What are those little holes in some meat?
My experience comes from making salami sandwiches... In order to allow myself the luxury of eating something so grease-filled without guilt, I'll zap it in the microwave for 20-30 seconds to render out the fat.  Since packing the salami with ground up fat is part of the process. Cooking the meat melts the fat globules and leaves little hole where the fat use to be.  This more apparent when done with sliced salami and bologna.

In chicken and certain cuts of beef, I think the holes are the result of cutting across veins.  Also, I know that from preparing raw chicken for cooking, I am sometimes able to pull out lengths of veins that leave behind perfect little holes where they came out from.