34 THE BRAIN OF THE TIGER SALAMANDER 



analysis. This is a close-meshed web of finest axons, within which all 

 cell bodies are imbedded. It is everywhere present, providing con- 

 tinuous activation (or potential activation) of every neuron, summa- 

 tion, reinforcement, or inhibition of whatever activities may be going 

 on in the more superficial layers and affecting the general excitatory 

 state of the whole central nervous system. It receives fibers from all 

 sensory and motor fields and seems to be the basic apparatus of in- 

 tegration. In man this type of tissue survives in the periventricular 

 gray of the diencephalon, and, as suggested by Wallenberg ('31), it 

 probably plays an important part in determining the disposition and 

 temperament of the individual ('34rt, pp. 241, 245). This unspecial- 

 ized tissue may serve the most general totalizing function. From it 

 there have been derived the complicated mechanisms for synthesiz- 

 ing the separate experiences and organizing them in adaptive pat- 

 terns — a process of differentiation which culminates in the human 

 cerebral cortex. The relations of neuropil to reflex arcs are discussed 

 in chapter vi. 



The relative abundance of myelinated fibers is a rough indicator of 

 the relation between stable and labile types of performance. Thus 

 the myelinated white substance is relatively greater in the spinal 

 cord than in the brain, and the ratio of myelinated to unmyelinated 

 tissue diminishes as we pass forward in the brain, as is well illustrated 

 by a published series of Weigert sections drawn from a single speci- 

 men (no. IIC) from the olfactory bulb to the spinal cord ('10, figs. 

 8-21; '25, figs. 2-9; '44&, figs. 1-6). This is because the myelinated 

 fibers, most of which are long conductors of through traffic, tend to 

 be compactly arranged, with relatively scanty collateral connections 

 with the neuropil. 



In the gray substance there is a similar increment m relative 

 amount of neuropil as we pass forward from spinal cord to hemi- 

 spheres, the higher levels being specialized for correlation, associa- 

 tion, conditioning, and integration and the lower levels for stabilized 

 total activities and reflexes. In the phylogenetic series this principle 

 takes a form which can be quantitatively expressed as Economo's 

 coefiicient of the ratio between total gray substance and the mass of 

 the nerve cells contained within it. The lower the animal species in 

 the scale, the greater is the mass of the cells compared with the gray 

 substance. This law may be expressed in the converse form: Higher 

 animals have a larger proportion of neuropil in the gray substance, 

 thus giving them capacity for conditioning and other individually 



