THE RHOMBENCEPHALON. 



cated in the lower half of the medulla. About four-fifths of the 

 pyramid crosses over through it, and becomes the lateral pryam- 

 idal tract of the spinal cord. The lower level of the pyramidal 

 decussation marks the boundary between medulla and cord. The 

 medial fillet, the anterior and the posterior external arcuate fibers 

 all rise in the nucleus funiculi gracilis and nucleus funiculi cuneati. 

 The two former run forward and cross over in the middle of the 

 medulla, forming the fillet decussation (decussatio lemniscorum}, 

 after which they separate (Fig. 94). The medial fillet bends 

 upward and ascends between the inferior olives, hence its name 

 in the medulla, stratum interolivare lemnisci. The anterior ex- 

 ternal arcuate fibers, continuing their ventral direction, issue 

 from the anterior median fissure and anterior surface of the 

 medulla (Fig. 93), then arch backward around the medulla to 

 the restiform body, through which they enter the cerebellum. 

 The posterior external arcuate fibers run through the restiform 

 body to the cerebellum without decussating. The external arcu- 

 ate fibers, like the medial fillet, conduct impulses of the tactile 

 and muscular senses. The cerebello-olivary tract is a large one 

 (Fig. 92). It comprises most of the internal arcuate fibers of the 

 medulla. Its origin is in the cerebellar cortex, and perhaps in the 

 cerebellar ganglia; it terminates in the opposite inferior olivary 

 nucleus. This tract probably belongs to the indirect motor path, 

 though Cunningham believes it to be ascending in direction. 



(2) The dorso-ventral fibers of the medulla are found in five 

 situations: (a) In the median raphe, (b) in either half of the me- 

 dulla between the anterior and lateral areas, running in a linear 

 series of ten or twelve fascicles toward the anterior lateral sulcus, 

 and (c) in several bundles not exactly in linear series, which run 

 inward or outward through the posterior lateral sulcus between 

 the lateral and posterior areas of each side. 



(a) The dorso-ventral fibers of the raphe are the anterior exter- 

 nal arcuate fibers (Fig. 92). These can be traced to the cortex 

 of the cerebellum through the restiform body. Their origin is 

 in the nuclei funiculi gracilis and funiculi cuneati (Fig. 94). 

 Some of them seem to be interrupted in the arcuate nucleus on 

 the anterior surface of the pyramid. 



