THE NERVE FIBER TRACTS. 89 



alon, which is in turn connected by special tracts with the olfactory lobes, hem- 

 ispheres, and cerebellum. 



e. In both classes, the nerve roots are arranged in two distinct series, neural 

 and haemal; each series may contain both motor and sensory elements. 



In Limulus, the haemal roots enter the brain toward the haemal surface and 

 extend horizontally, through the crus and the haemal commissures, to the main 

 nucleus or cell cluster on the other side of the median line. But many fibers 

 end in dendrites on the same side the nerve enters. 



In vertebrates, a similar condition prevails in the ventral or haemal nerves, 

 for according to Johnston " It is a noticeable peculiarity in the origin of the nerve 

 (i.e., the third nerve of vertebrates) that a large part of the fibers arise from the 



FIG. 69. Four cross-sections of the brain in the vagus region of an adult scorpion, showing the enormous vagal 

 lobes, the central canal, and the cephalic portion of the middle cord, or lemmatochord. 



nucleus of one side, and cross to enter the root of the opposite side. The same 

 arrangement is found in the roots of other ventral nerves, but to a much less 

 degree." 



On the other hand, the neural nerves are associated with special ganglia, which 

 arise independently of the brain, and which are attached to its neuro-lateral 

 margin. In Limulus, these fibers end, or originate, on the same side the nerve 

 enters the brain; very few, if any, fibers of a neural nerve arise from cells located 

 on the opposite side of the median line. 



/. In both classes, the floor of the brain is divided lengthwise into two main 

 columns, a median and a lateral one, by an important series of vertical fibers; 

 arcuate fibers, vertebrates, B and , fibers, arachnids. The prolongations of 

 these fibers run lengthwise in the haemal (and neural ?) tracts and crosswise in 

 the commissures. 



g. In Limulus, there is in the vagus region an important decussation of im- 

 pulses coming from the trunk (See section on Physiology, p. 191) ; and there is also 



