http://design.tutsplus.com/tutorials/how-to-paint-realistic-eyes-in-adobe-photoshop--cms-23054

Anatomy. It seems so simple, yet it’s so complex. It is the bane of every artist—you just cannot go over a certain level of professionalism without having at least some notion of anatomy. Most people never thoroughly study it, and that leads to shaky art foundations that leave them permanently unsure of their own artistic strength.

That is why it is wiser to dedicate a little bit of your daily artistic time to studying anatomy. It might seem overwhelming at first, but if you break it down into pieces it becomes actually fun and easy to do!