The oxygen in the air is appropriated by the blood and is made use of by the circulatory system. The prana in the air is appropriated by the nervous system, and is used in its work. And as the oxygenated blood is carried to all parts of the system, building up and replenishing, so is the prana carried to all parts of the nervous system, adding strength and vitality. If we think of prana as being the active principle of what we call "vitality," we will be able to form a much clearer idea of what an important part it plays in our lives. Just as is the oxygen in the blood used up by the wants of the system, so the supply of prana taken up by the nervous system is exhausted by our thinking, willing, acting, etc., and in consequence constant replenishing is necessary. Every thought, every act, every effort of the will, every motion of a muscle, uses up a certain amount of what we call nerve force, which is really a form of prana. To move a muscle the brain sends out an impulse over the nerves, and the muscle contracts, and so much prana is expended. When it is remembered that the greater portion of prana acquired by man comes to him from the air inhaled, the importance of proper breathing is readily understood. CHAPTER V. THE NERVOUS SYSTEM. It will be noticed that the Western scientific theories regarding the breath confine themselves to the effects of the absorption of oxygen, and its use through the circulatory system, while the Yogi theory also takes into consideration the absorption of Prana, and its manifestation through the channels of the Nervous System. Before proceeding further, it may be as well to take a hasty glance at the Nervous System. The Nervous System of man is divided into two great systems, viz., the Cerebro-Spinal System and the Sympathetic System. The Cerebro-Spinal System consists of all that part of the Nervous System contained within the cranial cavity and the spinal canal, viz., the brain and the spinal cord, together with the nerves which branch off from the same. This system presides over the functions of animal life known as volition, sensation, etc. The Sympathetic System includes all that part of the Nervous System located principally in the thoracic, abdominal and pelvic cavities, and which is distributed to the internal organs. It has control over the involuntary processes, such as growth, nutrition, etc. The Cerebro-Spinal System attends to all the seeing, hearing, tasting, smelling, feeling, etc. It sets things in motion; it is used by the Ego to think--to manifest consciousness. It is the instrument with which the Ego communicates with the outside world. This system may be likened to a telephone system, with the brain as the central office, and the spinal column and nerves as cable and wires respectively.