816 



THE NERVOUS SYSTEM 



through the pons, the lemnisci gradually become spread horizontally, and beyond the pons 

 their then more lateral portions are further displaced and come to course in the lateral borders 

 of the isthmus rhombencephah and mesencephalon, while the medial portions remain nearer 

 the mid-line. This lateral spreading of each lemniscus produces the lateral lemniscus and the 

 medial lemniscus, distinguished in transverse sections of the superior pons and mesencephalic 



Fig. 641. — Transverse Section of Medulla Oblongata at the Level of the Decussation 



OF THE Pyramids. 



Central grey substance 



Central canal 

 \ 



Nucleus of fasciculus gracilis 

 • Funiculus cuneatus 



.•.\ ^-Substantia gelatinosa (Rolandi) 

 •'••^fy Spinal tract of trigeminus 



j:^^L' Dorsal spino-cerebellar 

 !j,' ' fasiculus 



0\ 



Gowers' tract 



» Lateral cerebro-spinal fasciculus 

 I \ Ventral horn 



\ Decussation of pyramids 



Pyramid 



regions of the brain stem (fig. 660). The lateral lemniscus is contributed very largely by the 

 cell-bodies of the nuclei of termination of the cochlear nerve of the opposite side. 



The reticular formation of the medulla and pons region is considerably more abundant 

 than in the spinal cord. As in the spinal cord, it consists of grey substance through which 

 nerve-fibres, singly and in small bundles, course in all directions, and more sparsely than in 

 other regions. In the medulla it is traversed by the internal arcuate fibres. It may be con- 



FiG. 642. 



-Transverse Section of Medulla Oblongata at Level of the Decussation of 



THE Lemnisci. 



Posterior median fissure 

 Central grey substance ^ 

 Nucleus of hypoglossus \ 



Internal arcuate fibers 



Nucleus of fasciculus gracilis 



> Commissural nucleus of ala cinerea 



' ," Nucleus of fasciculus cuneatus 



/Dorsal external arcuate fibres 



Nucleus of spinal tract of 

 trigeminus 



Spinal tract of trigeminus 



- • -' Restiform body 

 Nucleus lateralis 



Root filum of hypoglossus 

 Nucleus of inferior oliv 



Medial accessory olivary nucleus ' , 



Raphe' 



bstantia reticularis 

 ntral external arcuate fibres 



Decussation of lemnisci 



Pyramid 



sidered an enlarged continuation of tlie middle portion of the grey column of the cord, dispersed 

 by numerous fibres, giving it the reticulated appearance which suggests its name. Its numer- 

 ous nerve-cells belong, for the most part, to the association and commissural systems of the brain 

 stem, and, therefore, the fibres arising in it correspond largely to the fasciculi proprii of the 

 spinal cord. As in the cord, inost of the fibres an; of short course, serving to associate different 

 portions of tlie same level and adjacent hivols witli each other. Those of long course show a 

 tendency to collect into a small, well-iiiarkod bundle which courses one on each side close to 

 the mid-line, ventral to the central canal in the closed part of the medulla, and near the median 

 sulcus of the floor of the fourth ventricle, in tlie open part. In the mesencephalon this bundle is 

 again situated closely ventral to the aquuiductus cerebri. 



