Final answer:
Descartes' belief that the soul traveled through hollow tubes to control muscle movement is outdated; those tubes are actually nerves, which along with the brain, control voluntary and involuntary muscle systems.
Step-by-step explanation:
Descartes believed that muscle movements were controlled by the soul traveling through hollow tubes in the body. We now know these hollow tubes to be nerves. The skeletal muscle tissue that Descartes referred to forms skeletal muscles, which attach to bones and are responsible for locomotion and movements that can be consciously controlled. These are also known as voluntary muscles. In contrast, smooth muscle tissue is found in the walls of hollow organs such as the guts and blood vessels, and these are typically involuntary muscles not directly controlled by conscious thought.
Initially, Descartes thought that it was through the pineal gland where the soul contacted the brain, but modern science has discredited this theory, showing that thought and muscle movement are actually controlled by the brain's complex structure and functioning, specifically the skeletomotor system utilizing the corticospinal tract for voluntary movement.