RESPIRATION 57 



gerated reflexes and general failure of nervous coordination in 

 "neurasthenia," fatigue, and "shock," we seem to render these 

 conditions more intelligible. Thus the great general nervous ir- 

 ritability, exaggeration of circulatory reflexes, tendency to sweat- 

 ing, and occasional instability of temperature, as observed in 

 "neurasthenia/' are probably analogous to the exaggerated re- 

 flex restriction in the depth of breathing and the inability to hold 

 a breath. All these symptoms seem to be due to what Hughlings 

 Jackson called "release of control." 



In the causation of military neurasthenia the nervous over- 

 strain of war, and the shocks to the nervous system in connection 

 with various incidents of warfare and gross bodily injuries had 

 evidently played a prominent part; but it was equally evident 

 that infections of different sorts were also in part responsible for 

 the condition, the nervous system being apparently weakened by 

 toxic influences. In the same way ordinary fatigue of the respira- 

 tory center or other parts of the nervous system may be due not 

 merely to extra work, but also partly to want of oxygen (as will 

 be shown later) , or to other chemical influences. Neurasthenia may 

 thus be regarded as only a more lasting and persistent form of 

 ordinary fatigue or exhaustion. It will be shown later that a very 

 important secondary effect of the shallow breathing characteris- 

 tic of neurasthenia or fatigue of the respiratory center is im- 

 perfect oxygenation of the blood. 



The readiness with which a given resistance to breathing pro- 

 duces signs of fatigue of the breathing varies greatly in different 

 individuals. In some persons a comparatively small resistance 

 suffices to produce shallow breathing and rapid exhaustion of the 

 respiratory center, though in other quite healthy persons a very 

 considerable resistance is needed. Men with symptoms of neu- 

 rasthenia are, as might be expected, particularly sensitive to re- 

 sistance. This matter is, of course, important in connection with 

 the design of respirators, etc. A respirator causing any consider- 

 able resistance may easily disable a man for muscular exertion. 



The threshold alveolar CO 2 pressure at which the respiratory 

 center begins to be excited may be altered by various abnormal 

 conditions which will be discussed further in later chapters. The 

 threshold may be lowered by want of oxygen or by the presence 

 in the blood of an abnormally low proportion of available alkali, 

 or by certain drugs, including, as Yandell Henderson 11 has pointed 



11 Yandell Henderson and Scarbrough, Amer. Journ. of Physiol., XXVI, p. 

 279, 1910. 



