The Afferent and Efferent Neurons 



71 



beyond the general epithelial surface, and bears from six to eight stiff 

 cilia (the olfactory hairs) [Fig. 59]. From the other, deeper, end the cell 

 sends an axon, which passes in through orifices in the bone (the 'cribri- 

 form plate ' of the ' ethmoid ' bone) and arborizes in the olfactory glome- 

 ruli of the olfactory bulb {bulbus olfactorius) [Fig. 60]. 



AFFERENT CHAINS OF THE OPTIC NEURONS. 



In the eye, we have to consider not a single afferent neuron, but a series 

 of three, forming an afferent chain [Fig. 61]. The outermost neurons are 



Fig. 59. Olfactory cells and sustentacular cells, schematic. Magnified about 800 

 diameters. (Merkle-Henle, Anatomic.) O, olfactory cell; S, sustentacular cell. 



specialized receptor cells of the two types: rod cells and cone cells. The 

 rods and the cones are the outermost portions of these cells, and are the 

 structures which receive stimulation from the light waves. The rods in the 

 human retina are approximately 60/n in length and 2/x in diameter. The 

 cones are about 35a* in length, the ' outer segment ' being approximately 

 of the diameter of a rod, and the ' inner segment ' about 7^ in diameter. 



The rods and cones are packed closely together with their long axes 

 perpendicular to the surface of the retina. In the fovea centralis there 

 are only cones : in the macula lutea surrounding the fovea, each cone is 

 encircled by a row of rods. Farther away from the fovea (». e., in the 



