84 HUMAN PHYSIOLOGY 



Destruction of this spot causes immediate death because 

 breathing stops (hence the spot is called the nceud vital). 



Some authors suppose that this spot is a nerve trunk which 

 unites the nuclei of the 5, 9, 10 and n cranial nerves with the 

 nuclei of the motor respiratory nerves. It has also been advanced 

 that the real respiratory centre lies not in the medulla but in the 

 spinal cord. 



The centre is composed of an inspiratory and an expiratory 

 centre which act alternately; it is bilaterally double, but the 

 two parts are connected by commissural fibres so that they 

 are always stimulated simultaneously. 



The nerves from the respiratory centre to the motor nuclei 

 of the nerves of respiration run in the two lateral columns of 

 the spinal cord (respiratory bundle]. 



The stimulation of the respiratory centre can be brought 

 about directly and indirectly. 



i. Direct stimulation. Normally the respiratory move- 

 ments take place involuntarily because the respiratory centre 

 is continually and directly stimulated. The normal stimula- 

 tion is automatic, not reflex for the centre retains its activity 

 after all the centripetal nerves which can stimulate it have 

 been severed. 



The normal stimulation is the lack of oxygen and the 

 accumulation of carbon dioxide in the blood. As the arterial 

 blood contains so little oxygen and so much carbon dioxide, 

 the centre is stimulated even by the arterial blood. This 

 brings about the normal quiet breathing which is called 

 eupnoea. 



If the blood is well aerated by deep respiration, so that it 

 contains much oxygen and little carbon dioxide, the respira- 

 tory centre is not stimulated. Hence, breathing is suspended 

 apnoea. 



Apncea is, indeed, partly due to a stimulation, by the expansion 

 of the lungs, of centripetal inhibiting vagus fibres (see below) ; 

 for, after section of the vagi, it is more difficult to produce apnoea. 



The embryo in uterus is in apnoea, the blood of the mother 

 causing a sufficient gas-exchange in the placenta. If the circula- 

 tion of the umbilical cord (by compression, for example, of the 



