Innervation of the rabbit rectococcygeus muscle and the functional relationship of the muscle to the terminal large intestine.

1. The rectococcygeus muscle is supplied by cholinergic motor fibres from the parasympathetic division of the autonomic nervous system. 2. The histofluorescence technique reveals a sparse adrenergic innervation. 3. When a maintained state of partial contraction is induced by carbachol, stimulation of the extrinsic sympathetic nerves produces a relaxation of the muscle which is usually followed by contraction. 4. The rectococcygeus muscle may also receive nerves which are neither adrenergic nor cholinergic. 5. Ganglion cells have been found close to and, in some preparations, within the muscle; the role of these ganglion cells is not clear. 6. It would appear that the rectococcygeus muscle and the longitudinal muscle of the rectum are not linked by a myogenic mechanism. The rectococcygeus muscle would, however, appear to be linked by nerve fibres to neighbouring structures.