But the United States also possesses police power. The line of demarcation between the State and the national police power follows closely, if not precisely, the line of demarcation between State power and national power to regulate commerce.[138] The State has power to protect itself,—that is, to guard its people against contagious or infectious diseases, as is exemplified in laws for the inspection of foods, for forbidding the pollution of streams, for securing the accuracy of weights and measures, the peace and good order of communities, the comfort of the inhabitants,—and, in a word,—to exercise such authority as, were no such authority exercised, the State would cease being the State. 54. The power granted to Congress to regulate commerce is not a power granted to the States; it pertains to the United States only. Therefore Congress has no power to regulate commerce that is not “with foreign nations, and among the several States, and with the Indian tribes.” Practically this deprives the State of police power over foreign and interstate commerce, and deprives the United States of police power over commerce that is, as to the State, completely internal. To what extent a State can protect itself from the entrance of paupers, insane or diseased persons, is a question for determination by the Courts. If such persons are “commerce” their entrance is a matter within the jurisdiction of Congress. But the welfare of the people of the United States is essentially the welfare of the people of the States, and Congress, in considering that welfare, avoids possible conflict with State legislation. Thus the immigration laws—all of which are national—include, or seek to include, these provisions for inspection which a State would prescribe, in the exercise of its police power for the health, safety, and general welfare of its own citizens. But here, too, a dominant principle prevails; The government of the United States, within the scope of its powers, operates upon every foot of territory under its jurisdiction. It legislates for the whole nation, and is not embarrassed by State lines. Its peculiar duty is to protect one part of the country from encroachments by another upon the national rights which belong to all.[139] Tested by this principle, any State laws conflicting with national immigration laws are unconstitutional.