Final answer:
The Cricothyroid muscles act to tense the vocal cords, thereby increasing the pitch of the voice by stretching and thinning them.
Step-by-step explanation:
The action of the Cricothyroid muscles is to tense the vocal cords (true vocal folds). This tension increases the pitch of the voice. The Cricothyroid muscles achieve this by tilting the thyroid cartilage forward, therefore stretching and thinning the vocal cords. This stretching action alters the tension in the vocal cords, which in turn modifies the pitch at which they vibrate as air passes through during phonation. This is part of the laryngeal function that alters the shape and length of the glottis, the space between the vocal cords, to produce different sounds.