Nervous System 



357 



Fig. 132. Nerve growth in "alternate-choice" experiment (from Weiss and Taylor, '44"). Top- Diagram of 

 operation. Proximal nerve stump (left) is introduced into stem of Y-shaped artery; one branch of artery Is 

 sealed off distally, the other branch contains degenerating nerve as supposed "lure." Middle: Experimental 

 case, 20 weeks after operation, showing both the blind and the connected branches filled with regenerated 

 nerve fibers. Bottom: Detail from same case at bifurcation at higher magnification. 



ciprocal fiber tracts in the brain, etc.). Thus, 

 the only known electric effects are the inhibi- 

 tory ones mentioned above (p. 356). 



Chemical "attractions" have been postu- 

 lated largely on the strength of Cajal's ('28) 

 varied experiments demonstrating a tendency 



of regenerating nerve fibers to converge upon 

 the open end of any degenerated nerve stump 

 (Fig. 131, middle), as if the latter were a 

 source of "neurotropically" active substances. 

 While the observations were correct, the in- 

 terpretation was not. The nerve fibers are 



