The organs of respiration are the nose, throat, larynx, windpipe or trachea, and the two lungs. On the outer walls of the nasal cavities are three shelves known as the turbinated bones, the surfaces of which contain blood-vessels to heat the air as it passes through the nose. The mucus which constantly forms on the lining membrane of the nose and the little hairs in the nostrils, act as screens, preventing dust being breathed into the lungs. The pharynx is the cavity behind the nose, mouth and larynx. The larynx forms a prominence in the throat known as the “Adam’s Apple.” It contains the vocal cords, the vibrations of which, as air from the lungs passes through them, give rise to voice sounds. The epiglottis is a cartilaginous curtain above the larynx which blocks up its entrance when food is being swallowed. The trachea or windpipe is a continuation of the larynx. Shortly after entering the chest it divides into two main branches, the right and left branches, which lead to all parts of the lungs. The lungs, two spongy, air-filled organs, take up most of the space in the chest-box or thorax. The smallest end-branches of the bronchial tubes open into numerous tiny sacs known as the air vesicles, in the walls of which the end-branches of the capillaries ramify. Here the impure gases in the blood escape through the vessel walls into the air vesicles, while the oxygen breathed into the lungs is taken up the same way by the blood in the vessels.

HOW THE HUMAN BODY IS CONTROLLED BY THE BRAIN

CORD WITH
DURA MATER

THE ARRANGEMENT OF
THE DURA MATER

[Large illustration] (375 kB)

The nervous system consists of (1) the brain; (2) the spinal cord; (3) the nerves which run off from these structures; and (4) the sympathetic system. The chief mass of the brain is known as the cerebrum, or fore-brain, the small mass at the lower part being termed the cerebellum, or little brain. From the brain, which is contained within the bony skull, twelve pairs of cranial nerves proceed. The most important of these are the first or nerve of smell, the second (sight), eighth (hearing), and twelfth (taste). The fifth, one of the most important nerves of sensation, has three main branches running to the orbit and forehead, the jaws and teeth, and the skin of the face. Six of the twelve pairs of cranial nerves govern the movements of different parts (motor nerves), others have to do with the special sense organs, taste, smell, hearing, and sight (sensory nerves), and others are a combination of motor and sensory nerves. The spinal cord is a continuation of the brain, and is contained in the hollow canal running through the vertebræ of the spine. From it thirty-one pairs of nerves originate. The nerves which run to the arm are collected in a network called the brachial plexus. In the same way the great nerves to the leg come together in the lumbar plexus. The sympathetic nervous system consists of a main nerve trunk running downward along the spine from the skull to the coccyx. This sympathetic system communicates indirectly with the brain and spinal cord, and also with all the great arteries and other important structures in the abdomen.

The dura mater is the strong external cranial membrane which adheres to the skull and also penetrates into the cavities of the brain, dividing it into partially separate compartments. These dividing portions of the dura mater may be seen at A, A, in the diagram above. B marks the various venous blood sinuses of the brain, which receive blood from veins in the different parts of the brain, and, merging into one large sinus (seen at lower right of diagram), afterwards become the jugular vein. C is the great cerebral vein. The Roman numerals mark the great cranial nerves.