Parallel spindle systems in the small muscles of the rat tail.

1. The examination consisted of a histological analysis using paraffin sections, gold chloride staining and osmic acid techniques.2. The parallel spindle system found in the rat tail muscles is compared with that found in frog muscles. The rat spindle system occasionally included extrafusal fibres.3. It was evident that there are basic Class differences between the frog and rat tail parallel spindles, but it is suggested that they may serve a similar proprioceptive role in muscles having almost isometric contractions.4. Other short muscles of the rat that might have an analogous function to the tail muscles were examined, but only the capitus and deep masseter muscles showed parallel spindle systems, and none contained extrafusal fibres.5. Of other mammals examined, the muscles of the tails of the cat, rabbit, mouse and guinea-pig contained parallel spindles, while the dog and mole showed none in the small sample taken.6. Nerve trunk fibre size histograms, including de-efferented trunks, indicated a high proportion of sensory nerve fibres from the tail (55%), a low motor unit ratio (1:60) and a beta axon motor supply both to the spindles and to the small muscles of the tail.7. A generalized schema of the rat tail anatomy is presented, and a tabulation of the large sensory endings expected to be found in any particular small tail muscle.8. It was concluded that the rat tail has a spindle density enough to provide a proprioceptive sensitivity equivalent to that of a caudal digit.