THE SENSE OF SIGHT. 



645 



fibers pursue a radial or meridional direction. Taking their origin from the 

 junction of the sclera, cornea, and iris, they pass backward to be inserted 

 into the chorioid coat opposite the ciliary processes. The inner portion of 

 the muscle is interrupted by bundles of fibers which pursue a circular 

 direction. (Fig. 303.) They collectively constitute the annular or ring 

 muscle of Miiller. The ciliary muscle in common with the circular fibers 

 of the iris receives its nerve-supply direct from the nerve-cells in the ciliary 

 ganglion. Contraction of the 

 ciliary muscle tenses the cho- 

 rioid coat, and for this reason 

 it is frequently termed the ten- 

 sor chorioidecB. 



The Retina. The retina 

 is the internal coat of the eye, 

 extending forward almost to 

 the ciliary processes, where it 

 terminates in an indented bor- 

 der, known as the or a s errata. 

 In the living condition it is 

 clear, transparent and pink 

 in color. After death it be- 

 comes opaque. The retina is 

 abundantly supplied with 

 blood-vessels, derived from 

 the arteria centralis retina, a 

 branch of the ophthalmic, 

 which pierces the optic nerve 

 near the sclera, runs forward 

 in its center, to the retina, in 

 which its terminal branches are distributed. The veins arising from the 

 capillary plexus leave the retina by the same route. 



In the posterior portion of the retina, at a point corresponding with the 

 axis of vision, there is a small oval area about 2 mm. in its transverse and 

 about 0.8 mm. in its vertical diameter. From the fact that it presents a 

 yellow appearance, it is known as the macula lutea. This area presents in 

 its center a depression with sloping sides, known as the fovea centralis. 

 About 3.5 mm. to the nasal side of the macula is the point of entrance of the 

 optic nerve. 



The retina is remarkably complex in structure, presenting an appearance, 

 when viewed microscopically, something like that represented in Fig. 304, 

 indicating that it is composed of different cellular elements arranged in 

 layers. These have been named, from behind forward, as follows: 



1. The layer of pigment cells. 



2. The layer of rods and cones, or Jacobson's layer. 



3. The external limiting membrane. 



4. The outer nuclear or granular layer. 



5. The outer molecular or reticular layer. 



6. The inner nuclear or granular layer. 



7. The inner. molecular or reticular layer. 



i. Pigment-layer (not shown). 



2. Layer of rods and cones. 



3. External limiting membrane. 



4. Outer nuclear layer. 



5. Outer molecular layer. 



6. Inner nuclear layer. 



7. Inner molecular layer. 



8. Layer of ganglion cells. 



9. Layer of nerve-fibers. 



FIG. 304. VERTICAL SECTION OF HUMAN RETINA. 

 (Schaper.) 



