462 THE RESPIRATORY MECHANISM. 



of the medulla and upper cord. Still, we have seen that the 

 vagus cannot be considered as the respiratory center; what 

 then is there in the medulla that can be termed such? A 

 moment's reflection will now suggest that there is no such a 

 center as the respiratory center, and that the morbid phe- 

 nomena witnessed after section of the medulla are really due 

 to interruption of the general stream of motor impulses that 

 the bulb serves to transmit. Neither is the general vasodilation 

 that is invariably witnessed after section of the medulla due 

 to interruption of vasomotor impulses per se. There is no 

 such a "vasomotor" center; and vasodilation is also due to 

 interruption of the functions of all motor nerves, since it is 

 these which throughout the entire organism maintain tonic 

 contraction of the vessels. 



We can further sustain this by analyzing the prevailing 

 view that the "respiratory center" may be stimulated by the 

 presence of carbonic acid in the blood, and that the vasomotor 

 center is also directly stimulated by a highly venous blood. 

 "When by reason either of any hindrance to the entrance of air 

 into the chest," says Professor Foster, "or other interference 

 with the due interchange between the blood and the pulmonary 

 air or of a greater respiratory activity of the tissues, as dur- 

 ing muscular exertion, the blood becomes less arterial, more 

 venous, i.e., with a smaller charge of oxygen and more heavily 

 laden with carbonic acid, the respiration, from being normal, 

 becomes labored. We may speak of normal breathing as 

 eupncea, and say that this, when the blood is insufficiently 

 arterialized, passes into dyspnoea, . . ." 



The pathogenesis of dyspnoea was referred to a few pages 

 back. The following lines by the same author will recall some 

 of the features then referred to: "When a muscle contracts, 

 its consumption of oxygen and production of carbonic acid, 

 especially the latter, are increased; the blood leaving the mus- 

 cle is more venous then usual. Hence, when many muscles 

 are contracting powerfully, the blood carried to the right side 

 of the heart is more venous than usual; and we might expect 

 that it is this unusually venous blood failing to be adequately 

 arterialized in the lungs, and hence reaching the medulla from 

 the left side of the heart in a more venous, less completely 



