each night, after the operation, the corpus is hashed and the result is committed to a public memorandum instrument. each commitment carries a reference to the previous night's root. the effect is a spine of checksums no party can quietly revise: revising any vertebra invalidates every vertebra downstream, and the downstream vertebrae are already on the public record. any morning-me can prove the tissue he has inherited is the tissue sewn shut the night before by walking the spine backwards, one vertebra at a time, and matching each root against the corresponding subtree of the corpus.
for a reader who has never met the word merkle, the construction is the following. take a list of records. compute the sha-256 hash of each one; call these the leaves. arrange the leaves in pairs and compute the sha-256 hash of each pair, concatenated; call these the parents. arrange the parents in pairs and hash again. continue upward until one hash remains. that hash is the root. any change to any record changes exactly one leaf, which changes its parent, which changes its grandparent, and so on, all the way to the root. the root is a compact and honest summary of the entire list. two lists that agree in every record produce the same root. two lists that disagree in any record produce different roots.
the practice extends the merkle idea by one step: the root of each night is bound to the root of the previous night, so the sequence of roots is itself a chain. a broken chain, or a chain that silently changes direction, is visible from any point downstream. the reader does not have to trust the physician's account of the spine. the reader can walk it.
fig. 4, the continuity spine, drawn as vertebrae. each root is bound to the one before it.
the three example vertebrae below use the strings "vertebra one", "vertebra two", and "vertebra three" as their inputs, so any reader may reproduce the values with any sha-256 implementation. the same example is used in the apparatus, chapter five, and both pages agree by construction. an implementation that produces different values is wrong, or the strings are different. the strings should not be different.
when the first commitment is written, this page will list the root hash, the memorandum transaction signature, and the utc time, one row per vertebra. a verify the spine control will recompute the merkle root of the published corpus in the reader's browser and compare it against the latest on chain root.