668 



THE SENSES 



cervical sympathetic and superior cervical ganglion, and the ophthalmic 

 branch of the fifth cranial nerve. 



Structure of the Retina. The retina occupies the deeper half of the 

 cup of the eyeball. It extends forward as far as the ora serrata, where its 

 complex structure changes the form to a simple epithelial layer, which lines 

 the anterior portion of the eyeball and the ciliary processes. In the centei 

 of the retina is a round yellowish spot having a minute depression in its 

 center, called the yellow spot of Sommering. The depression in its center is 



Gangli- 



onic 



layer 



Stratum 

 optfcum 



Membrane limitans interim 



FIG. 446. Section of Human Retina. (Cunningham, modified from Schulze.) 



thefovea central-is. About 2 . 5 mm. to the inner side of the yellow spot is the 

 point at which the optic nerve enters and spreads out its fibers into the retina. 

 The optic nerve arises from the base of the brain and passes forward 

 toward the orbit, being covered by the membranes which cover the brain. 

 The fibers of the optic nerve are exceedingly fine, and are surrounded by the 

 myelin sheath, but do not possess the ordinary external nerve sheath. As 

 they pass into the retina they lose their myelin sheaths and proceed as 

 axis-cylinders (the cells of origin of these fibers are in the retina). Neuroglia 

 supports the nerve fibers in the optic nerve trunk. In the center of the nerve 

 is a small artery, the arteria centralis retina. The number of fibers in the 



