22 THE ACTIVE FORCES OF LIVING ORGANISMS 



is carried on under conditions of excessive oxidation 

 and of loss. 

 The reia- In respect to the nature of metabolism in the cere- 



tively 



small bral and other nerve centres, one feature deserves 

 oxygen in Special attention. The composition of the chief com- 

 braicom- pounds which go to make up the brain and spinal 

 ter^hfef^" cord, or those parts more properly called nervous 

 oflhSr''''^ matter— of lecithin, that is to say, and protagon— is 

 iSn''^°' such that, however rapid metabolism may be, the 

 actual degree of oxidation in a healthy subject is very 

 limited. As already remarked, the amount of oxygen 

 compared to the other elements which they contain 

 is, indeed, so small that they are incapable of absorb- 

 ing much of that gas when forming, or of producing 

 much carbonic acid when breaking down. When the 

 venous blood from the brain is compared with that 

 proceeding from a muscle, it is found to contain much 

 less carbonic acid. Both the white matter of the 

 brain and muscle contain a large proportion of water, 

 the amount being 70 per cent, in the former and 

 75 per cent, in the latter. But the chief constituents 

 of cerebral matter, such as protagon, cerebrin, and 

 lecithin, contain more carbon in proportion to the 

 amount of oxygen which enters into their composition 

 than does the proteid of muscle ; they also contain 

 vastly more hydrogen, and, excluding the nuclei of 

 muscular tissue, more phosphorus, though probably 

 less nitrogen. The amount of oxygen in proportion 

 to the carbon present being more in muscle than in 

 the substances just mentioned, it is natural that the 



