INNERVATION OF THE RESPIRATORY MOVEMENTS. 695 



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 

 reflex stimulation. The further question arises as to the nature of 

 the automatic stimulus. Inasmuch as the activity of the center 

 controls the gaseous exchanges of the blood, it was natural perhaps 

 for physiologists to look to the gases of the blood for the origin of 

 the internal stimulus. Experiments show beyond question that 

 the condition of the gases in the blood has a direct and marked 

 influence upon the activity of the center. If for any reason the 

 blood supplying the center becomes more venous, the respirations 

 are increased in force or rate or both, and indeed the activity of the 

 center is in a general way increased in proportion to the venosity 



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

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



