Structure of the Brain Stem

The brain stem or truncus cerebri as defined here comprises the derivatives of two of the three primary brain vesicles, viz. the mesencephalon and the rhombencephalon. Both of these brain parts are well developed in the frog. Their importance appears immediately from the fact that they harbor the chief centers of origin and termination of all of the cranial nerves, with the exception of I. In addition, the brain stem of the frog contains a fairly well differentiated reticular formation and a number of sensory and motor relay nuclei with their associated ascending and descending connexions.