how to convert SIFT to one dimensional matrix?

hello i want to extract SIFT features from each human face . When i am running the code given at official SIFT website :

         [image, descriptors, locs] = sift('1.pgm'); where 1.pgm is one image

I am getting three matrices

             image 58x128 double
             descriptors 112x92 unit8
             locs 58x4 double

What should I choose a feature vector? Or how to convert the descriptor matrix to 1xN matrix ?I would like to aply it to my neural network