The doctor is saying that the loss of speech is worse than the loss of sight. He believes this because he makes the connection between language and the heart. Why doesn't he say "tongue"? I believe it's because being able to speak allows a person to express his/her thoughts and feelings. The doctor feels that it's easier to adapt to being blind because our need to express ourselves through speech is part of what makes us human. Speaking from the heart even makes us different from other human beings. This is why the doctor feels that the inability to express ourselves is worse than the ability to speak.
He means that language is more powerful then even vision. It can affect your heart more then light can affect your eye.
It was trying to suggest losing the speech of language is lots worse than losing the sense of sight. He is trying to say that speech is valued more importantly than sight and it holds a greater power and influence over each other. The "speech" can make people express their thoughts and desires to wide masses of audience and to the whole world. It is better to become "blind" then to become "speechless" as this would break down one form of vital communication and a portal of the interflowing of thoughts and feelings. They have to isolate themselves into the world of their own and they would be more reserved and self-centered, not willing to open, which can lead to detrimental results and consequences. This form of communicating make us different from one another, makes use "unique" to other people and make us a normal human being. If you are blind, you still can get own with life as per normal except you are swarmed with blackness and you don't have a sense of direction. But, if you are speechless, you can't communicate your emotions and feelings to people and you would not be well-accepted by your friends. So, losing sight is better than losing speech.