The three bellies of the canine cricothyroid muscle.

The cricothyroid muscle is capable of multiple functions, including vocal cord elongation and adduction. In addition, the cricothyroid can lengthen the glottis as well as provide posture to the cricothyroid joint. The purpose of the present study is to demonstrate that the many functional capabilities of the cricothyroid are a reflection of the existence of separate bellies within the muscle. Microdissection revealed three distinct muscle bellies within the cricothyroid: rectus, oblique, and horizontal. These differ in fiber orientation (rectus 79 degrees, oblique 48 degrees, horizontal 7 degrees) and are separated by fascial planes. Histochemistry showed that these three bellies are composed of different percentages of slow twitch fibers (rectus 31.3%, oblique 43.3%, horizontal 39.6%). Finally, electromyography demonstrated differences in the electrical activity patterns of the three bellies. It is concluded that the cricothyroid muscle is composed of three distinct muscle bellies that probably play separate roles in the complex function of this muscle.