First of all, this is still the 'closed' part of the medulla oblongata. Which means it should be close to the section seen earlier.

The dorsal column area retains its general form. However, note that the whte matter is being replaced by grey matter. This indicates that the fibres of the two fasciculi are terminating in their respective nuclei. You can see that the gracile tubercle at this level is almost entirely grey matter, whereas the cuneate tubercle has a shell of white with a core of grey matter.

The neurones of these nuclei are second neurones on the pathway; and their axons cross to the opposite side. The sweeping curves of these fibres are seen around the blue lines. These fibres are called internal arcuate fibres and continue as the medial lemnisci.

The area in the orange outline is where one sees the spinal tract and nucleus of the trigeminal nerve. (What are the other nuclei of this nerve - where are they located and what are their functions?)

In this and the next two sections of the medulla oblongata one sees an area of white matter (tracts) close to the midline. The tracts forming this white matter retain their relationship in the more rostral sections of the medulla; and can also be logically traced into the pons and the midbrain. As you see in this picure, these tracts are the pyramidal (P), the medial lemniscus (ML) and the medial longitudinal fasciculus (MLF).

You have reviewed the pyramidal and lemniscal fibres. What is the function of the MLF? Which brainstem nuclei does it connect?

This, then, is a section through the medulla oblongata at a level rostral to the previous one. It is still through the 'closed' medulla. We can say this is a section through the lemniscal deccusation. The structure at 'A' gives a clue to the exact level. 'A' is the lowest part of the inferior olivary nucleus.

In the grey matter in the centre (now closer to the gracile and cuneate tubercles) is the central canal (not clear due to poor photographic contrast). Just ventral to the central canal, close to the midline, would be the nucleus of the hypoglossal nerve. Ventrolateral to it would be the nucleus of the cranial accessory (ventrolateral, because it is a branchiomotor or SVE nucleus). We have already mentioned the nucleus of the spinal tract of nerve V.

In the region lateral to the arcuate fibres, between the olivary nucleus and the orange outline are the spinothalamic and spinocerebellar tracts. The dorsal spinocerebellar tract will soon make its exit from the medulla through the inferior cerebellar peduncle.

Note also that we have seen two major deccusations in the two sections so far. The next section also shows a major deccusation.