STRUCTURE OF MEDULLA OBLONGATA 



81? 



This bundle is known as the medial longitudinal fasciculus (posterior longitudinal 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 retains its medial position and 

 thus 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 or ventral mesencephalo-spinal 

 fasciculus, pass through the medulla in the medial longitudinal fasciculus. The nuclei of 

 termination of the vestibular nerve are said also to contribute many fibres to it. 



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 pi-ominences, the olives 

 of the exterior, and constitute the most conspicuous and striking isolated masses of grey sub- 

 stance in sections of the medulla. They appear as crenated laminae 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 corrugated cup with the opening or hilus on the side toward 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 capsules. 



Fig. 643. — Transverse Section of Medulla Oblongata Through Nuclei of Vagus and 

 Htpoglossus and Through the Middle of the Olives. 



Medial longitudinal fasciculus 

 Chorioid tela of fourth ventricle 

 Nucleus of hypoglossus 



Medial nucleus of vestibular nerve *>., 

 Descending (spinal) nucleus n 



\ 



of vestibular nerve 



Nucleus of ala cinerea Urigonum vagij 

 / Dorsal efferent nucleus of vagus 

 ' / Solitary tract 



Nucleus of solitary tract 

 / 



Internal arcuate 

 fibres 



Nucleus ambiguus 



Dorsal accessorj 

 olivary nucleus 



Nucleus of inferior 

 olive 



Root filum of hypo- 

 glossus 



Nucleus of fasciculus 



cunatus 



Nucleus of spinal tract 



of trigeminus 



Restiform body 



Spinal tract of 



trigeminus 

 Cerebello-olivary 



fibres 

 Root filum of vagus 

 Nucleus lateralis 



Thalamo-olivary tract 



_ External arcuate 

 fibres 



' ^ PyrarDid 

 Lemniscus Raphe 



There are several small detached portions of the olivary nucleus known as the accessory 

 olivary nuclei. These are named according to their position with reference to the chief portion 

 or ohve proper. They are plates less corrugated than the chief nucleus, and appear rod-like 

 in sections. The largest is the dorsal accessory olivary nucleus. The medial accessory olivary 

 nucleus is widest at its inferior end, which extends a little below the inferior extremity of the 

 chief nucleus. The lateral accessory olivary micleus is the smallest. In serial sections the 

 accessory nuclei are found to be plates of grey substance usually continuous with one another. 



The olivary nuclei are mainly cerebellar connections. By both ascending and descending 

 fibres each cerebellar hemisphere is connected with the olivary nucleus of the same and opposite 

 sides. Serial sections of a human brain witli congenital absence of one cerebellar hemisphere, 

 described by Strong, show that the chief connection of a hemisphere is with the olive of the oppo- 

 site side. These fibres necessarily pass between the cerebellum and the olives by way of the 

 restiform body, and, in so doing, form an obliquely coursing bundle in the lateral border of 

 the medulla known as the cerebello-olivary fibres (fig. 643). The olivary nuclei also comprise a 

 secondary relay between the spinal cord and the cerebellum by way of the spino-olivary fas- 

 ciculus of the cervical cord, and it will be noted that they receive fibres from the thalami. The 

 latter fibres, the thalamo-olivary tract, approach the olive at its lateral periphery, while upward 

 through the brafn-stem the tract courses in a more medial position. This tract comprises one of 

 the cerebro-cerebellar paths. Arising in the thalamus and terminating in the olive, its impulses 

 reach the opposite cerebellar hemisphere by way of the cerebeUo-olivary fibres. 



The arcuate fibres are referred to as internal and external, according as they course dorsal 

 or ventral to the inferior olivary nucleus. 



The internal arcuate fibres comprise fibres destined for both the cerebellum and cerebrum, 

 and also for the association of the tegmental grey substance of the two sides in which they course. 

 Certain of the fibres passing between one restiform body (cerebellar hemisphere) and the 

 olive of the opposite side course internal to the ohve of the same side, and thus form the ventral 

 portion of the internal arcuate fibres. As noted above, the internal arcuate fibres consist in 



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