image processing - what is the starting point of a contour as given by cvConvexityDefect() function in OpenCV

i have to distinguish between palm and elbow in my contour(I'm using cvconvexitydefect for that purpose). Here's the image:

I'm taking contour of left image.

the 2 topmost points in right image are shoulder(detected by other method)

bottom left 2 are darker than bottom right 2 (to distinguish between left and right hand)

using the code defect.start is darker than defect.end(but for left hand defect.start is palm while for right hand defect.start is elbow)

I have 2 questions:

1. So how does cvconvexity defect get it's defect.start?


2. In any case how to differentiate palm from elbow?

To be clear i thought that i could make out the palm from elbow just by thinking that start might be upper most point of a contour or leftmost point of that but it ain't so.

here's the relevant part of code:

cvFindContours( HeadLess1, mem, &contours, sizeof(CvContour), CV_RETR_CCOMP, CV_CHAIN_APPROX_SIMPLE, cvPoint(0,0));

for(ptr=contours;ptr!=0;ptr=ptr->h_next){
    tem=cvConvexHull2(ptr,0,1,0);
    tem=cvConvexityDefects(ptr,tem,mem);

    CvConvexityDefect defect,cd;
    defect.depth=0;

    for(int l=0;ltotal;l++){
        cd=*(CvConvexityDefect*)cvGetSeqElem(tem,l);
        if(cd.depth>defect.depth)defect=cd;
    }

    if((*(defect.start)).y>(*(defect.end)).y){
        elbow=(*(defect.start));palm=(*(defect.end));

        //a bad inference for the case when palm is above elbow
    } else { 
        elbow=(*(defect.end));palm=(*(defect.start));
        //a bad inference same case
    }

    if(elbow.x<320)
    {
        q=100;
    }

    else
    {
        q=200;
    }

    cvCircle(HeadLess1,*(defect.start),10,cvScalarAll(q),CV_FILLED);
    cvCircle(HeadLess1, *(defect.end), 10, cvScalarAll(q+50), CV_FILLED);

}

Headless1 one is my image. Thanks in advance.