1136 



PHYSIOLOGY 



spreads throughout the whole nervous system, beginning in the bulbar 

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

 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 

 exhaustion of the nerve centres, accompanied or preceded by exhaustion 

 and dilatation 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. 



Fia. 521. Dissociation curve of oxyhaemoglobin in defibrinated cats' blood. 



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

 of increasing poverty in oxygen; 4, cat II, at beginning of experiment; 3, cat II, 

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



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

 brought 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 

 and cannot be recovered in a vacuum. Pfliiger therefore suggested that it was these 

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

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

 starvation there was an excessive production of lactic acid in the body, and we have 

 seen that the same is true for the isolated muscle, and that to these substances has been 

 ascribed the excitation of the respiratory centre which takes place in violent muscular 

 exercise (Zuntz and Gepport). Haldane has suggested that in the hyperpncea and con- 

 vulsions, which occur as the result of breathing mixtures with very low percentages of 

 oxygen, the effective stimulus is also lactic acid. Experiments were carried out by 

 Ryffel on individuals who had been subjected in a respiratory chamber to veiy low 

 oxygen tensions, sufficient to cause cyanosis, so that their oxygen alveolar tension was 

 only about 6 per cent. After an experiment lasting four hours, there was a definite 

 increase of lactic acid in the blood of the forearm (up to 23*6 mg. lactic acid per 100 c.c.). 

 After one lasting only fifteen minutes, in which the oxygen shortage became very 

 marked, no increase could be detected. When we expose an animal such as a rabbit to 

 low percentages of oxygen, the hyperpncea so produced disappears almost immediately 



