CENTRAL AUDITORY MECHANISMS 



589 



tional features of cochlear nuclei in describing the 

 efferent fibers which leave them, particularly as the 

 latter display a very great range of diameters (75). 



Efferent Fibers from Coe/ileiir .Suelei 



Three principal groups of fibers emanate from the 

 cochlear nuclei (13, 60). /) The dorsal (or superior) 

 acoustic stria leave the dorsal nucleus to pass through 

 the reticular formation under the medial longitudinal 

 fasciculus, and, upon crossing the mid-line, pass 

 ventrolaterally to the vicinity of the superior olivary 

 nucleus. 1^) The intermediate stria arises from the 

 dorsal part of the ventral cochlear nucleus, passes 

 over the restiform body and crosses the reticular 

 formation to the opposite side in an intermediate 

 position, j) Fil:)ers, which e.xceed in number the com- 

 bined total of the other two striae, arise in the main 

 body of the ventral cochlear nucleus, pass directly 

 medially \cntral to the restiform body, traverse the 

 ventral part of the reticular formation and cross the 

 mid-line as the trapezoid body (or \'entral stria). 



The three striae tend to draw together in the \icin- 

 ity of the contralateral superior olivary nucleus where 

 many of them terminate. In the course of their passage 

 from origin to that point, there is a considerable dimi- 

 nution of fibers even before the mid-line is crossed 

 due to termination of some fibers in the reticular 

 formation and others in the ipsilateral superior olivary 

 nucleus (13). The latter are of .special interest insofar 

 as they provide an essential part of an anatomical 

 basis for ipsilateral rostral projection and possibly for 

 reflex connections at the medullary level. 



A few fibers emanating from the cochlear nuclei 

 fail to be interrupted by synapses in either the ipsi- 

 lateral or contralateral superior olivary nucleus (13) 

 but turn rostrally and ascend through the pontine 

 medulla in company with third order fibers which 

 arise from the superior olivary nucleus, the combined 

 elements being called the lateral lemniscus. Appar- 

 ently all of the second order fibers which ascend in the 

 contralateral lateral lemniscus terminate in either 

 the nucleus of the lateral lemniscus or the inferior 

 coUiculus (13). 



Lateral Lemniseus and its Nueleus 



This tract ascends from the region of the superior 

 olivary nucleus to the inferior colliculus and, in part, 

 beyond, as the inferior quadrigeminal brachium, to 

 the medial geniculate body in the thalamus. Between 

 superior olive and inferior colliculus, the tract is com- 



posed of at least two, and probably more, different 

 components, /) fibers having origin in the contra- 

 lateral cochlear nuclei (13) and 2) fibers taking origin 

 from the ipsilateral superior olivary nuclei (13, 33, 

 69, 71. 75)- It should be noted that since the superior 

 oli\e receives second order fibers from both ipsi- and 

 contralateral cochlear nuclei, the two components of 

 the lateral lemniscus listed above can actually be 

 sui^divided into three with respect to cochlear origin 

 of excitation carried by each: /) contralateral second 

 order, 2) contralateral third order and 3) ipsilateral 

 third order. 



At this point it may be pointed out that our use of 

 'second order' and 'third order' is valid only if we 

 assume a single synapse in each successive nucleus, 

 for each chain of conduction as represented at one 

 single point by a fiber of the lateral lemniscus. This 

 a.s.sumption is neither necessary nor likely in view of 

 the complexity of the nuclei so far encountered. It 

 would seem more likely that a variable number of 

 links in such chains of conduction might be introduced 

 by the patterns of intranuclear conduction. Evidence 

 on conduction time to the cochlear nucleus, trapezoid 

 body and lateral lemniscus (5) indicates that there are 

 at least some conduction chains in the system which 

 are as direct as would be implied in speaking of second 

 and third order fibers in the lemniscus; however, the 

 protraction of the response to a very brief stimulus 

 would also make one suspect the presence of chains 

 with greater numbers of synapses. 



The nucleus of the lateral lemniscus, unlike the 

 other nuclei so far discussed, is neither compact nor 

 does it show any recognizable organization. It con- 

 sists of scattered groups of cells lying among the fibers 

 of the tract. Some few of the tract fibers apparently 

 terminate in synapse with these cells, and in turn they 

 send their axons upward with the tract to termina- 

 tions in the inferior colliculus, probably both ipsi- 

 lateral and contralateral (by way of the commissure 

 of Probst). 



Some fibers of the lateral lemniscus, of third order 

 or higher, pass lateral to the inferior colliculus and, 

 becoming part of the inferior quadrigeminal bra- 

 chium, continue with it to terminations in the medial 

 geniculate body (3, 51, 75). The greater number of 

 lemniscal fibers terminate in the inferior colliculus 

 (13. 75)- 



Inferior Colluulus 



The inferior colliculus (or posterior corpus quadri- 

 geminum) receives a few fibers which project without 



