Comparative anatomy of cranial nerve motor nuclei: with a comment on evolution of the craniofacial region.

Changes of cranial nerve nuclei are investigated at three crucial points of phylogenesis in frogs, lizards and rats. Profound changes can be observed in the trigeminal and facial motor nuclei parallel to transformation of the primary mandibular joint into the secondary mandibular joint. New muscles appear for mastication and facial expression and they are innervated by new types of neurons. The muscular tongue appears first in tetrapods coinciding with terrestrial life. It can be shown that an entirely new type of neuron develops at the medullospinal junction for the innervation of the intrinsic tongue muscles. These muscles and their neurons are not homologous to somatic muscles and spinal cord neurons. The external eye muscles and their innervation are present from the early craniates through apes in virtually the same pattern of organization. They evolved coincidently with the head and are not homologous to any other structures. It is concluded that if we accept the 'neogenesis' view in head evolution, all contradictions imposed, by the 'segment theory', upon the interpretation and classification of cranial nerve nuclei can be circumvented.