RESPIRA TION 1 37 



Animals which do not sweat pant after work in order to get 

 rid of the surplus heat by warming a larger volume of air. This 

 question will be considered again under Animal Heat, but the 

 possible mechanism involved may here be glanced at. It is 

 conceivable that the congested condition of the skin may send 

 impulses to the respiratory centre, but this will not account for 

 the panting of animals in ' show condition.' Here there is pre- 

 sumably no increase in the carbon dioxide in their alveoli, and 

 the stimulation to increased respiratory activity would appear 

 to be obtained reflexly from the skin, though the increased tem- 

 perature of the circulating blood in the medulla may account for 

 it, as appears to be the case in fever. 



Influence of the Phrenic Nerves on Respiration. — The phrenic 

 is essentially an inspiratory nerve. We have referred to the 

 cutting off of the respiratory centre by dividing the cord above 

 the phrenic. If the cord be divided below the point of exit of 

 the phrenics, the channel between the respiratory centre and 

 lungs via the spinal cord is not interfered with, but the resulting 

 paralysis of the abdominal and intercostal muscles necessitates 

 that the action of the diaphragm should be more powerful. If 

 one phrenic nerve be divided, half the diaphragm is paralysed ; 

 if both be divided, the whole diaphragm is paralysed, and in 

 most animals causes death. In the horse division of the phrenic 

 nerves is not fatal ; it leads to difficulty in breathing, increased 

 heart action, and the collection of faeces in the rectum ; but in 

 about twenty-four hours these symptoms pass away, and if the 

 animal be worked, no appreciable difficulty in breathing is sub- 

 sequently observed. 



We are not prepared to offer any explanation of this re- 

 markable exception to experimental division of both phrenic 

 nerves. Sometimes the fact may be demonstrated in surgical 

 practice, for though as a rule in the horse fracture of either 

 of the four upper cervical vertebrae — viz., above the origin of 

 the phrenic nerves — is immediately fatal, yet there are many 

 exceptions to the rule, and death may be delayed for some 

 time. 



The phrenic nuclei in the cord are connected by crossed fibres, 

 so that, if the cord be half cut through above the nuclei, both 

 sides of the diaphragm are still able to contract, the explanation 

 being that the impulses cut off from one half of the diaphragm 

 are transmitted through the crossed channel. 



Division of Seventh Pair. — Colin has shown that if the 

 seventh pair of nerves be divided in the horse, and the animal 

 worked, asphyxia results. This nerve dilates the nostrils ; 

 when divided, the paralysed flaccid nostrils are drawn inward 

 at each inspiration, and so close the opening. 



