346 



PHYSIOLOGY 



CHAP. 



to light the pigment descended and the cones contracted, even in 

 the eye that remained in the dark. He further noted that the 

 same effect could be produced in both eyes on exposing the back 

 and the hind-limbs of the frog to light, while the eyes were kept 

 in darkness. This reflex action of the nervous system does 

 not occur with the visual purple which only changes in the 

 eye that is directly illuminated, and undergoes no appreciable 

 alteration in the retina that is kept in the dark. 



The reflex action on the cones and pigment-cells of the retina 

 may also be seen in decapitated and bloodless frogs, provided the 



FIG. 169. Vertical section of temporal half of retina of A'a?ia esculenta, fixed in Flemming's fluid. 

 (P. Chiarini.) A, after keeping the animal in direct sunlight for 6 hours ; B, after keeping it 

 in the dark for 24 hours. 



brain is left intact ; once the brain is destroyed, the light only 

 affects the illuminated eye. To explain the transmission of 

 excitation from the retina or from the skin to the brain, and 

 thence to the "myoid" elements of the retina, Engelmann 

 assumed the presence in the optic nerve of centrifugal or retino- 

 motor conducting paths which, as we have seen, were directly 

 demonstrated by Cajal (Fig. 159). 



Fick disputed Engelmann's conclusions, but they were con- 

 firmed by Nahmacher (1893), who observed movement of the 

 pigment and contraction of the cones in frogs kept in darkness, 

 on stimulating either the chiasma or the optic nerve with sodium 



