Synechdochical Accusative

One of the more idiomatic uses of the Latin accusative is a part for whole construction, the synechdochical (συνεκδοχή) accusative, wherein the accusative subject specifies the range of the verb or adjective. This is also called the Greek Accusative, or the Accusative of Specification.

Caput nectentur: they shall be bound at the head.
Nūda genū fuit: she was bare to the knees.
Femur trāgulā ictus vēnit: he arrived wounded in the thigh by a dark.

The Essential AG: 397b

Alternatives to the Ablative of Agent

There are two: per + accusative and operā + genitive.

Recall that operā (sans ā/ab) is operating as an Ablative of Instrument
These are constructions where the agent/instrument sense breaks down: with per, animate agents seems to work as instruments, and with operā, the (instrumental) labor of an agent take the place of that agent.

Agents with Per Construction

Caesar was informed by scouts: Caesar ab explōrātōribus certiorātus est.
Cesar was informed by scouts: Caesar per explōrātōrēs certiorātus est.

He was trampled by charioteers: ab origīs compressus est.
He was trampled by charioteers: per origās compressus est.

Agents with Operā Construction

Caesar was informed by scouts: Caesar ab explōrātōribus certiorātus est.
Caesar was informed by the work of scouts: Caesar operā explōrātōrum certiorātus est.

The walls were cleaned by a slave: mūrī ab famulō lōtī sunt.
The walls were cleaned by slave’s labor: mūrī operā famulī lōtī sunt.

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While we’re at it, note the odd forth principle part of wash: lāvō, lāvāre, lāvī, lōtum.

Really, it varies: it can also be lautum or lāvātum, depending on the author.

The Essential AG: 405b