The mesencephalon, or midbrain, is a term used to describe the upper part of the brain stem. It is this part of the brain that is responsible for, among others, the functioning of the reflexes, pupil dilation and hearing. In its structure three distinct parts can be discerned.

The tectum: composed out of two colliculi superiors (responsible for the integration of optic signals) and two colliculi inferiors (which integrate acoustic signals).
The tegmentum, which is involved in many unconscious homeostatic and reflexive pathways, and
The aqueduct of Sylvius, or the cerebral aqueduct, which connects the third and fourth cerebral ventricles.

The Hindbrain

The hindbrain is divisible into two structures, the metencephalon and the myelencephalon or medulla. The metencephalon, in turn can be divided into the cerebellum and the pons.

The Cerebellum

The cerebellum is also known as the arbor vitae (or the tree of life) and is that part of the metencephalon that is responsible for the coordination of movements (this is one of the areas that is affected when you are drunk, which explains why controlled movements become increasingly difficult after a couple of beers) and conditioning. Three distinct structures shape the cerebellum.

The frontal lobe
The posterior lobe
The lobus floccundularis

The Pons

This part of the midbrain regulates the sleep processes, dreaming and the fight- or flight response. The structure is also called the bridge of Varol, since it connects the ‘big brain’ (the cerebrum) with the ‘small brain’ (the cerebellum). Furthermore, it is a part of the formatio reticularis, a network of closely intertwined nerve cells that runs through the brain stem and regulates the activation condition of the cortex.

The Myelencephalon

The myelencephalon, or medulla oblongata (literally, the extended marrow) is the most basal part of the brain. Here, the nerve paths cross. This part of the brain regulates some very vital, basal functions such as the heartbeat, breathing, blood pressure and digestion.

The Brain Stem

Some of the aforementioned structures form the brain stem. This is mainly comprised out of the midbrain and the hindbrain without the cerebellum. So, the brain stem is formed by:

The medulla oblongata (or myelencephalon)
The pons
The formation reticularis
The tectum, and
The tegementum

The brainstem is a very important part of the brain, as it regulates the internal condition of the human body. It uses the thalamus as ‘switch station’ and also contains the twelve cranial nerves, which control about every facial muscle function.

Figure 1: Brain Stem

Image source

Figure 1: http://allaboutthebrain.tripod.com/id13.html