In the structure of a fully formed human, whether adult or child, the midbrain cannot be seen without slicing the brain open and disappears under the forebrain before a regular human fetus is five months old. Comparatively, birds have large midbrains, scientists speculating that targeting prey from long distances requires an especially adapt midbrain.
The midbrain regulates auditory and visual stimuli. Sections appear to be responsible for eye movement and in the cases which apply, perking floppy ears. Unlike other animals, mammals have small midbrains because the forebrain has evolved to take responsibility for the majority of a mammal’s auditory and visual needs. Additionally, the midbrain works with the hindbrain to coordinate simple movements.
In some cases, infants are born with only hindbrains and midbrains. This abnormality, known as anenephaly, enable the infants to complete basic movements, such as sucking, yawning, crying, and following visual stimuli. However, they cannot learn and frequently appear drowsy if not stimulated. Frequently the infant is born without skin covering their hind or midbrains. Generally infants only containing midbrains and hindbrains die anywhere from a few hours to several months after they are born due to cardiorespitory arrest. Because a human only containing a hind and midbrain will never attain consciousness, anenephaly has a poor prognosis and medical staff will not attempt to save babies suffering from the disease. Rather, doctors attempt to make babies with anenephaly comfortable and allow them to quietly die. Ultrasounds will reveal whether a fetus has ananephaly before they are born.
Though causes for anenephaly are not agreed upon, it is thought that if women of child bearing age introduces a regular supply of folic acid to their daily diet their children are less likely to be born with only a hind and midbrain. Additionally, some scientists believe nickle, lead, chromium and mercury may be causes.