Home >> Anatomy Of The Brain >> Arteries The Cerebral Circulation to Or Sensory Paths Afferent >> Gray Matter of Medulla_P1

Gray Matter of Medulla

nucleus, arcuate, nuclei, anterior, column, migration and lateral

Page: 1 2 3 4 5

GRAY MATTER OF MEDULLA The gray matter of the medulla is composed (I) of that con tinuous with the nucleus pontis, called the arcuate nucleus and the ponto-bulbar nucleus; (2) of the sheet of gelatinous gray substance next the ventricle and the nuclei of the reticular substance continuous with the reticular formation and ven tricular gray substance of the pons; and (3) of special nuclei of the medulla, viz., the inferior olivary nuclei and the nuclei of the funiculus gracilis and funiculus cuneatus.

. Nucleus Arcuatus (Figs. 123 and 124) .—The arcuate nucleus is found only in man. It forms a large crescentic mass on the ventral and medial surface of the pyramid. Running over and through it there are the anterior external arcuate fibers, which are reinforced by a small tract from the arcuate nucleus. The two nuclei are sometimes fused across the median line and are usually continuous with the nucleus pontis. The cells of the arcuate nucleus are large and stellate like those of the nucleus pontis, as should be expected because of their common origin.

They are emigrants from the fertile rhombic lip but belong to a dis tinct migration which Essick calls the olivo-arcuate migration; it seems impossible to clearly distinguish the migration that forms the arcuate nucleus from that which gives rise to the inferior olivary nuclei. The olivo arcuate migration begins early in the second month in a 20 mm. embryo. At first it is intramedullary, but later in an 8o mm. foetus a superficial migration occurs among the roots of the vagus nerve, from the rhombic lip to the anterior surface of the medulla. The stream is but one cell deep and two or three cells broad at first; it is much larger in the 96 mm. foetus (about three months). The streams flow across the median raphe to the region of the opposite pyramid like the pontine streams and intermingle with each other. For a time the two nuclei remain continuous; but, as the migration ceases at the 143 mm. stage (a little under four months) and the pyramidal tracts continue to enlarge for at least one year after birth, the nuclei are frequently broken apart and are separate in the adult. The axons of the arcuate nuclei reinforce the anterior external arcuate fibers; they run chiefly through the opposite restiform body to the cortex of the vermis cerebelli. The cerebral relation of the arcuate nuclei is undeter

mined. According to Essick, the arcuate nuclei are peculiar to man (Am. Jour. Anat., Vol. 13).

2. The gray substance in the substantia reticularis and in the immediate floor of the fourth ventricle (Figs. 123 and 124) is continuous with the same in the pons and mid-brain above and is represented in the spinal cord by the H-shaped column of gray matter. By the posterior and lateral expansion of the neural canal in the upper half of the medulla and in the pons which forms the fourth ventricle the posterior column of the H-shaped column of gray matter are pushed outward to a trans verse direction and the entire bases of the anterior column are brought into the floor of the ventricle. The expansion of the canal, together with the decussation of the lateral pyramidal tracts through the anterior columnw and of the medial fillets through the posterior and anterior columnar, disposes the H shaped column as follows: Anterior Columna.—From the base of the anterior columna two columns of cells are derived—a lateral, called the nucleus intercalates (of Streeter) and a medial the hypoglossal nucleus (Figs. 123 and 124) which is two-thirds of an inch in length and extends along the median raphe in the upper medulla beneath the eminentia medialis. It is continued into the lower medulla as far as the pyramidal decussation. By commissural fibers it is joined to the nucleus of the opposite side according to KoHiker. The nucleus is purely somatic: its cell-bodies possess the characteristic somatic structure pointed out by Malone, namely, abundant, tigrous cytoplasm; and its axones supply only striated, voluntary muscles (Anat. Rec., Vol. 7). It gives origin to the hypoglossal nerve proper and probably to a small fasciculus which by way of the.medial longitudinal bundle joins the facial nerve and supplies the orbicularis oris. The hypoglossal axones run in linear series forward through the medulla to the anterior lateral sulcus whence they emerge be tween the pyramid and the olive (Fig. II). They separate the anterior from the lateral column. The main body of the anterior columna is broken up into the nucleus lateralis inferior, the nucleus ambiguus and the motor part of the nucleus of the ala cinerea.

Page: 1 2 3 4 5