The intrinsic muscles of the tongue are contained completely within the tongue and act to change the shape of the tongue. There are four muscles that are found in relatively distinct layers which will be described starting at the most superior layer and working down to the most inferior layer. The descriptions will be brief as whoever named them made it easy to figure out which direction the fibers are running and therefore what the action is. Remember from above these are all innervated by the hypoglossal nerve.
Superior Longitudinal Muscle
The superior longitudinal runs from the distal and a septum in the center of the tongue called the median septum and inserts in the outer portion of the tongue. It acts to mainly both curl the tip of the tongue and lateral surfaces of the tongue superiorly and shorten the tongue if used with the inferior longitudinal muscle.
The transverse muscle proceeds from an origin along the median septum out to the lateral borders of the tongue. This muscle acts to narrow the tongue which in effect lengthens and helps protrude it.
The vertical muscle fibers intertwine in the intermediate muscle layer of the tongue with the fibers of the transverse muscle. They run vertically within the tongue and when contracted will flatten it and by doing that will effectively also widen the tongue.
Inferior Longitudinal Muscle
The inferior longitudinal runs parallel to the superior longitudinal intrinsic muscle from the base of the tongue to the tip. It acts to mainly both curl the tip of the tongue and lateral surfaces inferiorly and shorten the tongue if used with the superior longitudinal muscle.