Medulla Olongata and Pons Varolii


Medulla Olongata and Pons VaroliiExternal Appearences - The medullaoblongata, about an inch in length, is directly continuous with the upper end of the spinal cord. In form it resembles a truncated with its base upwards and applied to the pons.

The pons Varollii presents the appearance of a prominence composed of transverse strands which narrow towards either side and are continued into the cerebellum. A broad shallow median groove extends down its anterior surface (Fig. 43).

Along the anterior surface of the medulla the anterior median fissure runs from the spinal cord up to the margin of the pons. The posterior median fissure is continued for about halfway up the medulla; at this level the central canal, which has been gradually approaching the posterior surface, opens up by the divergence of the posterior columns, and becomes the cavity of the fourth ventricle (Fig. 44).

On either side of the anterior median fissure is a pear-shaped swelling the pyramid - which contains in its upper part the undivided pyramidal tracts. When it is traced downnwards towards the spinal cord the decussating portions the tracts can be seen interlacing across the fissure. To the outer side of the pyramid is an oval swelling known as the olive; it contains some masses of grey matter - the inferior olivary nuclei. On the posterior surface of the lower part of the medulla the continuations of the columns of Goll and of Burdach can be seen. They are rendered more evident here by the fact that in each there is a nucleus of grey matter giving rise to a distinct surface elevation. These elevations are known respectively as the gracile tubercle on the column of Goll, and the cuneate tubercle on the column of Burdach.

When traced upwards the columns of opposite sides diverge widely from one another, and disappear from surface view. Their place is taken by a rope-like elevation - the restiform body - which superficially seems to be their continuation, but is not really so. The restiform body lies immediately behind the olive and passes upwards and outwards to the cerebellum. The rest of the posterior surface of the medulla, together with the posterior surface of the pons, form the floor of the fourth ventricle. From the anterior and lateral surfaces of the pons and medulla the fifh to the twelfth cerebral nerves emerge as seen in Fig. 55.

Internal Structure. - The main feature of the internal structure of the medulla and pons is that the grey matter is no longer column round the central cavity as in the spinal cord, but is cut up into small independent portions or nuclei scattered amongst interlacing fibres. Some of these nuclei correspond to definite portions of the grey matter of the spinal cord, but a considerable number of new nuclei appear which are not apparently represented in the cord. The three great factors instrumental in breaking up the regular arrangement of the grey matter are: (1) The motor decussation; (2) the sensory decussation; (3) the expansion of the central canal into the fourth ventricle.

(1) To facilitate the understanding of this region it is convenient to trace the pyramidal tracts upwards from the spinal cord, although it must be borne in mind that the impulses which they convey travel downwards. Just beyond the spinal cord, at the region of the pyramidal or motor decussation, the crossed pyramidal tracts run forwards and inwards and cross the anterior median fissure to join the direct pyramidal tracts of the opposite sides. In their passage they cut through the anterior horns of grey matter, separating off completely the apices of these, but leaving the basal portion in the floor of the central canal (Figs. 45 and 46).

The apex of the horn, thus pushed aside, forms a nucleus termed the nucleus ambiguus; the axons of its cells take part in the formation of several of the cerebral nerves, viz., part of the spinal accessory (xi), and the motor portions of the vagus (x) and gloss opharyngeal (ix). Further forward in serial continuity with it are the nuclei of the motor portions of the facial (vii) and trifacial (v).

(2) With the posterior horn of the grey matter on either side two masses are connected. One, situated in the gracile tubercle on the column of Goll, is known as the nucleus gracilis; around its cells the fibres of the column of Goll terminate by arborisations. The other, in the cuneate tubercle on the column of Burdach, is termed the nucleus cuneatus; it forms a similar terminus for the fibres of the column of Burdach (Fig. 47). From the cells of these nuclei new fibres arise; some of them pass, either directly or after decussation, into the restiform body and thence to the cerebellum; the majority, however, run towards the middle line, where they decussate with those of the opposite side, and are then continued up as a flattened band situated immediately dorsal to the pyramid, and termed the fillet.

By this, which is designated the sensory decussation, the apices of the posterior horns of grey matter are cutoff and become the tubercles of Rolando; the basal portions of the horns remain in contact with the central canal (Fig. 48).

Fibres of the antero-lateral basis bundles of the spinal cord are continued up into the medulla, behind the fillets, as two strands (one on either side) known as the posterior longitudinal fasciculi. They retain their spinal characteristics, i.e. they consist of short fibres linking up the various grey nuclei.

(3) When the central canal of the lower part of the medulla opens up into the fourth ventricle the continuations of the basal parts of the anterior horns come to the surface in the floor of the ventricle immediately adjacent to the middle line. From them the nuclei of the hypoglossal (xii) and abducent (vi) nerves are formed. The continuations of the basal parts of the posterior horns are displaced to the outer sides of these. They are resolved into the sensory nuclei of the vagus (x), glossopharyngeal (ix), facial (vii), and trifacial (v) nerves (Figs. 49 and 50).

The main additional grey nuclei which are present in the medulla and pons are: (1) the inferior olivary nuclei, the axons of some of the cells of which run to the cerebrum, others to the cerebellum; (2) the superior olives; (3) a group of small nuclei in the pons termed the nuclei pontis; (4) the accessory nuclei of the auditory nerves.