VESTIBULAR MECHANISMS 



559 



the endolymph which causes a deviation of the 

 cupula, this time in the opposite direction. The post- 

 rotatory nystagmus occurs, and lasts as long as the 

 cupula needs to return to its starting position through 

 its elastic recoil. 



Thus we have seen that the impulses from the laby- 

 rinth are able to act on the different ocular muscles in 

 an extremely precise manner. However, the details of 

 the reflex arcs are as yet obscure. The sl ow pha se of 

 nystagmus is initiated from the l abyrinth and has its 

 center in the vestibular nuclei from which impulses 

 are propagated, in part at least, through the inedial 

 longitud inal bundles to the eve muscles. The quick 

 component is entirely central. Its neural mechanism 

 must lie in the b rain stem between and including the 

 nuclei foi't he third nerves and th e_y estibular nuclei . 

 for nystagmus occurs after tran.sections of the brain 

 above and below these levels (i8, 60, 61). It is not 

 abolished by ablation of the cerebellum. Lorente de 

 No has located the cejUfiT for the rapid phase in the 

 re ticular formatio n in the region of the abducens 

 nucleus. It has also been found that nystagmus could 

 still be produced after section of both medial longi- 

 tudinal bundles (60, 61, 98). This finding is supported 

 by experiments upon monkeys by Bender & Wein- 

 stein (9). There may be a double pathway from the 

 vestibular nuclei to the nuclei of the ocular nerves — 

 through the medial longitudinal bundle and through 

 the reticular formation. 



CORTICAL PROJECTION. It Was previouslx' implied that 

 the vestibular apparatus had only subcortical projec- 

 tions. Recently, however, it has been well established 

 by the work of a number of investigators using electro- 

 physiological methods that the organ is represented 

 by a projection area in the cerebral cortex of the cat, 

 dog and monkey. Adequate stimulation (37, 39, 99, 

 1 01) — which is not easily graded or measured, nor 

 brief enough for mapping out the exact boundary of 

 the area — does not, in the light of more recent work, 

 seem to be useful. The use of brief electrical stimula- 

 tion of the vestibular nerve, in order to elicit a dis- 

 crete evoked cortical response, has been of greater 

 value (2, 54, 79, 117). The receiving area lies in the 

 anterior e ctosylvian gyru s_and_tb£_ptQ§ terior bank of 

 the anterior suprasy lvian g yrus. The projection is 

 princitj allv contralater al, but stimulation of the 

 ipsilateral nerve activates a part of the same region. 

 The response to electrical stimulation of the peripheral 

 nerve occurs after a latency which suggests that the 



projection is direct from the thalamic relav nuclei 

 (79)- " "' ^ ' 



The orderly features of the vestibular innervation 

 and the projection of the vestibular fibers in the 

 primary nuclei (i, 47, 103) have prompted the postu- 

 lation that each vestibular receptor organ has its own 

 exclusive representation on the cerebral cortex. 

 Cortical respon.ses to liminal electrical stimulation of 

 three accessible vestibular branches can be recorded 

 only from a more limited portion of the projection 

 area as a whole (fig. 8). Stimulation of the nerve from 

 the utricle of the cat evoked responses from the dor.sal 

 part of the area. Below and anterior to the latter 

 focus, responses to stimulation of the ner\e from the 

 crista of the horizontal semicircular canal were re- 

 corded, and above it the cortical projection of the 

 nerve from the superior crista was found (2) (fig. 9). 



It has been demonstrated by neurotomy that the 

 corti^al__re sponse to sti mulation of the vestibular 

 apparatus r equires neithe r an intact cerebellum n or 

 an intact _medial lon git udinal fascicul us (4, 87). This 

 proves that there are other ascending \estibular path- 

 ways conducting the impulses. According to Wallen- 

 berg (i 16) a vestibulocortical pathway appears to run 

 parallel to the acoustic fibers (10). It should be noted 

 that available results also suggest the existence of a 

 corticovestibular connection, although it has been 

 impossible to trace one (33, too). 



B 



FIG. 8. Site of electrical stimulation ot branches of the ves- 

 tibular nerve. Ventrolateral view of the left \estibule, A before 

 and B after rcmo\ing the membranous labyrinth. Vestibular 

 nerve branches from the ampulla of the superior canal, /; 

 lateral canal, 2; and utricle, 4. The utricle is marked, 3, and 

 the saccule, j. In B three silver wires (black lines') are placed as 

 stimulating electrodes and are held in place by dental cement 

 attaching them to the cut edge of the bulla. [From Andersson 

 & Gernandt (2).] 



