1893.] the Normal State of the Knee Jerk is altered. 453 



I trust, therefore, we are justified in concluding that the failure of 

 the spinal centres, to which the knee jerk is related, to perform their 

 functions is due in large measure to their being starved of oxygen ; 

 for this same starvation occurs in asphyxia when nitrogen or nitrous 

 oxide is inhaled, and when anaemia of the lumbar cord is produced ; 

 in all these conditions abolition of the knee jerk being the ultimate 

 result observed. 



"With regard to the part played by carbonic acid in helping to bring 

 about this effect, what has already been said with reference to the 

 preliminary exaltation of the knee jerk is wholly applicable ; there- 

 fore nothing further need be said on this subject. 



The behaviour of the knee jerk when it reappears after being 

 absent for a time, taken in connexion with its behaviour when first 

 deprived of its normal supply of oxygen, and before it was abolished, 

 suggests the probability that not merely the presence of oxygen is 

 necessary for the lumbar cells to functionate normally, but a certain 

 definite quantity of that gas. And this normal balance appears to 

 be upset either by an insufficient quantity of the gas, or by too 

 much of it ; for when pure oxygen is inhaled the same exaltation is 

 observed. 



II. The Effects of the Inhalation of certain Gases. 



The results obtained with nitrogen and nitrous oxide, coinciding 

 as they do with those obtained in asphyxia, lend strength to the 

 hypothesis which has been advanced in explanation of the phenomena 

 related to asphyxia ; for when either of these gases is inhaled, to the 

 entire exclusion of all atmospheric air, the blood is principally 

 deprived of its oxygen, and so also the lumbar centres. Here, again, 

 it is therefore probable that the absence of oxygen is to be looked 

 on as the prime factor which brings about the alterations in the knee 

 jerk, which have been already detailed, the exact form of alteration 

 apparently depending on the amount of oxygen left in the blood. 

 When the amount of oxygen in the blood is small, or only moderate, 

 the lumbar centres become more excitable, while when it is absent, 

 or only present in very minute traces, inaction of these centres 

 results. 



III. The Effects of Ether and Chloroform. 



That the results obtained with these drugs are due to their direct 

 action on the lumbar centres is shown by the fact that the results 

 are almost identical, whether these centres have or have not been cut 

 off from their connexion with higher centres by complete transverse 

 section of the cord in the dorsal region. It is a little curious that 

 the lumbar centres should show the same increased excitability when 



