In the skull is situated the great nerve center of the animal, the brain, and from this through the vertebral column passes the great nerve distributor, the spinal cord. From the spinal cord, nerves are sent to all the muscles of the body, to the skin andto those organs, like the eye and the ear, which transmit to the brain impressions received from without the body. These nerves givethe stimulus which causes the muscles to thicken or contract. In fact, all the voluntary movements of animals are controlled from the brain just as the movements of trains on a railroad are controlled from the despatcher"s office.
106.Respiration. -All animals must
have a way to breathe, or energy cannot be supplied to carry on the activities of thebody. Different animals breathe in different ways, but in the higher vertebrates and inTHE NERVOUS SYSTEM OF MAN.
Notice how the nerves are distributed to all parts of the body.
man it is the same. Respiration in man will, therefore, be taken as the type.
Air enters the body through the nose or mouth, and passes down through the windpipe into the lungs. In order to keep out dust andgerms, the opening of the nose is
supplied with a large number of hairs projecting from the mucous membrane which lines the whole nasal chamber. These hairs and the secretion from the membrane catch and hold most of the harmful particles. At the back of the mouth the windpipe and the throat comeTHE LUNGS.
They are here pulled aside to show the heart.
together.
When food is being swallowed, the passage into the windpipe must be closed, and this is done by the little valvelike epiglottis. If, in swallowing, the epiglottis is not able to close quickly enough, something may pass into the windpipe and cause choking. The windpipe, at the upper part of the chest, branches into two parts, one branch going to each of the lungs.
The lungs fill the upper part of the chest and enfold the heart. In them the air tubes divide again and again, forming a vast network of tubes which grow smaller and smaller until they end in little air sacks. Interlacing with these air tubes are veins and arteries which carry the blood. The tiniest parts into which the blood vessels are divided, the capillaries, form close networks within the linings of the air sacks. The air and blood are thus separated by an exceedingly thin animal tissue, which allows an exchange of soluble materials. Thus the blood is able to take up the oxygen needed and to rid itself of the carbon dioxide and other waste products which it has accumulated.
The air-tight thoracic cavity in which the heart and lungs are situated is inclosed and protected by the ribs and at the lower part by a dome-shaped muscle called the diaphragm. Air enters the lungs because the muscles of the chest pull the ribs so that they move upward and outward and the muscles of the dome-shaped diaphragm cause it to move downward. These two actions enlarge the thoracic cavity. The air enters in the same way that it enters a hollow rubber ball that has been compressed and then set free. When the ribs move downward and the diaphragm upward, the air is expelled as in the rubber ball when compressed.
There are then two ways in which air can be made to enter the lungs, the "raising of the chest" and the movement of the diaphragm. In the proper kind of breathing these two movements go on together. The lungs are filled throughout and not simply at either the top or bottom. If this is to be accomplished, the body must be free and not restricted by tight clothing about the chest or the lower part of the trunk of the body, the abdomen. Not only is the right kind of breathing necessaryfor properly supplying the blood with oxygen, but also that the lung tissues themselves may be properly nourished and cared for. We should be particularly careful about this now that infectious diseases of the lungs are so prevalent.
107.Circulation.
Experiment 116. -If a compound microscope can be procured, tie a string tightly around the end of a clean finger, and when it has become full of blood, prick it quickly with a sterilized needle. Rub the drop of blood that comes out on a glass slide and quickly examine under the microscope. Notice the great number of round disk-like bodies, red corpuscles. Try to find an irregular- shaped body which, while the blood remains fresh, slowly changes its shape, a white corpuscle. These are rather difficult to find, but can be seen if the drop of blood is thoroughly examined quickly enough.
In order that all parts of the body may be provided with the materials used in building their cells and in doing the work necessary for continued existence there must be a distributory system. This is necessary wherever diversified work is to be carried on. This necessity has brought into effect the railway and canal systems of the world. The body is a little world by itself, and it has a most complete and wonderfully adapted system for supplying the material needed and for removing the waste. The center and motive power of this system is the heart. The medium of circulation is the blood.
When the blood is examined, it is found to consist of a watery liquid, called the plasma, a great number of little disk-shaped bodies, the red corpuscles, and some irregular whitish bodies, the white corpuscles (Fig. 108). The white corpuscles a r e p r o t o p l a s m i c c e l l s h a v i n g various functions and possessingthe power of movement and even Fig. 108.
of working their way out of the blood vessels. The main function of the red corpuscles is to carry oxygen from the lungs to the different living cells of the body. They contain a pigment, h?moglobin, which carries the oxygen and gives the blood its color. The plasma, an exceedingly complex fluid, is composed largely of water, but contains the nutrient and waste materials supplied by the different organs of the body.