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Resins

respiration oils carbon resin

RESINS. A resin is a secretion formed in special resin canals or passages of plants, from many of iihich, such as, for example, coniferous trees, it exudes in soft tears hardening into solid masses in the air. Otherwise it may be obtained by making incisions in the bark or wood of the secreting plant. Resin can also be extracted from almost all plants by treatment of the tissue with alcohol, and it is formed by the oxidation of essential oils, many authorities being of opinion that all true resins, which are in chemical composition oxidized hydrocarbons, result primarily from the action of oxygen on essential oils. Resinous substances are further produced hy the dry distillation of numerous organic compounds and by the drying of fatty drying oils. Certain resins are obtained in a fossilized condition, amber being the most notable instance of this class, and African copal and the kaurie gum of New Zealand are also procured in a semi-fossil condi-tion. The resins which are obtained as natural exudations are in general compound bodies containing more than one simple resin and varying proportions of essential oil. These compounds when soft are known as oleo-resins, and when imperfectly fluid they are called balsams. Other resinous products are in their natural condition mixed with gum or mucilaginous substances and known as gum-resins. Vary-ing in constitution as these bodies do, they also differ widely in physical properties ; but the general conception of a resin is a noncrystalline body, insoluble in water, mostly soluble in alcohol, essential oils, ether, and hot fatty oils, combining with alkalies to form resin soap, soften-ing and melting under the influence of heat, not capable of sublimation, and burning with a bright but smoky flame. A typical resin is a transparent or translucent mass, with a vitreous fracture and a faintly yellow or brown colour, inodorous or having only a slight turpentine odour and taste. Many compound resins, however, from their admixture with essential oils, are possessed of distinct and characteristic odours. A series of gradations among resins may be traced from the hard glassy transparent copals through soft elemis and oleo-resins, semi-fluid balsams and fluid wood oils, to the most limpid essential oils. The hard transparent resins are principally used for varnishes organism and the emission of carbon dioxide from it as the complemental portions of one process of respiration. Although such a combined consideration is not strictly philosophical, inasmuch as it leaves out of view the intro-duction of the carbon into the organism, yet it is extremely convenient because Ole two processes referred to do, in all classes of the animal kingdom from the highest to the lowest, involve the same organs and tissues in their per-formance. Respiration may therefore be defined as the aggregate of those processe,s which are concerned in the introduction of oxygen into the system and the escape of carbon dioxide from it.
RESINS
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Respiration in such an organism as an amceba is ex-tremely simple. The medium surrounding it contains a practically unlimited supply of oxygen, and is so vast that the carbon dioxide put out into the medium is quickly removed from the neighbourhood of the organism. The interchange of oxygen and carbon dioxide takes place at the surface of the organism, so far as wo know, continu-ously. In the higher animals, constituted as they are of a vast numberof structural units accurately packed together, each resembling more or less in its physiological instincts the unicellular amceba, respiration presents a much more intricate problem. The fine interstices which exist between the structural elements do indeed contain a small quantity of a fluid medium which serves the function of the water bathing the amceba ; but the store of oxygen in the medium would speedily become exhausted, and the emitted carbon dioxide would quickly accumulate to a dangerous degree, if the medium were not continually restored to its original purity. This revival is effected by the circulating blood which is brought by its capillaries into the neighbour-hood of the remotest cells of the body. But even the mass of the blood is small compared with that of the cells it nourishes ; unless it be itself purified and restored in turn the interstitial juiees which depend upon it for their purification must soon fail to support the respiration of the cells. Such restoration of the blood takes plaee in eertain organs called lungs or gills, where the blood acquires a fresh store of oxygen and parts with its excess of carbon dioxide.

Respiration in the higher animals may therefore be divided into (1) internal respiration, or the interchange of oxygen and carbon dioxide between the cells of the body and the fluid drenching them, and (2) external respiration, or the gaseous interchange taking place in the special re-spiratory orga,ns (lunga, gills). The first is really a part of NErTRMON (q.v.); the second, or respiration proper, is the subject of the present article.

It will be evident on reflection that the proeess of respiration naturally falls to be described under two divisions, the first of which is concerned with the move-ments of the chest in inspiration and expiration and the manner in which they are brought about, and the second with the 'interchange of gases which takes place between the blood and the a.ir in the lungs.

Structure of the Organs of Respiration.

In order to understand the movements it is necessary first to know the structure of the air passages and thorax.

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