Mean of Multiple Matrixes

I have 9 matrixes all the same size (90x144), say they're called z1,z2,z3...z9

How do I take the mean of all 9 matrixes

so that the result is 1 matrix with the averages at each of the 90x144 data points?

When I try

mean(z1+z2+z3...z9)

I get a 1x144 matrix

and when I try

 mean(z1,z2,z3..z9),

I get an error message

   Error using mean>parseInputs (line 187)
Trailing string input must be one of 'double', 'native', 'default' or one of 'omitnan', 'includenan'.
Error in mean (line 66)
    [flag, omitnan] = parseInputs(flag, flag2, isFlag2Set);

Thank you for your help

Answer 1

[Roger Stafford]

   m = (z1+z2+z3+z4+z5+z6+z7+z8+z9)/9;  % The mean of each element