for a physician; but she should gain a general knowledge of first principles, as a guide to her judgment in emergencies when she can rely on no other aid.
With this end in view, in the preceding chapters some portions of the organs and functions of the human body have been presented, and others will now follow in connection with the practical duties which result from them.
On the general subject of health, one recent discovery of science may here be introduced as having an important relation to every organ and function of the body, and as being one to which frequent reference will be made; and that is, the nature and operation of cell-life.
By the aid of the microscope, we can examine the minute construction of plants and animals, in which we discover contrivances and operations, if not so sublime, yet more wonderful and interesting, than the vast systems of worlds revealed by the telescope.
By this instrument it is now seen that the first formation, as well as future changes and actions, of all plants and animals are accomplished by means of small cells or bags containing various kinds of liquids. These cells are so minute that, of the smallest, some hundreds would not cover the dot of a printed i on this page. They are of diverse shapes and contents, and perform various different operations.
[Illustration: Fig 48.]
The first formation of every animal is accomplished by the agency of cells, and may be illustrated by the egg of any bird or fowl. The exterior consists of a hard shell for protection, and this is lined with a tough skin, to which is fastened the yelk, (which means the yellow,) by fibrous strings, as seen at a, a, in the diagram. In the yelk floats the germ-cell, b, which is the point where the formation of the future animal commences. The yelk, being lighter than the white, rises upward, and the germ being still lighter, rises in the yelk. This is to bring both nearer to the vitalizing warmth of the brooding mother.
New cells are gradually formed from the nourishing yelk around the germ, each being at first roundish in shape, and having a spot near the centre, called the nucleus. The reason why cells increase must remain a mystery, until we can penetrate the secrets of vital force—probably forever. But the mode in which they multiply is as follows: The first change noticed in a cell, when warmed into vital activity, is the appearance of a second nucleus within it, while the cell gradually becomes oval in form, and then is drawn inward at the middle, like an hour-glass, till the two sides meet. The two portions then divide, and two cells appear, each containing its own germinal nucleus. These both divide again in the same manner, proceeding in the ratio of 2, 4, 8, 16, and so on, until most of the yelk becomes a mass of cells.
The central point of this mass, where the animal itself commences to appear, shows, first, a round-shaped figure, which soon assumes form like a pear, and then like a violin. Gradually the busy little cells arrange themselves to build up heart, lungs, brain, stomach, and limbs, for which the yelk and white furnish nutriment. There is a small bag of air fastened to one end inside of the shell; and when the animal is complete, this air is taken into its lungs, life begins, and out walks little chick, all its powers prepared, and ready to run, eat, and enjoy existence. Then, as soon as the animal uses its brain to think and feel, and its muscles to move, the cells which have been made up into these parts begin to decay, while new cells are formed from the blood to take their place. Time with life commences the constant process of decay and renewal all over the body.
[Illustration: Fig. 49.]
The liquid portion of the blood consists of material formed from food, air, and water. From this material the cells of the blood are formed: first, the white cells, which are incomplete in formation; and then the red cells, which are completed by the addition of the oxygen received from air in the lungs. Fig. 49 represents part of a magnified blood-vessel, a, a, in which the round cells are the white, and the oblong the red cells, floating in the blood. Surrounding the blood- vessels are the cells forming the adjacent membrane, bb, each having a nucleus in its centre.
Cells have different powers of selecting and secreting diverse materials from the blood. Thus, some secrete bile to carry to the liver, others secrete saliva for the mouth, others take up the tears, and still others take material for the brain, muscles, and all other organs. Cells also have a converting power, of taking one kind of matter from the blood, and changing it to another kind. They are minute chemical laboratories all over the body, changing materials of one kind to another form in which they can be made useful.
Both animal and vegetable substances are formed of cells. But the vegetable cells take up and use unorganized or simple, natural matter; whereas the animal cell only takes substances already organized into vegetable or animal life, and then changes one compound into another of different proportions and nature.
These curious facts in regard to cell-life have important relations to the general subject of the care of health, and also to the cure of disease, as will be noticed in following chapters.
THE NERVOUS SYSTEM.
There is another portion of the body, which is so intimately connected with every other that it is placed in this chapter as also having reference to every department in the general subject of the care of health.
The body has no power to move itself, but is a collection of instruments to be used by the mind in securing various kinds of knowledge and enjoyment. The organs through which the mind thus operates are the brain and nerves. The drawing (Fig. 50) represents them.
[Illustration: Fig. 50.]
The brain lies in the skull, and is divided into the large or upper brain, marked 1, and the small or lower brain, marked 2. From the brain runs the spinal marrow through the spine or backbone. From each side of the spine the large nerves run out into innumerable smaller branches to every portion of the body. The drawing shows only some of the larger branches. Those marked 3 run to the neck and organs of the chest; those marked 4 go to the arms; those below the arms, marked 3, go to the trunk; and those marked 5 go to the legs.
The brain and nerves consist of two kinds of nervous matter—the gray, which is supposed to be the portion that originates and controls a nervous fluid which imparts power of action; and the white, which seems to conduct this fluid to every part of the body.
The brain and nervous system are divided into distinct portions, each having different offices to perform, and each acting independently of the others; as, for example, one portion is employed by the mind in thinking, and in feeling pleasurable or painful mental emotions; another in moving the muscles; while the nerves that run to the nose, ears, eyes, tongue, hands, and surface generally, are employed in seeing, hearing, smelling, tasting, and feeling all physical sensations.
The back portion of the spinal marrow and the nerves that run from it are employed in sensation, or the sense of feeling. These nerves extend over the whole body, but are largely developed in the network of nerves in the skin. The front portion of the spinal marrow and its branches are employed in moving those muscles in all parts of the body which are controlled by the will or choice of the mind. These are called the nerves of motion.
The nerves of sensation and nerves of motion, although they start from different portions of the spine, are united in the same sheath or cover, till they terminate in the muscles. Thus, every muscle is moved by nerves of motion; while alongside of this nerve, in the same sheath, is a nerve of sensation. All the nerves of motion and sensation are connected with those portions of the brain used when we think, feel, and choose. By this arrangement the mind knows what is wanted in all parts of the body by means of the nerves of sensation, and then it acts by means of the nerves of motion.
For example, when we feel the cold air on the skin, the nerves of sensation report to the brain, and thus to the mind, that the body is growing cold. The mind thus knows that more clothing is needed, and wills to have the eyes look for it, and the hands and feet move to get it. This is done by the nerves of sight and of motion.
Next are the nerves of involuntary motion, which move all those parts of the head, face, and body that are used in breathing, and