The internal cervical os might be covered completely, partially, or marginally, and this can bleeding that's painless and bright red, and can increase the likelihood of a preterm delivery and fetal hypoxia.
A preposition is often used to express the relationship between an object or an event and a location, although sometimes the relationship denoted by a preposition can be very abstract.
Now, the spinothalamic tract is a bit different, because it crosses over to the opposite side of the spinal cord 1 or 2 spinal segments above, where the 1st order neurons enter.
It could be some movement towards something, or it could be to do with being in a location and your relationship to that location. So, " at" , " I am at the supermarket."