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　　The nervous system of vertebrates is characterized by a hollow, dorsal nerve cord that ends in the head region as an enlargement, the brain. Even in its most primitive form this cord and its attached nerves are the result of evolutionary specialization, and their further evolution from lower to higher vertebrate classes is a process that is far from fully understood. Nevertheless, the basic arrangements are similar in all vertebrates, and the study of lower animals gives insight into the form and structure of the nervous system of higher animals. Moreover, for any species, the study of the embryological development of the nervous system is indispensable for an understanding of adult morphology.

　　In any vertebrate two chief parts of the nervous system may be distinguished. These are the central nervous system (the nerve cord mentions above), consisting of the brain and spinal cord, and the peripheral nervous system, consisting of the cranial, spinal, and peripheral nerves, together with their motor and sensory endings. The term autonomic nervous system refers to the parts of the central and peripheral systems that supply and regulate the activity of cardiac muscle, smooth muscle, and many glands.

　　The nervous system is composed of many millions of nerve and glial cells, together with blood vessels and a small amount of connective tissue. The nerve cells, or neurons, are characterized by many processes and are specialized in that they exhibit to a great degree the phenomena of irritability and conductivity. The glial cells of the central nervous system are supporting cells collectively termed neuroglia. They are characterized by short processes that have special relationships to neurons, blood vessels, and connective tissue. The comparable cells in the peripheral nervous system are termed neurilemmal cells.

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