182 VETERINARY PHYSIOLOGY 



granular layer. The deeper layer becomes differentiated 

 into a layer of polymorphic cells, and outside of this a layer of 

 fibres, the layer of Baillarger. Outside the granular layer 

 a layer of j^yramidal cells appears below the outer layer of 

 fibres. 



Histology. — At birth, the cortex in the neopallium, 

 from without inwards, consists of (fig. 92) — 



1 . Outer layer of fibres. 



2. Layer of pyramidal cells. 



3. Layer of granules. 



4. Inner layer of fibres (Baillarger's layer). 



5. Layers of polymorphic cells. 



It is the supra-granular layers which increase as develoj)- 

 ment advances. The adult mole has a cortex like that of 

 the six-month human foetus (fig. 90). 



The cells of the cortex send dendritic processes up 

 towards the surface, Avhere they form a complicated series of 

 synapses, and they also send axon-processes downwards into 

 the white substance of the brain. 



From these fibres, collaterals come off which connect 

 different parts of the cortex of the same side, and which also 

 connect the cortex of one side with that of the other, and 

 with the basal ganglia (fig. 93). 



In the rhinencephalon three layers develop in the cortex 

 — (1) the outer layer of fibres; (2) the layer of granules, 

 often curiously broken up into nests ; and (3) the layer of 

 polymorphic cells. 



2. Functional Development. 



An animal at birth is little more than a reflex machine, 

 and the functions of its cortex cerebri are still in abeyance. 

 But the ingoing tracts from the visual, auditory, tactile and 

 other receptors are fully developed, and hence from the first 

 a stream of stimuli pours in upon the cortex. 



Although these may not at first be properly integrated, 

 they influence and colour one another, giving rise to the 

 pleasurable state of consciousness in the creature signified by 



