200 HUMAN PHYSIOLOGY. 



Origin of the Afferent Fibers. The afferent fibers have their origin in 

 nerve-cells in the Gasserian ganglion. From each cell a short process 

 develops which soon divides into two branches, one of which passes centrally, 

 the other peripherally. The centrally directed branches form the so-called 

 large root; the peripherally directed branches collectively constitute the three 

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

 the inferior maxillary. 



Distribution. The centrally directed branches enter the pons Varolii 

 on its lateral aspect. After pursuing a short distance, these fibers arborize 

 around nerve-cells in the gray matter of the pons and medulla. 



The peripherally directed brances are distributed as follows: 



1. The ophthalmic branches to the conjunctiva and skin of the upper eyelid, 

 the cornea, the skin of the forehead and the nose, the lachrymal gland and 

 the mucous membrane of the nose. 



2. The superior maxillary branches to the skin and conjunctiva of the lower 

 lid, the nose, the cheek and upper lip, the palate teeth of the upper jaw 

 and the alveolar processes. 



3. The inferior maxillary branches to the external auditory meatus, the side 

 of the head, the mouth, the tongue, the teeth of the lower jaw, the alveolar 

 processes and the skin of the lower part of the face. 



Properties. The trigeminal nerve, composed mainly of afferent fibers, 

 is the most acutely sensitive nerve in the body, and endows all the parts to 

 which it is distributed with general sensibility. 



Stimulation of the large root, or of any of its branches, will give rise to 

 marked evidence of pain; the various .forms of neuralgia of the head and 

 face being occasioned by compression, disease, or exposure of some of 

 its terminal branches. 



Division of the large root within the cranium is followed at once by a com- 

 plete abolition of all sensibility in the head and face, but is not attended by 

 any loss of motion. The integument, the mucous membranes, and the eye 

 may be lacerated, cut, or bruised, without the animal exhibiting any evidence 

 of pain. At the same time the lachrymal secretion is diminished, the pupil 

 becomes contracted, the eyeball is protruded, and the sensibility of the tongue 

 is abolished. 



The reflex movements of deglutition are also somewhat impaired, the im- 

 pression of the food being unable to reach and excite the nerve center in the 

 medulla oblongata. 



Origin of the Efferent Fibers. The efferent fibers have their origin in 

 nerve-cells in the gray matter of the pons Varolii beneath the floor of the 

 fourth ventricle. 



