THE BRAIN. 515 



When this nerve is paralyzed the patient cannot turn the eye out- 

 ward and downward ; the action of the superior oblique is, there- 

 fore, to turn the eye outward and downward. If the head is not 

 turned toward either side when this nerve is paralyzed, the only 

 thing observable is that the patient sees double when he looks 

 downward, and the image perceived by the affected eye is oblique 

 and below that seen by the eye that is affected. For a further 

 discussion of the ocular muscles see p. 556. 



Trigeminus. This nerve, which is also called " trifacial," has 

 received its names from the fact that it has three subdivisions, 

 and its latter name from the fact that is distributed in the main to 



FIG. 304. General plan of the branches of the fifth pair : 1, lesser root of the 

 fifth pair : 2, greater root, passing forward into the Gasserian ganglion ; 3, placed 

 on the bone above the ophthalmic division, which is seen dividing into the supra- 

 orbital, lacrimal, and nasal branches, the latter connected with the ophthalmic 

 ganglion ; 4, placed on the bone close to the foramen rotundum, marks the superior 

 maxillary division ; 5, placed on the bone over the foramen ovale, marks the inferior 

 maxillary division (after a sketch by Charles Bell). 



the parts about the face. It arises by two roots, anterior and 

 posterior. The anterior root, the smaller, is purely motor; the 

 posterior root, the larger, is sensory, and is characterized anatomi- 

 cally by having upon it the Gasserian ganglion. The nerve leaves 

 the brain at the side of the pons Varolii. The real origin of 

 the motor root is a nucleus in the floor of the fourth ventricle ; 

 the sensory root arises from a nucleus on a level with the middle 

 of the superior peduncle of the cerebellum, just internal to the 

 margin of the fourth ventricle. Some authorities give it a more 

 extensive origin, from the pons through the medulla and as far as 

 the posterior cornua of the gray matter of the spinal cord. 



