ARCHITECTURE AT MATURITY. 187 



One general constructional feature in the mammalian 

 system is the crossing of the middle line by the neurons 

 of many cells. This arrangement brings the cells 

 located on one side of a median vertical plane into 

 connection with the structures located on the other 

 side. This capacity for decussation is confined to the 

 elements of the medullary system, whereas the out- 

 growths of the ganglionic system remain on the same 

 side of the body as that on which they originate. The 

 fact that these decussations are completed rather late in 

 the growing period suggests why they should be highly 

 variable, and thus at times contribute to asymmetry in 

 form. Within the central system the decussations of 

 the optic fibres and the pyramidal tracts are the most 

 familiar examples. The different cranial nerves ex- 

 hibit this arrangement to varying degrees, and while 

 the decussation is complete in the case of the fourth 

 nerve and absent in the case of the sixth, it is partial 

 in the third. (Fig. 31.) 



Outside of the central system, decussations are rare, 

 but have been described for the peripheral nerves going 

 to the electric organ in the torpedo (Fritsch) and in the 

 superior laryngeal nerve of the tortoise (Weir Mitchell). 

 As illustrating the variability in these decussations, 

 Flechsig's study of the pyramidal tracts in man may be 

 cited. 1 It is known that at the crossing of the pyramids 

 certain groups of fibres pass the middle line, and ulti- 

 mately enter the lateral column on the opposite side of 

 the spinal cord. Others do not decussate at this point, 

 but pass into the ventral columns, ultimately crossing at 

 lower levels. Flechsig found that practically the two 

 extreme cases conceivable might be present, viz., that 

 all the fibres might pass into the lateral column or 



1 Flechsig, Leititngsbahnen im Gehirn und Riickenmark, Leipzig, 

 1876, 



