THE HIND-BRAIN OR RHOMBENCEPHALON 



777 



of the medulla spinalis, and may be regarded as dorsal projections of this, each 

 being covered superficially by the fibers of the corresponding fasciculus. On 

 transverse section (Fig. 694) the nucleus gracilis appears as a single, more or less 

 quadrangular mass, while the nucleus cuneatus consists of two parts: a larger, 

 somewhat triangular, medial nucleus, composed of small or medium-sized cells, 

 and a smaller lateral nucleus containing large cells. 



Decussation of 

 superior pe- 

 duncle 



Nucleus of 

 lateral 

 lemniscus 



Decussation of 

 lemniscus 



Internal arcu- 

 ate fibers 





FIG. 690. Deep dissection of brain-stem. Ventral view. 



The fibers of the fasciculus gracilis and fasciculus cuneatus end by arborizing 

 around the cells of these nuclei (Fig. 692). From the cells of the nuclei new fibers 

 arise; some of these are continued as the posterior external arcuate fibers into the 

 inferior peduncle, and through it to the cerebellum, but most of them pass forward 

 through the neck of the posterior column, thus cutting off its head from its base 

 (Fig. 693) . Curving forward, they decussate in the middle line with the correspond- 

 ing fibers of the opposite side, and run upward immediately behind the cerebro- 

 spinal fibers, as a flattened band, named the lemniscus or fillet. The decussation 

 of these sensory fibers is situated above that of the motor fibers, and is named 

 the decussation of the lemniscus or sensory decussation. The lemniscus is joined by 

 the spinothalamic fasciculus (page 762), the fibers of which are derived from the 

 cells of the gray substance of the opposite side of the medulla spinalis. 



The base of the posterior column at first lies on the dorsal aspect of the central 

 canal, but when the latter opens into the fourth ventricle, it appears in the lateral 

 part of the rhomboid fossa. It forms the terminal nuclei of the sensory fibers of 





