THE ENCEPHALIC OR CRANIAL NERVES 



623 



Function. The function of the trochlear nerve is to transmit nerve 

 impulses to the superior oblique muscle and to excite it to contraction. 



THE FIFTH NERVE. THE TRIGEMDSTAL 



The fifth cranial nerve, the trigeminal, consists of both afferent and 

 efferent axons which for the most part are separate and distinct. The 

 afferent axons constitute 

 by far the major portion, 

 the efferent fibers the 

 minor portion, of the 

 nerve. 



Origin of the Affer- 

 ent Axons. The afferent 

 axons have their origin 

 in the monaxonic cells in 

 the ganglion of Gasser, 

 which rests on the apex of 

 the petrous portion of the 

 temporal bone. The cells 

 of this ganglion give origin 

 to a short process which 

 soon divides into two 

 branches, one of which 



passes centrally, the other 



n J f^. ., N FIG. 269. SCHEME OF ORIGIN AND CONSTITUTION OF THE 



penpnerally (rig. ^ 209). TRIGEMINAL NERVE, i. Centrally coursing fibers. 2, 3, 

 The Centrally directed 4- Peripherally coursing fibers of the cells of the ganglion of 

 branches rollertivplv fnrm Gasser - R > N - Nuclei of origin of the efferent fibers. 6. 



lively lorm Motor root Central terminations of the large root 

 the so-called large or sen- 

 sor root; the peripherally directed branches collectively constitute the three 

 main divisions of the nerve: viz., the ophthalmic, the superior maxillary, 

 and the inferior maxillary. Branches of the carotid plexus of the sym- 

 pathetic enter the nerve in the neighborhood of the ganglion of Gasser and 

 accompany some of its branches to their terminations. 



Distribution. -i. The Central Branches. The axons of the large root 

 pass backward into the pons Varolii on its lateral aspect. After entering 

 the pons each axon divides into two branches, one of which passes upward 

 a short distance, the other passes downward, descending as far as the 

 second cervical segment. Both branches give off a number of collaterals, 

 some of which terminate in fine end-tufts around nerve-cells in the sub- 

 stantia gelatinosa. 



2. The Peripheral Branches. The peripheral axons emerge from the 

 peripheral end of the ganglion of Gasser in three distinct and separate 

 branches, each of which is distributed to a different region of the face 

 and head. 



i. The ophthalmic branch passes forward and subdivides into three large 

 branches, the frontal, the lachrymal, and the nasal. The ultimate 

 termination of the branches of these nerves is as follows: viz., the 

 conjunctiva and skin of the upper eyelid, the cornea, the skin of the 

 forehead and the nose, the lachrymal gland and caruncle, and the 

 mucous membrane of the nose. 



