I have happened upon the sentence with the phrase of outside that I cannot understand in Crash Course Anatomy & Phisiology. It is at around 4 minut and 36 second. Here it goes:
Your digestive tract is really one unbroken, insulated tunnel of outside that just happens to run through your body.
It seem that the word outside was used there as an adverb,and not a noun because there is no article. Could you please rephrase the sentence for me?