What are the masses of gray matter in each hemisphere that control voluntary muscle movements called?

The term for the masses of gray matter in each hemisphere that control voluntary muscle movements is known as the basal ganglia. These structures, located deep within the cerebral hemispheres, play a critical role in motor control, influencing a variety of functions including the regulation of voluntary movements, procedural learning, and routine behaviors. The basal ganglia help initiate movements and ensure they are executed smoothly, which is essential for activities requiring coordinated muscle actions, such as running or playing an instrument.

The basal ganglia consist of several nuclei, including the caudate nucleus, putamen, and globus pallidus, all of which work together to facilitate the planning and coordination of movement. The dysfunction of the basal ganglia is associated with various movement disorders, such as Parkinson’s disease and Huntington’s disease, further emphasizing their importance in voluntary movement regulation.

The other options mentioned do not fulfill this specific function: gyri refer to the folds of the brain's surface; the corpus callosum is a bundle of nerve fibers connecting the two hemispheres; and the medullary body does not specifically pertain to voluntary muscle movement control. Thus, the basal ganglia are the key structures responsible for managing voluntary motor activity.