92 A Manual of Veterinary Physiology. 



elastic recoil of the chest wall and lungs ; division of one 

 phrenic nerve leads to paralysis of one half of the dia- 

 phragm, and of both nerves to complete cessation of its 

 action, inspiration being carried on by the muscles which 

 elevate the ribs. Division of both vagi in the neck leads 

 to a modification of the respiratory process though it still 

 continues, and division of the cord below the medulla does 

 not interfere with the facial and laryngeal movements. 



Sussdorf states that division of the phrenic nerves in the 

 horse leads to increased respiration, difficulty in breathing, 

 the latter being chiefly thoracic ; increased heart's action, 

 and the fieces collect in the rectum. In about 24 hours 

 these symptoms pass away, and if the animals be worked 

 no appreciable difficulty in breathing is observed. The 

 result of the operation is fatty degeneration of the muscular 

 portion of the diaphragm. 



The respiratory centre is considerably affected, not only 

 by the nervous influences conveyed to it, but by the quality 

 of the blood circulating through it. The violent respira- 

 tory efforts observed in the second stage of asphyxia are 

 said to be due to the deoxidized condition of the blood 

 circulating through the medulla. At one time it was 

 supposed that the excess of carbonic acid in the blood 

 stimulated the expiratory centre, whilst a deficiency of 

 oxygen stimulated the inspiratory movement ; this view is 

 not generally held at the present day, or only partly believed 

 to be true. The changes in the blood no doubt lead to 

 stimulation of the respiratory centre, but whether these 

 changes are due to the difference in the composition of the 

 blood gas, or to some other substance circulating in the 

 fluid, is not known. It has been suggested that sarco- 

 lactic acid, which is produced as the result of muscle 

 activity, may increase the respiratory movements and 

 explain the increased respiratory activity which, as we 

 know, accompanies rapid work. 



The Quantity of Air breathed. — The amount of air passing 

 into a horse's lungs in an hour during perfect repose 

 amounts to about 80 cubic feet; it is sometimes much 



