HISTOLOGICAL STRUCTURE 37 



complexity in some reptiles, as described by Slianklin ('3.'3) and others. In the mam- 

 mals it retains its individuality as the chief terminus of basal optic fibers, though 

 other fibers of this system are rather widely spreatl in the surrounding tegmentum 

 (Gillilan, '41). The history of the visceral and other components of the amphibian 

 ventrolateral neuropil has not yet been written. 



In my recent description ('42) of the optic system of Amblystoma, special at- 

 tention was given to the central distribution of thick and thin fibers from the retina, 

 and this was followed ('42, p. 295) by some speculations about its physiological sig- 

 nificance. In development the thick fibers appear first, and the number of thin 

 fibers is enormously increased in later stages, particularly at the time of meta- 

 morphosis. Thick fibers conduct more rapidly than thin fibers, and this time factor 

 may play an important part in the central analysis of mixed retinal excitation. 



Both thick and thin fibers end in the thalamus, pretectal nucleus, and tectum; 

 and in each of these fields their terminals are mingled, not segregated. The optic 

 fibers to the basal peduncular neuropil are moderately thick; upon retinal stimula- 

 tion this field, accordingly, is the first to be activated, and its excitation of the under- 

 lying peduncular neurons may precede any influence upon these cells from the longer 

 paths by way of the tectum and thalamus. Figure 22 shows in continuous lines the 

 major optic tracts and the motor paths from the peduncle, and in broken lines some 

 internuncial connections. The thick fibers which descend from the tectum and 

 thalamus are myelinated; some of them are crossed in the posterior commissure 

 and the commissure of the tuberculum posterius, and some are uncrossed. They 

 connect primarily with the descending pathway in the ventral tegmental fascicles 

 (f.r.t.). The thinner correlating fibers take other pathways, and they may make 

 synaptic connection with peduncular neurons in any one or all of the four layers of 

 the neuropil. The efl'erent path may be to low er motor centers through the ventral 

 tegmental fascicles or to the muscles of the eyeball through the III nerve. 



Figures 24 and 93 are diagrammatic transverse sections at the level of the III 

 nuclei and the middle of the tectum, illustrating some of the tecto-peduncular con- 

 nections. These fibers are all of medium or thin caliber and, for the most part, 

 unmyelinated. The dendrites of the peduncular neurons ramify widely throughout 

 the entire thickness of the brain wall, and few, if any, of them are in physiological 

 relation with any single one of the afferent systems of fibers. Each of them may be 

 activated by any or all of these systems. The only significant localized specialized 

 tissue here seems to be in the neuropil, where the texture is different in the four layers 

 and where all peduncular neurons spread their dendrites in all these layers. Since 

 the movements which are activated from this motor pool are not disorderly con- 

 vulsions, it is evident that the discriminative and well co-ordinated responses which 

 follow its excitation are not ordered primarily by the arrangement in space of the 

 motor elements. There are differences in the structural arrangements of the synaptic 

 junctions, though these are not so pronounced as in most other animals, and there 

 may be chemical and other factors yet to be determined. As I have elsewhere 

 pointed out, a time factor can be recognized by physiological experiment, and its 

 structural indicator can be identified histologically because large fibers have a faster 

 rate of conduction than small fibers and the interpolation of synaptic junctions in a 

 nervous pathway retards transmission. 



In the structural setup before us it may be inferred that the first result of a retinal 

 excitation is the activation of the entire tectum, pretectal nucleus, and dorsal thala- 

 mus through the tliick myelinated fibers of the optic nerve and tract and also of the 

 ventrolateral neuropil of the peduncle through the basal optic tract. This is pre- 

 sumably a generalized nonspecific effect, and it will come to motor expression, 

 first, through the basal tract, for this is the shortest path. The resting state is 



