UPPER PART OF THE MEDULLA OBLONG ATA. 



53 



cells, but from the cells of the Gasserian ganglion (His), and secondly, if the sensory root of 

 the fifth is cut at its exit from the pons, the fibres of the so-called ascending root degenerate, 

 but the cells of the adjacent gelatinous substance remain unaffected (Bechterew). There may, 

 however, be a physiological connection established with these cells by means of collateral 

 fibres, as in the case of the fibres of the posterior columns of the cord and the substance of 

 Rolando of the posterior horn. 



The grey matter of the base of the posterior horn undergoes a considerable in- 

 crease as we trace it upwards in sections. Portions of grey matter are soon found 

 to extend from it into the funiculi graciles and cuneati, forming the so-called nuclei 

 of those columns (fig. 43, n.g., n.c.). These nuclei are at first narrow in trans- 

 verse section ; but as the central canal approaches the posterior surface of the 

 medulla they appear as comparatively thick masses, which produce externally the 



Fig. 45. SECTION ACROSS THE MEDULLA OBLONGATA A 



LITTLE BELOW THE POINT OF THE CALAMUS SCRIP- ,, 



TORIUS (Lockhart Clarke). 



c, central canal; /, anterior median fissure ; /.</., 

 funiculus gracilis; f.c., funiculus cuneatus; t.R., tuber- 

 cle of Eolando ; o, olivary body ; a.p., pyramid ; XI, 

 XII, spinal accessory and hypoglossal nerves ; XI'. XII', 

 their nuclei. 



eminences of the clava and the cuneate 

 tubercle. Outside the nucleus of the funi- 

 culus cuneatus an accessory or external nu- 

 cleus becomes formed (fig. 43, n.c.). From 

 this nucleus fibres pass directly into the resti- 

 form body. The nerve-cells of the gracile 

 nucleus are for the most part larger than 



those of the cuneate nucleus, but those of the accessory cuneate are larger than 

 either (50/i to 80/x). The accessory cuneate nucleus is supposed to represent a 

 continuation of Clarke's column of the cord (Blumenau), while in the grey matter 

 of the nucleus gracilis and principal nucleus cuneatus most of the ascending fibres 

 of the posterior column of the cervical cord become lost. 



From the lower parts of these nuclei fibres are seen to emerge and to sweep forwards 

 and inwards in a curved manner (internal arched or arcuate fibres) towards the raphe 

 or septum which unites the two halves of the medulla oblongata. Having here 

 intercrossed with those from the opposite side in a decussation which lies above 

 that formed by the fibres of the pyramids (decussation of the fillet), they form 

 a considerable bundle of longitudinally coursing fibres which lies just dorsal to the 

 pyramid and is known as the fillet (lemniscus) . Its fibres receive their myelin 

 much earlier than those of the pyramid. On section of the fillet higher up some of 

 its fibres degenerate downwards and the degeneration also affects internal arched 

 fibres connecting them with the opposite nuclei. These therefore have their origin 

 from cells higher up in the brain. 



Upper or ventricular part of the medulla oblongata. When the slit-like 

 upper end of the central canal opens out into the fourth ventricle, the small remaining 

 portion of the base of the anterior horn, which in the closed part was ventro-lateral 

 to the central canal, comes to the surface at the floor of the ventricle, and as the 

 sections are traced upward increases gradually in size, producing the prominence of 

 the funiculus teres. In it, both in the lower part of the bulb where the canal is 

 still closed and above where it has opened out, a group of large nerve-cells (n.XII.) 

 is seen in all transverse sections. From this group (column) of cells the successive 

 bundles of the roots of the hypoglossal or twelfth cranial nerve arise and pass obliquely 

 through the substance of the bulb to leave it on its anterior aspect. The tract of 

 nerve-cells is accordingly known as the hypoglossal nucleus. 



