32 THE BRAIN OF THE TIGER SALAMANDER 



axonic neuropil, within the meshes of which dendrites of neurons 

 ramify widely. These axons are unmyelinated, and every contact 

 with a dendrite or a cell body is a synaptic junction. Eveiy axon con- 

 tacts many dendrites, and every dendrite has contact with many 

 axons, and these may come from near or from very remote parts. 



All neuropil is a synaptic field, and, since in these amphibian brains 

 it is an almost continuous fabric spread throughout the bram, its 

 action is fundamentally integrative. But it is more than this. It is 

 germinative tissue, the matrix from which much specialized structure 

 of higher brains has been differentiated. It is activated locally or 

 diffusely by every nervous impulse that passes through the substance 

 of the brain, and these impulses may irradiate for longer or shorter 

 distances in directions determined by the trend of the nerve fibers of 

 which it is composed. This web of conductors is relatively undifferen- 

 tiated, but it is by no means homogeneous or equipotential, for each 

 area of neuropil has characteristic structure and its own pattern of 

 peripheral and central connections. Many of these fibers, which take 

 long courses, may connect particular areas of gray substance either 

 in dispersed arrangement or assembled as recognizable tracts. In 

 fact, we find all gradations between an almost homogeneous web of 

 neuropil and long well-fasciculated tracts of unmyelinated or mye- 

 linated fibers. Many of these long fibers have collateral connections 

 throughout their length; others are well-insulated conductors be- 

 tween origin and termination. 



The web of neuropil shows remarkable diversity in different re- 

 gions. In some places it is greatly reduced, as, for instance, in the 

 ventral funiculi, where the alba is densely filled with myelinated 

 fibers; in other places there are local concentrations of dendrites and 

 finest axons so dense that in ordinary haematoxylin or carmine prepa- 

 rations they appear as darker fields, the Punksuhstanz of the early 

 histologists and the glomeruli of the olfactory bulb and inter- 

 peduncular nucleus (chap. xiv). When some of these fields are ana- 

 lyzed, it is found that their fibrous connections conform with those of 

 specific "nuclei" of higher brains. In Ambly stoma such a field is not a 

 "nucleus," for it contains no cell bodies; but examination of the cor- 

 responding area in anurans and reptiles may show all stages in the 

 differentiation of a true nucleus by migration of cell bodies from the 

 central gray outward into the alba ('27, p. 232). Such a series of 

 phylogenetic changes can be readily followed in the corpus striatum, 

 the geniculate bodies, the interpeduncular nucleus, and many other 



