THE ACOUSTIC NERVE 89 



medulla oblongata. A new relay of fibres arises in these 

 nuclei and, although some ascend through the same side of 

 the brain-stem and do not undergo decussation, the majority 

 cross the median plane. Many of the latter lie on the dorsal 

 surface of the pons and form transverse ridges, termed the 

 striae acousticae, which can be seen in the floor of the fourth 

 ventricle (Fig. 32). Having crossed the median plane, the 

 acoustic fibres turn upwards and form a tract, known as the 

 lateral fillet. This tract ascends through the medulla oblon- 

 gata and the pons to terminate in the lower acoustic centres 

 the medial geniculate body (p. 19) and the inferior corpus 

 quadrigeminum (p. 18). From these centres new fibres arise 

 which at once enter the posterior limb of the internal 

 capsule. In this situation they are mingled with ascending 

 sensory fibres from the opposite side of the body, but they 

 lie in front of the visual fibres and behind the motor fibres 

 for the lower limb. After leaving the internal capsule the 

 acoustic fibres pass laterally to reach the higher centres, 

 which are situated in the cortex of the superior temporal 

 gyrus (p. 9). 



SUPRA-NUCLEAR LESIONS of the acoustic tract never cause 

 complete deafness unless they are bilateral, on account of the 

 connexions of the cochlear nuclei with the cortex of both 

 cerebral hemispheres. 



CORTICAL LESIONS may give rise to word-deafness^ when they 

 occur in the superior temporal gyrus of the left side in right- 

 handed individuals. In this condition words can be heard 

 as sounds but they cannot be understood. 



Occasionally, in lesions affecting the posterior limb of the 

 internal capsule, some degree of unilateral deafness may be 

 associated with hemi-anaesthesia. Cases of hemi-anaesthesia 

 accompanied by complete unilateral deafness are always 

 hysterical in origin. 



In INFRA-NUCLEAR LESIONS the cochlear and vestibular nerves 

 are often involved together (vide infra^ p. 90). In some cases 

 the cochlear nerve may be involved alone. It is then necessary 



