Myeloarchitecture of the cerebellum of the chicken (Gallus domesticus): an atlas of the compartmental subdivision of the cerebellar white matter.

A myeloarchitectonic atlas of the longitudinal (or mediolateral) subdivision of the cerebellum of the chicken (white Leghorn) was prepared from serial Häggqvist or toluidine-blue-stained sections of five animals. This myeloarchitectonic subdivision is based on the alternate occurrence of large fiber accumulations (LFAs) and small fiber areas (SFAs) in the cerebellar white matter and allows the distinction of a number of parasagittal fiber compartments, each of which consists of a medial LFA and a lateral SFA. The compartmental subdivision of the cerebellar white matter in mammals and birds derives its importance from the fact that essentially it corresponds to the organization of the afferent and efferent connections of the cerebellar cortex. The simple structure of the avian cerebellum makes it ideally suited for a complete description of its compartmental subdivision and may serve as a natural system of coordinates in future anatomical and physiological studies. The number of fiber compartments that can be counted in the chicken cerebellum on either side of the midline varies from six (in the narrowest folium I) to nine (in the widest folia IX and X) and is approximately the same as in mammals, in which a maximum of eight or ten compartments can be recognized. On the basis of the organization of its myeloarchitecture and the otherwise relatively scarce data on the organization of the connections of its cortex, it can, therefore, be postulated that the avian cerebellum is the homologue of the entire mammalian cerebellum. In addition, the present knowledge of the connections of the cerebellar cortex in birds indicates that the avian compartments 1-3 may correspond to the mammalian compartments A1, A2, and A3 (or X), whereas the avian compartment 4 or 5 (or both) may represent the mammalian B compartment. Lack of further anatomical data so far precludes conclusions on a possible homology between the avian compartments 6-9 and the mammalian C and D compartments.