162 Basic Structure of Vertebrates 



I. Olfactory, consisting of fibers developed as processes of the 

 sensory cells in the lining of the olfactory (nasal) chamber and passing 

 into the olfactory lobe of the telencephalon. Sensory. Closely associ- 

 ated with each olfactory nerve is a relatively small nervus terminalis 

 (Fig. 136), which passes to the olfactory surface, but its fibers are de- 

 rived from cells in a small ganglion borne by the nerve. Apparently 

 sensory, the nature of the function of the nerve is not known. 



II. Optic, consisting mainly of sensory fibers derived from cells in 

 the retinal layer of the eye. Most, if not all, of the fibers pass through 

 the diencephalon to primary optic centers in the roof of the mesen- 

 cephalon. Except in fishes, however, some optic fibers pass directly to 

 centers in the thalami. These thalamic connections are most numerous 

 in mammals. A small minority of the optic fibers have origin from 

 nerve cells in the mesencephalon and grow forward and outward along 

 the optic stalk into the retina. The function of these centrifugal fibers is 

 problematic. At the base of the diencephalon, the right and left optic 

 nerves meet and some or all of the fibers of one nerve cross those of the 

 other, so that fibers of either nerve pass to optic centers in the opposite 

 side of the brain. The region of crossing, the optic chiasma, occasions 

 more or less of a prominence on the ventral surface of the diencephalon 

 (Figs. 140, oc, 530). In most vertebrates the crossing (decussation) is 

 complete, but in mammals the decussation is incomplete, each nerve 

 distributing fibers to centers on both sides of the brain. 



It is to be emphasized that the optic nerve differs from all other 

 cranial nerves in that the exteroceptors to which it is related are them- 

 selves a product of the embryonic brain. The entire mechanism of 

 vision is a pair of highly specialized lateral protrusions of the brain. 



Nerves III, IV, and VI innervate the small muscles which rotate 

 the eyeball in its orbit (Fig. 148). In all vertebrates there are six of these 

 extrinsic muscles of the eyeball. Four of them constitute a rectus 

 group — according to the positions of their points of attachment to the 

 eyeball, a dorsal, a ventral, an anterior, and a posterior rectus muscle. 

 These four muscles converge to their attachment at the posterior re- 

 gion of the orbit. The two others, a dorsal and a ventral oblique 

 muscle, pass obliquely forward to attach at the anterior region of the 

 orbit. 



III. Oculomotor, consisting mainly of somatic motor fibers 

 coming from a motor center in the base of the mesencephalon and in- 

 nervating four of the muscles of the eyeball, the dorsal, ventral, and 

 anterior recti, and the dorsal oblique muscle. A small branch goes 

 outward to supply the muscle which raises the upper eyelid, the levator 



