The motor nuclei of the hindbrain can be divided into those that supply striated ("voluntary") muscle fibers and those which supply smooth muscle fibers or secretory glands. The latter, the general visceral efferent nuclei, comprise the Edinger-Westphal nucleus, the dorsal motor nucleus of the vagus, and the salivatory nuclei. The brainstem nuclei which supply striated muscle can be divided into those that supply muscles which are believed to have arisen from the early vertebrate branchial (gill) arch system and those which supply muscles of purely somatic origin. The phox2b gene is expressed in the branchial motoneurons during development, and not in somatic motoneuorons. The expression of phox2b in the oculomotor and trochlear nuclei is something of a surprise, since they are traditionally classified with the somatic motor nuclei. Motor nuclei in the brainstem and spinal cord are often subdivided into motor neuron clusters, each of which supplies a different muscle or muscle group. This is referred to as the somatotopic or musculotopic arrangement of motor neurons. The facial nucleus is an outstanding example of musculotopic organization. Almost all of the branchial and somatic motor neurons supply muscles in the ipsilateral side of the head; the major exceptions are the oculomotor neurons which supply the contralateral superior rectus and the trochlear motor neurons which supply the superior oblique muscles of the eye.