RESPIRATION. 283 



piratory centre, which acts less generally, inasmuch as ordinary tranquil 

 expiration is seldom more than an elastic recoil, and not a muscular act 

 to any marked degree. 



Assuming this view of the double centres to be correct, of their exact 

 mode of action there is some difference of opinion ; it is now thought that 

 they may act automatically, but normally are influenced by afferent im- 

 pulses from the periphery, as well as by impulses passing down from 

 the cerebrum. The centre is, in other words, both automatic and re- 

 flex. It will be simplest to discuss its reflex function first of all. 



Action of Afferent Stimuli. (a) Action of the vagi. If both vagi 

 be divided in the neck, the respirations become much slower and deeper; 

 this may be the case, but to a less marked degree, if one of the nerves 

 is divided instead of both. If the central end of the divided nerve be 

 stimulated with a weak interrupted current, the most constant effect is 

 that the respirations are quickened, and if the stimuli are properly reg- 

 ulated, the normal rhythm of respiration may be resumed. If the stimuli 

 be repeated with sufficient quickness, after a while the breathing is 

 brought to a stand-still at the height of inspiration by tetanus of the 

 diaphragm. Sometimes, however, stimulation of the central end of the 

 divided vagi produces still greater slowing than that which follows 

 the division, so that if it be continued, the respirations cease, with the 

 diaphragm in a condition of complete relaxation. Marckwald considers 

 that the differences in the effects of vagus stimulation are due to the 

 stimulus being applied to the nerve at different periods in the respira- 

 tory cycle, and that the action of the vagus may be to call forth either 

 inspiration or expiration the impulses passing up the vagi being neces- 

 sary to the production of the normal respiratory rhythm. The fibres 

 of the vagus are used under the following circumstances, those fibres 

 which tend to inhibit expiration and to stimulate inspiration are stim- 

 ulated at their distribution in the lung when the lung is empty and in 

 a condition of expiration, and the fibres which tend to inhibit inspira- 

 tion and to promote expiration are stimulated when the lung is fully ex- 

 panded. The afferent impulses are the results of mere mechanical 

 stimulation, and do not depend upon the chemical nature of the gases 

 within the pulmonary alveoli. The vagus always acts upon the centres 

 as a stimulator of discharge, or exciter of catabolism. 



(b) Action of the superior laryngeal nerves. If the superior 

 laryngeal branch of the vagus be divided, which usually produces no 

 apparent effect, and the central end be stimulated, the effect is very 

 constant, respirations are slowed, but there is a tendency toward expi- 

 ration, as is shown by the contraction of the abdominal muscles. Thus 

 if the vagus fibres contain fibres which stimulate inspiration and inhibit 

 expiration, as well as other fibres which have the reverse effect, the su- 

 perior laryngeal fibres inhibit inspiration and stimulate expiration. 



