A coordinating and integrating system which functions in the adaptation of an organism to its environment. An environmental stimulus causes a response in an organism when specialized structures, receptors, are excited. Excitations are conducted by nerves to effectors which act to adapt the organism to the changed conditions of the environment. In animals, humoral correlation is controlled by the activities of the endocrine system. This article considers the morphology, histology, and embryology of the nervous system, including the brain and cranial nerves, and embryology of the sense organs.