172 VETEPwINARY PHYSIOLOGY 



from their free border ; Srd. The inner rods of Corti, each of 

 which may be compared to an ulnar bone attached by its 

 terminal end, and fitting on to the heads of the outer rods ; 

 Mh. The outer rods of Corti, each resembling a swan's head 

 and neck — the neck attached to the basilar membrane, and 

 the back of the head fitting into the hollow surface of the inner 

 rods. The two sets of rods thus form an arch ; oth. Several 

 rows of outer hair cells, with some spindle-shaped cells among 

 them ; 6th. The outer supporting cells ; 7th. Lying over the 

 inner and outer hair cells is the membrana reticularis, 

 which resembles a net, and through the meshes of which 

 the hairs project ; 8th. Lying upon this organ, with the 

 cilia of the hair cells embedded in it, is a homogeneous 

 membrane — the membrana tectoria, the inner margin of 

 which is firmly attached to the denticulate lamina as shown 

 in fig. 85 and fig. 87. 



The terminal neurons both to the vestibule and to the 

 cochlea end in dendrites among the hair cells. 



5. Connections with the Central Nervous System. 



The dorsal or auditory part of the VIII. nerve is the true 

 nerve of hearing, (a) Its fibres (fig. 86) (Goch.R.) begin in 

 dendrites between the hair cells of the organ of Corti, and have 

 a neuron -cell upon their course. When they enter the 

 medulla, they branch into two divisions, which end either 

 in the tuberculum acusticum or in the nucleus accessorius 

 (N.Acc), where they form synapses. 



(6) From these cells axons pass across the middle line 

 in the strife meduUares and corpus trapezoideum, and, 

 turning sharply upwards, run as the lateral fillet to the 

 posterior colliculi of the tectum of the opposite side. Here 

 many of the fibres form synapses, and fresh fibres pass to the 

 oculo-motor mechanism. When the cochlea is destroyed in 

 young animals, the opposite posterior colliculus atrophies. 



(c) The rest of the fibres run onwards to the medial 

 geniculate body of the thalamus where synapses are formed. 



(d) From these fresh fibres, which get their white sheath 

 early, course outwards to the cortex cerebri in the superior 

 temporo-sphenoidal lobe. 



