254: NERVOUS SYSTEM. 



on tlie circulation. The direct inhibitory action of the pneu- 

 mogastrics operates through filaments derived from the spinal 

 accessories. Galvanization of the " depressor-nerves " retards, 

 or may arrest the pulsations of the heart, by reflex action. 

 This occurs only when the central ends of the divided nerves 

 are stimulated. Galvanization of the central ends of these 

 nerves also diminishes the pressure of blood in the large ves- 

 sels. This is due to reflex action through the splanchnic 

 nerves, by which the vessels of the intestines are dilated. 

 No such effect is produced when the splanchnic nerves have 

 been divided. There is no entirely satisfactory explanation 

 of the influence of the pneumogastrics on the heart. 



The action of the pulmonary branches has been studied 

 chiefly by observations on the pneumogastrics in the cervical 

 region. Division of the pneumogastrics in this situation, in 

 young animals, produces almost instant death by closure of 

 the glottis in inspiration. In animals full-grown, death oc- 

 curs in from two to five days, and the respiratory acts are 

 very much diminished in frequency. "When death occurs in 

 this way, the lungs are found partially or completely " car 

 nified." This is due to mechanical causes. The small pul- 

 monary vessels are ruptured by the excessively deep inspira- 

 tions, and blood is gradually effused and coagulates. The 

 pneumogastrics have but little to do in conveying to the 

 nerve-centres the sense of want of air which gives rise to 

 the respiratory movements. Galvanization of the central 

 ends of the pneumogastrics divided in the cervical region 

 has the following effects : A very feeble excitation accele- 

 rates, and a more powerful excitation retards respiration. 

 A sufficiently powerful excitation arrests respiration. Gal- 

 vanization of the peripheral ends has no effect on respira 

 tion. 



The cesophageal branches supply only the lower third of 

 the oesophagus. The upper portion receives branches from 

 the inferior laryngeals, and the middle portion is supplied 

 by branches from the posterior pulmonary nerves. The sen- 



