CRANIAL NERVES. 197 



nerve-cells in the gray matter of the pre-callosal part of the gyrus fornicatus, 

 the gyrus hippocampus and the gyrus uncinatus. 



Properties. The olfactory nerves do not give rise to either motor or 

 sensor phenomena when stimulated. When stimulated at their periphery by 

 odorous particles, nerve impulses are developed which, when conducted to the 

 brain, evoke the sensation of smell. Destruction of the olfactory nerves, the 

 bulb or tract, is followed by a loss of the sense of smell. 



Function. Presides over the sense of smell. Conducts impulses to the 

 cerebrum which give rise to sensations of odor. 



Second Nerve. Optic. 



Origin. The optic nerve arises from large nerve-cells in the anterior part 

 of the retina. From this origin the nerve-fibers turn backward and converge 

 to form a well-defined bundle (the optic nerve) which passes out of the eyeball, 

 through the orbit cavity as far as the sella turcica. At this point there is a 

 union and partial decussation, in man at least, of the fibers, forming what is 

 known as the optic chiasm. From the posterior portion of the chiasm there 

 passes backward on either side a bundle of nerve-fibers, the optic tract. Each 

 tract contains nerve-fibers which come from the temporal two thirds of the 

 retina of the same side and the nasal third of the retina of the opposite side. 

 The fibers of the optic tract arborize around nerve-cells in the external genic- 

 ulate body, the pulvinar, and the anterior quadrigeminal body. By means 

 of the optic radiation, the nerve-cells in these different ganglia are brought 

 into relation with the visual center, the cuneus. 



Properties. The optic nerves are insensible to ordinary impressions, and 

 convey only the special impressions of light. Division of one of the nerves is 

 attended by complete blindness in the eye of the corresponding side. 



Hemiopia and Hemianopsia. Owing to the decussation of the fibers in 

 the optic chasm, division of the optic tract produces loss of sight in the outer 

 half of the eye of the same side, and in the inner half of the eye of the opposite 

 side, the blind part being separated from the normal part by a vertical line. 

 The term hemiopia is applied to the loss of function or paralysis of the one 

 half of the retina; as a result of this, there will be an obliteration of the field of 

 vision on the opposite side to which the term hemianopsia is given. If, for 

 example, the right optic tract be divided, there will be hemiopia in the outer half 

 of the right eye and inner half of the left eye, thus causing left lateral hemian- 

 opsia, and as the two halves are affected which correspond in normal vision, 

 the condition is known as homonymous hemianopsia. Lesion of the anterior 

 part of the optic chiasm causes blindness in the inner half of the two eyes. 



