Salivary reflexes, in which the sight of a fine dinner or the smell of it causes the flow of saliva; coughing, sneezing, vomiting reflexes and deglutition reflexes are complicated, but, knowing the nerve supply of the parts involved, the student should try to trace the impulses.
3. Spino-cerebral Reflexes.—Impulses received by the spinal cord through the afferent fibers of its nerves are transmitted by the medial longitudinal bundle, the fasciculi proprii and forma tio reticularis to the nuclei of motor cerebral nerves. Thus is brought about the movement of the head and eyes toward the source of impulse, a change of facial expression to agree with the painful or pleasing character of the impulses, etc.
Some long spino-cerebral reflex arcs are formed by the af ferent neurones which terminate in the quadrigeminal bodies, red nucleus, thalamus and lentiform nucleus, and the efferent neurones whose cell-bodies are located in those nuclei. The arc of the red nucleus will be considered with the cerebellar reflexes, as its afferent limb passes through the cerebellum. The reflex arc of the quadrigeminal bodies (tectum) is formed by afferent nerves and the spino-tectal fibers of Gowers's tract on the afferent side, and the anterior and lateral tecto-spinal tracts and motor nerves on the efferent side. The arc of the thalamus is formed in part by the spino-thalamic tract and the thalamo spinal tract, completed by the sensory and motor nerves. The thalamus also lies in the lenticulo-spinal path and in the cere bellar reflex arcs. The lentiform nucleus receives impulses through the spino-lentiform fibers of Gowers's tract, through the thalamo-striate and hypothalamo-striate fibers; it originates impulses that descend to motor nuclei by way of the thalamus, red nucleus, hypothalamic nucleus and substantia nigra, as its own neurones do not reach motor nerve nuclei. The thalamo spinal and tracts establish its chief connection with the spinal cord. According to Kinnier Wilson, the len ticulo-spinal impulses steady the activity of the lower motor neurones; they prevent hypertonicity, rigidity and tremor.
4. Cerebrospinal Reflexes.—Of these there are Let us notice three.
Respiratory Reflex.—Any obstruction or irritation in the laiynx or trachea sends an impulse through the vagus nerve to its sensory nucleus and, through its T-branched axones to the nucleus ambiguus, nucleus of the phrenic nerve and nuclei con trolling the accessory muscles of respiration. The connection of
the sensory nucleus of the vagus with these motor nuclei is probably established through the medial longitudinal bundle and the fasciculi proprii. The afferent impulses from the larynx first inhibit the inspiratory act and close the larynx by contraction of all the constrictors; then there follows a sudden and powerful expiratory effort which drives a column of air through the partially opened glottis, expelling the foreign body and producing a cough. The reflex producing a sneeze is simi lar; but the afferent limb of its arc is formed by the trigeminal nerve, the fibers supplying the nasal mucous membrane. A powerful reflex inspiration always precedes the sneeze, and the soft palate is brought down against the tongue so that the col umn of air is forced through the nose.
Equilibrium Reflex (vestibulo-spinal reflex).—The simplest arc of equilibrium between the eighth cerebral nerve and the spinal nerves is formed by the neurones of the vestibular ganglia (Scarpa's), the vestibulo-spinal tract and the motor neurones of the anterior columna of the spinal cord. A more complicated arc includes the fibers of the vestibular nerve that run through restiform body to the nucleus fastigii, and the nucleo-cerebellar fibers which run from the vestibular nuclei through the same body to the cerebellar cortex. Both these fasciculi belong to the afferent limb of the reflex arc. The cortico-nuclear axones of Purkinje's neurones form the first link in the efferent limb. The cerebello-tegmental tracts form the second link, connecting the cerebellar nuclei with the nuclei of the brain-stem; they run through the brachium conjunctivum to red nucleus, thalamus, and the motor nuclei of the mid-brain, pons and medulla, and through the restiform body to the nucleus of Deiters and motor nuclei in the medulla. The third link of the efferent limb extends from the terminations of the cerebello-tegmental tracts to the nuclei of motor nerves, and more especially the spinal nerves. This third link is a threefold one, composed of the thalamo spinal, rubro-spinal and vestibulo-spinal tracts. The motor nerves complete the efferent limb of the vestibular arc of equi librium. By this arc it is possible for the movements of labyrin thine fluid to preserve equilibrium.