a necessary condition to insure accuracy. The writer's observa- 

 tions were carried out on horses which actually performed, under natural 

 conditions, the various paces. These, however, are rejected as not suffi- 

 ciently representing the respiratory changes occurring during work, owing 

 to the samples of air being collected for too brief a period. 



