ORIGIN AND SIGNIFICANCE OF THE CEREBRAL CORTEX 227 



reflexes, rather than simple sensory elements. In the thalamic 

 centers these reflex systems in turn become the units involved 

 in a further process of coordination, the resultant responses 

 being still further removed from the rigidly determinate type 

 of response characteristic of the simple reflex arc. The efferent 

 neurons of the thalamus, like those of the midbrain, discharge 

 into the cerebral peduncles and lower motor centers, but the 

 reflex and instinctive responses thus brought to pass are of a 

 more complex sort than those of the primary and secondary 

 correlation centers of the medulla oblongata and midbrain, and 

 are far more easily modifiable by experience and by variable 

 physiological states. 



The thalamus of lower vertebrates (say all species below the 

 frog) is probably the organ of the highest associations of which 

 these animals are capable. These are mostly on the reflex and 

 instinctive plane, though of course a limited psychic factor 

 cannot be excluded. The cerebral hemisphere of fishes is domi- 

 nated by the olfactory system, as the midbrain is by the optic 

 system, and, so far as may be inferred from the anatomical 

 evidence, is by no means so efficient an associational mechanism 

 as the thalamus. There is nothing in these animals which can 

 be compared, when physiologically considered, with the mam- 

 malian cerebral cortex, though the primordia from which that 

 cortex has been derived in higher animals can be readily iden- 

 tified in them. It is indeed clearly established that the hippo- 

 campal formation (archipallium) has been differentiated from 

 the dorso-medial segment of the wall of the primitive cerebral 

 hemisphere, while the rest of the cortex (neopallium) was elab- 

 orated from materials found in the lateral wall of the hemis- 

 phere, the somatic area of Johnston. But in fishes and the 

 lower amphibians there is nothing here which conforms to our 

 ideas of cerebral certex, either structurally or functionally 

 considered. 



It is a far cry from an identification of the topographic 

 sources of the structural material from which the cerebral cor- 

 tex has been gradually elaborated to an adequate understand- 

 ing of the functional factors which have effected that differen- 

 tiation. As well might one say that the discovery of the quarries 

 from which the materials for the Parthenon were dug would 

 give an adequate explanation of that architectural masterpiece. 



