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Reflex Paths

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. REFLEX PATHS There is no visible limit to the number of reflex paths. Hence no attempt will be made to give them completely, but a few examples of various kinds will be given which may assist the student to trace others and be suggestive of their great multi plicity and importance. Under certain unquestion ably, the sensory and motor paths that have been traced are but the afferent and efferent limbs of reflex arcs.

Reflex arcs are formed (1) by the sensory and motor fibers of spinal nerves associated in the gray matter of the cord; (2) by the sensory and motor fibers of cerebral nerves which are connected in the brain; (3) by afferent spinal fibers connected by the ascending fibers of the fasciculi proprii, with efferent cere bral fibers; (4) by afferent cerebral and efferent spinal nerve fi bers, the two being associated by the anterior and lateral tecto spinal bundles, the reticulo-spinal tracts, the fasciculi proprii, the spinal tract of the fifth nerve, the vestibulo-spinal tract, etc.; and (5) coordinated cerebellar reflexes through spinal and cerebral nerves.

I. Spinal Reflexes (Figs. 72, 154 and 155).—In the simplest spinal reflexes, the afferent fibers of the arc arborize about the cell-bodies whose axones constitute the efferent fibers; the afferent and efferent fibers are connected by one or more sets of intervening neurones in the next grade of reflex arc. The intercalated neurones connect the posterior columna of gray matter with both anterior columnw, in the same segment; and, by means of T-branched axones in the fasciculi proprii, they connect a single segment of the posterior columna with many segments of the anterior columnx, above and below the seg ment receiving the afferent limb of the reflex arc. Among these are the skin and muscle reflexes, such as the plantar, the patellar, the gluteal and the cremaster reflexes, the involuntary with drawing of a part from a source of irritation, etc.

More complicated spinal reflexes are those of defecation, mic turition, parturition, vasomotor reflexes, cardio-accelerator reflexes, etc. The impulses traverse at least three neurones in these reflexes; because all efferent white rami communicantes terminate in some ganglion proximal to the organ supplied. As an example, trace a defecation reflex.

Defecation Reflex.—The rectum is supplied by the third and fourth sacral nerves and by branches of the inferior mesenteric and hypogastric plexuses. Irritation of the sensory endings in the mucous membrane is caused, normally, by the presence of feces. The impulses caused thereby run to the special defeca tion center in the lumbar enlargement of the spinal cord, either by way of the sacral nerves or through the sympathetic plexuses, the ganglionated cord, and the rami communicantes to the lum bar nerves, through the posterior roots of which they reach the center in the cord. From the defecation center the impulses pursue two courses: (a) They descend through the third and fourth sacral nerves and cause inhibition in the circular fibers of the rectum and contraction of the longitudinal muscle. (b)

This action is immediately followed by impulses which pursue thesympathetic course, through the anterior roots of the lumbar nerves, the rami communicantes, the ganglionated cord, and the inferior mesenteric and hypogastric plexuses, to the rectum. They cause, in succession from above downward, contraction of the circular muscle of the rectum. The two series of impulses thus open a way for the passage of fecal matter and then force it through the opening unless prevented by the voluntary con traction of the external sphincter.

2. Cerebral Reflexes.—The simplest of these reflexes are such as spasm of the muscles of mastication caused by a bad tooth, in which both limbs of the arc are formed by the tri geminal nerve. Again, the facial expression of pain due to the same cause. In this the impulses traverse the trigeminal nerve and by the collaterals of its root-fibers reach the nucleus of the 29 facial. Through the facial they cause contraction, of certain muscles of expression. Facial spasm in tic douloureux is due to the same reflex. The involuntary expansion of the nostrils upon the detection of a faint odor is due to an olfactory-facial reflex. The connection of the terminal nucleus and cortical center of the olfactory nerve with the gehetic nucleus of the facial nerve is very much involved; it may be established as follows with facial and other motor nuclei: (1) by the hippo campo-mammillary fasciculus of the fornix, the mammillo thalamic bundle and the thalamo-spinal tract; the olfacto amygdalate fasciculus, forming stria terminalis, the strio-fugal tracts and their continuations down the brain-stem and cord; (3) the hippocampo-mammillary fasciculus of the fornix, the mammillo-tegmental bundle and the mammillary peduncle, which also runs to the tegmentum, and the dorsal longitudinal bundle of Schutz; (4) the hippocampo-habenular fasciculus of the fornix and the olfacto-habenular bundle, the two forming stria medullaris thalami, the habenulo-peduncular fasciculus (fasciculus retroflexus), and the interpedunculo-tegmental bun dle; and (5) the olfacto-mesencephalic fasciculus (basal bundle of Wallenberg), which runs from the cortex of the olfactory tract to tuber cinereum, mammillary body, tegmentum of mid brain, pons and medulla, and even into spinal cord. The last tract in each of the above five groups terminates in connection with the motor nuclei of cranial and spinal nerves. Squinting, due to bright light, is produced by an arc composed of the visual path, the corticifugal part of the occipito-thalamic radiation, the anterior tecto-spinal bundle and the facial nerve. Substitute the oculomotor nerve for the facial and add neurones of ciliary ganglion and we have the arc for papillary contraction under the same conditions.

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