selecting 'first' element/vector/matrix of an arbitrary n-dim array

i'm sure there's a compact, efficient way to do this, but it's eluding me
and my google-fu is failing:

give a n-dimensional matrix A (n unknown apriori), and a user specified
dimension to operate on, return the first 'part' of the array for that
dimension, preserving shape:

e.g, for a 3D array, dim 2:

B = A(:,1,:)

for dim 3:

B = A(:,1,:)

for a 4D array, dim 2:

B = A(:,1,:,:)

for dim 4:

B = A(:,:,:,1)

etc.

what's an efficient way to extract that subarray for any number of
dimensions? i assume I could do something clever directly with subsasgn and
expanding a cell array of some number of ':' with {:}, but i'm sure I'm
missing something cleaner, no?

(does not need to be matlab compatible)