My teacher tried to describe this question to me, but I just don't get it.

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    $\begingroup$ If you have anything specific to ask after looking at my answer below, please refine your question. Note that I voted to close because of the textbook-nature of the question and lack of research effort. See the help center on homework for assistance in asking good homework questions. $\endgroup$ – AliceD Mar 30 '16 at 14:11

See the figure below:

enter image description here

source: Human Anatomy

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