THE NERVE FIBER TRACTS. 87 



opposite the pedal ganglia, and between them they are reduced to narrow bands. 

 (Fig. 56. l.tr, left side). At the anterior end of the crura, they appear to pass 

 outside (neural) the gustatory tracts, onto the posterior neural surface of the 

 tween-brain commissure. (Fig. 49, l.tr.) 



The general cutaneous tracts are two large, continuous columns of 

 neuropile on the median side of each crus. They lie between the gustatory and 

 haemal tracts, and extend from the tween-brain to the last vagus neuromere, 

 (Fig. 56 G.c.tr.) 



They usually have a slightly different color and appearance from the lateral 

 ones, from which they are separated by numerous bundles of vertical fibers. The 

 latter are arranged with considerable regularity, the more important sources being 

 neurones B, sending axones haemally, and neurones E, sending them neurally. 

 (Figs. 56 and 65, B and E.) The principal constituents of the tracts are the 

 crossed and uncrossed terminals of the cutaneous nerves, h.n. The majority 

 of the , uncrossed fibers and the optic fascicles, op.tr, extend lengthwise of the 

 tracts. 



Comparison of the Fiber Tracts of the Arachnid and Vertebrate Brain. — 

 A comparison of the fiber tracts in Limulus with those in the vertebrates presents 

 great difficulties. These difficulties are partly due to our imperfect knowledge 

 of the brain of arachnids and of the lower vertebrates, and partly to the fact that 

 the latter is generally studied for the purpose of explaining the structure of the 

 higher types of brain, not for comparison with an invertebrate brain, that its 

 own structure might be better understood. When we know more about the 

 brain of arthropods, and less emphasis is laid on the artificial system of classify- 

 ing cranial nerves, now in vogue among American neurologists, many points are 

 likely to be cleared up which are now obscure. 



Nevertheless the facts, so far as we understand them, indicate that the arachnid 

 and vertebrate brain are in essential agreement in the distribution and relations 

 of their main fiber tracts. The agreements to which we would call attention are : 



a. In both classes, important longitudinal tracts containing the principal 

 motor fibers extend along the haemal surface of the brain and cord. 



b. In both classes, conspicuous sensory tracts lie near the median neural 

 surface, coming from segmentally arranged taste organs and from other sense 

 organs, i.e., tactile, temperature, or auditory, having a less precisely determined 

 function. 



c. In these sensory columns, there are local enlargements, or lobes, corre- 

 sponding with special local functions; i.e., in Limulus, the flabellar lobes of the 

 eighth and ninth neuromeres; in the scorpion, the pectinal lobes of the tenth, 

 eleventh, and twelfth vagus neuromeres; in vertebrates, the auditory and the 

 vagal lobes of their respective neuromeres. 



d. In both classes, the numerous gustatory fascicles and those from the 

 vagus neuromeres form a very conspicuous median neural tract, extending the 

 whole length of the brain floor. It terminates in a special center, in the dienceph- 



