‘Brain’ and ‘CENTRAL NERVOUS SYSTEM’ are not synonyms. The brain is one part of the central nervous system; the other component is the SPINAL CORD. The brains of all VERTEBRATES appear to be built to a common plan (see NEURODEVELOPMENT). Among the MAMMALS there are remarkable similarities between brains across SPECIES, both in structure and functional specialization.
It is clear however that two major changes can take place: (1) neurons within a given structure often become complex with, for example, greater ARBORIZATION of their dendrites and more complex patterns of branching in the axons (see DENDRITE; NEURON). While the neurons may fulfil similar functions, a greater complexity evidently can develop in higher species. (2) Structures themselves change in their relative size. For example, the OLFACTORY BULB of a rat is very large compared to the remainder of its brain, whereas in the human, while being much bigger than in the rat, it is relatively small in relation to the remainder of the human brain. The comparison between brains of different species and their relative ENCEPHALIZATION is discussed in NERVOUS SYSTEM.
