As English (oddly) uses "back of the tongue" to refer to its top surface, how can I refer to the back part of the tongue, opposite to the front part of the tongue where its tip is? "back part of the tongue" ?
The top surface of the tongue has a front, a middle, and a back. You will find this terminology used, for example, in oral medicine and discussion of flavours.
Excess bitterness in your beer is usually perceived on the back of the tongue
Median rhomboid glossitis is a condition that can affect your tongue if you have oral thrush. It causes a red, smooth patch or lump to develop in the middle of the top part of your tongue, which can be sore.
The front two-thirds of the tongue that you can stick out is called the oral tongue
Generally speaking, referring to the surface or the top of the tongue means the part of the tongue with the tastebuds on it.
The back of the tongue refers to the surface of the tongue near the throat, while the front of the tongue refers to the surface of the tongue near the lips.
The underside of the tongue is the part without tastebuds.