86 MINUTE STRUCTURE OF THE BRAIN AND CORD OF ARACHNIDS. 



ing to the opposite side in the anterior haemal commissures, the other turning 

 backward into the haemal tracts. (Figs. 61 and 62.) 



In the cerebral neuromeres, the haemal tracts contain similar fibers, derived 

 from similar cells. The latter may be seen in small clusters between the roots 

 of the ganglia, on the neural surface of the crura. Their fibers pass vertically 

 through the crus, joining the longitudinal tracts and the cross commissures. (Figs. 

 56, 65, 66, B, 6, and b.') 



The lateral margin of the haemal tracts of the crura receives conspicuous 

 fibers that cross in the haemal commissures with the roots of the haemal nerve. 

 Near the lateral margin of the crura they turn backward and join the lateral 

 margin of the haemal tracts (Fig. 66, left side). A similar, but less conspicuous, set 

 of fibers forms on the median side of the tract. 



The lateral margin of the haemal tracts also receives a considerable number of 

 large fibers from the third and fourth lobes of the optic ganglion, op.g 3 ' 4 , and 

 from the crossed and uncrossed fibers of the large lateral neurones of the olfactory 

 lobes, ol.c'. 



The median margin receives fibers from the large central cells of the olfactory 

 lobes, ol.c?, and from the giant association neurones in the median lobe of the 

 hemispheres. (Fig. 49, H. as. tr.) 



A remarkable band of fine fibers comes from the optic nerve, passing through, 

 or over, the lateral margin of the first two optic lobes, along the lateral margin of 

 the optic stalk, through the tween-brain, and along the median haemal surface of 

 the crura to the beginning of the cord. (Fig. 66, op.tr.) A similar band of fine 

 fibers extends along the entire lateral margin of each haemal tract of the cord. 

 The two bands are united by a narrow commissure extending across the anterior 

 margin of each neuromere. (Fig. 62, op.tr. ?) It is not clear whether these fine 

 fibered bands of the cord are continuations of the optic bands in the brain or not. 



The great majority of the longitudinal fibers of the crura that are directed 

 forward appear to terminate on the haemal surface of the forebrain commissures. 



The Longitudinal Neural Tracts. These tracts lie close to the median 

 line, on the neural surface of the cord. They receive all the fibers of the first 

 root of the haemal nerves, many fibers from the nucleus at the root of the pedal 

 ganglia, and large unbranched fibers whose origin is unknown, that pass through 

 the cord over several neuromeres. 



In the crura the neural roots of the rui-mal nerves appear to be absent, and 

 the other constituents of the neural tracts could not be certainly identified. 

 But we may recognize the following tracts which may or may not be modifications 

 of those already described. 



The lateral or pedal ganglion tracts are large and exceedingly complex, 

 consisting of a confused mass of interlacing fiber bundles which form the lateral 

 margins of the crura; most of their fibers come in roughly parallel bundles from 

 the roots of the pedal ganglia (Fig. 56, /,/;-., right side). On reaching the tract, 

 the fiber bundles take on a longitudinal trend. The tracts are greatly enlarged 



