62 



PSYCHOBIOLOGY 



epithelium) the connective tissue sheath, then the medullary sheath, and 

 finally the neurilemma. Then, nearer the epithelium, the branches of sev- 

 eral fibers form a ' skein ' or network of fibrils called primary plexuses. 

 From these plexuses branches are given off which form secondary plexuses, 

 still nearer the epithelium. Finally, fibrils proceed from the secondary 

 plexus and ramify among the epithelial cells. The actual nerve endings, 

 or endings of the ultimate branches, are ' free varicose fibrils '. 



TACTILE DISCS. 



In some cases (in deep layers of stratified epithelium) the fibrils termin- 

 ate in flattened or saucer-shaped plates, tactile discs [Fig. 46], applied 

 to epithelial cells (called tactile cells) as is the cup to an acorn. 



The tactile discs in the human being are especially numerous in the skin 

 over the thighs and abdomen. 



THE AUDITORY AND GUSTATORY ENDINGS. 



The dendrites of the neurons located in the spiral ganglia of the cochlea 

 pass out, through the spiral lamina, and their branches are applied to the 



Membr. tector. 



Lam. spir 



Membr. basil. 



Vaa apir. * 



Fig. 47. Cross-section of the organ of Corti, showing nerve endings in the cochlea. 

 Magnified probably 500 diameters. (Modified from Merkel-Henle, Anatomic.) The 

 fibers (/4) are seen emerging from the lamina spiralis, some terminating synaptically 

 about the inner hair cells, (3), and the remainder crossing the 'tunnel of Corti' (B) 

 to terminate around the outer hair cells (6). The black dots (*) represent cross- 

 sections of the convolutions of the nerve fibers. 



hair cells [Fig. 47]. These hair cells are columnar epithelial cells, from 

 the free extremities of which bundles of cilia (auditory hairs) project. 

 These cells are specialized receptors for auditory stimuli, and pass the 



