CHAPTER III 

 HISTOLOGICAL STRUCTURE 



GENERAL HISTOLOGY 



IN AMPHIBIAN brains the histological texture is generalized, 

 exhibiting some embryonic features; and it is at so primitive a 

 level of organization as to make comparison with mammals difficult. 

 Most of the nerve cells are small, with scanty and relatively undif- 

 ferentiated cytoplasm. There are some notable exceptions, such as 

 the two giant Mauthner's cells of the medulla oblongata and related 

 elements of the nucleus motorius tegmenti. With the exceptions just 

 noted, Nissl bodies are absent or small and dispersed. 



Almost all bodies of the neurons are crowded close to the ventricle 

 in a dense central gray layer, with thick dendrites directed outward 

 to arborize in the overlying white substance (figs. 9, 99). The axon 

 usually arises from the base of the dendritic arborization, rarely from 

 its tip, and sometimes from the cell body; it may be short and much 

 branched or very long, with or without collateral branches. The 

 ramifications of the short axons and of collaterals and terminals of 

 the longer fibers interweave with dendritic arborizations to form a 

 more or less dense neuropil, which permeates the entire substance of 

 the brain and is a synaptic field. Some of the nerve fibers are 

 myelinated, more in the peripheral nerves, spinal cord, and medulla 

 oblongata than in higher levels of the brain. Both myelinated and 

 unmyelinated fibers may be assembled in definite tracts, or they may 

 be so dispersed in the neuropil as to make analysis difficult. The ar- 

 rangement of recognizable tracts conforms with that of higher brains, 

 so that homologies with human tracts are in most cases clear. These 

 tracts and the gray areas with which they are connected provide the 

 most useful landmarks in the analysis of this enigmatic tissue. 



In the gray substance there are few sharply defined nuclei like 

 those of mammals, but the precursors of many of these can be recog- 

 nized as local specializations of the elements or by the connections of 

 the related nerve fibers. In most cases the cells of these primordial 

 nuclei have long dendrites, which arborize widely into surrounding 

 fields (figs. 9, 24, 61, 66), so that the functional specificity of the 



