While observing cardiac muscle under a microscope for the first time, jennifer noticed lines running perpendicular to the direction of the muscle. these lines are called?

The correct answer is intercalated discs.

Explanation:

Specific structural conformations located in between the myocardial cells of the heart are known as intercalated discs. They perform an essential function in connecting the cells of the cardiac muscle together and in conducting the signals between the cells.  

The terminals of each cardiac muscle cell produce intercalated discs, where they come in contact with the adjacent cells of cardiac muscle.