Midbrain
The midbrain is the short, thick stalk which connect the hinbrain, in the posterior craninal fossa, with the cerebrum superior to the tentorium cerebelli. It traverses the tentorial notch, and is about 2.5cm long and slightly more in width. The narrow tubular cavity which traverses it (the cerebral equeduct) connects the fourth ventricle inferiorly to the third ventricle superiorly. Identify the inferior end of the aqueduct, and pass a fine probe through it. The tectum is the smaller part of the mibrain posterior to the aqueduct. It consists of the four colliculi. The large part, anterior to the aqueduct, can be divided into right and left halves, the cerebral peduncles. In the undissected brain, the tectum is overlapped by the anterior lobe of the ceebellum, pineal body, and splenium of the corpus callosum, but the anterior part of the cerebral peduncles, the crura cerebri, can be seen on the base of the brain where they from the posterolateral boundaries of the interpeduncular fossa.
Substantia Nigra
This curved plate of grey matter lies between the crus cerebri and the tegmentum. it contains many large nerve cells with a considerable amount of melanin in their cytoplasm. These cells have connection with the tegmentum of the midbrain and with the corps striatum in the centre of the corresponding hemisphere. They play an important part in the control of muscle tone and activity, and produce dopartant part in the control of muscle tone and activity, and produce dopamine which is passed along their axons to the corpus striatum. The substantia nigra extends from the superior border of the pons into the inferolateral part of the hypothalamus, superior to the midbrain.