| DRINKS AND|| 3. Possible Disturbances produced by Smoking. |_ NARCOTICS.|| 4. Influence upon the Nervous System. || 5. Is Tobacco a Food? ||_6. Influence of Tobacco on Youth. | _ | | 1. _Description._ | 3. Opium............| 2. _Physiological | |_ Effects._ | 4. Chloral Hydrate. | 5. Chloroform. |_6. Cocaine. THE NERVOUS SYSTEM. [Footnote: The organs of circulation, respiration, and digestion, of which we have already spoken, are often called the vegetative functions, because they belong also to the vegetable kingdom. Plants have a circulation of sap through their cells corresponding to that of the blood through the capillaries. They breathe the air through their leaves, which act the part of lungs, and they take in food which they change into their own structure by a process which answers to that of digestion. The plant, however, is a mere collection of parts incapable of any combined action. On the other hand, the animal has a nervous system which binds all the organs together.] STRUCTURE.--The nervous system includes the _brain_, the _spinal cord_, and the _nerves_. It is composed of two kinds of matter-- the _white_, and the _gray_. The former consists of minute, milk-white, glistening fibers, sometimes as small as 1/25000 of an inch in diameter; the latter is made up of small, ashen-colored cells, forming a pulp-like substance of the consistency of blancmange. [Footnote: In addition to the cells, the gray substance contains also nerve fibers continuous with the white fibers, but generally much smaller. These form half the bulk of the gray substance of the spinal cord, and a large part of the deeper layer of the gray matter in the brain.--LEIDY'S _Anatomy_, p. 507.] This is often gathered in little masses, termed ganglions (_ganglion_, a knot), because, when a nerve passes through a group of the cells, they give it the appearance of a knot. The nerve fibers are conductors, while the gray cells are generators, of nervous force. [Footnote: What this force is we do not know. In some respects it is like electricity, but, in others, it differs materially. Its velocity is about thirty three meters per second.--_Popular Physics_, p. 244, Note.] The ganglia, or nervous centers, answer to the stations along a telegraphic line, where messages are received and transmitted, and the fibers correspond to the wires that communicate between different parts. FIG. 50. [Illustration: _The Nervous System._ A, _cerebrum_; B, _cerebellum._]