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LATERAL AND MESIAL FILLETS. 497 
nucleus may, by the posterior longitudinal fasciculus, reach the facial nerve, and through it the 
orbicularis oris. In this manner, therefore, 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 posterior longitudinal bundle would render 
the passage of fibres from one to the other a matter which could easily be understood. Another 
interchange of fibres through the posterior longitudinal bundle has been described by Duval 
and Laborde. According to these authorities, fibres from the 
abducent nucleus ascend in the posterior longitudinal bundle 
into the mesencephalon, and, crossing the mesial plane, pass  oravron, 
out with the opposite oculo-motor nerve for the supply of the see 
internal rectus muscle. If this view be correct, it affords a 
ready and simple anatomical explanation of the harmonious 
action of the external and internal recti muscles'in producing 
movements of the two eyeballs simultaneously to the right and 
to the left. 
ANTR COM: 
ors Mig, 
-FOUNTAIN 
DECUSSATION 
Lateral Fillet (lemniscus lateralis)—The lateral 
fillet is a definite tract of longitudinal fibres, which 
extends upwards through the lateral part of the teg- 
mental substance of the upper portion of the pons and 
the mesencephalon. It is formed by the fibres of the Mirco 
corpus trapezoides in- the lower part of the pons, “@ 
abruptly turning upwards and taking a course towards 
the quadrigeminal region. Entering into its consti- 
tution, therefore, are fibres from several sources, viz. 
(1) from the terminal cochlear nuclei of the opposite 
side; (2) from the terminal cochlear nuclei of the 
same side; (3) from the superior olivary nuclei. As 
the tract proceeds upwards a continuation of the gray 
matter of the superior olivary nucleus is carried up in 
connexion with it to form the nucleus of the lateral 5, 3¢¢ GON 
fillet, and from this fibres are also added to the strand. NEXIONS OF THE PusTERIOR 
In the mesencephalon the fibres of the lateral fillet = LoNeirupinan Buypre (after 
3 ' Se ula: is ; : Held—modiftied). 
end in the nucleus of the inferior quadrigeminal body 
(p. 495) and in the gray substance of the corpus geniculatum internum, whilst a 
few are carried into the superior quadrigeminal body. The fibres which go to the 
inferior quadrigeminal body sweep outwards round the outer side of the superior 
cerebellar peduncle, and to some extent appear on the surface of the mesencephalon 
(p: 491). 
But the lateral fillet cannot be considered as a tract composed entirely of ascending fibres 
belonging to the auditory system. It also contains descending fibres, the connexions and 
ime gone of which are not fully understood. These have been ‘traced by Ferrier and Turner 
through the pons and medulla into the lateral column of the cord. 
Mesial Fillet (lemniscus mesialis)—The mesial fillet has already been followed 
through the medulla and pons, and its position in each of these portions of the 
brain-stem has been defined (pp. 456 and 467). In the tegmentum of the lower 
part of the mesencephalon it is carried up in the form of a more or less flattened 
band on the ventral aspect of the decussating superior cerebellar peduncles. To 
its outer side, and forming an angle with it (as seen in transverse section), is the 
lateral fillet (Figs. : 367 and 368), and at this level there is no clear demarcation 
between these two tracts. In the upper part of the mesencephalon the appearance 
of the red nucleus in the tegmentum causes the mesial fillet to take up a more lateral 
and dorsal position, so: that it now comes to lie subjacent to the corpus geniculatum 
internum (Fig. 366, p. 495). At this level it exhibits a crescentic outline in trans- 
verse section, and the lateral fillet has toa large extent disappeared from its outer side. 
The mesial fillet takes origin in the lower part of the medulla oblongata from 
the gracile and cuneate nuclei of the opposite side (p. 457). Seeing that the 
posterior column of the cord ends in these nuclei, the fillet may be considered to 
continue that column upwards into the brain. In the mesencephalon a consider- 
able contribution of fibres is given by the mesial fillet to the superior quadrigeminal 
body, and then the remainder of the tract proceeds through the subthalamic teg- 
mental region into the hinder part of the lateral nucleus of the optic thalamus. 
9 
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