The mammalian pons, olive and pyramid

The evolution of the pons, olive and pyramid have been studied in a series of mammals. The pons in the Monotremata is post‐trigeminal. In Marsupialia and most species of lower Eutherian orders, the pons is pre‐trigeminal and relatively constant in size and simple in form. In the Primates it undergoes a remarkable increase in size, and progressively extends caudally to cover the corpus trapezoideum. The pons in the giant anteater, hippopotamus, rhinoceros, elephant, bear, otter, Pinnipedia and Cetacea is unusually well developed. The olive is not visible in the Monotremata and Marsupialia. In the Eutheria it is seen only in the Carnivora, Cetacea, elephant and Primates. The olive in the elephant, bear, otter, Pinnipedia and Cetacea is unusually prominent. The pyramids show the same evolutionary trends as the pons, except in the Artiodactyla, Perissodactyla, Proboscides, Pinnipedia, otter and Cetacea where they are unusually small. The size and form of the pons, olive and pyramid are in keeping with the development of the cerebral cortex,1 basal ganglia and cerebellum. These structures increase in size and complexity, achieving their maximum development in man. Their relative sizes indicate the functional significance of the cortico‐pontine, striatopontine and pyramidal pathways to that animal. Variations from the well‐defined pattern of evolution of these structures in mammals may be correlated with specific adaptations of individual species to terrestial and aquatic life.