Extracting numerical vectors from an array - use of multhist (plotrix)

I am trying to create a histogram containing several data sets (I would
like to see how the dependent variable is distributed for the different
groups). This should be possible using the function multhist contained
in the plotrix package. (I am using R 2.2.1, in windows XP).
I have an array with 

dim(risetime)

[1]  5  8  4  4 79

And I would like to plot subsets of this array with multhist. As far as
I could understand multhist can only use a list of numerical vectors,
and that is why it can not process something like:

multhist(list(risetime[,,,1,],risetime[,,,2,],risetime[,,,3,],
risetime[,,,4,],breaks="FD")

On the other hand, if I try:
multhist(risetime[,,,1,],risetime[,,,2,],
	breaks="FD",xlab="rise time (sec)",ylab="counts",
	xlim=c(0,8))