Contrary to popular belief, most people are introverts(well if you want to be technical about it 99.9% of people are introverts). This idea works off of the basis of information metabolism. Internal thoughts are introverted and expression is extroverted. Some people though, are more extroverted than others. Over the course of evolution, man evolved into a being that is composed mostly on the basis of their own thoughts. If people were naturally extroverted, ESTps would either kill everyone in an attempt to take over every situation, or ESTps would be exterminated, either way, the world would be a lot different. Over the course of evolution, man because more of the mind. The extroverted functions that existed may have been directly responsible for the evolution of introverted functions. -Te evolved into +Ti; +Te evolved into -Ti,etc. -Te and +Ti will be always linked together, because they have the same origin, in theory, they are parts of the same function.
Introverted data must always have an extroverted mode of expression. Initial perception is always extroverted, perception as a whole though is a combination of extroverted and introverted functions working. All functions in model A must have an introverted and extroverted function of the same function type in each functional sub-block. These two functions(which are theoretically the same thing) communicate with each other extroverted<---->introverted to relate information to and from the external and internal world. Also, the conscious and unconscious functions communicate with each other. Contrary to some theory, I believe that a conscious and unconscious function work at the same time in metabolizing data. One data enters one function, the data has to be simultaneously processed by a functions shadow function.