1876.] Organ of Corti in Mammals. 349 



The bristles of the hair- cells penetrate into this membrane, which itself 

 is analogous to the otolith mass of the vestibule. 



The organ of Corti is not found at the two extremities of the lamina 

 spiralis. 



The whole organ increases regularly in breadth from base to apex of 

 the spiral, measuring in the cat ^i-g- inch near the base, and j^ inch near 

 the apex. 



The height, taken at the rod-heads, varies very little ; but in the region 

 of the outer hair-cells it again regularly increases from base to apex. 



The component parts are further separated from each other on pro- 

 ceeding up the spiral. The rods, as already stated, increase in length, 

 the heads widen, and the processes elongate. The hair-cells increase in 

 height and breadth, and, lastly, the cilia or bristles themselves regularly 

 elongate from base to apex of the cochlea. 



In man and monkeys (but, curiously enough, in no other animals) a 

 further change takes place ; the number of the outer hair-cells increases 

 from three to four, and at the apex to five rows. 



These observations of mine are original, except as regards the increase in 

 the organ as a whole. 



Part II. — The Innervation of the Organ or Corti. 



The cochlear nerve in the substance of the modiolus, in passing upwards, 

 gives off branches at right angles, which traverse a spiral ganglionic mass, 

 the ganglion spirale, and then enter the lamina spiralis. This ganglion 

 is situated in the lamina at the base, and gradually becoming more central 

 in position, is found entirely in the modiolus at the apex ; its cells are 

 bipolar fusiform, averaging y-^^ of an inch in man and most mammals, but 

 are much larger in the porpoise, averaging -^jj inch. The nerve-fibres run 

 in bundles, which form a plexus, to the end of the lower lip of the limbus, 

 where they perforate its upper surface, and, losing their dark borders, 

 reach the organ of Corti. The filaments are now divided into two 

 groups — an inner, passing to the parts internal to the rods, and an outer, 

 passing to the opposite side of those bodies. The inner principally enter 

 and appear to terminate in the adjacent mass of nuclear cells (the granular 

 layer of Waldeyer) ; one large filament, passing through one of the nuclear 

 cells, goes to the inner hair-cell and enters its lower extremity. The 

 outer group may be divided into three sets, which all pass between the 

 rods and traverse the triangular tunnel. The upper filament passes along 

 the roof of the tunnel, and on reaching the other side of the outer rod 

 traverses a nuclear cell, and then passes on to the first of the outer hair- 

 cells. The middle filament traverses the centre of the tunnel and enters 

 the same nuclear cell, besides sending a branch downwards to another of 

 the nuclear cells. The third or lower filament runs along the floor of the 

 tunnel, being connected with the two nuclear cells in the lower angles of 



