On the first page of Moby Dick, by Herman Melville, the author uses allusion twice.
The first line is "Call me Ishmael." This is an allusion to the Biblical story of Abraham and Sarah who want a child. When Sarah cannot conceive, she suggests that Abraham marry their slave Hagar and have a child with her. He does so, and the child is called Ishmael. Later, Isaac is born to Abraham and Sarah, and ultimately, Sarah convinces Abraham to send Ishmael and Hagar (his mother) away. The sense of "call" used in the novel is that Ishmael may not be the narrator's real name, but an alias.
Another allusion is found in the line:
With a philosophical flourish Cato throws himself upon his sword...
This is an allusion to the practice of the Romans of throwing themselves on their swords to commit suicide rather than be captured by the enemy.
Cato refers to Marcus Porcius Cato Uticensis, or Cato the Younger, born in 95 BC in Rome. He was well-known as a speaker, politician and statesman in the Roman Empire. We learn of Cato's fate, as he chooses to kill himself rather than live under the rule of Caesar.
In Utica, Cato did not participate in the battle and, unwilling to live in a world led by Caesar and refusing even implicitly to grant Caesar the power to pardon him, he committed suicide in April 46 BC.
The novel, Moby Dick, starts out with Ishmael explaining why he decides to go to sea. It is a result of depression that he begins to recognize in himself. And rather than "pausing before coffin warehouses" or "bringing up the rear of every funeral I meet," he decides to take steps to bring him out of his doldrums. Ishmael humorously continues that when he finds himself in such a state as to cause physical injury to others, he knows it is time to head seaward:
...and especially whenever my hypos** get such an upper hand of me, that it requires a strong moral principle to prevent me from deliberately stepping into the street, and methodically knocking people's hats off--then, I account it high time to get to sea as soon as I can.
To paraphrase his statement regarding Cato falling on his sword, the allusion is a parallel that means that Ishmael will do the unpleasant thing that must be done to save himself from a worse fate, such as those listed above. It is figurative language which means he does not intend to actually harm himself; he is simply choosing the inevitable action that will save him from himself, in much the same way that a Roman soldier would choose the sword rather than a more unpleasant fate.
** hypos - may refer to a "low state" - according to Webster's New International Dictionary, 1927