882 



COMPARATIVE ANATOMY 



tory nerve is very short, but the olfactory lobe is drawn out (Fig. 478), 

 while the distal end of the lobe is enlarged into an olfactory bulb con- 

 taining the glomeruli, extending proximally from this slender olfactory 

 tract. In these cases, the olfactory bulb lies very close to the olfactory 

 epithelium. 



In some fishes, amphibia, lizards, and turtles, the nerve is long, 

 while the olfactory lobe is shortened. 



It has recently also been found 

 that in all vertebrates there is a termi- 

 nal nerve, sometimes called the pre- 

 optic nerve, which leaves the brain near 

 the base of the olfactory nerve. It has 

 a ganglion upon it but its functions and 

 connections are as yet unknown. 



II. The Optic Nerve. This arises 

 in the eye and extends to the floor of 

 the diencephalon. Branches from it 

 are distributed over the entire inner 

 surface of the retina, the ganglion cells 

 lying in the inner layer. A fiber from 

 the right eye passes to the left side of 

 the brain, and those from the left eye 

 to the right side of the brain (Fig. 495). 

 The crossing of the fibers forms what 

 is called the optic chiasma. The fibers, 

 after crossing, extend dorsally and 

 backward into the optic lobe. The optic 

 chiasma is imbedded in the brain of 

 cyclostomes. In other vertebrates it 

 may be plainly seen from the outside. 

 In mammals the crossing in the 

 chiasma is incomplete, some of the 

 fibers not crossing. A clear understanding of the growth and develop- 

 ment of both olfactory and optic nerves can only be had from an under- 

 standing of their embryological development. 



There is also a small thalamic nerve which arises between the 

 diencephalon and the mesencephalon. This has so far, however, been 

 seen only in some embryonic fishes. It shortly disappears, and little is 

 known of its function. 



The third, fourth, and sixth nerves are the oculo-motor, trochlear, 

 and abducens nerves, or the eye-muscle-nerves, all of which assist in 

 moving the eye in its socket (Fig. 494). The oculo-motor nerve arises 

 from the ventral surface of the midbrain. It supplies the superior, mid- 

 <ile and inferior rectus, and the inferior oblique muscles (Fig. 466). The 

 abducens arises from the inferior surface of the medulla and supplies the 



Fig. 495. 



Diagram of the central connections of 

 the optic nerve and optic tract to show 

 crossing of the fibers. (From Cunningham's 

 Anatomy. ) 



