Part 26

Attacks vary much in intensity, but during the first three weeks so-called mild cases may suddenly develop more severe symptoms. Convalescence is slow and protracted, as in severe cases emaciation and debility are marked. The chief complications during the acute stage of the illness are perforation of an ulcer through the bowel, demanding immediate surgical interference, and intestinal bleeding resulting from haemorrhage from an ulcer. During convalescence relapses are frequent, brought on by indiscretions of diet, chill, and undue exposure, or some unknown cause. The most common _sequelae_ arising from the disease are thrombosis of a vein, usually in the thigh; bronchitis; one-sided parotitis; outbreaks of boils and superficial abscesses; and more occasionally heart weakness and disease of bone.

In its early stages enteric fever is difficult to diagnose, and confusion may arise between it and lobar- and broncho-pneumonia, influenza, diarrhoea associated with septic infection, typhus fever, appendicitis, or septicaemia.

In treatment, good nursing is of first importance. The patient must have suitable nourishment and stimulation, and requires to have the greatest care, whatever special form of treatment is being carried out, while constant watch must be kept for the appearance of any complication. Great differences in the treatment are observed in different countries and by different schools of medicine.

ENTERI'TIS (Gr. _enteron_, intestine) is inflammation of the intestines. It varies from a mild intestinal catarrh, causing slight symptoms, and yielding to treatment in a few days, to cases of severe vomiting and diarrhoea with extreme prostration. These severe forms are most frequently seen in infants and young children during the summer months, and frequently prove fatal. Removal of the cause of irritation and complete rest to the intestines are to be aimed at, as far as possible, in the treatment of the condition.

ENTEROMORPHA, a genus of Green Algae, similar to Ulva, but with a tubular thallus. _E. intestinalis_ is common in fresh and brackish waters.

ENTERTAINMENTS TAX, first imposed by the Finance (New Duties) Act, 1916, is an _ad valorem_ duty on payments for admission of persons as spectators or members of an audience to any entertainment. Entertainment is defined as including any exhibition, performance, amusement, game, or sport to which persons are admitted for payment. The tax is collected by means of stamped tickets of admission, or (in respect of places of regular entertainment, and in other cases on special cause shown) on the basis of returns furnished to the Board of Customs and Excise by arrangement previously made with the Board. Admission by complimentary ticket is not taxable if no indirect payment is a condition of such admission.

Exemption from the tax may be claimed in respect of any entertainment (1) where the gross receipts are entirely devoted to philanthropic or charitable purposes; or (2) where the purpose is the amusement of children, and the charge for admission does not exceed one penny per head; or (3) which is provided by or on behalf of a school or other educational institution for the furtherance of an object connected therewith, and at which the performers are children under sixteen years of age who have been or are in attendance thereat; or (4) which is wholly educational, or, being partly educational and partly scientific, is conducted by an association not established or carried on for profit, or, having for its aim the revival of national pastimes, is provided by such an association founded for that purpose.

Repayment of the tax may be claimed when the net proceeds of an entertainment are devoted to philanthropic or charitable purposes, and the total expenses met from the receipts do not exceed one-fifth of the receipts.

When admission to an entertainment is dependent upon payment of a contribution or subscription to a club, society, or association, tax is payable thereon, and where such payment carries with it the right of admission, the tax is due whether the right is exercised or not. If, however, the payment confers the right to other privileges besides admission to an entertainment, e.g. to the use of library, reading-room, &c., only such proportion thereof as the Board determines to represent the right of admission is taxable.

The amount yielded by the tax for the fiscal year 1916-7 was over L3,000,000, for 1917-8 nearly L5,000,000, while for 1919-20 it was approximately L10,485,000.

ENTOMOL'OGY, the branch of zoology which treats of insects, the name being from Gr. _ent[)o]ma_, animals 'cut in', the transverse division or segmentation of the body being their most conspicuous feature. The true insects are those animals of the phylum Arthropoda distinguished from the other classes of the phylum by the fact that the three divisions of the body--the head, thorax, and abdomen--are always distinct from one another. There are never more than three pairs of legs in the perfect insect, and these are all borne upon the thorax. Each leg consists of from six to nine joints. The first of these is called the 'coxa', and is succeeded by a short joint called the 'trochanter'. This is followed by a joint, often of large size, called the 'femur', succeeded by the 'tibia', and this has articulated to it the 'tarsus', which may be composed of from one to five joints. Normally two pairs of wings are present, but one or other, or both, may be wanting. The wings are expansions of the sides of the second and third sections of the thorax, and are strengthened by narrow thickenings called 'nervures'. In the beetles the anterior pair of wings becomes hardened so as to form protective cases for the posterior membranous wings, and are called in this condition 'elytra' or 'wing-cases'. The fore-wings are similarly transformed in the Orthoptera, while in many of the Hemiptera they are horny except at the tip. Respiration is effected by means of air-tubes or tracheae, which open on the surface of the body by lateral apertures called 'stigmata' or 'spiracles', and ramify through every part of the body. The head is composed of several segments amalgamated together, and carries a pair of feelers or 'antennae', a pair of eyes, usually compound (and often simple eyes in addition), and the appendages of the mouth. [Illustration: A, Sucking mouth of a Butterfly: _p_, proboscis; _l_, labium; _l_1_, labial palps; _m_, mandibles. B, Piercing mouth of a Mosquito: _a_, labial palps; _b_, labium epi-pharynx; _c_, labium; _d_ and _e_, maxillae; _f_, labrum; _g_, antennae; _h_ and _i_, ocelli: _hy_, hypopharynx; _j_, labial palp.] These last include an upper lip (labrum), and three pairs of jaws (mandibles, first maxillae, second maxillae), the third pair being more or less fused into a lower lip (labium). The thorax is composed of three segments, also amalgamated but generally pretty easily recognized. The abdominal segments are usually more or less freely movable upon one another, and never carry locomotive limbs; but the extremity is frequently furnished with appendages connected with generation, which in some cases serve as offensive and defensive weapons (stings). The organs of the mouth take collectively two typical forms, the masticatory and the suctorial, the former exemplified by the beetles, the latter by the butterflies, in which the mouth is purely for suction. The alimentary canal consists of the oesophagus or gullet, a crop, a gizzard, a stomach, and an intestine, terminating in a cloaca. There is no regular system of blood-vessels; the most important organ of the circulation is a contractile vessel situated dorsally and called the 'dorsal vessel'. The nervous system consists of a pair of cerebral ganglia (brain) in the head, these being the thickened upper part of a nerve-ring which encircles the gullet and passes below into a double ventral nerve-cord dilated into ganglia at intervals. The sexes are in different individuals, and most insects are oviparous. Reproduction is generally sexual, but non-sexual reproduction also occurs. (See _Parthenogenesis_.) Generally the young are very different from the full-grown insect, and pass through a 'metamorphosis' before attaining the mature stage. When this metamorphosis is complete, it exhibits three stages--that of the larva, caterpillar, or grub, that of the pupa or chrysalis, and that of the imago or perfect winged insect. Insects have been divided into three sections--Ametab[)o]la, Hemimetab[)o]la, and Holometab[)o]la, according as they undergo no metamorphosis, an incomplete one, or a complete one. The young of the Ametab[)o]la differ from the adult only in size. They are all destitute of wings; the eyes are simple and sometimes wanting. The Hemimetab[)o]la undergo an incomplete metamorphosis, the larva differing from the imago chiefly in the absence of wings and in size. The pupa, here termed a nymph, is usually active, or, if quiescent, capable of movement. In the Holometab[)o]la the metamorphosis is complete, the larva, pupa, and imago differing greatly from one another in external appearance and habits. The larva is worm-like and the pupa quiescent. The section Ametab[)o]la includes the order Apt[)e]ra (tassel-tails and spring-tails). The section Hemimetab[)o]la comprises the orders Hemipt[)e]ra (cicadas, bugs, plant-lice, &c.), Orthopt[)e]ra (cockroaches, crickets, grasshoppers, locusts, earwigs, &c.), and Neuropt[)e]ra (dragon-flies, may-flies, white-ants, &c.). The Holometab[)o]la comprises the orders Dipt[)e]ra (gnats, bot-flies, gad-flies, mosquitoes, house-flies, fleas, &c.), Lepidopt[)e]ra (butterflies and moths), Hymenopt[)e]ra (bees, wasps, and ants), and Coleopt[)e]ra (lady-birds, glow-worms, cockchafers, weevils, and all of the beetle tribe). A division is sometimes made into Mandibulate and Haustellate groups, the oral apparatus of the former being adapted for mastication, the latter for imbibition of liquid food. Both types are, however, sometimes modified, and occasionally combined.

ENTOMOPH'AGA ('insect eaters'), a term applied to (1) a group of hymenopterous insects whose larvae feed upon living insects; (2) a tribe of marsupials, as the opossums, bandicoots, &c., which are insectivorous, though not exclusively so; (3) a section of the edentates, as the ant-eater and pangolin.

ENTOMOPHTHORINEAE, a curious family of Fungi, group Zygomycetes, all parasites on insects. The best known is _Empusa Muscae_, which attacks the common house-fly. After thoroughly permeating and finally killing the infected fly, the fungus sends numerous hyphae to the surface, from the ends of which conidia are shot forth by an 'explosive' process, to spread the infection if they alight on living flies. In autumn, flies killed by this parasite may often be found adhering to window-panes, surrounded by a halo of ejected conidia. It has been proposed to utilize this fungus in combating the fly nuisance.

ENTOMOS'TRACA, a sub-class of crustacea, including forms which are mostly small, and comprising four orders: (1) Branchiopoda, brine-shrimps and water-fleas; (2) Copepoda, including the freshwater Cyclops, and numerous marine species contributing to the floating surface population (plankton); (3) Cirripedia, barnacles; (4) Ostracoda, types such as Cypris enclosed in a bivalve shell.

ENTOZO'A, a general name for the various parasitic worms that infest the bodies of other animals. Some are found in the intestines, others in the liver, brain, muscles, and other tissues. They pass through different stages in their development, and at each stage may occupy a different organ (or tissue), and usually a different animal. Thus the cystic or bladder-worm, whose presence in the brain of sheep causes staggers, is the immature form of a tapeworm of the dog, &c. The number of species is being reduced as the relations of the different forms are studied. They belong to the two phyla Platyhelminthes (flat-worms) and Nemathelminthes (thread-worms, &c.). The former embraces flukes (Trematoda) and tapeworms (Cestoda); while the latter includes thread-worms and round-worms (Nematoda), and spiny-headed worms (Acanthocephala).

ENTRENCHMENTS. The employment of entrenchments, or earthworks, in connection with military operations dates from the earliest times. Entrenched camps, made by the Romans, are still to be seen in many parts of the country. The primary object, in early designs, was to offer a material obstacle to the assaulting enemy; the defenders manned a high parapet overlooking a formidable ditch, and a hand-to-hand conflict decided the issue. The development of fire-arms, and especially of artillery, depreciated the value of entrenchments as affording material obstacles, but gave them a gradually increasing value as a means of protecting the defenders from missiles. The provision of cover, that is to say, concealment from view or protection from fire, ultimately became the dominant factor in the design of earthworks. Obstacles were still necessary, but they had to be provided by other means. Successive improvements in firearms altered the nature of the cover which it was necessary and practicable to provide. A bullet-proof parapet has always been an essential feature, but whereas a few inches of earth sufficed to stop a musket-ball, the modern rifle-bullet will penetrate a thickness of nearly four feet. The introduction, during last century, of shrapnel shell, the bullets from which descend at a steep angle, was followed by the adoption of overhead cover as a standard feature in trench design. This persisted for several years, until the great increase in the power of high-explosive shell made it impossible to construct any form of roof which would withstand bombardment, even by the lighter artillery accompanying an army in the field, and still permit of the defenders using their rifles from below it. Modern fire-positions are made open, i.e. without overhead cover.

All past wars have proved that victory can only be won as the result of offensive action. Nevertheless, in any campaign it will not be possible to attack at all times and in all places. The provision of the strongest possible force at the vital point necessitates a defensive attitude on other parts of the front. Although entrenchments presuppose a defensive attitude locally, they play an important part in offensive operations. The ultimate aim is that, by a skilful use of entrenchments, a commander may be able to reduce to a minimum the strength of his force in actual combat with the enemy, and thus to retain at his disposal a reserve of troops for offensive action.

Entrenchments may be either hasty or deliberate. Hasty entrenchments are those made on the actual field of battle: by the attackers, to secure the ground won prior to another bound forward; by the defenders, to hold up the attack pending fresh dispositions of troops in rear. The amount of digging that can be done is necessarily small; existing cover must be utilized to the utmost. This may consist of ditches, hedges, sunken roads, railway embankments and cuttings, buildings, woods, shell-holes, &c. All of these are readily convertible into strong defences. The test of battle has proved, over and over again, that troops well trained in adapting natural cover to defence are very difficult to dislodge, once they have dug themselves in. Deliberate entrenchments are employed in the gradual building up of a trench system when once the opportunity of manoeuvre has ceased to exist; or in the preparation of a defensive position somewhat remote from the scene of active operations.

The following are the salient features in the design of modern entrenchments: (1) A parapet 18 inches high, and upwards of 5 feet thick, in front of every fire-trench. (2) Longitudinal division of every trench, either by projecting buttresses of earth or by bends, so that no straight portion exceeds 10 yards in length. The effect of this is to give protection against flanking or enfilade rifle-fire, and to localize the burst of shell. (3) A parapet on the rear side (parados), to shield the defenders from the back-blast of shell which burst beyond the trench. (4) Wide trenches (6 feet at the top), to minimize the risk of men getting buried during bombardment. (5) Accommodation, in dug-outs and other refuges, for a proportion of the troops.

Arms of precision have, during the past twenty years, compelled careful attention to the concealment of entrenchments. Although systems of trenches cannot now be hidden from the eye of the aeroplane-camera, yet the enemy can be kept in doubt, by correct design and careful siting of trenches, as to the strength in which the various portions of a position are held. It is for this reason that the deep fire-trench, with a low parapet in front, has been universally adopted, despite the obvious objection that minor undulations restrict the field of fire because the rifle is brought nearer to the ground. Earthworks on a sky-line, or those seen by the enemy against a distant background, violate the first principles of siting.

Modern entrenchments are arranged in depth. The 'lines' of Wellington's time have given place to a broad belt of mutually supporting defences, organized in three zones. These merge imperceptibly into one another, and each zone extends upwards of a mile from front to rear. The foremost fringe of the outpost zone is in contact with the enemy; it is the high-water mark on which the troops advancing during the last action have come to a standstill and dug themselves in. Very lightly manned, as befits an area where heavy shelling is rife, this zone, nevertheless, plays an important r[^o]le. It harbours the forward artillery observation posts, which control and direct the fire of the guns in rear. It furnishes a 'jumping-off' place for attacks. It takes the first shock of a hostile attack, and, although not strong enough to repulse a serious offensive, contributes to the enemy's ultimate defeat by depriving his onslaught of momentum. The battle zone is more elaborately organized, and capable of being very heavily manned. Within this area the defence intend to bring to a standstill the most determined offensive. It lies sufficiently far back from the fringe of the outpost zone to be reasonably immune from destructive bombardment. Lying still farther back is the third zone, which serves for the accommodation of reserves of troops, and is also prepared for defence, as a last resort, in case the enemy should penetrate the battle zone.

ENTREP[^O]T ([.a][n.]-tr-p[=o]; Fr.), a port where foreign merchandise which cannot enter the interior of a country is deposited in magazines under the surveillance of the custom-house officers till it is re-exported; also, any place where goods are sent to be distributed wherever customers are found.

ENTRE RIOS (en'tre r[=e]'os; 'between rivers'), a province of the Argentine Republic, lying between the Uruguay and the Paran['a]; area estimated at 29,240 sq. miles; pop. 425,370. The province is largely pastoral. Capital, Paran['a], with a pop. of 25,000

ENTRO'PION is the inversion or turning in of the eyelid. It may be congenital, or arise as the result of some inflammatory process or burn of the conjunctiva. Entropion affecting the lower lid appears also as the result of extreme photophobia (intolerance to light).

ENTROPY, a term introduced into physics by Clausius as the name of one of the two important thermodynamical properties of a substance which depend on its 'state'. Suppose we have 1 lb. of water at atmospheric pressure and 212deg F., say, and suppose we apply heat to the water and change it into 1 lb. of steam at 212deg F. The temperature does not change during this process, while the heat which must be added is the latent heat of the steam, namely, about 960 British Thermal Units. The increase of entropy from the first state to the second state is got by dividing the heat given to the substance, namely, 960 B.Th.U., by the absolute temperature at which that heat was given to the substance, namely, 461 + 212 = 673 degrees absolute, i.e. the increase of entropy is 960/673 = 1.43 units. If the temperature changes with the addition of heat, as it would usually do, we have to imagine the heat to be supplied in small quantities, and to take the average temperature of the body at which these tiny quantities of heat are supplied. The quotient heat / temperature is taken for each small quantity of heat, and the results are added together. The summation is defined as the increase in entropy between the initial and the final states.

In mathematical language the increase in entropy between state A and state B is given by ([phi]_B - [phi]_A) = Integral[dQ/T] from A to B, where [phi] stands for the entropy, Q for the heat received by the body, and T for the absolute temperature at which the heat is received. The importance of the entropy function, namely, the integral Integral[dQ/T], in thermodynamics is due to the fact that it is, like the internal energy of the substance, a function of the state of the substance only, and consequently in any temperature cyclic change in which the final state of the substance is the same as its initial state A, the total change, either of its internal energy or of its entropy, is zero. The fact that the total change in the internal energy of the substance is zero is practically equivalent to the _First Law of Thermodynamics_, while the fact that the total change of the entropy of the substance is zero is equivalent to the _Second Law of Thermodynamics_.--BIBLIOGRAPHY: J. H. Poynting and J. J. Thomson, _Textbook of Physics_ (vol. 3): _Heat._

EN'VELOPES, the paper covers that enclose letters or notes. They became common shortly after the introduction of the penny postage system; were at first made chiefly by hand, but are now not only shaped, but folded and gummed, by machinery.