484 



ORGAN SYSTEMS OF MAN 



rises, as it does in exercise or emotional 

 stress, the impulses are quickened. If, on 

 the other hand, the carbon dioxide is dimin- 

 ished, the impulses will likewise be farther 

 apart. If the phrenic nerve is severed or 

 destroyed through disease such as polio- 

 myelitis, the impulses fail to reach the 

 diaphragm and this muscle ceases its con- 

 tractions. The same is true of the other 

 motor nerves going to the rib muscles. It is 

 possible by taking a series of deep inhala- 



braothing 

 cantar 



One of the problems of breathing that 

 was not understood for a long time was 

 how the respiratory center could send out 

 these bursts of impulses in a rhythmic fash- 

 ion. It has been discovered by clever sur- 

 gery that if all of the sensory nerves leading 

 to the respiratory center are severed, in- 

 spiration will occur but not expiration. In 

 other words, the muscles of the diaphragm 

 and of the ribs will contract but will not 

 relax, because a steady stream of impulses 



trachea 



alveoli 



diapbrogm 



Fig. 18-8. The rhythmic movements in breathing are controlled by the breathing center 

 also. Sensory nerve endings lying in the alveoli send impulses to the center which 

 inhibit its action vi/hen the alveoli are stretched on inspiration. During the period of 

 inhibition, no motor impulses come from the center to the muscles, hence they relax, 

 resulting in expiration. 



tions and exhalations to reduce the carbon 

 dioxide content of the alveoli to a point 

 where breathing can be held in abeyance 

 until the gas again reaches a sufficient con- 

 centration to stimulate the respiratory cen- 

 ter. For the same reason, a newborn child 

 will take its first breath only after the 

 carbon dioxide has reached a certain level. 

 When its placental circulation is cut off the 

 gas begins to build up in the blood. Occa- 

 sionally, even then breathing will not start. 

 This can be remedied oftentimes by forc- 

 ing air containing 10 per cent carbon di- 

 oxide into the lungs. 



continue to come to them. This means that 

 the respiratory center must be inhibited 

 periodically so that the bursts of impulses 

 coming to the breathing muscles are inter- 

 rupted. This is accomplished by sensory 

 endings in the walls of the alveoli and other 

 parts of the lungs (Fig. 18-8). When air is 

 drawn into the lungs, the stretching of the 

 walls presses on the nerve endings, causing 

 impulses which inhibit the respiratory cen- 

 ter. Such is the device by which the lungs 

 are alternately filled and emptied. 



The respiratory center is also inhibited 

 by impulses coming to it from many other 



