222 THE MICROSCOPIST. 



lated outer covering. In addition, certain globular or 

 lenticular refractive bodies, of different shape and color 

 in different animals, are found in these structures (Plate 

 XXIY, Fig. 178), which doubtless are designed to give 

 the rays of light such a direction for final elaboration in 

 the outer segment as they could not receive from the 

 coarser refractive apparatus in the front of the eye. 



e. Organs of Hearing. These are most intimately con- 

 nected with mental functions, because of language, which 

 is the highest sensual expression of mind. Hence the 

 structure of these organs is most delicate and complex. 



The labyrinth is the essential part of the organ, con- 

 sisting in man of the vestibule, the semicircular canals, 

 and the cochlea. Sonorous undulations are propagated 

 to the fluid in the labyrinth through the tympanum and 

 chain of otic bones. 



The auditory nerves are distributed to the ampullre and 

 sacculi of the vestibule, and to the spiral plate of the 

 cochlea. At the terminal filaments in the sac of the 

 vestibule, crystals, called otoliths, of shapes differing in 

 various animals, are inclosed in membrane. Hasse con- 

 siders them to be vibrating organs, but Waldeyer regards 

 their function to be that of dampening sound. 



As we distinguish in sounds the various qualities of 

 pitch, intensity, quality, and direction, it is probable that 

 there is a special apparatus for each, but histology has 

 not yet established this fully. Kolliker thinks the gan- 

 glionic termination of the cochlear nerve renders it proba- 

 ble that it only receives sonorous undulations. The ex- 

 periments of Flourens seem to show that the semicircular 

 canals influence the impression of direction of sound. 



In the sacs of the vestibule and ampullae, the nerve- 

 fibres are confined to a projection of the walls called the 

 septum nerveum. Here are found cylinder- and fibre-cells, 

 with rods, basal-cells, and nerves. But it is in the lamina 

 spiralis of the cochlea that the most elaborate organ, 



