When a wounded deer is moving, blood droplets coming into contact with the ground will have a forward motion. Each droplet will have a tail, with the tail pointing in the direction the animal was traveling. Since the “fat” part of the drop hits the surface first, and the droplet has momentum from the running deer, the tail will travel longer than the fat part, therefore landing last and in the direction the deer was traveling. You can test this by simply running and dripping water. There are a few exceptions when the drops hit all the varying surfaces in the woods, but it is good for a general rule.