The brain stem is the posterior section of the brain that includes the pons and the medulla oblongata, which is continuous with the spinal cord. The midbrain is usually included as part of the brain stem and includes the corpora quadrigemina (shown partially on the photo as the corpos colluculi.) The midbrain joins the diencephalon, which consists of the thalamus and hypothalamus.
The brainstem plays an important role in the regulation of the cardiac cycle and respiratory function. It also plays a role in regulation of the sleep cycle, in conjunction with hormones secreted by the pineal gland.