Feb. i3, 1883.] 



KNOWLEDGE 



115 



except at any spot where a new surface is made by the 

 close adhesion to the glass of any refracting body. Alany 

 beaded structures seen in this way look smaller — nearer 

 their true size — than they appear with an achromatic con- 

 denser and larger aperture. 



A vinegar plant, then, is a compound organism, in 

 which alcoholic ferments and acetous ferments live toge- 

 ther in harmonious co-operation. The first convert the 

 sugar into alcohol and carbonic acid, and the second oxidise 

 the alcohol into vinegar, obtaining the oxygen from the 

 air. The process may be a little more complicated, but 

 this is the result 



When a vinegar plant has once been obtained, it is easily 

 kept in a dark place in a jar with a little sugar and water. 

 When fresh vinegar is to be made, it is placed in tlie fluid 

 intended to be acetified in a warm cupboard. Between G0° 

 and 70° is a sufficient temperature, and about six weeks 

 the time required for two or three pints. At the end of 

 the process the plant will be found greatly thickened, and 

 it readily splits into two. The solution employed may 

 vary according to taste. If pale vinegar, like that made 

 from white wine, is required, good moist sugar and a little 

 golden syrup treacle will do, with enough water to make a 

 decidedly sweet, but not syrupy, solution. The vinegar 

 obtained in this way smells like dilute acetic acid. The 

 smell and Havour of common brown vinegar is produced if 

 coarse brown sugar and darker treacle is used. When the 

 process is finished, it is well to bring the vinegar rapidly 

 near the boiling-point, to kill any ferments that may re- 

 main in it. Prolonged heating would dissipate the acetic 

 acid, and must be avoided. 



The vinegar plant seems to be a vegetable instance of 

 what Van Benedon calls commensalism, the two sets of 

 micro-ferments are fellow-boarders. Neither can be said 

 to be a parasite upon the other, and it is probable that the 

 minute acetic-ferments form the gelatinous mass in which 

 the friends dwell like brothers in unity. A very small 

 piece of an old plant is sufficient to start a new one ; and 

 it is an interesting experiment to put a bit the size of a 

 pea into a two-ounce wide-mouthed bottle of clear glass, 

 filled with a solution of lump sugar. The growth of the 

 plant can then be easily watclied with the help of a hand- 

 magnifier, and the plant occasionally transferred to a 

 shallow cell, and placed under the microscope. It will be 

 found much more transparent than those developed in 

 a brown sugar solution. The tendency of the growth is 

 at first chiefly horizontal ; but when it has reached the 

 edges of the containing vessel, it thickens rapidly, if well 

 scpplied with food. While the fermentations arc active, 

 the plant floats ; but when the sugar is exhausted, no fresh 

 supply of carbonic acid buoys it up, and it falls to the 

 bottom. AVhen it is desired merely to preserve a plant for 

 future action, it should be kept in a cool place, and 

 moderately supplied with sugar solution, which should not 

 be allowed to dry up, or the plant will be spoilt, and blue 

 mould will f'row. 



"OUR BODIES:" 



SHORT PAPERS ON PHYSIOLOGY. 



By Dr. Andrew Wilsox, F.R.S.E., &c. 



No. VI.— THE TRCXK AND LIMB.S. 



THE tntnh of the body (minus the head or skull, already 

 described), consists of the spine, chest, and the 

 "girdles " of the limbs. These " girdles " are the series of 

 bones which attach the limbs to the trunk. Their compo- 

 sition we shall note presently. 



The chest or thorax is formed by the spine and ribs 

 behind, by the ribs at the sides, and by the ribs and 

 their carlilai/cg (or grixtlij cmln), and breast-bone in front 

 The ribs number twelve pairs in man, and are attached 

 behind to the twelve dorsal or back-vertebr;v, each by a 

 movable joint, which is utilised in the breathing move- 

 ments. Occasionally thirteen pairs of ribs are developed 

 in the human subject The thirteenth pair in such a case 

 is usually borne by the first lumbar vertebra'. Sometimes, 

 however, we find a thirteenth rib attached to the seventh or 

 last neck- vertebra'. These odd or extra ribs in man are, per- 

 haps, best explained as survivals of the conditions of lower 

 life, and as ancestral marks. The gorilla has normally 

 thirteen pairs of ribs, and crocodiles have a wliolc series of 

 neck-ribs. What is unusual in human anatomy, may thus 

 be perfectly normal in lower existences. The first seven 

 ribs are attached in front to the stent iim or brea-st bone each 

 by a rih cartUatje. Each cartilwje is simply a bar of 

 gristle, of highly clastic nature. We can see that with the 

 front of the chest thus rendered elastic, the movements of 

 breathing Piioidd be readily accomplished. In a crowd, our 

 ribs are never so readily broken, if we meet the pressure 

 from before backwards, so that the cartilages of the ribs 

 act as buffers. If, on the contrary, the same pressure 

 were applied at the sides of the chest, the ribs would most 

 likely be fractured. The eighth, ninth, and tenth pairs of 

 ribs are attached in front to the cartilage of the seventh rib. 

 Tlie eleventh and twelfth ribs are often called Jloatimj ribs 

 — a bad name, because they do not " float " in anything. 

 We should call them " free " ribs, and thus express the 

 fact that they are short, and that whilst springing from 

 the spine behind, they are not attached to anything in 

 front. The sternum, or breast bone, is a flat bone, shaped 

 somewhat like a sword. It is broad above, and narrow 

 below, where it ends in a piece of gristle. In old age, the 

 cartilages of the ribs and breast-bone tend to become of 

 bony nature. The ribs in old persons are thus more 

 brittle and more readily broken than those of the young. 

 Lime appears to bo deposited in the rib cartilages in old 

 age, and this fact, which is named a " degenerative 

 process," is also represented in the bloodvessels as well as 

 in the rib-cartilages. 



The limbs in Vertebrate or " backboned " animals never 

 exceed four in number. They are always developed in pairs, 

 and alwaysposscss an internal bony axis or skeleton, to which 

 the mnsc/es of the limbs are attached. Vertebrate animals 

 may have no limbs at all. Some fishes, 7nost serpents 

 (some snakes have small and rudimentary hind limbs), and 

 a few lizards are limbless. Some fishes, such quadrupeds 

 as the whales and sea-cows, and a few lizards have only 

 one pair of limbs. 



Each limb, as already noted, is joined to the body or 

 trunk by a series of bones forming the limb ijirdh. If we 

 take this yirdle into account, we shall find a limb to be 

 composed of six parts. The vpper (or fore) limb is attached 

 to the trunk by the shoulder-" girdle." In man, two bones 

 form this " girdle." In a bird or reptile, we should find at 

 least three distinct bones. One of these three bones, man 

 possesses in a modified condition. 



The .scapula, or shoulder-blade, is the first of the two 

 shoulder-bones. It is a triangular, flattened bone, lying on 

 the back and upper part of the chest Outside it has a 

 strong ridge (the spine of the shoulder-blade) for the 

 attachment of muscles. To the outer side, there are seen 

 two prominent projections. The spine of the bone ends in 

 one of these, the acromion process, and to this process the 

 collar-bone of that side is attached. The other process is 

 the coracoid process. This, in the bird or reptile, is a 

 I distinct bone, and forms, in fact, the chief support of the 



