Nervous System

The nervous system shows the greatest development in vertebrates. There is an increase in centralization with increasing elaboration of the brain with areas with specific functions. The central nervous system includes the brain and a dorsal (upper) spinal cord encased and protected by the skeletal system. The central nervous system is connected to the rest of the body through a peripheral nervous system that includes the nerves connecting the brain and spinal cord with receptors such as the ear and eyes and effectors such as the muscles in the body. In the evolution of vertebrates from fish to mammals, the most significant changes have occurred in the structure of the brain. Even in the earliest vertebrates, the brain had three divisions: the hindbrain, midbrain, and forebrain. In fish, the hindbrain is dominant and concerned mainly with motor reflexes. The largest section of the fish brain is the optic lobes in the midbrain, with the anterior of the brain (forebrain) composed of the olfactory lobes and the cerebrum. In the progression from fish to mammals, the hindbrain becomes less and less prominent, and the area of the brain used for receiving and integrating information becomes greater and greater as shown by an increase in the size and development of the cerebrum. The cerebrum is the part of the brain involved in learning voluntary movement as well as the interpretation of sensation. Birds and mammals have the largest brain mass relative to body size with the largest ratio found in man and porpoises. In man the brain weighs approximately 3 lb (1.4 kg) with the cerebrum making up 80% of the total brain mass.