Matlab Simple Diffusion outer bounds!

I am attempting to run this simple diffusion case (I understand that it isn't ideal generally), and I'm doing fine with getting the inside of the solid, but need some help with the outer edges.

if true
global M
size=100
M=zeros(size,size);
M(25,25)=50;
for diffusive_steps=1:500
oldM=M;
newM=zeros(size,size);
for i=2:size-1;
  for j=2:size-1;
  %we're considering the ij-th pixel
  pixel_conc=oldM(i,j);
  newM(i,j+1)=newM(i,j+1)+pixel_conc/4;
  newM(i,j-1)=newM(i,j-1)+pixel_conc/4;
  newM(i+1,j)=newM(i+1,j)+pixel_conc/4;
  newM(i-1,j)=newM(i-1,j)+pixel_conc/4;
  end
end
M=newM;

end % code end It's a pretty simple piece of code, and I know that. I'm not very good at using Octave yet (chemist by trade), so I'd appreciate any help!