[R] How to keep plot title same as column name using apply/sapply?

[C W]

Dear list,
I am trying to plot histogram of a 10 by 5 matrix by columns.

What is a good way to paste the column names?

dput(a)
structure(c(0.891856482875103, 0.249593821948295, 0.0385066520473322,
0.109098868876336, 0.238035894186719, 0.971470380855763, 0.168018536530906,
0.941457062296419, -0.285381460315397, -0.0229335863796271, -20138.175683257,
28190.7238887329, 8521.44473371867, 37565.8599592035, -61036.6139527803,
-57278.42539968, 24284.2860193156, 57470.8275786857, -5035.05919672665,
-48223.3060594833, 0.105003428645432, 0.0385466025676578, 0.130895042931661,
0.0682065344415605, 0.155879093753174, 0.0598988083656877, 0.121624303748831,
0.120134906983003, 0.159434637567028, 0.0144033632241189, 6133.33965969139,
5363.20408479208, 22432.8473630982, 10627.5697498379, 273.481263455154,
8326.07386291277, 133.117356408579, 4108.23352922685, 643.558745779806,
5532.10081652847, -4512.80115728638, -5839.1178791698, 7341.05048720629,
-1299.37511688102, -13151.5558568303, -6442.78100296569, -975.590624649323,
298.440121450997, 10808.9562034117, 3210.22497745543), .Dim = c(10L,
5L), .Dimnames = list(NULL, c("Tmrw", "hRm2", "vm", "Fluid",
"SM")))
par(mfrow=c(3, 2))
apply(a, 2, function(x){hist(x, main="column name here")})

This plots, but doesn't plot title.

But,
> apply(a,2,function(x){colnames(x)})
NULL

Maybe the column names is lost?  But, I see column names here.

> apply(a,2,function(x){is.vector(x)})
 Tmrw  hRm2    vm Fluid    SM
 TRUE  TRUE  TRUE  TRUE  TRUE

My understanding is that, apply() takes data matrix column by column,
and pastes names back on at the end?  But I need it for plot title,
how should I change my code?

Thanks,
Mike