SENSE ORGANS AND NERVOUS COORDINATION 577 



Cornea.' 



•Iris 



Anlerior chamber 

 Posterior' cTiamter 



Ciliary body 



Ciliary 

 process 



Retina.- 



Choroid 

 coat 



Conjunctiva. 



Canal op Schlemm 

 Ciliary muscle 



Extrinsic 

 muscle 



fL Fibers oF 

 Coptic nerve 



QanSlion 

 cells 



Bipolar 

 cell 



-Blind spot 

 Optic nerve and sheath 

 A 



Figure 29.2. The mammalian eye. A, Diagram of a section through the eye; B, 

 diagram of the layer of the retina containing the receptor cells, rods and cones, and 

 the neurons. (Modified after Walls.) 



this respect the iris is analogous to the iris diaphragm of a camera or 

 microscope. The thickened portion of the choroid around the base of the 

 iris is the ciliary body. A number of zonule fibers extend from it to the 

 lens and help to hold it in place. Muscles within the ciliary body are 

 concerned with focusing the eye. 



The retina is the innermost layer of the eyeball. It consists of a 

 pigmented layer, intimately associated with the choroid, and a nerv- 

 ous layer, which contains millions of receptor cells, the rods and 

 cones, and afferent neurons that continue through the optic nerve 

 to the brain. The rods and cones lie in the surface of the nervous 

 layer that faces the choroid, and light must pass through most of the 

 retina before it can stimulate them. This apparently illogical arrange- 

 ment is explained by the mode of development of the eye. The retina 

 develops from an outgrowth of the brain, which in turn develops 

 from an infolding of the surface ectoderm (Fig. 29.3). What was the 

 outer surface of the ectodermal cells becomes the inner surface of the 

 nervous layer of the retina. The polarity of the cells is retained dur- 

 ing their various developmental gymnastics. The fact that the retina 

 and optic nerves are developmentally parts of the brain also explains 

 why at least two afferent neurons (bipolar and ganglion cells) are 

 involved in transmitting impulses from the rods and cones. Chains 

 of neurons are common in brain tracts, but in most nerves only one 

 neuron extends from a receptor cell to the brain or spinal cord. 



Rods respond to light of much lower intensity than cones and 

 are particularly efficient in dim light. It is not surprising, therefore, 

 that they are abundant in the eyes of nocturnal animals. Cones are 

 more efficient in brighter light, and they also distinguish between 

 colors in some way not yet understood. One theory is that there are 

 three types of cones, each type sensitive to light of the wavelength of 

 one of the three primary colors. Each cone typically activates a single 



