346 



THE RESPIRATION 



The question now arises as to 'how the chief center functionates. Is it 

 purely reflex in the sense that it depends for its activity entirely on the 

 transmission to it of nervous impulses from elsewhere, or is it automatic 

 in the sense that it can work independently of such impulses? The au- 

 tomaticity of the heart makes it seem not improbable that the center 

 which controls the co-ordinate action of the respiratory muscles would 

 also have an inherent or automatic power. The activity of such an auto- 

 matic respiratory center would, of course, be subject to great variation 

 as a result of changes in the composition of the blood supplying it, and 

 the fact that it was automatic would not remove it from the influence of 

 nervous impulses. Indeed it is possible to conceive of the automaticity 

 of the center as being of a low order, with its normal functioning 

 dependent upon afferent nerve impulses. Its automaticity might, then, 



Medulla 



Fig. 119. Diagram to show certain positions in the medulla and upper cervical cord, where 

 sections may be made without seriously disturbing the respirations. Sections made separately will 

 not disturb the respiration, nor interfere with the effect of vagus stimulation. If both sections 

 are made at once, however, breathing will be seriously interfered with on the side of the 

 hemisection, and this side will not respond to vagus stimulation. 



be merely a factor of safety called into play only when the influences 

 ordinarily controlling the center were for some reason removed. 



The question which at present confronts us, however, is whether the 

 center may or may not act automatically. Many experiments have been 

 undertaken to test this point, the nature of all of them depending upon 

 the isolation of the center as completely as possible from afferent nerve 

 paths. The most successful experiment has been performed as follows: 

 The influence of the higher nerve centers was removed by cutting across 

 the peduncles of the cerebrum or the pons. The influence of afferent im- 

 pulses traveling up the spinal cord was removed by completely severing 

 the spinal cord below the level of the phrenic nerves and sectioning all 

 the posterior or sensory spinal roots of the cervical cord above the level 

 of this section. The vagi were also cut to remove the impulses traveling 



