450 H. H. DONALDSON 



Avith a water content of about 48 per cent. Moreover, the myelin 

 must be regarded as a more or less extraneous substance, having 

 but little significance for the characteristic activities of the 

 neurons. 



If we compare the loss of water in the case of the nervous 

 system with that in the muscular system, which also contains 

 a large proportion of fat (Tribot '05), we find that while the two 

 systems lose about the same percentage of water between birth 

 and maturity (Lowrey '13), yet in the case of the nervous system 

 alone is this lipoid (or non-protein substance) accumulated out- 

 side of the cell. From this it is seen that the neuron is peculiarly 

 able to maintain its early water-solids composition and that it 

 accomplishes this by throwing out the material, which in the 

 muscles is retained within the cells. 



As the diminution in the percentage of water in the central 

 nervous system is preeminently a function of age, and as it 

 appears to be due almost entirely to the formation of the myelin, 

 it follows that the myelin formation is also a function of age 

 (Donaldson '11). A glance at the graphs in Chart 1 shows that 

 the most active production of myelin, as indicated by the rapid 

 loss in the percentage of water, occurs early, i.e., during the first 

 forty days of rat life, in the brain, and during the first hundred 

 days, in the spinal cord. 



