**Core Concept:** Spinal anaesthesia is a technique used to induce anaesthesia by injecting a local anaesthetic agent into the subarachnoid space around the spinal cord. The drug is deposited either intrathecally or epidurally, depending on the desired level of anaesthesia.

**Why the Correct Answer is Right:** In spinal anaesthesia, the drug (local anaesthetic) is deposited between the **dural tubes**, which are the two layers of the arachnoid membrane that surrounds the spinal cord. This results in the local anaesthetic diffusing into the subarachnoid space, blocking sensory and motor nerves, and providing anaesthesia for the lower limbs and sometimes the lower half of the trunk.

**Why Each Wrong Option is Incorrect:**

A. Depositing the drug **intramuscularly** is incorrect as it would not result in spinal anaesthesia.
B. Depositing the drug **intravenously** is incorrect as it would quickly distribute throughout the body, causing systemic toxicity.
C. Depositing the drug **epidurally** would not result in spinal anaesthesia, as epidural anaesthesia involves injections outside the dural tubes.
D. Depositing the drug **intracisternally** is incorrect as it would not specifically target the subarachnoid space around the spinal cord.

**Clinical Pearl / High-Yield Fact:** Understanding the correct placement of the local anaesthetic is crucial for achieving desired depth and level of anaesthesia in spinal anaesthesia. Misplacement can lead to inadequate anaesthesia or complications such as systemic toxicity due to intravenous administration or neurological deficits from epidural placement.