Why the stock exchange is called the pulse of the nation?

I didn't know it was called that.

Assuming this was assigned for homework, please see the guide for posting:
https://www.askmehelpdesk.com/finance-accounting/announcement-font-color-ff0000-u-b-read-first-expectations-homework-help-board-b-u-font.html

I suspect this was meant to be researched. Asking on a forum isn't "research."