THE NERVOUS SYSTEM 109 



nerves also play an important part. The sensory nerve 

 chiefly concerned is the tenth cranial or vagus which is 

 in part distributed directly to the lung tissue, but al- 

 most any sensory nerve may convey impulses to this 

 center. The vagus appears to carry two sets of fibers, 

 inspiration stimulating the inhibitory fibers by expan- 

 sion of the lung, while the partial collapse of the lung at 

 expiration stimulates the inspiratory fibers. That the 

 sensory nerves of the skin affect the center anyone may 

 prove by dashing cold water over the person and noting 

 the "gasp for breath" which immediately follows. The 

 sensory nerves of the face, breathing and swallowing 

 passages (fifth and ninth) can inhibit inspiration. This 

 is a protective arrangement whose action can be shown 

 by swallowing when the reflex through the ninth tem- 

 porarily arrests respiratory movements; by breathing 

 or attempting to breathe, any irritating gas, like am- 

 monia, the inhibitory impulses in this case following the 

 fibers of the fifth in the nose. 



At birth the first inspiration of the newborn child 

 seems to be caused mainly by the accumulation of C0 2 

 in the infant's blood as a result of cutting off its connec- 

 tion with the mother through the placenta; though a 

 contributing stimulation is the exposure of the skin, 

 with its sensory nerves, to the air. Obstetricians must 

 often take advantage of this to start the inspirations 

 which do not always begin at once after prolonged and 

 difficult labor. 



Apnea and Dyspnea. The first of these terms means 

 absence of breathing literally, but is employed in physi- 

 ology to describe a condition of respiratory rest when 

 the lungs and blood are full of oxygen. A fleeting con- 

 dition of apnea is produced by a very full inspiration. 



Dyspnea, difficult or labored breathing, is the condi- 



