380 OF KBSPIRATION. 



cord just below its exit from the cranium ; whilst if the injury take 

 place below the origin of the phrenic nerve, life may be prolonged for 

 some time. 



685. The- Respiratory movements, like other reflex actions ( 394), 

 depend upon a stimulus of some kind, originating at the extremities of 

 the nerves, propagated towards the centre by the afferent trunks, an 

 giving rise to a motor impulse, which is transmitted along the efferent 

 or motor nerves to the muscles, and which occasions their contraction. 

 Now the importance of the respiratory function to the maintenance of 

 life which has already been sufficiently pointed out, necessitates an 

 ample provision for its due performance ; and thus we find that the 

 stimulus for the excitement of the movements may be transmitted 

 through several channels. Its chief source, no doubt, is in the lungs ; 

 and arises from the presence of venous blood in the capillaries, and of 

 carbonic acid in the air-cells. Under ordinary circumstances, that is, 

 when the blood is being duly aerated, and the air being properly renewed, 

 the impression thus made upon the nerves of the lungs is so faint, that 

 we cannot perceive it, even when we specially direct our attention to it. 

 But if we suspend the movements for a moment or two, we immediately 

 experience a sensible uneasiness. The Par Yagum is obviously the 

 channel, through which this impression is conveyed to the nervous cen- 

 tres ; and it is found that, if the trunk of this nerve be divided on both 

 sides, the respiratory movements are greatly diminished in frequency. 

 Hence it is undoubtedly one of the principal exciters of the respiratory 

 movements. 



636. But the sensory nerves of the general surface, and more parti- 

 cularly the sensory 'portion of the Fifth pair, which supplies the face, 

 are most important auxiliaries, as excitor nerves ; the inspiratory move- 

 ment being peculiarly and forcibly excited by impressions made upon 

 them, especially by the contact of cold air or water with the face. 

 The power of the impression made by the air upon the general surface, 

 and particularly upon the face, in exciting the inspiratory movement, 

 is well seen in the case of the first inspiration of the new-born infant^ 

 which appears to be excited solely in this manner. An inspiratory 

 effort is often made, as soon as the face has emerged from the Vagina 

 of the mother; whilst, on the other hand, if the face be prevented 

 from coming into contact with cool air, the inspiratory effort may be 

 wanting. When it does not duly take place, it inay often be excited 

 by a slap with the flat of the hand upon the nates or abdomen ; a fact 

 which shows the special influence of impressions upon the general 

 surface, in rousing the motor impulse in the Medulla Oblongata, and 

 in causing its transmission to the muscles. The deep inspirations which 

 follow a dash of cold water upon the face, or the descent of the cold 

 douche or of the divided streams of the shower-bath upon the body, or 

 the shock of immersion in the cold plunge-bath, all testify to the 

 powerful influence of such impressions in the adult ; and the efficacy 

 of other kinds of irritation of the skin, such as beating with holly- 

 twigs, in maintaining the respiratory movements in cases of narcotic 

 poisoning, shows that the required impressions are not restricted to 

 the contact of cold air or water. It seems probable, from various facts, 



