When the sympathetic in the neck is cut, the pupil becomes considerably contracted. Hence, it has been argued that the nerves supplying the dilator are derived from the sympathetic.
These fibres are supposed to take origin in the gray matter of the cervical spinal cord. The sympathetic also supplies the walls of the vessels, and thus controls the amount of blood going to the iris, and this contraction of the pupil has been explained as due to vascular engorgement. It is argued that though the vasomotor mechanisms may cooperate in dilatation, they cannot be its only cause, as the widening of the pupil may occur in a bloodless eye. Since the other tissues are elastic and antagonize the sphincter, paralysis of that muscle would give rise to dilatation even in anaemic mydriasis.
The constricting nerve mechanism of the sphincter muscle is distinct, and more definitely understood. Its common action is reflex; the stimulus starts in the retina, and travels along the optic nerve as an afferent channel to the corpora quadrigemina, where there is a centre governing the contractions of both irides. The efferent impulses are sent by the third nerve to the lenticular ganglion, and thence by the short ciliary nerves to the eyeball.
In accommodation for near objects three muscles act together, their movements being "associated" by the central nerve mechanisms. The same voluntary effort that causes the ciliary muscle to contract, makes the sphincter of the iris contract, and also causes the internal rectus to move the eye inward. The voluntary nerve centre must be in intimate relation with the reflex centre, which keeps up the tonic action of the sphincter iridis.
We have then central governors for the ciliary and iris movements. The ciliary muscle and sphincter of the pupil are both caused to act by the will, and the sphincter alone is excited by means of a centre, which reflects the stimulus arriving from the retina by the optic nerve to the branches of the third nerve. The dilator of the pupil, if a muscle, is also kept in gentle tonic action by the impulses sent from the spinal cord with the vasomotor impulses, via the sympathetic; but some think that the blood supply and tissue elasticity explain the dilatation.
Further, from the undoubted facts (i) that some reflex contraction of the pupil may be produced by stimulating the retina even when the eye is cut off from the brain centres, and (2) that the local effect of atropia in dilating, and calabar bean in narrowing the pupil, seem in a measure independent of the central nerve organs, it has been concluded that there must also be some local nerve mechanism in the eye which is capable of reflecting nerve impulses, and is affected by these poisons.
The student must carefully bear in mind all the circumstances under which the pupils contract, namely: -
1. The application of strong light to either retina causes reflex stimulation of the ciliary nerves of both eyes.
2. Stimulation of the nasal or ophthalmic branches of the fifth afferent nerve reflexly excites the sphincter.
3. Contraction of the pupil is "associated" with accommodation for near objects.
4. Similar "associated" contraction always accompanies inward movement of the eyeball.
5. During sleep, or as the result of vasomotor disturbances in the brain (anaemia), the pupil contracts,
6. Under the influence of physostigmin, nicotin and morphia.
7. From any stimulation of the optic or third nerves or the corpora quadrigemina.
The circumstances in which the pupils are found to be dilated are equally important from a practical point of view, namely: -
1. In the dark or with insensitive retinae.
2. Irritation of the cervical sympathetic.
3. Under the influence of atropin, daturin, etc.
4. In asphyxia or dyspnoea from venosity of the blood.
5. Painful sensations from the skin, etc.