THE NERVES OP THE RESPIRATORY MUSCLES. 467 



be traced to it, or the nasal branch of the fifth which supplies 

 the levator muscle must be considered as a factor of the co- 

 ordinating system. As the latter's motor and sensory roots 

 are situated in the medulla, there is nothing to militate against 

 its being considered precisely as is the vagus elsewhere. In- 

 deed, we are led, by the strict delineations established between 

 various parts of the organism by the older anatomists, to at- 

 tach undue importance to names, and the nerve in question 

 may, while being called "trigeminus," merely because of its 

 physical attributes, be an inherent portion of a general sys- 

 tem of which other subdivisions called pneumogastric, hypo- 

 glossal, etc., may form part. 



The fact that "when the cord is divided just below the 

 medulla all thoracic movements cease" implicates all the mus- 

 cles of the neck and thorax, besides the intercostals, in the 

 co-ordinative process. The usual separation of these muscles 

 into two classes, one for normal respiratory movements, an- 

 other for excessive respiratory effort, the latter being brought 

 into action to reinforce the former, affirms the existence, 

 beyond mere co-ordination, of a function such as we have 

 ascribed to the vagus in various organs. The prevailing views 

 as to the way these muscles take part in the respiratory process 

 give no clue to the manner in which their functions are ad- 

 justed to the fluctuating needs of this process, the "respiratory 

 center" being supposed to supply the required impulses when 

 need be. 



The relationship we have suggested between the phrenic 

 nerve and the hypoglossal, in the case of the diaphragm, again 

 shows itself in this connection. As is well known, the phrenic 

 nerve is also termed the "internal respiratory" nerve by Bell. 

 If the origin of the "external respiratory nerve of Bell" (the 

 posterior thoracic) is examined, it will also be found so related 

 with its mate, particularly at its junction with the fifth cer- 

 vical, as to clearly indicate a common origin. It is an extensive 

 nerve, and supplies filaments to each digitation of the serratus 

 magnus: a "reinforcement" muscle. That ample communica- 

 tion with the medulla to satisfy the needs of perfect co-ordi- 

 nation exists is further shown by the fact that communications 

 also appear through a vagal branch (first cervical), three hypo- 

 so 



