How to find the the boundery of a 3D plot, and how to find the coordinates of the bounderys

Hi, I have made a 3D plot and I want to find some values on the x and y axis. I want to find the 4 points, where my function no longer change value on the Z-axis. Is there a command in matlab that can find these points and give me the x and y values at thise points? Thanks!

My script:

C = [23.875 15.7528;15.7528 93.9843];
mu = [1788.2 70.8];

figure
xtheta12bot = 1750; theta12top = 1810;
theta22bot = 10; theta22top = 100;
theta12 = xtheta12bot:1:theta12top;
theta22 = theta22bot:1:theta22top;
z2bot = 0; z2top = 5*1e-3;
[X12,X22] = meshgrid(theta12,theta22);
F2 = mvnpdf([X12(:) X22(:)],mu,C);
F2 = reshape(F2,length(theta22),length(theta12));
surf(theta12,theta22,F2);
caxis([min(F2(:))-.5*range(F2(:)),max(F2(:))]);
axis([xtheta12bot theta12top theta22bot theta22top z2bot z2top]);
xlabel('\theta1'); ylabel('\theta2'); zlabel('Probability density');
title('multivariate');