442 THE NERVOUS SYSTEM. 



rior pyramids, or the lower extremity of the olivary bodies, and is some- 

 what below the apparent origin of the pneumogastric nerves. 



Respiration accordingly is an act consisting of various associated 

 movements, which have their nervous centre in the medulla oblongata. 

 The movements themselves are involuntary in character ; for although 

 those of the chest and abdomen may be for a short time increased in 

 frequency, the surplus movements thus .performed are not necessary to 

 respiration, and soon produce a fatigue which prevents their continu- 

 ance. Respiration goes on with its natural rhythm, unaccompanied by 

 fatigue, under the influence of the medulla, from the moment of birth, 

 without necessary consciousness of its existence. If arrested by vol- 

 untary effort, the internal stimulus which prompts it grows gradually 

 more urgent, until the will can no longer withstand its demands; and 

 as soon as voluntary resistance is discontinued, the movement recom- 

 mences under the independent action of the medulla oblongata. 



The function of the medulla in respiration is usually regarded as a 

 reflex act. According to this view, its gray substance is sensitive to 

 a stimulus derived from the lungs and other vascular organs, which 

 gives notice of a commencing deficiency in respiration. This excites 

 in the medulla a motor impulse, which is reflected in the centrifugal 

 direction and calls into activity the respiratory muscles. In normal 

 respiration the reflex action of the medulla takes place without an 

 appreciable sensation. On the renewal of air in the lungs by inspira- 

 tion, the unconscious demand is satisfied, the muscles relax, and expi- 

 ration follows by passive collapse of the lungs and thorax. In a few 

 seconds, as the oxygen is consumed and carbonic acid accumulates, the 

 previous condition recurs and the action is repeated as before, thus 

 causing the rhythmical alternating movements of inspiration and expi- 

 ration. 



The evidence that the medulla acts in this way as a reflex centre for 

 respiration is mainly of two kinds. First, the sudden contact of an 

 external stimulus, such as a dash of cold water on the skin, or the appli- 

 cation of a pungent vapor to the nostrils, causes almost invariably an 

 involuntary inspiration. As the medulla is the sole nervous centre for 

 respiratory movements, the external impression in these cases must be 

 conveyed by centripetal fibres to its gray substance, exciting there the 

 special motor stimulus of respiration. Secondly, division of the pneu- 

 mogastric nerve, an operation which shuts off from the medulla all 

 influences derived from the lungs, causes immediate diminution in the 

 frequency of respiration ; a result which may be explained by supposing 

 that the most effective stimulus to the medulla as a respiratory centre 

 is received from the lungs through this nerve. 



A different view of the action of the medulla in respiration is taken 

 by Foster * and by Flint. f According to these writers, the medulla 



* Text-book of Physiology. London, 1879, p. 334. 



| American Journal of the Medical Sciences. Philadelphia, 1880, vol. Ixxx., p. 69. 



