Midbrain 69 



descending tract from the posterior colliculi, and concludes 

 that these cannot be regarded as reflex centres. Many of 

 these fibres decussate bet^\'een the two corpora quadrigemina 

 as the commissure of the posterior colliculi and Papez finds 

 these to end entirely in the opposite colliculus. Along with 

 the commissure of Probst and the incomplete decussation 

 of the trapezoid body, they provide for bilateral representa- 

 tion of auditory stimuli in the posterior colliculi and in the 

 medial geniculate bodies. Many fibres arising in each pos- 

 terior colliculus, moreover, enter its brachium (vide infra) 

 to ascend to the thalamus. 



Not all the fibres of the lateral lemniscus end in the 

 posterior colliculi. Some divide before terminating there 

 and send a branch into the anterior colliculus, and it may be 

 that a few such fibres end in the latter region without pre- 

 viously dividing. A considerable number of the fibres termi- 

 nating in the colliculi are, moreover, collaterals from axons 

 which continue their course farther up the brain along with 

 some which have no connection in the midbrain. These 

 form a distinct tract on the lateral aspect of the tectum, 

 which is the direct continuation of the lateral lemniscus, but 

 which is now known as the brachium of the posterior collicuhis 

 or posterior (inferior) quadrigeminal brachium {F\s.Xll.-X.y.). 

 The fibres which end in the tectum form the chief reflex 

 auditory pathway, according to the usual view, while the 

 brachium is the cortical pathway, up which pass auditory 

 impulses destined to enter consciousness. The fibres of the 

 latter group end in the medial geniculate body, whence new 

 axpns pass to the auditory cortex. The brachium contains 

 also fibres arising in the posterior colliculi, and some of 

 cortical origin which terminate in these centres. 



The nucleus and fibres of the mesencephalic root of the 

 trigeminal nerve, which lie in the lateral part of the tectum 

 at the outer border of the central gray matter, have already 

 been described. 



