SIGHT. 161 



What is the origin of the optic or second cranial nerve ? 



It arises by two roots one in the corpora quadrigenrina, and the 

 other in the optic thalamus with many fibres derived from centres 

 of vision in the posterior portion of the cerebral lobes. These 

 fibres unite and form a nerve-trunk (the optic tract) which en- 

 circles the crus cerebri, meeting the tract of the opposide side just 

 in front of the pituitary body. Here there is a fusion of the two 

 optic tracts in a partial decussation, some fibres going to the eye 

 of the same side, and some decussating. The fibres then pass to 

 each eye as the optic nerve, which is distributed by special nerve- 

 endings in the retina. 



Has the optic nerve any other physiological property than that of 

 conduction of special-sense impulses ? 



No. It has no common sensibility. It is, however, the con- 

 ductor of afferent impulses for the iris reflex. 



What is the effect of stimulation of the divided optic nerve ? 



None, for the peripheral end ; for the central portion, a sensation 

 of light and contraction of the pupil. 



What accessory apparatus is important to the protection of the 

 eye? 



Eyelids and lachrymal gland. 



Describe the eyelids. 



Each eye has two lids, an upper and lower. Each consists of a 

 thin plate of elastic tissue with a covering of loose skin and a 

 smooth lining of mucous membrane the conjunctiva which is 

 reflected upon the eyeball. Along the edges of the lids are a 

 number of short curved hairs which screen the eye from foreign 

 bodies. The extreme sensitiveness of the conjunctiva helps in 

 this by giving immediate warning when any foreign substance 

 gets in the eye. 



What is the lachrymal gland? 



It is a small racemose gland lodged in the upper and outer part 

 of the orbit. It has several ducts, which lead to the surface of 

 :the conjunctiva of the upper lid. The secretion of the gland is 

 usually just sufficient to keep the eye moist, but under the stimu- 

 lus of pain or intense emotion the secretion is increased, and ap- 

 pears in drops which flow from the eyes tears. Under ordinary 

 circumstances a slight excess of this moisture is drained into the 



11 Phy. 



