J 329 



animals than in (liat of large animals, very strikingly so in the brain 

 of the monse as compared with tiiat of the elephant '). 



Thus the total volume of the cell-bodies in large species and 

 individuals increased less than that of the nerve fibers and of the 

 medullary sheaths, the last being couimoidy regarded as an accessory 

 isolating envelope of the solely essential, although passive, cylinder 

 axis. This interpretation, has, however, not been confirmed ; the 

 results of Donat.uson and Hoke *) among others show that the 

 average volume of the tneduUary sheath, in all classes of vertebrates, 

 is equal to that of the enclosed cylinder axis, a fact distinctly 

 pointing to a less insignificant part played by the medullary sheath 

 than is commonly ascribed to it. This, however, does not account 

 for the seemingly inex|)licable combination of a regular increase of 

 the volume of the brain with the disproportionate increase of the 

 more passive fibrous parts and the constituents regarded simply as 

 enveloping and supporting interstitium in the brain with regard to 

 the cell-bodies contained in it. 



It has been tried to explain this disproportional increase of the 

 substantia alba, with increasing size of the body, by i\\e lengthenuuf 

 to which the nerve fibers are subjected in proportion to the longi- 

 tudinal dimension of the brain, while at the same i\n\Q {\\q\v nmnbev 

 increases in proportion to its volume, the nerve fibers being assumed 

 not to increase in diameter nor the cell-bodies, from which they 

 arise, in volume. *) 



According to this explanation we are obliged to accept that with 

 the supposed disproportion of the parts of the neurone, enforced on 

 it independently^ of its character by external circumstances, functional 

 disproportion too is enforced on it, even if we should continue to 

 hold the opinion of the passive part played by the nerve fiber. 



In the spinal cord something analogous is to be observed as in 

 the brain ; for in the cord too the* substantia alba increases much 

 more rapidly than the substantia grisea. ■*). 



A "disproportion" of the same kind exists necessarily between 

 the volume of the peripheral nerve fibers, which must lengthen 



') H Obersteiner, Die Kleinhirnrinde von Eleplias und Balaenoptera. Arbeiten 

 aus dem Neurologisclien Institute der Wiener Univeisitat, f'.)13, p. f53. 



') H. H. Donaldson and G. W. Hoke, On the Areas of the Axis Cylinder and 

 Medullary Sheath as seen in Gross Sections of the Spinal Nerves of Vertebrates. 

 Journal of Gomparalive Neurology and Psychology. Vol. XV. (1905), p. 1 — 16. 



^) Ernst de Vhies, Das Gorpus striatum der Saugeliere Anatomischer Anzeiger. 

 Bd. 39. Jena 1910, p. 387—388. 



'♦') This has been very clearly demonstrated by A. J. Hovy: On the relation 

 betv^^een the Quantity of White and Grey Substance in the Gentral Nervous System, 

 These Proceedings. Vol. XVI, (1913), p. 311-318. 



