The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. Only a few invertebrates such as sponges, jellyfish, adult sea squirts and starfish do not have a brain; diffuse or localised nerve nets are present instead. The brain is located in the head, usually close to the primary sensory organs for such senses as vision, hearing, balance, taste, and smell. The brain is the most complex organ in a vertebrate's body. In a typical human, the cerebral cortex (the largest part) is estimated to contain 15–33 billion neurons, each connected by synapses to several thousand other neurons. These neurons communicate with one another by means of long protoplasmic fibers called axons, which carry trains of signal pulses called action potentials to distant parts of the brain or body targeting specific recipient cells. 

Physiologically, the function of the brain is to exert centralized control over the other organs of the body. The brain acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. This centralized control allows rapid and coordinated responses to changes in the environment. Some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior based on complex sensory input requires the information integrating capabilities of a centralized brain. QUESTIONS:

Q1:
1. What body part serves as the center of the nervous system?
2. The center of the nervous system is controlled by what part of the body?
3. 
Q2:
1. What is the main purpose of the brain?
2. What is the brain's main function?
3. 
Q3:
1. What does the brain do to the overall system?
2. What function does the brain have on the overall system?
3. 
Q4:
1. Is the brain responsible for secreting anything?
2. Does the brain assist in secreting certain compounds?
3. 
Q5:
1. What does the brain help secrete?
2. What useful compounds does the brain secrete?
3. 
Q6:
1. What does the secretion of hormones by the brain assist in doing?
2. How does the brain's secretion of hormones assist the body?
3. 
Q7:
1. Are there parts of the body other than the brain that can help with regulating processes inside the body?
2. Can body parts excluding the brain assist in the regulation of bodily processes?
3. 
Q8:
1. Which parts of the body other than the brain can assist in regulating processes?
2. Which body parts excluding the brain assist in the regulation of bodily processes?
3. 
Q9:
1. What can the spinal cord or peripheral ganglia help with specifically?
2. Which processes can the spinal cord or peripheral ganglia help with?
3. 
Q10:
1. Is a brain present in all living things?
2. Are all living things in possession of a brain?
3. 
