1086 



PHYSIOLOGY 



The first phase in the phenomena of asphyxia is thus conditioned simply 

 by the changes in the carbon dioxide tension. A little later the gradual 

 exhaustion of oxygen in the blood round the centre begins to make itself 

 felt. The respiratory centre shares with the rest of the central nervous 

 system a sensitiveness to the absence of oxygen, deprivation of oxygen 

 having first an excitatory and later a paralytic effect. In asphyxia the 

 first centres to feel this effect are those of the cortex, and during the first 

 stage there is mental excitation terminating rapidly in abolition of con- 

 sciousness. During the second stage there is a discharge of energy, which 

 spreads throughout the whole nervous system, beginning in the bulbar 

 centres and causing a great rise of blood- pressure with slowing of the heart, 



it 



FIG. 508. Dissociation curve of oxyhaemoglobin in defibrinated cat's bleed. 

 1, cat I, after partial occlusion of trachea and fifteen minutes breathing of gas 

 l lnC L aSmg P. overtv in oxygen ; 4, cat II, at beginning of experiment ; 3, cat II, 

 ifteen minutes gas respiration ; 2, after twenty-one minutes ditto. 



and extending thence to all the spinal centres with the production of muscular 

 spasms. At this stage too there is a discharge of impulses giving contraction 

 of the pupil, and a discharge along the whole sympathetic system, producing 

 the various phenomena of vaso-constriction, erection of hairs, sweating, 

 salivation, which are generally brought about by stimulation of different 

 parts of this system. The phenomena of the third stage are due to exhaus- 

 on of the nerve-centres, accompanied or preceded by exhaustion and dilata- 

 tion of the heart, the circulation failing before the excitation of the lower 

 centres has entirely come to an end. In this third stage it is impossible 

 by the strongest stimuli to evoke any reflex. 



Considerable discussion has taken place as to the exact nature of the stimulation 



Dught about by want of oxygen. The blood of animals which have been killed by 



asphyxia is known to contain reducing substances, so that oxygen added to it disappears 



mnot be recovered in a vacuum. Pfliiger therefore suggested that it was these 



cmg substances themselves which were effective exciting agents. It was shown 



any years ago by Hoppe-Seyler and his pupils that in conditions of chronic oxygen 



