Final answer:
The missing term in the statement referring to the flow of CSF from the lateral ventricles to the 3rd ventricle is the 'interventricular foramina'. This term describes the openings that allow CSF to pass from each lateral ventricle to the third ventricle, located within the diencephalon.
Step-by-step explanation:
The term missing in the statement 'The lateral ventricle drains into the 3rd ventricle via the foramen of Monro' is the interventricular foramina. These are the openings between the lateral ventricles and the third ventricle allowing for the passage of cerebrospinal fluid (CSF). Each of the lateral ventricles has one of these openings, through which CSF flows into the third ventricle.
The third ventricle is located in the region of the diencephalon and is surrounded by structures such as the hypothalamus and thalamus. From the third ventricle, the CSF passes through the cerebral aqueduct into the fourth ventricle, found in the region of the brain stem. This ventricle opens into the subarachnoid space through the median and lateral apertures, where CSF can circulate over the surface of the brain and spinal cord.