I 



Phenomena of neurobiotaxis in thc optic system 291 



from ¡t. The sequence the activation of the neuroblasts follows the di- 

 rection of the growth of the activating bundle, causing the próxima! neu- 

 roblasts to be foremost in formirig their axonal offshoots (fig. 211. 



Also the wellknown perpendicular position of the coUaterals on their 

 neiirite, is a consequencc of this natural law. 



Thus the inítial direction is determined by the activating sti- 

 mulus. 



The end-point however of these new axons or collaterals obviously 

 cannot be defined by this vis a tergo, but probably is determined by 

 regions that are stimulatively connected with it, i. e. by cellgroups which 

 by some influence, are in stimulation at that moment. 



Thus \ve find also here correlative stimulation between the terri- 

 tory of origin and ending of these tracts, to be the chief factor. 



This stimulative relation appears to precede and to determine the 

 course of these tracts and explains many peculiarities of their end — point, 

 f i. the singular fact that central motor tracts as the spinal cord pyramids 

 do not end originally in motor — but sensory regions. 



The physico-chemical factors which direct these tropisms,the eiectric phenome- 

 na that accompany stimulation and which also determine the dynamic ( = neurob¡o- 

 tactic) polarisation of the neurone, have been disciissed in my IXth contribution 

 on Neurobiotaxis (Jotirn. of Comp. Neurol^ vol 27, 1917) and in a lecture read in 

 the University of London (BraÍ7i, vol. 41, 1921). 



The relation between the early nutritive tropism of the dendrites (as f. i. is 

 very clearly shown by the horizontal cells of Cajal). and their latcr stimulative 

 tropism is treated in I' Eucephale (No. i, 1922), where I pointed out that nutritive 

 and stimulative tropisms run paralel on account of the fact that also stimulation 

 is accompanied with oxvdation \\.. Hill) and thus an eprly nutritive tropism of 

 dendrites may l)e changed in a stimulative tropism without the principies of the 

 process being changed. (Heriiigs autonomic and allonomic metaboiism.i 



The great importance of this synchronic stimulative correlation tor 

 the arrangement of axonal fibertracts is also shown in the optic nerves, 

 viz. in the cause of its partial decussation so striking in mammals with 

 binocular visión. 



S. Ramón y Cajal gave an ingenious explanation of the semidecussa- 

 tion of the N. N. optici in mammals with binocular visión by pointing 

 out that, if the cortical centres are to reccive a naturally complete image. 

 the central projection must be such as to maUe the temporal retina ot 

 one side corres|3ond with the nasal retina ot the other side. 



