Science, the Veternarian and X-rays

So,  Joules had been limping for about a week and at the end of this, we saw he wasn't eating his dry food either. I made a mountain and it stayed a mountain.  So, this was Sunday of a long holiday, so on Monday night I emailed them requesting an appointment.  The next day , we bring Joules in and wait to be called. The nurse asks us what is wrong and weighs joules. He is 8.6 pounds.  Then, the veterinarian came in and he looked at Joules' leg and he felt a tiny bump. He let Megan and I feel for this.  Then, he said to rule out other possible problems, he was going to take an x-ray. He did and then he came back and opened it and let us see the bones of a cat. He discussed the femur and tibia. He explained that the bones were white and that the muscle takes on a grey appearance. He also showed us the knee-cap and the hip bones.  He used other parts of the bones of a cat as well, but I can not recall these. He explained that he doesn't have arthritis and showed the the bones next to each other on the knee and leg are smooth and have tendon going around them nicely.  If it was arthritic, you would see more jagged bones.  Then he showed us what might be hip arthritis, but it could be the odd way joules had to lay. He also discussed his backbone and said if this were causing the problem or related, the little discs in the backbone would be squashed together , but we could see equal spacing between the joints.