434 TEXT-BOOK OF PHYSIOLOGY 



lished by volitional efforts or by mechanic inflation, the respiratory move- 

 ments gradually return. At first they are feeble but soon increase in 

 amplitude and frequency until the normal is reached. At one time the 

 apnea that results from rapid ventilation of the lungs, whether volitional or 

 mechanical, was attributed, on the assumption that a deficiency of oxygen 

 in the arterial blood is the physiologic stimulus to the activity of the inspira- 

 tory center, to an excess of oxygen in the blood, the result of the forced 

 ventilation, complete saturation of the plasma and the hemoglobin, in 

 consequence of which the inspiratory center remained inactive. The 

 apneic state is at present attributed, on the assumption that carbon dioxid 

 in the arterial blood is the physiologic stimulus to the inspiratory center, to a 

 diminution in the percentage of the carbon dioxid in the alveolar air (to 4 per 

 cent, or less), in the blood, and therefore in the center, the result of the forced 

 ventilation. The increased ventilation eliminates the carbon dioxid to such 

 an extent that the percentage and pressure in the blood is insufficient to 

 arouse the center to activity. To the condition of the blood that results 

 from this rapid ventilation, viz.: a diminished percentage of CO 2 , the term 

 acapnia has been given. An apnea which is thus developed is termed apnea 

 chemica or apnea vera. As previously stated, stimulation of certain afferent 

 nerves, especially the vagus, will induce a similar cessation of the respiratory 

 movements. Thus if the central end of the divided vagus be stimulated 

 with induced electric currents of marked intensity, the thorax will come to 

 rest in the state characteristic of deep expiration from inhibition of the in- 

 spiratory center. Inasmuch as stimulation of the vagus causes an apnea 

 resembling that caused by rapid inflation of the lungs, it has been suggested 

 that in the development of apnea the inspiratory center is inhibited in its 

 activity simultaneously with the elimination of the CO 2, from the mechanic 

 stimulation of the pulmonic terminations of the vagus. An apnea caused 

 by stimulation of the vagus is termed apnea vagi or apnea inhibitoria. 



In the apnea that results from voluntary or mechanic inflation of the 

 lungs it is difficult to state in how far the condition is due to a diminution 

 in the pressure of the CO 2 and in how far to a stimulation of the vagus. 

 But inasmuch as apnea can be established, though not of such long duration, 

 after division of the vagus nerves, the probabilities are that the diminished 

 percenta^e_of_the_CO 2 is the main cause. 



^0ys^nea^ -Excessive and laborious respiratory movements constitute 

 a condition known as dyspnea. Movements of this character indicate that 

 the bloo4 contains a greater percentage of CO^^j^n^rmal^or a diminished 

 fin ' etffier case the'irntability of the inspiratory center 



is abnormally- heightened. Of the two conditions, the former is by far the 

 more common. While-iris true that a deficiency of oxygen in the arterial 

 blood gives rise to an increase in the rate and depth of the inspiratory move- 

 ments, this does not arise until the deficiency of the oxygen falls to about 

 one-third of the normal. On the other hand, an increase of even 0.2 per 

 cent, of CO 2 in the alveolar air will almost double the inspiratory activity. 

 These conditions of the blood may be caused: (i) By all those pathologic 

 conditions of the respiratory apparatus which limit the free entrance of oxygen 

 into and the free exit of carbon dioxid from the blood; (2) by those alterations 

 in the composition of the air and subsequently in the blood which arise when 

 the individual is confined in a space of moderate size with imperfect ventila- 



