70 PHYSIOLOGY CHAP. 



of the various tissues of the higher animals, we may refer to a 

 remarkable experiment of Pfliiger's on the frog. He placed two 

 of these animals in an atmosphere at C. which had scrupulously 

 been deprived of every trace of oxygen. After a quarter of an 

 hour they exhibited considerable dyspnoea, which, however, was 

 unaccompanied with convulsions. After five hours the frogs were 

 quiet and flaccid, but reacted to stimulation with a wire. After 

 nineteen hours they lay as if dead, and no longer reacted to the 

 strongest cutaneous stimuli, or showed any trace of respiratory 

 movement. After twenty hours, they were taken out of their 

 prison into the fresh air, but no sign of life could be elicited in 

 spite of repeated insufflation of air through the trachea. On 

 opening the thorax of one of the frogs, Pfiiiger was astonished 

 to see the heart still beating with great energy, while the 

 arteries contained bright red blood. But it was not till two hours 

 after the animal had been brought into the oxygenated atmo- 

 sphere that spontaneous muscular movements were exhibited, 

 followed by reflex movements and spontaneous respiration. The 

 more complicated voluntary movements, however, which depend 

 upon the higher nervous system never came back. 



To explain this long survival in an atmosphere wholly deprived 

 of oxygen, it must be admitted for vertebrates also that the living 

 protoplasm of the various tissues has the property (in different 

 degrees) of utilising the oxygen which is bound up in the 

 organic molecules. The cells of the central nervous system 

 are the most sensitive to deprivation of free oxygen ; other cells, 

 on the contrary, can live for a long while in a medium destitute of 

 oxygen, because they have the power of taking it from organic 

 combinations, and utilising the potential energy. 



The most interesting phenomenon, from the point of view of 

 anaerobic metabolism, is afforded by the group of bacteria which 

 are not only capable of living in the absence of oxygen, but die 

 in a medium that contains it, e.g. Tetanus and Anthrax bacilli. 

 Interesting phenomena, too, are exhibited by other bacteria, e.g. 

 the comma bacillus of Cholera, which is greedy of oxygen, and is 

 at the same time capable of living and multiplying enormously 

 in the intestine, where there is no trace of free oxygen, so that it 

 must necessarily utilise the combined oxygen of the alkaline salts. 



IV. In addition to food -stuffs, water and oxygen, which 

 penetrate into the body, and directly condition metabolism, 

 other conditions of a dynamic character are indispensable 

 in order that the vital functions may be accomplished. The 

 external temperature exercises a predominant influence on elemen- 

 tary organisms. Each cell demands a temperature oscillating 

 between given limits, beyond which the cell must die. For the 

 majority of plant and animal cells, the maximal limit of endurable 

 temperature lies between 40 and 47 C. Kiihne found that the 



