640 PHYSIOLOGY OF RESPIRATION. 



The expiratory center may or may not be located in the same 

 region as the inspiratory center, but the following general char- 

 acteristics may be assigned to it: In the first place, it is not auto- 

 matic; at least not under normal conditions. In the second place, 

 its activity must be dependent in some way upon that of the in- 

 spiratory center. Even our most violent respiratory movements 

 show an orderly sequence of inspiration and expiration, and we 

 may believe that the action of the expiratory center is conditioned 

 by the previous discharge of the inspiratory center, just as in the 

 heart the beat of the ventricle depends upon the previous systole 

 of the auricle. That an active expiration is not caused reflexly by 

 the mechanical expansion of the lungs seems to be demonstrated 

 by the fact that the most forcible voluntary inspiration is followed 

 by a passive, not an active expiration. Until our knowledge is 

 extended by further experimental work we may consider the ex- 

 piratory center as a group of cells connected by definite paths with 

 the expiratory muscles and capable of being stimulated in one of at 

 least four general ways: (1) In special reflexes, such as coughing. 

 (2) By voluntary control from the cerebrum, as in straining. (3) 

 By stimulation through afferent fibers from the skin, especially the 

 pain fibers. (4) By the action of an increased venosity of the blood. 

 Under the latter two conditions it is possible that the irritability 

 of the center is so increased that it becomes responsive to the in- 

 fluence of the inspiratory center. The relations of the inspiratory 

 and expiratory centers under the various conditions of artificial 

 stimulation are very complex, and although it is possible to rep- 

 resent these relations more or less completely by schemata of some 

 sort it does not seem advisable at present to seriously consider 

 such hypotheses. 



The Accessory Respiratory Centers of the Midbrain. Several observers 

 have called attention to the existence of a possible accessory respiratory center 

 in the midbrain at the level of the posterior colliculus. Martin and Booker 

 found that stimulations in this region caused a marked increase in the rate 

 of inspiratory movements and finally a standstill in inspiration, that is, 

 a complete tetanic contraction of the inspiratory muscles lasting during the 

 stimulation.* Lewandowskyf has shown that section of the brain stem 

 at or below the inferior colliculi causes an alteration in the respiratory rhythm 

 similar to that following section of both vagi. After cutting through the 

 inferior colliculi further sections more posteriorly do not add to the effect. 

 He considers that there is an automatic inhibitory center in the midbrain 

 which influences continually the automatic activity of the medullary center. 



The Nature of the Automatic Stimulus to the Respiratory 

 Center. We have accepted the view that the respiratory (inspira- 

 tory) center is essentially automatic, although very sensitive to 



* Martin and Booker, "Journal of Physiology," 1, 370, 1878. 

 f'Archiv f. Physiologic," 1896, 489. 



