260 NORMAL HISTOLOGY. 



of a layer of epithelium containing two sorts of cells : first, ciliated 

 cells, which are somewhat flask-shaped and are called " hair-cells " ; 

 second, epithelial cells, the " cells of Deiters," which surround and 

 enclose the hair-cells, except at their free ends, and reach the sur- 

 face of the mucous membrane, where their ends are cuticularized. 

 These cells of Deiters extend from the surface of the membrane to 

 the basement-membrane, while the hair-cells extend only for a por- 

 tion of that distance. 



The dendrites of the auditory nerve are distributed among these 

 cells, but are not in organic union with them (Fig. 242). In this 

 respect the auditory apparatus diifers from the olfactory and resem- 

 bles the tactile. The nervous dendrites are processes of bipolar 

 ganglion-cells situated in the ganglia on the branches of the auditory 

 nerve. The neurites from those cells presumably carry the nervous 

 stimuli to the cerebrum. The bipolar cells are, therefore, analogous 

 to the posterior root ganglion-cells of the spinal nerves. Whether 

 this single neuron carries the nervous stimulus directly to the cere- 

 bral cortex cannot be stated, but it is probable that there is an inter- 

 mediate neuron in the tract of transmission, perhaps in the medulla 

 oblongata. 



5. Sight. The receptive nervous organ of vision is the retina. 

 This has an extremely complicated structure, which may be divided 

 into the following nine layers : 



1. The layer of pigmented epithelium, which lies next to the 

 choroid coat of the eye, and is, therefore, the most deeply situated 

 coat of the retina ; 2, the layer of rods and cones ; 3, the external 

 limiting membrane ; 4, the outer granular layer ; 5, the outer molec- 

 ular layer ; 6, the inner granular layer ; 7, the inner molecular 

 layer ; 8, the ganglionic layer ; 9, the layer of nerve-fibres. 



Internal to the ninth layer is the internal limiting membrane, 

 which separates the retinal structures from the vitreous humor 

 occupying the cavity of the eyeball. The general character and 

 associations of these layers are shown in Fig. 243. 



1. The layer of pigmented epithelium is made up of hexagonal 

 cells, which are separated from each other by a homogeneous 

 cement and form a single continuous layer upon the external sur- 

 face of the retina. They are in contact with the rods and cones 

 of the next layer, and send filamentous prolongations between those 

 structures. The pigment lies within these filamentous processes 

 and the portion of cytoplasm continuous with them, but its position 



