252 COLOUR VISION 



Hering thus arrived at six primary sensations, white, black, red, 

 green, yellow and blue, from which all other visual sensations are derived. 

 His theory consists essentially in the arrangement of these primaries 

 into three pairs, the members of each pair being antagonistic to each 

 other and dependent upon antagonistic physiological processes. The 

 physiological processes are assumed to affect three different hypothetical 

 visual substances, white-black, red-green and yellow-blue substances. 

 These substances exist somewhere in the sub-cortical visual paths : 

 their exact position is not defined, and it is indeed unnecessary to 

 predicate three separate substances, except for the sake of clearness^. 

 The opponent physiological processes are expressed in terms of an- 

 tagonistic directions of metabolic change. 



Hering- supposes that when a living substance is protected from 

 external stimuli it undergoes spontaneous autonomous metabolic 

 changes. Some molecules break down or undergo dissimilation (or 

 katabolism), fresh ones are built up or undergo assimilation (or anabol- 

 ism). When the two processes balance each other the substance is in 

 a state of autonomous equilibrium. It is to be noted that autonomous 

 equilibrium does not necessarily mean physiological inactivity. Fresh 

 formative matter (Beale) may be brought from the blood in the exact 

 quantity necessary to replace the formed matter which is poured out 

 into the blood. If the substance is acted upon by an external stimulus 

 allonomous metabolic changes are set up. They may be either anabolic 

 or katabolic, but they induce a spontaneous tendency in the opposite 

 direction so as to re-produce autonomous equilibrium, i.e., allonomous 

 katabohsm, for example, induces autonomous anabohsm. With con- 

 stant stimulation the autonomous anabolism becomes equal to the 

 allonomous katabolism, and a new condition of equilibrium at a lower 

 potential is set up, which is called allonomous equilibriutn. Upon 

 removal of the stimulation autonomous anabolism will prevail for a 

 time until autonomous equilibrium is again set up. 



Hering's metabolic theory of the activities of living substances has 

 been elaborated and put into more concrete form by Verworn^. It has 

 had a considerable influence upon modern physiology. The funda- 

 mental principles have been adversely criticised by McDougalH. 



Applying Hering's hypotheses to the visual sensations^, black, green 



1 Hering, Sitz. d. Wiener Akad. xcviii. 3, 73, 1889. 



- Vorgdmje der lehenden Materie, Prag, 1888 : translated in Brain, xx. 232, 1897- 

 3 AUgemeine Physiologie, Jena, 1901. * Brain, xxvi, 153, 1903. 



s Rivers, Schdfefs Physiology, p. 1112, 1900. 



