716 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



therefore the discussion of the general powers of the nervous system for per- 

 formance must never leave this factor unconsidered. 



Constituents of the Central System. Calculation shows that the cell- 

 bodies probably contribute less than 10 per cent, of the entire weight of the 

 central system, so that the remainder must be made up of neurons and other 

 tissues. 



In the central system there are present, besides the nerve-elements proper, 

 the sustentacular tissues and the nutritive vessels the channels for blood and 

 lymph. Just what fraction of the total weight of the central system is thus 

 represented has not been exactly determined, but it must be nearly equal to 

 that of the nerve cell-bodies alone. 



The weight of the brain is the weight of these several constituents. 



Of course a brain congested with blood would weigh more than one from 

 which the blood had been largely withdrawn, but there is no way of controlling 

 this condition directly. Previous to weighing, the brain is sometimes sub- 

 divided and even cut into large sections, in which case of course much of the 

 blood and lymph has the opportunity to drain away. In some cases too the 

 brain is weighed without, and in others with, the pia. 



"Weight of the Pia and Fluid. Broca's table for the weight of the pia in 

 males is as follows : l 



20-30 years 45 grams. 



31-40 " 50 " 



60 " 60 " 



The cast of the ventricles, as made by Welcker, displaces 26 cubic centi- 

 meters of water, so that the fluid filling these cavities would weigh a trifle 

 over 26 grams. 



Percentage of Water. In man the percentage of water in the gray matter 

 of the cerebrum is 81.8 per cent., and in the white matter 70 per cent. 2 



Specific Gravity. According to calculation, the specific gravity of the 

 entire encephalon is 1036.3 in the male and 1036.0 in the female. Ober- 

 steiner 3 found the specific gravity of the cortex to gradually increase from 

 frontal to the occipital lobe. It was further found that while the outermost 

 layer of the cortex had a specific gravity of 1028, that of the middle layers 

 was 1034 and of the deepest layers 1036, thus indicating a progressive increase 

 from the most superficial to the deepest layers an increase to be associated 

 with the larger proportion of medullated fibres in the deeper layers. 



Weight of the Encephalon and Spinal Cord. As a result of the pre- 

 ceding statement it follows that when the weight of any portion of the nerv- 

 ous system is taken, the final record represents, in addition to the weight of 

 the nerve-tissues proper, that of the supporting and nutritive tissues, together 

 with the enclosed blood and lymph. It is, however, assumed that under 

 normal conditions the relation between the nervous and non-nervous tissues is 



1 Broca, quoted by Topinard : Elements d' Anthropologie generate, 1885. 



2 Halliburton : Journal of Physiloogy, 1894. 8 Centralblatt filr Nervenheilkunde, 1894. 



