SENSE AREAS AND ASSOCIATION AREAS. 193 



fibers in the cord, especially those of the posterior columns, after 

 ending in .the nucleus gracilis and nucleus cuneatus of the medulla. 

 This path is represented in a schematic way in the accompanying 

 diagram (Fig. 84). The second sensory neurons arise in the nuclei 

 mentioned. For the most part, at least, these new neurons run 

 ventrally, as internal arciform fibers, cross the mid-line, and then 

 pass forward or anteriorly. The crossing occurs mainly just in 

 front of that is, cephalad to the pyramidal decussation, forming 

 thus a sensory decussation,,, which explains the crossed sensory 

 control, as the pyramidal decussation explains the crossed motor 

 control of the cerebrum in relation to the body. After this decussa- 

 tion the sensory fibers form a longitudinal bundle -on each side 



Fig. 85. Cross-section through midbrain (Kdlliker) to. show the position of the fillet (L, L): 

 ." . Nr, The red nucleus; Sn t the^substantia nigra; Pp, the crus. 



' 



known as the jnedian fillet, lemniscuSi or laqueus, which in the pons 

 lies just dorsal to the pyramidal fibers. m 



The fillet fibers may be traced' forward (see Fig. 85) as far as 

 the anterior colliculus of the corpora quadrigemina and the thal- 

 amus, the important termination being in the thalamus (ventral 

 or lateral nucleus) . Those neurons that end in the thalamus 

 are continued forward by a third set of neurons, which end in the 

 parietal lobe of the cerebrum (see Fig. 76, C). On its way 

 through the medulla and pons -the fillet tract is believed to receive 

 accessions of sensory fibers from the sensory nuclei of the cranial 

 nerves of the opposite side. The course of .the fillet has been 

 traced by various means, but especially by the method of myeliniza- 

 13 



