Section 3. Public corporations may be divided into two classes: The first are known as municipal corporations and the second class come under the head of public quasi-corporations, which include counties, townships, school districts and the like. There is also a third class of corporations denominated quasi-public corporations, combining the elements of both public and private. Although organized for private profit, they are compelled by law or contract to render public service. They are distinguished as having in view some public enterprise in which the public interests are involved; these species of corporations owe certain duties to the public, as railroad and canal companies.8