NERVOUS SYSTEM 873 



II. Tunica Media. 







1. Choroid coat. 



2. Iris. 



3. Ciliary body. 



III. Tunica Interna. 



1. Retina. 



2. Pigment membrane. 



The refracting media, or transparent media of the eye traversed by 

 a ray of light are : 



1. The cornea. 



2. Aqueous humor. 



3. Lens. 



4. Vitreous humor. 



Each of these layers is made up of other layers in turn and can be 

 best understood from a careful observation of Figure 18. The retina 

 is made up of several layers of ganglion and sensory cells. The sensory 

 cells lie toward the outside of the eyeball and have a rod or cone toward 

 their outer end. This is the real seeing-portion of the cell. The cells 

 themselves are called rod and cone cells (Fig. 490, C). 



The yellow spot at the center of the retina where vision is the most 

 distinct is called the macula lutea or fovea centralis. 



From what has been said in our general discussion of the central 

 nervous system, we have seen that the surface lining of the central canal 

 of brain and spinal cord is the sensory portion. This is called the 

 ependyma. Originally the rods and cones are on the primitive outer 

 surface, and the ganglion cells and nerve fibers are on the ventral sur- 

 face, of the ectoderm. The rods and cones, therefore, correspond to 

 other sensory organs, such as the organs of the lateral line, taste buds, 

 etc. Now for the light to get to the rods and cones, it is necessary that 

 such light traverse the whole retina and then the nervous impulses have 

 to travel back through the same layer to reach the optic nerve. 



It is well at this point to compare the vertebrate eye which we "are 

 now studying with the parietal eye of the reptiles. 



The space between lens and retina is filled with a semi-solid sub- 

 stance called the vitreous humor. What this vitreous substance is and 

 how it arises is still in dispute. 



The outer wall of the optic cup forms the pigmented epithelium of 

 the eye ; the black pigment developed in this region ultimately surrounds 

 and isolates the rods and cones, so that only light which falls directly 

 upon them can affect them. It is from the outer portion of this pig- 

 mented layer that the various tunics of the eye develop. These tunics 

 of the eye are mesenchynial in origin. The tunica vasculosa which 

 surrounds the retina is divided into a choroid and a ciliary portion. The 



