1UU THI: XKKVOUS SYSTKM AND ITS CONSERVATION 



pons docs not stop the respiration. If the injury is ex- 

 tended backward into the medulla, breathing is suspended 

 permanently when certain cell-clusters are destroyed or 

 when they are cut off from communication with the 

 spinal cord. The French workers who first demonstrated 

 the absolute dependence of life upon the integrity of 

 this very limited region called it the "vital knot." It is 

 generally referred to as the respiratory center. It is so 

 related to the remainder of the nervous system that it is 

 involved in almost every reflex act of any magnitude. 

 Its ordinary action seems to be determined by the condi- 

 tion of the blood which is sent to it. If the dissolved 

 carbon dioxid is increased, the center is excited to cause 

 deeper and quicker breathing. If the same gas is reduced 

 to an unusually low concentration in the blood, the stimu- 

 lation of the center is suspended and there is a pause in 

 the breathing (apnea). 



