Because once a nerve fiber is severed, there is no way in heaven or on earth to regenerate life back into it.
13 seconds sound clip from the Young Frankenstein (1974) movie soundboard.
You can hear this line at 00:09:29 in the Blu-ray version of the movie.
Quote context
[...]
- In conclusion, it should be noted...
- That any more than common injury to the nerve root is always serious.
- Are there any questions before we leave?
- Uh, Dr. Frank... Fronkensteen.
- Yes?
- Are you speaking of the worm or the spaghetti?
[...]