162 S L O S M A orange, yellow, or white. There are also structural differences in the skulls, as in the amount of inflation of the pterygoid bones, which indicate real differences of species ; but the materials in our museums are not yet sufficient to correlate these with external characters and geographical distribution. The habits of all are apparently alike. They are natives of Guiana, Brazil, and Peru, and one if not two species (B. infuscatus and B. castaneiceps) extend north of the Isthmus of Panama as far as Nicaragua. Of the former of these Dr Seeman says that, though generally silent, a specimen in captivity uttered a shrill sound like a monkey when forcibly pulled away from the tree to which it was holding. 1 Genus Cholcepus. Teeth f ; the most anterior in both jaws separated by an interval from the others, very large, caniniform, wearing to a sharp, bevelled edge against the opposing tooth, the upper shutting in front of the lower when the mouth is closed, unlike the true canines of heterodont mammals. Vertebrse : C 6 or 7, D 23-24, L 3, S 7-8, C 4-6. One species (C. didactylus} has the ordinary number of vertebrae in the neck ; but an otherwise closely allied form (0. hoffmanni) has but six. The tail is very rudimentary. The hand generally resembles that of Bradypus ; but there are only two functional digits, with claws, those answering to the second and third of the typical pentadactyle manus. The structure of the hind limb generally resembles that of Bradypus, the appellation "two-toed" referring only to the anterior limb, for in the foot the three middle toes are functionally developed and of nearly equal size. C. didactylus, which has been longest known, is commonly called by the native name of Unau. It inhabits the forests of Brazil. C. hoffmanni has a more northern geographical range, extending from Ecuador through Panama to Costa Rica. Its voice, which is seldom heard, is like the bleat of a sheep, and if the animal is seized it snorts violently. Both species are very variable in external coloration. ( W. H. F. ) SLOUGH, an urban sanitary district of Buckingham- shire, England, is situated on the Great Western Railway, 18 miles west of London and 2 north of Windsor. Within recent years it has largely increased, and it con- tains a number of good shops and villas. It is supplied with water from artesian wells. The parish church of St Mary, erected 1837, has been recently enlarged. Among other public buildings are the British orphan asylum, the Eton union workhouse, and the reading-room and literary institute. Sir William Herschel, the astronomer, resided at Slough, and there constructed his telescope. The population of the urban sanitary district, which embraces parts of the parishes of Stoke Poges and Upton-cum-Chal- vey, in 1871 was 4509, and in 1881 (area, 401 acres) it was 5095. SLOVAKS. See SLAVS. SLOVENES. See SLAVS. SLUG. See SNAIL. SLUTSK, a district town of Russia, in the government of Minsk, situated on the Stutcha river (tributary of the Pripet), 123 miles south of Minsk. This old town is men- tioned in the 12th century as a dependency of Kieff, and, like other towns of the region, was devastated by the Tatars, and later suffered in the wars between Russia and Poland. It is now merely a large village, whose inhabitants are chiefly engaged in agriculture, Avith a little trade in corn, timber, and wooden wares. The immense marshy and woody tracts of the Polyesie (see MINSK) surround it on all sides, the Stutcha being its chief means of communica- tion. Its population remains almost stationary and was 19,000 in 1883. SMALLPOX, or VARIOLA (varus, " a pimple "), an acute infectious disease characterized by fever and by the appear- ance on the surface of the body of an eruption, which, after passing through various stages, dries up, leaving more or less distinct cicatrices. Few diseases have been so destruc- tive to human life as smallpox, and it has ever been re- garded with horror alike from its fatality, its loathsome accompaniments and disfiguring effects, and from the fact that no age and condition of life are exempt from liability to its occurrence. Although in most civilized countries its ravages have been greatly limited by the protection afforded by vaccination, yet epidemic outbreaks are far 1 Godman and Salvin's Biologia Centrali- Americana, p. 184. from uncommon, affecting especially those who are un- protected, or whose protection has become weakened by lapse of time. Much obscurity surrounds the early history of smallpox. It appears to have been imported into Europe from Asia, where it had been known and recognized from remote antiquity. The earliest accounts of its existence reach back to the middle and end of the 6th century, when it was described by Procopius and Gregory ol Tours as occur- ring in epidemic form in Arabia, Egypt, and the south of Europe. In one of the narratives of the expedition of the Abyssinians against Mecca (c. 550) the usual miraculous details are combined with a notice of smallpox breaking out among the invaders. 2 Not a few authorities, however, regard these accounts as referring not to smallpox but to plague. The most reliable statements as to the early exist- ence of the disease are found in Rhazes (see vol. xv. p. 805), by whom its symptoms were clearly described, its pathology explained by a humoral or fermentation theory, and directions given for its treatment. During the period of the crusades smallpox appears to have spread exten- sively through Europe, and hospitals for its treatment were erected in many countries. But at this period and for centuries afterwards the references to the subject include in all likelihood other diseases, since no precise distinction appears to have been made between the different forms of eruptive fever until a comparatively recent date. Small- pox was known in England as early as the 13th century, and had probably existed there before. It appears to have been introduced into America shortly after the discovery of that continent, and there, as in Europe and throughout the known world, destructive epidemics were of frequent occurrence during succeeding centuries. The only known factor in the origin of smallpox is con- Con- tagion, this malady being probably the most contagious of tagion all diseases. Its outbreak in epidemic form in a locality may frequently be traced to the introduction of a single case from a distance. The most direct means of com- municating smallpox is inoculation (see below). By far the most common cause of conveyance of the disease, however, is contact with the persons or the immediate surroundings of those already affected. The atmosphere around a smallpox patient is charged with the products of the disease, which likewise cling tenaciously to clothing, furniture, &c. The disease is probably communicable from its earliest manifestations onwards to its close, but it is generally held that the most infectious period extends from the appearance of the eruption till the drying up of the pustules. Smallpox may also readily be communicated by the bodies of those who have died from its effects. No age is exempt from susceptibility to smallpox. In- fants are occasionally born with the eruption or its marks upon their bodies, proving that they had undergone the disease in utero. Dark-skinned races are said to suffer more readily and severely than whites. One attack of smallpox as a rule confers immunity from any recurrence, but there are numerous exceptions to this rule. Over- crowding and all insanitary surroundings favour the spread of smallpox where it has broken out ; but the most in- fluential condition of all is the amount of protection afforded to a community by previous attacks and, especi- ally in the present day, by vaccination. Such protection, although for a time most effectual, tends to become ex- hausted, unless renewed. Hence in a large population there is always likely to be an increasing number of indi- viduals who have become susceptible to smallpox. This probably explains its occasional and even apparently 2 See Noldeke, Geschichte der Perser . . . aus Tabari (Leyden, 1879), p. 218. Noldeke thinks that this notice may be taken from genuine historical tradition, and seems to find an allusion to it in an old poem.
