800 



THE NERVOUS SYSTEM 



lemniscus, distinguished in transverse sections of the superior pons and mesen- 

 cephalic regions of the brain-stern (fig. 609). 



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 in- 

 ternal arcuate fibres. It may be considered an enlarged continuation of the middle 

 portion of the grey column of the cord, traversed by numerous fibres, giving i 

 reticulated appearance which suggests its name. Its numerous nerve-cell 

 for the most part, to the association and commissural systems of the brain-stem, and. 

 therefore the fibres arising in it correspond to the fasciculi proprn of the spinal cord. 

 As in the cord, most of the fibres are of short course, serving to associate different 

 portions of the same level and adjacent levels with each other. Those of long course 

 show a tendency to collect into a small, well-marked 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 floor of the fourth ventricle, in the open part. In the mesencephalon 

 this bundle is again situated ventral to the aquseductus cerebri. 



FIQ 535. TRANSVEKSE SECTION OF MEDULLA OBLONGATA THROUGH NUCLEI OP VAGUS AND 

 HYPOGLOSSUS AND THROUGH THE MIDDLE OF THE OLIVES. 



MEDIAL LONGITUDINAL FASCICULUS 

 TEL A CIIORIOWEA OF FOURTH VENTRICLE 



NUCLEUS OF IIYPOGLOSSUS v i 



MEDIAL NUCLEUS OF VESTIIi- \ \ '. 



ULAR NERVE \ 



DESCENDING ROOT OF 



VESTIBULAR NERVE 

 INTERNAL ARCU- ^^ 

 A TE FIBRES 



NUCLEUS AH- -- 

 BIG U US 



DORSAL ACCESSORY 

 OLIVARY NUCLEUS 



SUf'LF.VS OF IN- , 

 FERIOR OLIVE'-^ 



NUCLEUS OF ALA C1XKKEA (.TRlOOyi'X V H.V) 



/ NUCLEUS OF SOLITARY TRACT 

 ,' I SOLITARY TRACT 



OF FUNirr- 



LUS Cl'NEATUX 

 .NUCLEI'S HI-' >//- 

 1 .V.I/. Tl: M'l 111-' 

 TRIGEMINUS 



ROOT FILUM OF HY- ' 

 POGLOSSUS 



SPINAL TRACT nF 



TRIGBMINVS 

 CEREBELLO-OLIVARY 



ROOT riLi'M < IF r.i'. 

 'S L.I TI:KALIS 



EXTERNAL MlfTATE 

 FIBRES 



LEMNISCUS 



RAPIIE PYRAMID 



This bundle is known as the medial longitudinal fasciculus (posterior longi- 

 tudinal bundle). It corresponds more nearly to the ventral fasciculus proprius of 

 the spinal cord than to others of the fasciculi proprii. In the medulla it appears as the 

 dorsal edge of the lemniscus, but in the shifting of the position of the lemniscus in 

 the pons region, it becomes isolated. By position it is especially adapted for the 

 association of the nuclei of the cranial nerves. Evidence has been found that those 

 fibres which arise in the corpora quadrigemina and descend the spinal cord in its 

 sulco-marginal fasciculus, pass through the medulla in the medial longitudinal 

 fasciculus. 



The inferior olivary nucleus is an added structure in the medulla oblongata, 

 i. e., it has no homologue in the spinal cord. The two of them occupy the olivary 

 prominences, the olives of the exterior, and constitute the most conspicuous and 

 striking isolated masses of grey substance in sections of the medulla. They appear 

 as crenated laminse of grey substance folded so as to encup a dense mass of white 

 substance, and in actual shape the entire nucleus has the form of an irregular corru- 

 gated cup with the opening or hilus on the side towards the mid-line. The mass 

 is so crumpled that the diameter of the hilus is appreciably less than the length of the 

 nucleus, and thus transverse sections of either extremity of it appear as closed cap- 

 sules. 



