ABSTRACT
This chapter explains that direction of gaze, position of the eyelids, lens shape and the size of the pupils are controlled by cranial nerves 3, 4, and 6. It shows that the brainstem is attached to the spinal cord just above the level of C1 through the foramen magnum. The chapter observes that some people experience phantom limb pain following the loss of a limb. It suggests that neurons consume about 20% of the body's oxygen and if this supply is interrupted for more than a few minutes, cell death occurs, resulting in temporary or permanent loss of function. Myelin sheaths are formed by Schwann cells in the peripheral nervous system and by oligodendrocytes in the central nervous system. Since the peripheral nervous system is mainly composed of axons, there are only two types of support cell: satellite cells and Schwann cells. Myelin sheaths consist of myelinated segments, separated by small gaps or interruptions called nodes of Ranvier.
