Proprioceptive afferent fibers in the cranial nerves III, IV and VI.

The purpose of the present works was to clarify whether the cranial nerves III, IV and VI carry proprioceptive afferent fibres from the extrinsic ocular muscles. In sheep the picture is now clear. The cranial nerves III, IV and VI carry many large proprioceptive fibres (12-16 micrometer) to the central nervous system. These nerves also contain many small fibres of the y-range (2-6 micrometer) which innervate the intrafusal muscle fibres in the spindles. In man the picture is still vague: most of the spindles are not typical, the large proprioceptive fibres (12-16 micrometer) and the small y-fibres (2-6 micrometer) are very few in the cranial nerves III, IV and VI. It is to be concluded that in sheep the cranial nerves III, IV and VI are not purely motor nerves to the extrinsic ocular muscles, but they also carry many of the large fibres of the proprioceptive function. In man, such large fibres are not found and the pathway of proprioceptive afferents from the orbital muscles is still not certain.