As stated below, the Left crus cerebri was damaged (see what it should normally look like below). This contains the corticospinal tract. Since the corticospinal tract decusates at the medulla, below the midbrain section we're looking at, you would see Contralateral (Right) Spastic Hemiparesis
Is it appropriate to ask a question about the structure which is absent on the picture? It was super confusing! How can you say it was not a defect of the tissue cut?
It affected the corticospinal tract in the crus cerebri on the L
The corticospinal tract runs from the cerebral cortex (Pre-Central Gyrus), through the posterior limb of the internal capsule, and into the posterior portion of the brainstem where they form highly myelinated medullary pyramids. These travel down the spinal cord, decussate and synapse of lower motor neurons in the ventral horn of the spinal cord.
A past history of cerebral infarction must have affected this patient's left middle cerebral artery and through Wallerian degeneration, the left medullary pyramid degenerated.
Destruction of the left corticospinal tract before decussation leads to contralateral spastic hemiparesis (since this is an upper motor neuron lesion.
The damage is in the L midbrain in the area affecting the corticospinal tract. Because it is in the midbrain, decussation in the pyramids (medulla) so it will show ipsilateral dysfunctional motor signs.
Photo of midbrain and important areas: shorturl.at/myHLR