Pupillary Reflexes.—Pupillary dilatation belongs to the cere brospinal group of reflexes. The cilio-spinal center is in the cervical enlargement of the spinal cord. It receives optic im pulses through both the tecto-spinal bundles from the corpora quadrigemina. The superior quadrigeminal colliculi receive those impulses by two routes: First, directly through the fibers of the lateral root of the optic tract and second, indirectly through corticifugal fibers in the occipito-thalamic radiation, and the brachium superius. By the latter route the optic impulses which have reached the visual area of the occipital lobe by way of the intrinsic retinal neurones and the optic nerves, tracts and radiation, are returned to the lateral genicu late and superior quadrigeminal bodies. Thence reaching the cilio-spinal center through both the tecto-spinal bundles, the impulses take the following course: They leave the spinal cord through the anterior roots of the upper thoracic nerves and run in succession through the rami communicantes, the cervical cord of the sympathetic, the cavernous plexus, and the short cil iary nerves to the radiating fibers of the iris, causing dilatation of the pupil.
For pupillary constriction, the impulses run directly from the superior quadrigeminal colliculus to the oculomotor nucleus, traversing the anterior tecto-spinal bundle only through the dorsal tegmental decussation (Meynerti). Then through the visceral fibers of the third and the axones of the ciliary ganglion, which form the short ciliary nerves, they reach the sphincter papillae muscle.
Accommodation for near, and distant vision is secured through the arcs just given. Through the arc of pupillary contraction, impulses reach the circular fibers of the ciliary muscle and, by their contraction, accommodate for near vision. The meridional fibers of the ciliary muscle being supplied by the cervical sym pathetic are controlled by the arc of pupillary dilatation. Impulses through this arc cause flattening of the lens and shortening of the polar axis of the eye so that the focus of an object at a great distance falls on the retina and the eye is thus accommodated for distant vision.
5. Cerebellar Reflexes.—The cerebellar reflexes connected with the vestibular nerve have been given under the head of equilibrium reflexes. Every motor nucleus receives impulses from the cerebellum. The impulses excited in the cerebellar cortex, besides toning up the muscles and augmenting the power of steady, tonic contraction, coordinate the muscles producing instinctive movements (running, flying, swimming, etc.) and the muscles performing acquired and educated movements; and, furthermore, control many of the motor and secretory func tions (sympathetic functions) of the great viscera. In lower
animals all these reflexes may be produced without the inter vention of the cerebellum.
Through Ventral Spino-cerebellar Tract.—Impulses of pain, heat and cold received by the gray crescent of the cord through the spinal ganglion neurones, decussate through the gray and white commissures of the cord and ascend Gowers's tract to the brain, running through the spino-thalamic and spino-tectal tracts to thalamus and tectum and through ventral spino cerebellar tract to cerebellar cortex. Those carried by the ven tral spino-cerebellar tract ascend the lateral surface of the cord and medulla, dorsal to the anterior root-line of the cord and anterior lateral sulcus of the medulla, and continue through the lateral part of the reticular formation of the pons to a point above the root of the trigeminal nerve; there, the tract flexes backward about 90 degrees, winds over the lateral and dorsal surface of the brachium conjunctivum into the velum medullare superius of the cerebellum, where some of its fibers decussate; it then proceeds through the corpus medullare cerebelli to the cortex of the superior vermis. As some fibers of Gowers's tract end in the inferior lateral nucleus of the medulla, some pain and temperature impulses may reach the cerebellum also through the restiform body traversing the reticulo-cerebellar fibers from the lateral nucleus. Having arrived in the cortex of the cere bellum, the pain and temperature impulses probably assist in the production of the impulse-complexes characteristic of the cerebellum; but it may be inferred that they are chiefly con cerned with the coordinations adapted to an escape from the offending object. Such cerebellar impulses are conducted from the cortex to the cerebellar nuclei by the cortico-nuclear fibers (axones of Purkinje's cells). Whether they proceed from the cerebellar nuclei through the restiform body to Deiters's nucleus, or through the brachium conjunctivum to red nucleus and thala mus, it has not yet been determined; but the brachium con junctivum is the more probable course. Rubro-spinal and th alamo-spinal tracts complete the conduction to the motor nuclei of the spinal nerves, and the spinal nerves produce the reflex contractions necessary to escape from the hot, cold or painful stimulus. Impulses traveling through the ventral spino-cerebellar tract may also reach the center of conscious ness for pain and temperature in the thalamus and be trans formed into sensations.