![]() The mesencephalon, metencephalon, and the myelencephalon comprise the brain stem. ![]() These three vesicles further differentiate into five subdivisions: telencephalon, diencephalon, mesencephalon, metencephalon, and the myelencephalon (Figure 1.1B). The brain and the spinal cord arise in early development from the neural tube, which expands in the front of the embryo to form the three primary brain divisions: the prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain) (Figure 1.1A). The adult spinal cord is approximately 40 to 50 cm long and occupies about 150 cc. It occupies a volume of about 1400 cc - approximately 2% of the total body weight, and receives 20% of the blood, oxygen, and calories supplied to the body. ![]() The adult human brain weighs between 1,200 to 1,500 g and contains about one trillion cells. Lateral view of human embryo at the beginning of the 3rd (A) and 5th (B) week of gestation.
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