Online Encyclopedia
Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.
GROUP XIX
Online EncyclopediaEncyclopedia Home :: GRA-GUI
del.icio.us it!
|
See also:GROUP XIX . Alkaloids.—This embraces a very large number of important pharmacological substances, which differ a See also:good See also:deal in the details of their See also:action, but they all See also:act upon muscle and See also:nerve See also:tissue. Some of them affect only certain portions of the See also:nervous See also:system, others have a much wider range of action; they may act in either See also:case as stimulants or as depressants, and hence the symptoms produced by them vary very greatly. 1. See also:Morphine and the other See also:opium alkaloids (codeine, narcotine, laudanine, &c.) have two prominent actions–a narcotic followed by a tetanic action. In morphine, on the higher animals at least, the narcotic action is very marked, the tetanizing action slightly so; while in thebaine there is little narcotic effect, but a tetanizing action like that of See also:strychnine. Morphine exercises its effects chiefly upon the cerebrum and the medulla oblongata in See also:man. It has in addition a markedly depressing action upon the See also:respiratory centre, it lessens all the secretions except the sweat, and diminishes bowel peristalsis and the See also:size of the See also:pupil. Men are much more affected by it than birds, rabbits, See also:dogs and most other animals. See also:Cats, however, show marked symptoms of cerebral excitement and increase of the reflexes. Compared with morphine, codeine and the other alkaloids are only slightly narcotizing. 2. Strychnine and See also:brucine very closely resemble each other in action, and under this heading curarine may also be included. These bodies stimulate the See also:grey See also:matter in the See also:spinal See also:cord and cause tetanic See also:convulsions. In the case of curare these are masked almost at once by See also:paralysis of the terminations of the motor nerves. 3. See also:Caffeine is the active principle in See also:tea, See also:coffee, See also:kola, See also:mate and See also:guarana; while thcobromine, a See also:body closely allied to it, is found in See also:cocoa and See also:chocolate. They both stimulate the grey nerve-cells in the See also:brain and cord, this being the See also:foundation of their dietetic value and their use as nervine stimulants. They also markedly increase the secretion of urine by stimulating the secreting cells of the kidneys. 4. See also:Cocaine is the active principle of the See also:coca See also:leaf, which is chewed as a stimulant-narcotic in See also:Peru and See also:Bolivia. Small doses excite the nervous system, while larger doses are depressing. The See also:chief action of cocaine from a See also:practical point of view is its See also:power of paralysing the terminations of sensory nerves. 5. Atropine, hyoscyamine, homatropine, duboisine, daturine and some other bodies have a paralysing action upon the ends of the motor and secretory nerves. They therefore lessen all the secretions, and among other actions dilate the pupil and increase the rapidity of the See also:heart by paralysing the vagus. In addition they have a stimulating action on the central nervous system. 6. See also:Nicotine, piturine and loheline are the active principles of See also:tobacco and other substances which are smoked as stimulant See also:narcotics. In large doses they are powerful nerve poisons, but as usually taken they exercise a gently stimulant effect upon the nervous system. See also:Pilocarpine has an action closely allied to that of nicotine, but as it is much less poisonous (the effects produced by small doses being chiefly excessive sweating and salivation), it is capable of being utilized in See also:medicine. Muscarine has a very See also:close resemblance in action to pilocarpine. 7. Physostigmine, the active principle of the See also:Calabar See also:bean, acts chiefly as a stimulant to voluntary and involuntary muscles, and at the same See also:time exercises a depressing effect upon the spinal cord. It contracts the pupil. 8. See also:Conine, gelseminine and sparteine all exert a paralysing effect on the terminations of the motor nerves, to the implication of which the weakened gait and other symptoms are due. 9. Aconitine, delphinine and many of their derivatives have a very widespread depressing action on muscle and nerve. 10. Apomorphine is essentially a muscle See also:poison, but owing to the fact that See also:minute doses stimulate the vomiting centre and cause emesis before any other symptoms are observable, its emetic action is the most prominent effect in man. ft. Emetine acts as a See also:gradual depressant to the nervous system in animals. In man its chief effect is its emetic action, which seems to be due entirely to See also:local irritation of the See also:stomach. 12. See also:Quinine. Several of the other alkaloids found in See also:cinchona See also:bet-lc act very much like quinine. They all depress the conducting pcwer and the grey matter of the spinal cord, and to a much less extent that of the brain. They lessen the See also:general See also:metabolism and See also:lower febrile temperature. The cinchona alkaloids have a specific-ally poisonous effect on the parasites of See also:malaria when See also:present in human See also:blood, and are poisonous to all See also:low organisms. 13. See also:Phenacetin, acetanilide, phenazone and many similar bodies act as antipyretics in virtue of an action on the See also:heat-regulating centres in the cerebrum. Additional information and CommentsThere are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML. Site content, images, and layout Copyright © 2006 - Net Industries, worldwide. |
|
|
[back] GROUP XIII |
[next] GROUP XVI |