THE RESPIRATORY SYSTEM. 
895 
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 
Resptratory centre 
in the medulla. 
Sptnal cord. —>| | 
\ 
‘Abdominal muscles. 
Brain above medulla from which 
impulses modifying respiration 
may proceed. 
Facial muscles. 
Cutaneous surface from which 
afferent impulses proceed di- 
rectly to brain. 
---Thoracie resp. muscles, 
Diaphragm with 
phrenic nerve. 
Cutaneous sur- 
face from which 
impulses reach res- 
ptratory centre by 
spinal cord. 
Fia. 318.—Diagram intended to illustrate nervous mechanism of respiration. Arrows indicate 
course of impulses. 
principal, is not demonstrably clear. 
When others function as 
afferent nerves, capable of modifying the action of the respira- 
