The brain is the most complex part of the nervous system. It is housed in the cranial cavity and is estimated to consist of some one hundred billion nerve cells, which communicate with each other via roughly one trillion connections. The brain is the center for sensory functioning and perception, as well as the initiator of motor commands which control the skelatal muscles. In addition to this, the brain also carries on such higher cognitive functions as learning, remembering, reasoning, and feeling emotions.
The "brain" refers to the sum of three anatomically separate regions: the cerebrum, the cerebellum, and the brain stem. Each region is subdivided into smaller regions with unique responsibilities. These subdivisions are discussed in the next card.