INTEENAL STEUCTUEE OF THE MESENCEPHALON. 



589 



Decussating 



fibres 



Nucleus of 



inferior 



colliculus 



Mesencephalic 



root of tri- 



geminal nerve 



Trochlear 



nerve 



Medial 



longitudinal 



bundle 



Lateral 

 lemniscus 

 Brachium 

 conjunctivum 



Medial 

 lemniscus 



portant functions of this strand is to bind together these nuclei, and thus enable 

 m to act in harmony one with the other. Fibres also enter the medial 

 gitudinal fasciculus from the vestibular nucleus of the acoustic nerve system, 

 e results obtained by degeneration would seem to indicate that, to a large 

 tent, it is formed of fibres which run a short course within it. 

 It is evident that it is a brain tract of high importance, from the fact that it 



a present in all vertebrates, and, further, that its fibres assume their medullary 



Deaths at an extremely early period. In fishes, amphibians, and reptiles, it is one 

 the largest bundles of the medulla oblongata. In man, its fibres medullate 



>etween the sixth and seventh months of foetal life, and at the same time as 



,he fibres of the fasciculus anterior proprius of the spinal medulla, with which 



t stands in connexion. 



According to van Gehuchten and Edinger, it extends upwards beyond the 



evel of the oculomotor nucleus, 



.nd in the thalamic region its 



ibres take origin from a special 



mcleus of its own in the gray 



natter of the third ventricle, im- 



nediately behind the level of the 



;orpora mamillaria. Fibres also 



mter the medial longitudinal 



)undle from a nucleus common to 



t and the posterior commissure of 



;he brain. This nucleus is placed 



n the anterior part of the central 



*ray matter of the mid-brain. Held 



isserts that numerous fibres, aris- 



ng from cells in the superior col- 



iculus, curve in an arcuate manner 



n the tegmentum outside the cen- 

 tral gray matter, to take part on 



:he ventral aspect of this in what 



S called the fountain decussation Fia -523. SECTION THROUGH THE INFERIOR COLLICULUS 

 . , AND THE TEGMENTUM OF THE MESENCEPHALON, AT A 



teaching the opposite Side, these SLIGHTLY LOWER LEVEL THAN FIG. 522. 



ibres turn downwards and join the 



nedial longitudinal bundle. The same authority considers that fibres from the 

 central part of the posterior commissure can also be traced downwards into the 

 medial longitudinal bundle. Edinger, on the other hand, places these fibres as 

 : i distinct tract on the ventral and lateral aspect of the medial longitudinal bundle, 

 although in apposition with it. 



Mendel believed that fibres from the oculomotor nucleus are carried down in 

 the medial longitudinal bundle, and, from this, into the facial nerve for the supply 

 of the orbicularis oculi and the corrugator supercilii, bringing these muscles, 

 therefore, under the control of the same nucleus as the levator palpebrse superioris 

 muscle. This view was adopted by many clinicians because this upper group of 

 facial muscles is often spared in cases of facial paralysis ; but Harman has adduced 

 reasons in support of the view that there is a superior prolongation of the facial 

 1 nucleus which innervates these muscles. It has been suggested further that fibres 

 from the hypoglossal nucleus may, by the medial longitudinal bundle, reach the 

 facial nerve, and through it the orbicularis oris. In this manner the same nucleus 

 would hold sway over the tongue and the sphincter muscle of the lips. The close 

 relation which exists between the ascending part of the intrapontine portion of 

 the facial nerve and the medial longitudinal bundle would render the passage of 

 fibres from one to the other a circumstance which could easily be understood. 

 But the balance of evidence now available inclines us to regard the facial nucleus 

 as the origin of the fibres innervating all the facial muscles. Another interchange 

 of fibres through the medial longitudinal bundle has been described by Duval and 

 Laborde. According to these authorities, fibres from the abducens nucleus ascend 

 in the medial longitudinal bundle into the mesencephalon, and establish connexions 



