![]() ![]() 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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