The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone, however two parts are more prominent: the cranium and the mandible. In humans, these two parts are the neurocranium (braincase) and the viscerocranium that includes the mandible as its largest bone. The skull forms the anterior-most portion of the skeleton and is a product of cephalisation—housing the brain, and several sensory structures such as the eyes, ears, nose, and mouth. In humans, these sensory structures are part of the facial skeleton.
Skull in situ
Anatomy of a flat bone – the periosteum of the neurocranium is known as the pericranium
Chimpanzee skull
Fish head parts, 1889, Fauna of British India, Sir Francis Day
The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. In vertebrates, a small part of the brain called the hypothalamus is the neural control center for all endocrine systems. The brain is the largest cluster of neurons in the body and is typically located in the head, usually near organs for special senses such as vision, hearing and olfaction.
It is the most energy-consuming organ of the body, and the most specialized, responsible for endocrine regulation, sensory perception, motor control, and the development of intelligence.
The brain of a chimpanzee
Cross section of the olfactory bulb of a rat, stained in two different ways at the same time: one stain shows neuron cell bodies, the other shows receptors for the neurotransmitter GABA.
Neurons generate electrical signals that travel along their axons. When a pulse of electricity reaches a junction called a synapse, it causes a neurotransmitter chemical to be released, which binds to receptors on other cells and thereby alters their electrical activity.
Fruit flies (Drosophila) have been extensively studied to gain insight into the role of genes in brain development.