90 ON THE NATURE AND ACTION OF THE 



The respiratory movements, besides being frequently altered in 

 form, are generally quickened after the introduction of the 

 poison ; then the number sinks to the normal or even below it ; 

 they become weaker and finally cease altogether. The blood, 

 being no longer aerated, becomes more and more venous, and, 

 by irritating either the respiratory centre itself or some nervous 

 centre closely associated with it, occasions general convulsions. 

 These disappear whenever artificial respiration is begun and the 

 blood again aerated ; while they reappear when the respiration 

 is discontinued and the blood regains its venous character. 

 This condition is to be observed in Experiment LXII. The 

 dependence of the convulsions on the venosity of the l)lood is 

 well shown by Experiment VIII of our former communication, 

 where the condition of the blood was indicated by the colour 

 of the fowl's comb, and as this became florid, or livid, the 

 convulsions disappeared or returned. After they have continued 

 a short while the convulsions cease ; for the venous blood does 

 not maintain the vitality of the nervous centres sufficiently Lo' 

 keep them in action ; but if artificial respiration be recom- 

 menced, the first effect of aerating the blood is to renew the 

 convulsions, by increasing the vitality of the nervous centres, 

 and rendering them again susceptible to the action of a 

 stimulus, though the convulsions disappear as soon as the- 

 arterialisation has proceeded sufficiently far. 



Increased rapidity of the respiratory movements may depend 

 either upon greater excitability of the respiratory centre in the 

 medulla, or upon stimulation of some oj" the afferent nerves 

 which have the power to accelerate it. The chief of these are 

 the pulmonary branches of the vagus, though there are probably 

 others proceeding from the cerebrum, through which the 

 emotions influence the breathing, and others from the general 

 surface of the body. 



In order to ascertain the cause of the acceleration of respira- 

 tion, several experiments were made. Experiment LXIII shows 

 that it is not due to the action of the poison on the cerebrum ; 

 for it occurs after the cerebral lobes have been removed. The 

 ultimate arrest of respiration is probably due, in part, to 

 paralysis of the medulla, and, in part, to paralysis of the motor 



