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TEXT-BOOK OF PHYSIOLOGY. 



center, and thereby the extent and frequency of the inspiratory movement; 

 the other of which when stimulated, excites or augments the discharge of 

 nerve energy from the inspiratory center and thereby leads to an increase 

 in the depth or amplitude of the inspiratory movement. According as 

 the one or the other of these two classes of fibers are excessively stimu- 

 lated, will the inspiratory center be inhibited or augmented in its activity 

 to such an extent that the chest-walls will come to rest in the first in- 

 stance in the state of expira- 

 tory standstill, in the second 

 instance in the state of inspira- 

 tory standstill. 



The stimulus adequate to 

 the excitation of the pulmon- 

 ary terminations of the vagus 

 nerve-fibers in the physiologic 

 condition was formerly be- 



Diaphragm. 

 Seconds. 



FIG. 200. POSITIVE VENTILATION (Head). Under 

 the influence of positive ventilation, the inspiratory lieved to be the chemiC action 

 contractions of the diaphragm become less and less of Carbon dioxid: it IS now 



believed to be a mechanic 



till they disapoear completely. 



action, the result of the alternate distention and collapse of the walls of the 

 pulmonary alveoli. Thus, it has been shown by Head that if the lungs 

 are actively inflated (positive ventilation) there will be produced an in- 

 hibition of the inspiratory and an augmentation of the expiratory movement 

 until the inspiratory muscles are completely relaxed as indicated by the 

 relaxation of the diaphragm, the movements of which are simultaneously 



Negative 

 ventilation. 



Diaphragm. 



Seconds. 



FIG. 201. NEGATIVE VENTILATION. (Head). At a negative ventilation was commenced. 

 The expiratory relaxation of the diaphragm is seen to become more and more incomplete, until it 

 finally enters into continued contraction. 



recorded (Fig. 200), a result similar in all respects to that produced by 

 stimulation of the superior laryngeal nerve. On the other hand, if the 

 lungs are collapsed by the artificial withdrawal of air (negative ventilation) 

 there will be produced an augmentation of the inspiratory and an inhibition 

 of the expiratory movements until the inspiratory muscles are in a condi- 

 tion of tetanic contraction as indicated by the contraction of the dia- 

 phragm (Fig. 201) and by the state of the thorax which is that characteristic 

 of extreme inspiration, a result similar in all respects to that produced by 

 moderate stimulation of the central end of the divided vagus. 



A satisfactory explanation of the action of the respiratory mechanism 



