702 PHYSIOLOGY OF RESPIRATION. 



give off its rhythmic discharges. There is no desire to breathe 

 and the animal lies quiet in a condition of apnea. Voluntary 

 forced respirations in man maintained for some minutes will 

 produce a similar condition. According to the interesting 

 account given by Haldane and Poulton* an apnea may be 

 produced in this way which will last for 100 to 150 seconds, and 

 before the individual begins to breathe again he may become 

 very blue in the face, owing to the loss of oxygen from the 

 blood. Henderson f has given experimental evidence to show 

 that a marked diminution in the pressure of the CO 2 in the blood, 

 brought about by forced respiration, may cause not only a condi- 

 tion of apnea but also a feeble rapid heart-beat; with fall of blood- 

 pressure and the symptoms of surgical shock. It would appear, 

 therefore, that a certain tension of CO 2 in the blood is necessary 

 for the normal irritability of the vasoconstrictor and cardio- 

 inhibitory centers as well as the respiratory center. { In the apneic 

 condition produced by rapid ventilation of the lungs all three 

 centers suffer a diminution in activity. In addition to the factors 

 discussed above, namely reflex inhibition or excitation through 

 sensory nerve fibers and variations in the carbon dioxid of the 

 blood, it is to be borne in mind that this irritable center may be 

 influenced in many other ways, for example, by the specific action 

 of various drugs or toxins, by variations in the inorganic elements 

 of the blood, etc. 



Innervation of the Bronchial Musculature. Numerous 

 investigators, using different methods, have demonstrated 

 that the bronchial musculature is supplied through the vagus 

 with motor and inhibitory fibers, bronchoconstrictor and 

 bronchodilator fibers, as they are usually called. Stimulation 

 of the constrictors causes a narrowing of 'the bronchi, and 

 therefore increases the resistance to the inflow and outflow 

 of air. Some observers state that these fibers are nor- 

 mally in a condition of tonic activity (Roy and Brown), but 

 others find little evidence for this belief. An artificial tonus 

 that is, a condition of maintained activity of the constrictor fibers 

 may be set up by the action of a number of drugs, such as muscarin, 

 pilocarpin, and physostigmin, which in this case, as in so many 

 other instances of autonomic fibers, are supposed to stimulate the 

 endings of the fibers in the lungs. Their effect is removed by the 

 action of atropin. These fibers are stimulated also during the ex- 

 citatory stages of asphyxia. Reflex stimulation of the constrictors 



* Haldane and Poulton, "Journal of Physiology," 37, 390, 1908. 

 t Henderson, "American Journal of Physiology," 21, 128, 1908. 

 j See Mathison, "Journal of Physiology," 42, 283, 1911. 

 I For references to literature, see Dixon and Brodie, "Journal of Physi- 

 ology," 29, 97, 1903. 



