Segment 2D image into 3 equal parts

I'm currently trying to draw 2 vertical lines at each lung, to divide them into three almost equal parts but I'm having some trouble.

To be really honest I'm not sure where to begin.

Could you please help me?

Thanks in advance,

J

[KALYAN ACHARJYA]

im=rgb2gray(imread('image11.jpg'));
[rows colm]=size(im);
%Find the blob properties and calculate the distance 
%Draw the lines at particlar column
imshow(im);

Is this you are looking for?

[João Mendes]

Yes! That's it! Thanks!!