THE TRIFACIAL NERVE. 307 



forming the optic commissure (or chiasm), and then proceed to certain ganglia, 

 from which the visual impressions are conveyed to the visual centers of 

 the occipital lobes. (Fig. 194, 16, optic chiasm.) 



The third, or oculo-motor (Fig. 197), is the mover of the eyes. 

 It proceeds from the base of the brain and enters the orbit, to sup- 

 ply four of the muscles of the eyeball, and also the elevator of the 

 upper lid. (Eye muscles thus supplied: Superior, inferior, and 

 internal recti, and inferior oblique. See p. 343, orbital muscles.) 



By the action of the first three the eye is turned upward, downward and 

 inward; the inferior oblique turns it upward and outward. 



& 7 to 



FIG. 197. NERVES or THE ORBIT. 



i, Optic nerve; 2, Oculo-motor (3d); 5, abducens (6th). Other figurer mark various 

 branches. 10, Ciliary ganglion. (Sappey.) 



The third nerve supplies also the circular fibers of the iris "which 

 contract the pupil of the eye, and the accommodation muscle by 

 which the eye is focussed for viewing objects at different distances. 



The fourth, or trochlear nerve, is so called because the tendon 

 passes through a loop of fascia and bends around like a rope on 

 a pulley or trochlea. It supplies the muscle which rolls the eye 

 downward and outward (the superior oblique muscle). 



The fifth, or trifacial (trigeminal), is the great sensory nerve 

 of the face, nose and throat. Some motor fibers for muscles of 

 mastication accompany the sensory fibers and the nerve is 

 described as having two roots; sensory and motor. 



The sensory root has a large ganglion, semilunar (or Gasserian) ganglion, 

 and in front of this it is in three divisions, called the ophthalmic, maxillary, 

 and mandibular nerves. The ophthalmic nerve lies in the orbit ; it is the nerve 

 of sensation of the structures contained therein; also of the eyelids and side 

 of the nose. The maxillary nerve appears at the infraorbital foramen. It 

 is the nerve of sensation for the upper teeth and the cheek and temple. The 



