CENTRAL NERVOUS SYSTEM 545 



are transmitted through thalamic relay-stations in an ever-increasing degree 

 to the cortex. This translation from a reflex to a highly integrative level 

 allows the development of the central nervous system to proceed along two 

 main lines — an advance from mass reflex reactions to more restricted but 

 complicated patterns of behaviour, and an advance from a fixed rigidity to 

 an extreme degree of plasticity and lability of resf)onse. 



It follows from the late projection of vision to the cerebral cortex that 

 ablation of this structure in the lower Vertebrates involves no visual 

 incapacity. The fore -brain of C'yclostomes is completely, and of Fishes and 

 Reptiles almost completely an olfactory brain and its removal has no visual 

 effects, and, indeed, entails little alteration in the locomotion and the general 

 behaviour of the animal (Magendie, 1824 ; Flourens. 1824). Fishes, 

 it is true, lose the faculty of responding to unusual stimuli with initiative, 

 become more purely reflex creatures than they already are, and are slower 

 in their reactions (Janzen, 1933 ; Hosch, 1936 ; Header, 1939 ; and 

 others) ; while Amphibians lose spontaneity and initiative in their 

 conduct and the conditioning of reflexes may fail (Diebschlag. 1934 ; 

 Aronson and Noble, 1945). Nevertheless, a decerebrate frog will catch flies 

 quickly and without difliculty (Schrader, 1887). It is not until Birds are 

 reached that removal of the cerebral hemispheres induces a general listless- 

 ness and a marked lack of response ; without a cortex the pigeon will main- 

 tain its bodily functions, will eat, mate and rear its young (Rogers, 1920-28), 

 it will avoid obstacles and select its food visuall}^ although some emotional 

 responses can be elicited to visual stimuli (Schrader, 1888). Thus a 

 decerebrate jjigeon shows some impairment of the higher faculties of 

 recognition and will not show the usual reactions to a threatening approach 

 (Visser and Rademaker, 1935). Blindness can only be caused by destruction 

 of the primary centres (Panizza. 1855 ; Schrader, 1887 ; Munk, 1890). 



It is only in Mammals that the conduct of the animal is seriously 

 disturbed by removal of its cerebral cortex, and even then it is only the 

 Primates that are rendered blind by this mutilation ; similarly it is only 

 among Mammals that cortical stinndation involves motorial responses 

 although the number of discrete movements that can be elicited in this 

 way are few among the more primitive representatives of this class (less 

 than 10 in Monotremes and Marsupials, v. Buddenbrock, 1937). In the 

 lower Mammals there is a considerable equipotentiality of function m the 

 cortex and, depending on the survival of incoming tracts, one part can 

 readily act as substitute for another. Even if the entire cortex is removed, 

 however, rabbits, after an initial period of blindness, can later differentiate 

 between light and darkness (ten Cate, 1935) and decerebrate dogs will react 

 and exhibit emotions to visual stimuli (Goltz. 1892 ; Pavlov, 1927). If, 

 however, the occipital cortex alone is removed from Rodents (rabbit, rat), 

 there is a loss of form vision only, while the faculties of perception of light 

 and spatial localization are maintained so that the animal can move around, 



