If you require brain histology performed, you should ideally send the whole brain, fixed in formalin, to the laboratory. If you need to take a fresh sample, you can remove a small portion no problem.
We receive submissions every now and then that consist of cerebral cortex only. If you are specifically checking for polio this type of sample should be adequate, but diseases like listeriosis will be missed.
The brain is not one uniform piece of tissue but highly variable. Several diseases are regional, affecting specific parts of the brain. Listeriosis, for example, affects the brainstem and especially the medulla. This is because the infection ascends in the cranial nerves and not via the vascular system (and those nerves enter the brain at the caudal brainstem).
It is ideal to fix brains in a large container such as a bucket, for at least 24 hours before submitting to the laboratory. Once fixed, brains can be sent to the laboratory in a plastic bag without formalin (this is preferred by us). Formalin loses it strength over time, and once you have fixed several brains in the same solution you will find it does not fix tissue very well. Discard it safely* and use fresh formalin.
If you are unsure, speak to a pathologist at your local laboratory .