464 THE RESPIRATORY MECHANISM. 



4- The general vasodilation witnessed after section of the 

 medulla is due to the interruption of the stream of general motor 

 impulses through which tonic contraction of the arteries is main- 

 tained, and which the medulla serves to transmit. 



THE INNERVATION OF THE RESPIRATORY MUSCLES. 



How, in the absence of a respiratory center, are the re- 

 spiratory movements governed? 



After reviewing the part played hy the muscles involved 

 in the respiratory mechanism, Foster says: "It is impossible 

 that all these so carefully co-ordinated muscular contractions 

 should be brought about in any other way than by co-ordinate 

 nervous impulses descending along efferent nerves from a co- 

 ordinating nervous center. By experiment we find this to be 

 the case. When in a rabbit the trunk of a phrenic nerve is 

 cut, the diaphragm on that side remains motionless, and res- 

 piration goes on without it. When both nerves are cut, the 

 whole diaphragm remains quiescent, though the costal respira- 

 tion becomes excessively labored." 



Even did a "respiratory center" exist, the co-ordinating 

 impulses upon which the diaphragm depends for its rhythmic 

 contractions would not be accounted for unless we grant the 

 phrenic nerve vagal properties: i.e., afferent, as well as efferent, 

 fibers. The origin of this nerve in no way indicates this to 

 be the case. In fact, it appears to us only as a secondary nerve, 

 such as any subdivision of the brachial plexus would be, since 

 it only arises from a branch of the third, fourth, and fifth 

 cervical nerves. Yet this very subdivision seems suggestive, 

 for, if the origin of the phrenic i.e., its connection with the 

 third cervical be closely examined, it will be found to meet, 

 or inosculate with, a communicating branch of the hypoglossal. 

 This would place the diaphragm on the same plane, as regards 

 nervous supply, as the lungs themselves, since it would then 

 be supplied (1) with general motor fibers, and (2) with hypo- 

 glossal fibers which immediately adjoin, in the fourth ventricle, 

 the origin of the vagus. This would fully account for the 

 co-ordination referred to, and which the phrenic as a mere 

 motor nerve would not explain were even a respiratory center 

 present. 



