The nerves of the third pair are efferent, being the chief motor nerves of the eyes. They arise from the gray matter on the floor and roof of the aqueduct of Sylvius, pass out of the brain substance near the pons from between the fibres of the peduncle, and run between the posterior cerebral and superior cerebellar arteries. They pass into the orbits in two branches, and are distributed to the following orbital muscles: (i) elevator of the eyelid, (2) the superior, (3) inferior, and (4) internal recti, and (5) the inferior oblique. They also contain fibres which carry efferent impulses to (1) the circular muscle of the iris, and to (2) the ciliary muscle. The latter branches reach the eye by a short twig from the inferior oblique branch, which goes to the ciliary ganglion, and thence enters the ciliary nerves.
The action of the orbital muscles is, in the main, under the control of the will, though they afford good examples of peculiar coordination and involuntary association of movements. The reflex contraction of the pupil by the action of the circular muscle (sphincter pupillae) is a bilateral act, the afferent impulse of which originates in the retina, passes along the optic nerves, and is transmitted, from the corpora quadrigemina, to both the third nerves. The central extremities of the third nerves must have an intimate connection with each other and with the optic nerves, for the diminution in size of both pupils follows accurately the increase in intensity of the light to which even one of the retinae is exposed. In retinal blindness and after section of the optic nerve the pupil becomes dilated from loss of the retinal excitation. The action of the ciliary muscle may be said to be voluntary, since we can voluntarily focus our eyes for near or far objects. Contraction of the sphincter pupillae and of the internal rectus is associated with the contraction of the ciliary muscle in accommodation.
Section of the third nerve within the cranium gives rise to the following group of phenomena: (i) Drooping of the upper lid (Ptosis). (2) Fixedness of the eye in the outer angle (Luscitas). (3) Dilatation and immobility of pupil (Mydriasis). (4) Inability to focus the eye for short distances.