Const int not const in function

For some unfathomable(to me) reason the 'w' var member in the SDL_Rect object 'Box' below is being set to '0' after the first complete row is drawn, despite it being initialised to Boxsize. i just cant see why it should change?

Code:

//header files

const int dwnMax = 60;     
const int acrMax = 60;     

const int Boxsize = 14;
const int Boxgap = 1;
const int Mapsize = 3600;

// other functions 

// other functions 


void DrawMap(SDL_Surface *screen, SearchArea mapnode[dwnMax][acrMax])
{
    int dwn = 0;
    int acr = 0;
    Uint32 color = 0;
    SDL_Rect Box;
    Box.w = Boxsize;                 //this keeps going back to 0 after each complete loop of 'acr'
    Box.h = Boxsize;                  
    Box.x = 0;
    Box.y = 0;

    Slock(screen);

    for(dwn = 0; dwn < dwnMax; dwn++)
    {
        for(acr = 0; acr < acrMax; acr++)
        {
            if(mapnode[dwn][acr].Walkstate)
            {
                color = SDL_MapRGB (screen->format, 128, 128, 255);
            }
            else color = SDL_MapRGB (screen->format, 0, 0, 255);

            if(mapnode[dwn][acr].STnode)
            {
                color = SDL_MapRGB (screen->format, 255, 128, 0);
            }

            if(mapnode[dwn][acr].TGnode)
            {
                color = SDL_MapRGB (screen->format, 0, 255, 128);
            }

         Box.x += (Boxsize + Boxgap);                                                                           

         SDL_FillRect (screen, &Box, color);
        }
        Box.x = 0;
                                                         //  Box.w = 14; adding this line is the only way i can get it working at moment
        Box.y += (Boxsize + Boxgap);
    }
    Sulock(screen);
    SDL_Flip (screen);
}

forget this post, fool that i am i was incrementing the Box.x value before i drew anything, i still dont see how that changed the original problem but it needed sorting and has fixed it so...

changed to this >

Code:

SDL_FillRect (screen, &Box, color);
Box.x += (Boxsize + Boxgap);