This nerve transmits both efferent and afferent impulses carried by two different strands of fibres. The motor part, which arises from a gray nucleus in the floor of the fourth ventricle, is much the smaller of the two, and has been compared to the anterior root of a spinal nerve. The large sensory division springs from a very extensive tract, which can be traced from the pons Varolii through the medulla to the lower limit of the olivary body, and on to the posterior cornua of the spinal marrow. This set of fibres has been linked to the posterior root of a spinal nerve, being somewhat analogous to it in origin, function, and the fact that there is a large ganglion on it within the cranium.

The distribution and peripheral connections of this nerve are somewhat complicated, and should be carefully studied when the manifold functions of its branches are being considered. The various impulses conveyed by the trifacial nerves may be thus enumerated: -

(I) Efferent Fibres

1. Motor

To the muscles of (1) mastication, viz., temporal masseters, both pterygoids, mylohyoid, and the anterior part of the digastrics; (2) to the tensor muscle of the soft palate; and (3) to the tensor tympani. (4) In some animals (rabbit) nerve filaments are said to pass to the iris, reaching the eyeball by the ciliary ganglion.

2. Secretory

The efferent impulses which stimulate the cells of the lachrymal gland to increased action pass along the branches of the ophthalmic division of this nerve.

3. Vasomotor

The nerves governing the muscles of the blood vessels of the eye, of the lower jaw, and of the mucous membrane of the cheeks and gums.

4. Trophic

On account of the impairment of nutrition of the eye and mucous membrane of the mouth, which occurs after injury of fifth nerve, it is said to carry fibres which preside over the trophic arrangements of these parts.

(2) Afferent Fibres

1. Sensory

All the divisions of the trifacial nerve may be said to be connected with cutaneous nerves, by which the ordinary sensory impulses are carried from - (1) the entire skin of the face, and the anterior surface of the external ear; (2) from the external auditory meatus; (3) from the teeth and periosteum of the jaws, etc.; (4) from the mucous membrane lining the cheeks, floor of the mouth, and anterior part of the tongue; (5) from the lining membrane of the nasal cavity; (6) from the conjunctiva, ball of the eye, and orbit generally; (7) and from the dura mater, including the tentorium.

2. Excito-Motor

Some of the fibres which have just been enumerated as carrying ordinary sensory impressions have special powers of exciting coordinated reflex motions. Thus the sensory fibres from the conjunctiva and its neighborhood are the afferent channels in the common reflex acts of winking and closing the eyelids; and the fibres from the nasal mucous membrane excite the involuntary act of sneezing.

3. Excito-Secretory

As in the case of reflex movements, secretion may be excited reflexly. Fibres carry afferent impulses to the medulla from the anterior part of the tongue, and excite activity of the salivary glands. Stimulation of the mucous membrane of the nose or eye causes impulses to pass to the secretory centre of the lachrymal glands, which are frequently thus reflexly excited.

Intense stimulation of almost any of the afferent nerves may excite these reflex phenomena. Thus the most stoic person will experience active secretions of saliva and lachrymal fluid, as well as spasmodic closure of the lids during the extraction of a tooth. Even the bold use of a blunt razor will cause tears to flow down the cheeks, by sending excito-secretory impulses along the branches of the inferior and superior maxillary division of this nerve.

4. Tactile Impulses

Tactile impulses are appreciated by the anterior part of the tongue with remarkable delicacy, and are conveyed by the lingual branch of the fifth nerve; most of the cutaneous fibres are also capable of receiving tactile stimulation.

5. Taste

The tastes appreciated by the anterior part and the edges of the tongue are carried by fibres which lie in the peripheral branches of this nerve. These belong chiefly, if not altogether, to the chorda tympani, and leave this lingual branch of the fifth to join the seventh nerve on their way to the trunk of the glosso-pharyngeal.

There are four ganglia in close relation to the branches of the fifth nerve which have certain points of similarity, and may, therefore, be considered together, although their positions show that they are engaged in the performance of very different functions.

We have not yet been able to ascertain the value of these little points of junction of motor, sensory, vasomotor, and secretory fibres, because, so far, we are unable to attribute to the cells of the ganglia either reflecting or controlling action, or any automatic power.

They all have efferent (motor and secretory) and afferent (sensory) connections with the nervous centres, and also connections with the main channels of the sympathetic nerves. These are spoken of as the roots of the ganglia. Their little branches are generally mixed nerves.