much more than the gvey substance (the difTerence being about 10''/j. 



It seems to me that the rule demonstrated liei'e has to be exphiined 

 by the factors pointed out by EriNst dk Vkies viz. that if a brain 

 increases in size, the number of myehnated fibres increases with the 

 number of cells (that is with the grey substance) but since each 

 fibre (each myeline sheath) becomes so much longer ^), the whole 

 quantity of white matter increases in a higher degree than the 

 quantity of grey matter. 



Apart from the differences mentioned above, I found also a difference 

 in the shape of the grey substance in small and large representants 

 of the same orders (see fig. 2). 



As a rule the butterfly-like figure representing the grey matter 

 has a clumsy form in small animals whereas in the large ones it 

 is much more gracile. Specially the grey commissure between the 

 left and right horns becomes more gi-acile as is strikingly demonstrated 

 by a comparison between the mouse and the elephant. It seems that 

 the increase of white substance in the posterior and anterior funiculi 

 causes an enlargement of the distance between the horns. 



Finally I want to remark that also the form of the cervical cord 

 as a whole seems to be modified in large animals in such a sense 

 that in small animals this form is more round, while in large 

 animals it is more oval (see fig. 2). 



It may be possible that this difference has to be ascribed to a 

 greater opportunity for bilateral extension in the cervical part of the 

 vertebral canal in large animals. It is however equally, if not more 

 possible that the increase of white substance in large animals does 

 not take place in the same degree in all the funiculi of the cord 

 and that special parts are favoured, so it may be that the considerable 

 enlargement of the posterior and anterior funiculi, which causes the 

 horns to lie at a greater bilateral distance of each other, causes at 

 the same time an enlargement of the cord in bilateral sense. 



1 do not believe as yet that this phenomenon may be considered 

 as a rule, but I wanted to call attention to it since R. Stern (I. c. 

 p. 322) has pointed out the opposite during the growth of the human 

 spinal cord, stating that the spinal cord of the infant has a more 

 oval, the one of the adult a more circular form. 



1) Moreover the longer myeline-sheaths are often a little thicker. This is 

 however not of great importance relatively and may practically be neglected. 



On the other hand the fact that in larger brains some cells are somewhat 

 larger, as has been demonstated still recently by OfiERSTKiNEri for the Purkinje cells 

 of the Cerebellum ( Arbeiten a. d. Neurologischen Inslitut der Universiliit Wien 

 Bnd XX, 1913), may be also neglected, since its influence on these relations is 

 relatively unimportant. 



