THE KBSPIRATORY SYSTEM. 393 



THE NERVOUS SYSTEM IN RELATION TO RESPIRA- 

 TION. 



We have considered the musciilar movements by which the 

 air is made to enter and leave the lungs in consequence of 

 changes in the diameters of the air-inclosing case, the thorax. 

 It remains to examine into the means by which these muscles 

 were set into harmonious action so as to accomplish the pur- 

 pose. The nerves supplying the muscles of respiration are de- 

 rived from the spinal cord, so that they must be under the do- 

 minion of central nerve-cells situated either in the cord or the 

 brain. Is the influence that proceeds outward generated within 

 the cells independently of any afferent impulses, or is it depend- 

 ent on such causes ? 



A host of facts, experimental and other, show that the cen- 

 tral impulses are modified by afferent impulses reaching the 

 center through appropriate nerves. Moreover, drugs seem to 

 act directly on the center through the blood. 



The vagus is without doubt the afferent respiratory nerve, 

 though how it is affected, whether by the mechanical movement 

 of the lungs, merely, by the condition of the blood' as regards 

 its contained gases, or, as seems most likely, by a combination 

 of circumstances into which these enter and are probably the 

 principal, is not demonstrably clear. When others function as 

 afferent nerves, capable of modifying the action of the respira- 

 tory center, they are probably influenced by the respiratory 

 condition of the blood, though not necessarily exclusively. 



But when all the principal afferent impulses are cut off by 

 division of the nerves reaching the respiratory center directly 

 or indirectly, respiration will still continue, provided the motor 

 nerves and the medulla remain intact. 



The center, then, is not mainly at least, a reflex but an auto- 

 matic one, though its action is modified by afferent impulses 

 reaching it from every quarter. 



It has been argued that there are both inspiratory and ex- 

 piratory centers in the spinal cord, but this can not yet be re- 

 garded as established. But, as we have pointed out, on more 

 than one occasion, we must always be on our guard in inter- 

 preting the behavior of one part when another is out of gear. 



The Influence of the Condition of the Blood in Respiration.— 

 If for any reason the tissues are not receiving a due supply of 

 oxygen, they manifest their disapproval, to speak figuratively. 



