CENTRAL NERVOUS SYSTEM 543 



In Cyclostomes and Fishes the entire telencephalon and much of the diencephalon 

 are devoted to olfactory activities. In Amphibians are seen the beginnings of the 

 emergence of non-olfactory systems into this region of highest integration ; although 

 no part of their cerebral hemispheres is free from olfactory connections, much of their 

 thalamus is devoted to other sensory mechanisms, and it is significant that the first 

 indication of a sensory cortical projection — the thalamo -cortical tract of Amphibians 

 — is not visual (Rubaschkin, 1903 ; Herrick, 1917). In Reptiles the ascending sensory 

 systems are greatly enlarged so that they monopolize areas of the corpus striatum and 

 the cortex, while in Mammals the sensory and somatic systems dominate the cortex 

 to an ever-increasing degree until in man the olfactory centres become insignificant 

 and are relegated to an obscure corner while the visual projections are prolific and 

 widespread. 



In Cyclostomes and selachian Fishes the entire fore-brain is represented 

 by an insignificant paleocortex with purely olfactory functions, which 

 persists as the pyriform lobes (the primary olfactory cortex) of the higher 

 Vertebrates. To this is added in Teleosteans the archicortex, still entirely 

 olfactory, which persists as the hippocampus (the secondary olfactory 

 cortex). It is interesting that in Cyclostomes, " ganoid " and teleostean 

 Fishes the fore-brain has a non-nervous (ependymal) roof. In Amphibians 

 some non-olfactory fibres reach the fore-brain and in Reptiles a true 

 cortex first appears. In Birds this structure is well developed and the 

 olfactory lobes have become small ; but its surface is still smooth, the roof 

 is still thin and its main mass is occupied by the relatively enormous corpus 

 striatum. The neopallium as we know it is a characteristic of Mammals, 

 serving as a receptor area of optic, auditory, tactile and other sensory 

 stimuli, an initiator of voluntary movements and a centre for associative 

 memory and eventually of conceptual thought. As the scale of mammalian 

 evolution is ascended this portion of the cerebrum becomes increasingly 

 important and the olfactory area less ; m Insectivores or the rabbit, for 

 example, the cortex is only slightly convoluted and does not cover the 

 cerebellum (Fig. 708) ; in the horse and the dog convolutions have become 

 prominent (Fig. 709), while in the Primates both anatomically and function- 

 ally it has become the master-tissue wherem afferent sensory impressions 

 are assessed and stored and are correlated through an intricate system of 

 association fibres with the complex activities of these animals. 



The evohdion of the visual pathways and centres will be readily understood 

 from this short sketch of the phylogenetic development of the brain (Figs. 

 710 to 715). In Cyclostomes (apart from the degenerate Myxinoids^), the 

 optic fibres from the retina are jDrojected into the superficial layers of the 

 tectum, here to come into relation with the bulbo- and spino-tectal fibres 

 arriving to the deeper layers (Fig. 710) ; in these a primitive anlage of the 

 lateral geniculate body may be present (Herrick and Obenchain, 1913). In 

 Fishes a few collateral fibres are given to the still very rudimentary lateral 



J p. 734. 



