In the Museum of Ethnology

[dukealien]

In the Museum of Ethnology  I almost wish the title contained a sly hint of what's to come... but in a way (how our tribe thinks of death) it does

A brush and ink drawing 
is on display,  this line seems a  bit weak... "stands" or "rests" or some more descriptive word, perhaps
protected by glass
in the Chinese room.

With its free-flowing mane  "its" might be enhanced... "his" or "her," for example
and feathery tail,
a horse in mid-gallop
is looking towards me, more showing could take place here... "looks sidelong," for example
glorious, wild, and joyful,
straining to leap 
from the paper.

Knowing how much
you love horses and art,
I think to myself,
I'll go to the gift shop,
that horse on a card
should be on your desk, perhaps "be" could be enhanced - "should frolic," for example
I'll post it tomorrow,
might take just a week
to reach your address.

A split atom later you're about to use "split" again shortly... could one of them be a different word?
the space-time continuum trembles, not sure "space-time" is necessary here
people and artefacts fade.

I implode,
feel molecules rearrange,
hear the splitting of stone, "hear stone split" or the like might do better, but this is fine
and a new vein of sorrow
cracks open 
in the granite rockface of grief.

What was I thinking?

For an almost moment forgetting  nice word ordering... perhaps a hyphen ("almost-moment")?
you no longer lived there.

Forgetting you no longer live.

With its free-flowing mane
and feathery tail,
a horse in mid-gallop
is looking towards me,
glorious, wild, and joyful,
straining to leap  perhaps (reluctant to suggest) "still strains to leap"
from the paper.

Oh, the times this has happened to me!  "Dad would love that," or "the brother-in-law would give that his arch chuckle" and then, the sad shock.  Very well done.

In general (along with the specifics above, which should be taken as very mild suggestions), subject the word "the" to suspicion at every instance - unless it's truly the only one (MY bishop, THE Pope) things tend to flow better without it, or with some more specific word/phrase that leaves your rhythm intact.

I found myself suspecting what the turn would be just before it appeared (on first reading, of course).  This is not a criticism, more an acknowledgement that the buildup was artfully done while leaving that nervous opening for the storm approaching.

Without over-decorating, a little more description would not hurt.  But this is very good as it stands; the choice of the horse (if you're not simply relating an actual experience, which would also be more than fine) is inspired.