Nervous Sy sterns 835 



paired olfactory nerves. In cyclostomes and elasmobranchs the primitive pal- 

 lium (cortex or cover) known as the paleocortex contains secondary olfac- 

 tory fibers, and this paleocortex is represented in higher vertebrates by the 

 piriform lobes, the primary olfactory region of the brain in higher vertebrates. 

 The archipallium of fishes contains tertiary olfactory neurones and becomes 

 the hippocampus of higher classes: Both the paleocortex and archicortex re- 

 ceive some non-olfactory fibers and have integrative functions to varying 

 degrees in different animal groups.^'' A neurological summary of the origin 

 of the cerebral cortex is given by Herdck.^*^" In the sluggish dipnoan fish, 

 relatives of the likely ancestors of amphibians, enlarged vascular choroid 

 plexuses insure a supply of ox7gen to the brain and the ventricles are di- 

 lated. The expanded hemispheric vesicles persist in amphibians, and there is 

 addition of some non-olfactory fibers entering the forebrain, but there is no 

 superficial gray matter. In reptiles a cortex appears, with penetration into 

 the pallium of localized thalamic projection fibers. In birds the olfactory sys- 

 tem is greatly enlarged; there is a massive corpus striatum and a limited prim- 

 itive neocortex. 



The neocortex is already well developed in the most primitive mammals; 

 it is non-olfactory in its connections and lies between the paleocortex and 

 archicortex. Higher mammals are characterized by an increase in complex- 

 ity of this cortex, six cellular layers normally being recognized in the mam- 

 malian neocortex. These six layers are about 100 cells (or 100 synapses) 

 thick in higher primates but are much thinner in lower mammals.^^'* 



As one goes from the primitive orders (monotremes and marsupials) to 

 the higher (primates), there is a gradual increase in total surface by fold- 

 ing and formation of sulci between gyri. The total number of neurones in 

 the human cortex has been estimated at 10^" and that of extrinsic ascending 

 or descending neurones at 10^.^^^ Most of the cortical connections, are there- 

 fore, within the cortex itself, horizontal, or vertical from one cellular level to 

 another. The svstem of relay paths must be exceedingly complex. If we as- 

 sume all of the properties of synaptic transmission to apply at each cortical 

 junction it is little wonder that the total configuration shows unique "psy- 

 chic" properties. 



Inside the hemispheres is the massive corpus striatum, present first as a 

 sohd structure in fishes. Beside the priniitive striatum develops in higher 

 vertebrates a neostriatum which contains the various basal nuclei. These 

 are intimately connected with the thalamus; for many functions the thalamus 

 is best considered a part of the forebrain. 



The function of regions of the cerebral cortex has been studied by va- 

 rious means: by ascertaining anatomical connections; by electrical and 

 chemical (strychnine) stimulation, with observation of motor and auto- 

 nomic responses; by extirpation, with observation of deficiencies in behavior; 

 sensory and strychnine stimulation. None of these methods is adequate to 

 of locating and seizing food is unaffected; the variability in frequency of 

 measure the functioning of the system as a whole. 



The forebrain of fishes is essentially an olfactory brain, although some 

 olfactory discrimination is said to remain when the forebrain is removed. 

 The forebrain shows some spontaneous electrical rhythms^" and responds 

 well to chemical stimulation of the olfactory organs. Removal of the fore- 



