This is a special usage of the word "the". It is listed in the MacMillan Dictionary under definition #2 as follows,
"used before a singular noun when making a general statement about people or things of a particular type"
They give two examples:
The average university student is not very interested in politics.
People have come to depend on the car as their only means of
Used in this way, "the + singular noun" means basically the same thing as that same noun pluralized:
the cow = cows
the English garden = English gardens
the computer = computers
This construction is very common in titles and introductory sentences within paragraphs and chapters because it has a certain air of scholarliness about it that the simple plural form just doesn't convey.