108 PHYSIOLOGY FOR NURSES 



They do not all cross at one point, but successively as 

 they pass down the spinal cord. 



The medulla is an important reflex nerve center dif- 

 fering somewhat from a similar action of the cord which 

 will be discussed later. 



Respiratory Center. Among these centers that pre- 

 siding over respiration is so important that the point at 

 which it is located has long been called the ' ' vital spot. ' ' 

 To carry on a function so essential as breathing, a mech- 

 anism must be employed which can act independently of 

 the will, which functions when one is asleep or uncon- 

 scious as in anesthesia. This center is located in the 

 lower part of the medulla and consists of two parts, one 

 on each side of the midline, each presiding over its own 

 side of the body. Its neurons descend in the spinal cord 

 and are connected through the gray matter witli the 

 spinal nerves at their points of origin at different levels. 

 Motor impulses, therefore, originate in the medullary 

 center and are distributed to the lower centers in the 

 cord, or to the centers of the vagus or facial nerve. Es- 

 sentially the center is automatic and is normally stim- 

 ulated by the amount of carbon dioxide in the blood ; i.e., 

 if the amount of C0 2 is small, the respirations will be 

 fewer, if the amount is increased the respiratory move- 

 ments will increase in number. This fact, and the ex- 

 tent of the control which the brain exerts over the cen- 

 ter, is illustrated in "holding the breath." One may 

 voluntarily cease to breathe for a time, but when C0 2 

 has sufficiently accumulated in the venous blood in the 

 center, an inspiration takes place regardless of one's 

 attempts to prevent it. 



The chief motor (efferent) nerve which carries the im- 

 pulses of the center is the phrenic, a branch of the cer- 

 vical plexus, though the intercostal, lumbar and other 



