236 STUDIES IN ADVANCED PHYSIOLOGY. 



individual breaths are, however, deeper. If, now, the cen- 

 tral ends of the vagi, that is those connected with the brain, 

 be stimulated with electrical stimuli, the number of breaths 

 begins to rise to the normal; Here is, of course, an evident 

 control of the respiratory movements. If these vagi are then 

 excessively stimulated there ensues sometimes a relaxation 

 of the respiratory muscles, that is a passive expiration; 

 at other times a standstill of the chest and lungs in the 

 position of an inspiration, in which the respiratory muscles 

 are in a state of tetanic contraction. Here we have then 

 apparently a double effect; sometimes relaxation, sometimes 

 greater contraction. This double effect shows that the vagi 

 have two kinds of nerve fibres in them going from the lungs 

 to the center. First, those that stimulate the center to 

 greater respiratory exertion; and, secondly, those that tend 

 to inhibit the center. 



The rather interesting question now presents itself: Un- 

 der what circumstances does each of the nerves act ? The 

 solution of this question is probably found in the following 

 experiment: If the lung of an animal whose chest has just 

 been opened, be forcibly distended by pressing air into it, 

 there is at once developed a strong desire on the part of 

 the animal to breathe out. An expiration is induced. If, 

 on the other hand, the air be sucked out of the lung and 

 the lung so tend to collapse, there at once follows a strong 

 inclination for an inspiration. In other words, when the 

 lung expands the center is inhibited, breathing movements 

 stop, and the muscles relax. But the lung expands in an 

 inspiration, consequently an inspiration induces an ex- 

 piration. But in an expiration the lung is compressed, and 

 this compression of the lung stimulates the center to act 

 more energetically; that is, the center tends to breathe, 

 but actively breathing means to inspire. Thus the ex- 

 piration serves to induce an inspiration. We find, then, in 

 these nerves a kind of self-regulating arrangement which 

 stated again is as follows: When the center sends an im- 

 pulse to the muscles of respiration and the chest is thus 



