1 24 INNERVATION. [CHAP. XX. 



nerve is of vast importance to the function of respiration. Section 

 of one nerve produces no effect upon the respiratory organs, either 

 structural or functional. Dr. Reid made careful examinations to 

 ascertain if, after cutting out a large portion of one nerve, the lung 

 of that side suffered any alteration in its texture, but he could not 

 detect any. But when both nerves have been divided above the 

 giving off of the pulmonary branches, the most severe dyspnoea 

 comes on, the respirations are generally much diminished in num- 

 ber, the animal breathes just like an asthmatic ; after a short time 

 the lungs become congested and oedematous, and the bronchial 

 tubes filled with a frothy serous fluid. When a piece has been cut 

 out of each nerve or the cut ends of the nerves are kept apart, 

 the animals never survive beyond three days, and during the whole 

 of that period they suffer severe dyspnoea. If the cut ends of the 

 nerves be kept in contact the animals will live ten or twelve days. 



It may be inferred from Dr. Reid's experiments that section of 

 the vagi nerves does not destroy that peculiar feeling of distress 

 (besoin de respirer), which is occasioned by the want of fresh air in 

 the lungs. He proved that animals, in which the vagi nerves had 

 been cut, struggled violently, and seemed to suffer greatly, when 

 the access of air to the lungs was cut off by compressing the 

 trachea. 



In such cases as that just described, the only channel through 

 which sensitive impressions could be conveyed from the lungs to 

 the brain is the sympathetic system, and it is to the afferent power 

 of the sympathetic nerves and possibly to the same power in the 

 cutaneous ramifications of the fifth and of the spinal nerves, that 

 we must attribute the imperfect excitation of the respiratory act, 

 which, under these circumstances, takes place. 



The phenomena which follow section of both vagi are doubtless 

 to be explained by the imperfect manner in which the centre of 

 respiration is excited, after the destruction of the influence of these 

 nerves consequent on their section. The movements, after the sec- 

 tion, are partly of the voluntary kind, produced by the sense of 

 distress occasioned by the imperfect supply. The asthmatic state 

 which also follows the section may perhaps be in part caused by the 

 irritation of the central portion of the nerve, exciting the medulla 

 oblongata and the extremities of the motor nerves of respiration. 



Lastly, the office of the gastric branches of the vagi nerves, ap- 

 pears from Reid's experiments to be chiefly to control the move- 

 ments of the muscular coat of the stomach. Mechanical irritation 

 of these nerves causes slow and vermicular contractions of this tunic. 



