Page:The New International Encyclopædia 1st ed. v. 13.djvu/399

* META. 365 METACHROSIS. and flows in a nojilipa.stcrly direct inn to its junction with the Orinoco, on the boundarj' be- tween Coloniliia and Venezuela (Map: Colombia, D 2). Its length is aljout 700 miles, for the greater part of which it is navigable for small steamers, though its channel is tilled with nu- merous islands. METABETCHOTTAN, m5t'a-bet-chCU-;in'. The princi]ial siiiith.Tn alliuent of Lake Saint John (q.v. I, Canada. Jt is 00 miles long and near its mouth occur its fine falls 236 feet high. METAB'OLISM (from Gk. ii£ra(3n?,ij,mct, metatallein, to change, from iiirii, iii'.ld, beyond -- AaAAtiv, Tjollein, to throw). The continual molecular or physico- chemical changes taking place in the protoplasm of organisms, during growth and throughout life. Upon it the life-processes rest. Verworn says that it is solely a process that distinguishes the living organism from the dead organism, and not from inorganic substance; it occurs also among inorganic bodies. Verworn defines it as the thing in which the living organism differs from the lifeless, and saj's that it consists in the con- tinual self-decomposition of living substance, the giving oil" to the outside of the decomposition- products, and, in return, the taking in from the cutside of certain substances, which give to the organism the material with which to regenerate itself and grow by the fornuition of similar groups of atoms, i.e. by 'polymerization.' This IS characteristic of all living substance. During the process of metabolism the living cell-sub- stance is being continually broken down and reformed by the continual giving off and taking in of material. The metabolism of living sub- stance, says Verworn, upon which all life is based, is conditioned by the existence of certain very labile compounds, which stand next to the proteids and on account of their elementary significance in life are best termed "biogens.' Their continual decomposition and reformation constitute the life-process. In plant physiology the term metabolism is used in essentially the same sense as in animal physiology-. Two .series of processes arc distin- guishable: (1) Processes which result in the up- building of complex substances, particularly such as contain a gi'cater amount of potential energy than the ones from which they are constructed. These processes are designated as 'constructive' metabolism or 'anabolism.' (2) Processes which result in the decomposition of complex substances into simpler ones, designated as 'destructive' metabolism or 'katabolism.' For special di.scus- sions of metaljolic processes see A.ssimilation in Plants; Digestion ix Plants; Fermentation; Nutrition; Photosynthesis; Respiration in Plants. Consult Verworn, General Physiology : An Outline of the Seience of Life (New York, METACENTRE (from Ok. iiitci, mrta. after + KeyTpov, h-entron, centre). It is shown in hydro- statics that a floating body is acted on by two equal forces in opposite directions ; one, vertically down through the centre of gravity of the body itself; the other, vertically up through the centre of gravity of the volume which was occupied by the liquid now displaced by the body. If the body is at rest, these two forces must lie in the same vertical line; but the question of the sta- bility of this equilibrium depends upon what happens when the floating body is tipped slight- ly. If the forces bring it back to its former posi- tion, the equilibrium is stable; if they make it tip still farther, the equilibrium is unstable. Imagine a line drawn in the floating body so as to pass through the centres of gravity' of the body and of the displaced liquid, when the body is in equilibrium ; this line is called the 'axis.' Xow imagine tlic body tipped slightly, thus mak- ing the axis inclined to the vertical; the line of action of the vertical upward force will intersect this line at a point called the 'metacentre.' If the metacentre lies above the centre of gravity of the body, the two forces will form a couple tending to restore the body to its former posi- tion, where the equilibrium was stable. If, on the other hand, the metacentre is below the centre of gravity of the body, the forces form a couple tending to tip the body farther, and so the equilibrium was unstable. An elongated floating body like a ship has a transverse metacentre and a longitudinal metacentre. The former is the one most commonly considered. The metacentric height is the vertical distance between the centre of gravity and the metacentre. It is evident that this must always be a positive quantity; for if the centre of gravity were above the metacentre there would be no force tending to keep the ves- sel upright and it would capsize. See Ship- building. MET'ACHRO'SIS (from Gk. fieraxpavvitvai, nietacli njiui yna i, to change color, from /itrdj meta, after + xp"''"'vi'ai. chronnymii, xp^^^i'". chrosein, to color, from ;i'pu(7(f, chrosis, color, from XP"'", chroia, xp^o-, e'l roa, skin, color) . Color-change, as that of the chameleon, in adaptation to surround- ings, and due to changes in the size of the pigment- cells of the inner layer of the skin. These special- ized pigment-cells are called 'chromatophores,' and the remarkable changes in the color of the skin of the chameleon, of the tree-toad, the squid, etc., depend on the distribution of these pigment-cells, which dilate (becoming highly ramified) and contract under certain kinds of irritation. The pigment (q.v.) varies in color in different species and in different parts of the body, being black, / « c a h CHROMATOPHORES OF A FROG. .7, Wholly contrai'tod : b, c, half relaxed ; d. wholly re- laxed ; e, wholly contracted (a capillary vessel); f, g, h, ex- panded color-cells. brow^n, yellow, and sometimes even red or green. In the goby Heineke found that the chromato- phores which are yellow or greenish-yellow when distended become orange-colored when contracted, wliile the orange or red ones when shrunk become brown or even black: and he detected in the goby a special kind of chromatophores which were filled with iridescent crystals of marvelous deli-