376 OF RESPIRATION. 



295. The reflectorial, as opposed to the automatic activity of the respira- 

 tory centres is chiefly excited through the Pneumogastric or vagal nerves, 

 and especially through the branches of these nerves distributed to the larynx 

 and lungs. Other sensory nerves, however, as the fifth, and the sensory 

 nerves of the skin generally, may convey impressions which are reflected at 

 the medulla to the muscles maintaining respiration. Under ordinary cir- 

 cumstances, that is to say, in tranquil breathing, it would appear that the 

 deficiency of oxygen or the accumulation of Carbonic acid and other prod- 

 ucts of disintegration in the blood, acts as a stimulus to the sentient ex- 

 tremities of the Vagus distributed to the lungs ; the excitation is conveyed 

 through the trunks of these nerves to the vagal centres in the medulla ob- 

 longata, from thence it is reflected through the spinal cord and chiefly 

 through its lateral columns 1 to the phrenic nerves, and the diaphragm con- 

 tracting an act of inspiration takes places. The Blood, now become oxygen- 

 ated, ceases to be a stimulus to the vagal branches, the vagal centres become 

 quiescent, the diaphragm relaxes, and the elasticity of the chest and lungs 

 expels the air, and the act of expiration is effected.' 2 The succession of these 

 phenomena constitutes normal or ordinary respiration, but if from any cir- 

 cumstances, as the performance of active exercise, or exposure to a high 

 temperature, 3 the consumption of oxygen and production of carbonic acid 

 are increased, the vagal centres and extremities are more strongly excited, 

 they transmit a more powerful influence, and other motor nerves (such as 

 the intercostals) besides the phrenics are excited. In cases where the blood 

 is still less perfectly arterialized, as when the supply of air is insufficient, 

 the vagal centres are still more strongly excited, and all the accessory 

 muscles are then called into play. 



296. The important part played by the Vagi in the respiratory acts may 

 be experimentally shown by their division, and the application of a stimulus 

 to their centric extremities. The trunks of the vagi are not endowed with 

 much sensibility, for if pinched or pricked the animal does not exhibit nearly 

 such acute signs of pain as when the trunks of the ordinary spinal nerves or 

 of the fifth pair are subjected to similar treatment. The effects of section of 

 the vagi appear to differ considerably in different animals, which is doubt- 

 less in part due to the fact that in some animals, as the Cat and Dog, the 

 vagus is united into one trunk with the sympathetic, the section of the latter 

 leading to vaso-motor complications; whilst in others, as in Man and the Rab- 

 bit, these nerves are separate. When the section is made on one side only 

 in the Rabbit the effects are not constant, 4 but if the division be made si- 

 multaneously on both sides, the number of respirations per minute, as was 

 shown by Dr. Reid, 5 diminishes from 120-140 to 40-50. The mechanism of 

 breathing is at the same time completely altered. Each respiration be- 

 comes much deeper than before, which is partly due to increased action of 

 the diaphragm, and partly to the participation of the accessory muscles in 

 the act of inspiration. The quantity of air breathed per minute is, however, 

 not materially altered, being as great after as before the section, the dimin- 

 ished frequency being compensated for by increased depth of the respiratory 

 act. Still it is to be particularly observed that although the vigor of the 

 respiratory movements does not appear to be materially impaired, the blood 



1 See SchiflF, Pfliiger's Archiv, Bd. iv, p. 225. 



8 Brouor, Sitz. <1. Wien. Akad., 1808, Heft xii, attempts to show that the mere 

 act of inspirntion lends to relaxation of the respiratory muscles, and is a stimulus to 

 the nerves supplying the expiratory muscles, and via 1 rcr*<"t. 



3 See Goldstein, Yorhamllunn Phys. Med. Gescll. zu Wurzburg, Bd. ii, 1871, p. 

 15G. 



4 Gilchrist, Mcd.-Chir. Rev., 1858, vol. ii, p. 495. 5 Op. cit. 



