Sentence:  ...trillions of bacteria, each equally an individual, which are found in a person's gut, his mouth, his scalp, his skin and all of the crevices and orifices that <WSD>subtend</WSD> from his body's surface. 
Meaning of the word marked with <WSD> tags:  To extend or stretch opposite something; to be part of a straight or curved line that is opposite to and delimits an angle.