788 PHYSIOLOGY. 



The optic tract arises in the retinal cells, which are its trophic 

 center. These retinal cells send in fibers which arborize around the 

 cells of the anterior corpora quadrigemina, pulvinar, and the lateral 

 corpus geniculatum. Kow, from the lateral corpus geniculatum and 

 pulvinar we have a second set of neuraxons running to the occipital 

 cortex, the center of vision. Here the lateral corpus geniculatum 

 and pulvinar are the relay centers in the path of visual impulses. 



THE VITREOUS HUMOR. The hyaloid membrane, a homogene- 

 ous capsule, encloses the vitreous humor. This hyaloid membrane 

 divides as it comes forward over the vitreous, one part going to the 

 capsule of the lens as the zonule of Zinn, and the other passing in 

 front of the vitreous. The free part of the hyaloid, stretching from 

 the capsule of the lens to the ciliary body, is termed the suspensory 

 ligament of the lens. Between these two layers of the hyaloid the 

 canal of Petit is formed, a lymphatic canal. In the center of the 

 vitreous is the canal of Stilling, which, in the foetal state, was the 

 pathway of the artery of Zinn to the posterior part of the capsule 

 of the lens. The vitreous has no blood-vessels, and is composed 

 chemically of water, 98.5 per cent., and salts, extractives, and traces 

 of proteid and nucleo-albumin. The vitreous has fine intercrossing 

 connective-tissue fibers, connective-tissue cells, and leucocytes. 



AQUEOUS HUMOR. This fluid contains about 2 per cent, of 

 solids, chiefly in the form of sodium chloride. It occupies the 

 anterior chamber in the space back of the cornea and in front of 

 the iris. The so-called posterior chamber lies between the back of 

 the iris and in front of the lens. 



When by ulceration of the cornea or accident the aqueous humor 

 escapes, it is found to be regenerated very rapidly. 



The secretion of the aqueous humor has been studied by fluore- 

 scin instilled into the fluids of the eyeball. It has been found that 

 the humor is secreted by the posterior surface of the iris and ciliary 

 body. It passes through the pupil into the anterior chamber. 



BLOOD-VESSELS OF THE EYE. There are two systems of blood- 

 vessels: the retinal and the ciliary system. These systems are 

 separate, and anastomose only at the place of entrance. The retinal 

 system is the central artery of the retina, which goes through the 

 axis of the optic nerve until it reaches the optic papilla, where it 

 divides into two branches, one running forward and the other in a 

 posterior direction. These vessels are seen with the ophthalmo- 

 scope. 



