The rat's tail is a spine in which the only Pressure on the dises is caused by muscle contractions. The tail is not weight-bearing and cannot be adequately compared with the human spine. However, the pressure exerted by muscles is greater than that exerted by body weight. In accordance with this, disc degeneration and rupture occur frequently in the spine of several animals.
Even if a comparison between the forces actig on the disc in animals' tails and in the human spine is justified, the conditions in these experiments were not sufficiently like normal physiological conditions for definite comparisons to be made between normal and experimental ruptures.
In order to obtain a more reasonable correspondence, experiments are now being made in which tails are curved and tied up in the same direction for a short period of time every day. Additional experiments are being made in which the tails are fixed for longer periods of time than any in the present series.
The only definite conclusions which can be drawn from the present results are as follows:
1. It is possible to produce degeneration and rupture of the annulus fibrosus of the intervertebral disc by constant compression of one side of the annulus.
2. This degeneration and rupture takes place without change in the nucleus pulposus.