Why do cat ears, and dogs as well for that matter, have those pockets toward the back? I spent some time on the Internet trying to learn the pockets' function. Browsing from cat-ear page to cat-ear page, I found a good bit of information. For example:

• Each cat ear contains 27 muscles, which allow the ear to pivot 180°
• Cat ears are recognized as capable of five different movements which provide better hearing and indicate emotional state:
  • RELAXED, ears forward and slightly outward
  • ALERT, ears erect and forward
  • AGITATED, ears twitching
  • DEFENSIVE, ears flattened against head
  • AGGRESSIVE, between alert and defensive, ears positioned so their backs are visible from the front

My browsings even turned up a webpage with pictures of four-eared cats. You can see that at http://www.messybeast.com/freak-ears.htm.

I never found a definitive statement about the function of those pockets, other than "probably help with hearing." However, since having input from two eyes enables the human brain to create binocular vision with depth perception, I can imagine that the pockets in our pets' ears serve as a second set of ears, possibly in the pets' brains creating a sensation like auditory depth perception, something poorly developed in humans. Maybe high-frequency, short- wavelength sounds bounce around in the pockets before being focused through the slit clearly visible in the picture, toward the eardrum. The difference in arrival times of sounds of different wavelengths at the eardrum could provide the basis of the sensation.

In the dog ears I've examined the pockets are less developed than in the photographed cat ear.