160 CHANGES OF AIR AND BLOOD IN RESPIRATION. 



these nerves sometimes and possibly always, in tranquil respiration convey 

 an impression to the respiratory nervous centre, which gives rise to the ordi- 

 nary automatic and periodical action of the muscles of inspiration. If such 

 an impression be conveyed from the lungs by the afferent fibres of the pneu- 

 mogastrics, it could not operate when both pneumogastrics are divided in the 

 neck. This operation, as is well known, profoundly affects the respiratory 

 movements. After division of both nerves, the respirations become slow and 

 unusually deep, without, as a rule, any evidence of respiratory distress. In 

 dogs, the number of respirations often falls to four or five per minute, and 

 their nervous mechanism seems to be modified. Any respiratory distress that 

 occurs is due to the arrest of the respiratory movements of the larynx, and 

 not to an exaggeration of the sense of want of air. When a feeble Faradic 

 current is passed through the nerves, the respiratory movements are increased 

 in frequency, but the movements are arrested by a relatively powerful current. 

 This action is reflex. 



In view of all the experimental facts bearing upon the question, it is proba- 

 ble that the respiratory movements are sometimes reflex and sometimes due 

 to direct excitation of the cells of the respiratory centre by the absence of 

 oxygen. 



In perfectly normal and tranquil respiration, an impression is probably 

 conveyed from the lungs to the respiratory centre by the pneumogastrics, 

 which stimulates this centre to excite movements of inspiration. This is 

 probably due to a gradual and progressive change in the character of the con- 

 tents of the air-cells, although experiments are wanting to show the exact 

 mechanism of this process. 



When this reflex action is abolished, as by section of both pneumogastrics 

 in the neck, the respiratory centre is stimulated only when the deficiency in 

 the supply of oxygen becomes considerable. This excitation of the respira- 

 tory centre is direct. It requires a certain time for its operation, and this 

 accounts for the slow respirations in animals after the pneumogastrics have 

 been divided. Under certain physiological conditions, this direct stimulation 

 may be added to the impression conveyed by the pneumogastrics, and it is 

 probable that this always occurs in dyspnoea. 



Sense of Suffocation. The respiratory sense must not be confounded 

 with the sense of distress from want of air, and its extreme degree, the sense 

 of suffocation. The first is not a sensation, but an impression made upon the 

 medulla oblongata, giving rise to involuntary respiratory movements. The 

 necessities for oxygen on the part of the system regulate the supply of air to 

 the lungs. Once in every seven or eight respirations, or when the respiratory 

 movements are restricted under the influence of depressing emotions, an 

 involuntary, deep or sighing inspiration is made, for the purpose of changing 

 the air in the lungs more completely. The increased consumption of oxygen 

 and a certain degree of interference with the mechanical process of respira- 

 tion during violent muscular exercise put one " out of breath," and for a time 

 the respiratory movements are exaggerated. This is perhaps the first physio- 

 logical way in which the want of air is appreciated by the senses. A defi- 



