Each of the twenty-five districts into which the Zone was divided, as far as the general sanitary work was concerned, was in the charge of a sanitary inspector. The sanitary inspector had under him a force of from twenty to one hundred laborers, with assistants and foremen as necessary. The districts varied considerably in the number of people living in them. Some of the districts had as many as eight or ten thousand people; some only a few hundred. The area of the district varied between fifteen and thirty-five square miles. The Zone extends for five miles on each side of the Canal, that is, a strip ten miles broad and fifty miles long. Most of the population was located on each side of the Canal, within about a mile of its axis, while a few houses and cabins were scattered through all parts of the Canal Zone. Sanitary work, generally speaking, was done only within a mile or so of the Canal itself. All brush and undergrowth were cleared within two hundred yards of houses and villages, and the ground carefully drained within the same area. There was no object in carrying sanitary work beyond the populated area. Even if mosquitoes bred where no human beings were living, no harm would be done, as there would be no one to infect. All told, the country under the jurisdiction of the Isthmian Canal Commission amounted in area to about five hundred square miles, and only a hundred of this was affected by the work of the Sanitary Department. The great work of the sanitary inspector was his anti-mosquito work. In the early days he paid most attention to the stegomyia mosquito, but after the fall of 1905 when yellow fever had been conquered, attention was concentrated on the anopheles, the malarial mosquito. For the elimination of the stegomyia, the inspector in these country districts took the same measures as described in Colon and Panama against these mosquitoes, though of course on a very much smaller scale. Against malaria, he had a sufficient number of laborers under one or more foremen, according to the size of the district, who cut the brush and undergrowth within two hundred yards of all villages, houses and dwellings, and who also cut the grass within this area whenever it reached a foot in height. This was done for several reasons. The adult mosquito is destroyed by wind or sunlight, and he seeks all sorts of shrubbery, grass and foliage for protection against both of these enemies. Therefore, if the brush, shrubbery and high grass is cleared off within two hundred yards around a dwelling, there is no shelter for the mosquito from either the wind or sunlight, and there are therefore no mosquitoes within this area.