“ One of the old forms of personal actions in common-law pleading, being used, under that system, to recover a sum certain, or capable of being ascertained by computation only. Formerly the declaration alleged the debet et detinet, on the theory that the defendant owed the money and was guilty of an unjust detention of the goods or whatever happened to be the consideration, but this fell into disuse and is no longer proper. Where the obligation is founded on a simple contract the declaration must allege the consideration, and if on a specialty or on a record it must be set out in full. ”