Clearing up and keeping clean the two centers of population was, however, the least of the work of sanitation. The whole Isthmus was a breeding place for the mosquitoes. Obviously every foot of it could not be drained clear of pools and rivulets, but the preventive campaign of the sanitation men covered scores of square miles adjacent to villages and the Canal bed, and was marvelously effective in reducing the number of mosquitoes. Away from the towns the campaign was chiefly against the malarial mosquito--the _anophelinæ_. The yellow fever mosquito, the _stegomyia_, is a town-bred insect coming from cisterns, water pitchers, tin cans, fountains in the parks, water-filled pans used to keep ants from the legs of furniture and the like. It is even said to breed in the holy-water fonts of the multitudinous churches of Panama, and the sanitary officials secured the co-operation of the church authorities in having those receptacles kept fresh. The malarial mosquito however breeds in streams, marshes and pools and will travel sometimes a mile and a half from his birth-place looking for trouble. As you ride in a train across the Isthmus you will often see far from any human habitation a blackened barrel on a board crossing some little brook a few inches wide. If you have time to look carefully you will see that the edges of the gully through which the brook runs have been swept clear of grass by scythe or fire or both, and that the banks of the rivulet are blackened as though by a tar-brush while the water itself is covered by a black and greasy film. [Illustration: _Photo by Underwood & Underwood_ THE CANAL COMMISSION HOSPITAL AT COLON BUILT BY THE FRENCH] This is one of the outposts of the army of health. Of them there are several hundred, perhaps thousands, scattered through the Zone. The barrel is filled with a certain fluid combination of oil and divers chemicals called larvacide. Day and night with monotonous regularity it falls drop by drop into the rivulet, spreads over its surface and is deposited on the pebbles on the banks. The mosquito larvæ below must come to the surface to breathe. There they meet with the noxious fluid and at the first breath are slain. Automatically this one barrel makes that stream a charnel house for mosquito larvæ. But up and down throughout the land go men with cans of the oil on their backs and sprinklers in their hands seeking for pools and stagnant puddles which they spray with the larvacide. So between the war on the larva at its breeding point and the system of screening off all residences, offices and eating places the malarial infection has been greatly reduced. It has not been eradicated by any manner of means. The Panama cocktail (quinine) is still served with meals. In one year 2307.66 pounds of the drug were served out. But if not wholly obliterated the ailment has been greatly checked. Dr. A. J. Orenstein, of the Department of Sanitation, says in summing up the results of the policy: