Cerebrum Surfaces

fillet, fibers, lateral, nucleus, superior, inferior, colliculus, brachium and medial

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The Fillet or Lemniscus (Figs. 59, 63 and 64).—Near the upper end of the pons, in the ventral part of the formatio reticu laris, the fillet, or lemniscus, forms a very broad band of fibers on either side of the median raphe. The fillet is equal in width to half the transverse diameter of the mid-brain. It continues into the ventral and lateral portions of the tegmentum in the form of a broad crescentic bundle made up of two fasciculi, viz., the medial fillet, and the lateral fillet. Farther forward a small bundle leaves the lateral part of the medial fillet and runs up to the superior quadrigeminal colliculus. That bundle is called the superior fillet.

Function.—The fillet forms a segment in the direct sensory tract. It carries spinal and cerebral impulses of the tactile and muscular senses to the corpora quadrigemina and thalamus, and auditory impulses to the inferior quadrigeminal colliculus.

Lemniscus { L. Medialis •i L. Superior. L. Lateralis Medial Fillet (Lemniscus medialis, Figs. 62 and 64).—The fibers composing the medial fillet rise chiefly in the nucleus funiculi gracilis and nucleus funiculi cuneati of the opposite side of the medulla oblongata. They cross over in the fillet decussa tion of the medulla; and, excepting a small bundle, terminate in the lateral nucleus of the thalamus. Fibers are added from the terminal nuclei of sensory cerebral nerves which cross the median plane and enter the opposite fillet. Thus connected with all common sensory nerves, and with the vestibular nerve, it enters the mid-brain and divides into two parts. A small bundle of fibers separating from the lateral part and running to the superior quadrigeminal colliculus, forms the superior fillet. It associates ocular movements with sensations from cerebral and spinal nerves. The medial fillet continues to the lateral nucleus of the thalamus, bearing impressions of the tac tile and the muscular sense. From the thalamus the impulses are carried by the cortical fillet to the sommsthetic area of the cortex.

The lateral fillet (lemniscus lateralis) forms an oblique ridge on the lateral border of the tegmentum (Fig. 41). It trends upward and inward over the brachium conjunctivum to the inferior quadrigeminal colliculi .where some of its fibers termi nate. A few fibers continue to the superior colliculus. Its function is auditory conduction. It rises chiefly from the ventral and lateral parts of the cochlear nucleus (principally the oppo site one) and ends in the medial geniculate body, the fibers being continued beyond the inferior colliculus through the inferior quadrigeminal brachium. The greater number of its fibers cross through the trapezoid body and medullary strie, some are uncrossed up to the inferior quadrigeminal colliculus; but the acustic path is entirely crossed above that level (Ferrier and Turner). It undergoes partial relay in the nucleus of the supe

rior olive and nucleus of the trapezoid body on both sides and the nucleus of the lateral fillet on the same side.

The acustic path is only partially relayed in the inferior col liculus of the corpora quadrigemina, being continued directly into the brachium inferius. Auditory conduction therefore proceeds from the inferior colliculus through the brachium in ferius to the medial geniculate body and then through the acus tic radiation to the temporal cortex. Thus the lateral fillet forms the second stage in the auditory conduction path. The acustic nerve constitutes the first stage, the fibers of the lateral fillet the second stage, the brachium inferius the third, and the acustic radiation the fourth stage. The last stage ends in the cortex of the superior and the transverse temporal gyri.

The spino-thalamic fasciculus (Figs. 63 and 64) is located in the region of the nucleus lateralis superior. It is a loose strand of fibers not isolated from surrounding structures. The spino thalamic tract rises in the gray substance of the spinal cord and in the terminal nuclei of the common sensory cerebral nerves. Though it sends some fibers to the quadrigerninal bodies, spino-tectal fasciculus, and to the substantia nigra and the lentiform nucleus, its chief termination is in the lateral nucleus of the thalamus. It is sensory. It conducts tactile, pain and temperature impressions.

Brachium Conjunctivum (Superior cerebellar peduncle).— The brachium conjunctivum forms a ridge on the surface near the median line of the isthmus, which ends above at the inferior quadrigeminal colliculus (Fig. 41). The lateral fillet winds inward over its upper extremity. It is joined to its fellow by a sheet of white matter, the superior medullary velum. The fibers of the brachia conjunctiva bend ventrally beneath the inferior colliculus of the corpora quadrigemina and, for the most part, decussate anterior to the cerebral aqueduct, through the median raphe (Fig. 62). These crossed fibers with the few uncrossed run forward toward the inferior surface of the thalamus, where they inclose the red nucleus, and help to form the stratum dorsale of the hypothalamic region (Forel). Many of the fibers terminate in the red nucleus and from it others rise and proceed forward to the thalamus, the rubro-thalamic fasciculus. Though most of the brachium conjunctivum rises in the cere bellum and forms a segment of an indirect sensory tract, it also contains a few visual fibers; probably from terminal nuclei of the optic nerve, which run through it to the cerebellum. This is the optic nucleo-cerebellar fasciculus.

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