In the line leading to vertebrates, several major evolutionary events occurred that established the basis for most parts of our nervous systems, including major gains in sensory system structure and the motor neuron-musculature system (Butler & Hodos, 1996; Nieuwenhuys, ten Donke-laar, & Nicholson, 1998). The brain and spinal cord were greatly enlarged, and the vertebrate sensory systems—including olfactory, visual, somatosensory, auditory, vestibular, gustatory, and lateral line (mechanosensory and elec-trosensory)—were gained (Northcutt, 1996). Motor neuronal pools for the muscles of the face and throat (pharynx) regions and, subsequently, for eye muscles and then jaw and neck muscles were gained. In the ancestral line leading to tetrapods, sets of paired appendages and a muscular tongue were gained along with their respective motor nuclei, while the lateral line system was lost.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.