We very well know, when we work hard at anything, we are likely to "get warmed up." Although a certain amount of this heat is necessary to our bodily health, too much of it is very dangerous.
Just as it is best for the temperature, or heat, of a room to be at about a certain level, somewhere from 60 deg.

to 70 deg.

F., so it is best for the interior of our bodies to be kept at about a certain heat.
This, as we can show by putting a little glass thermometer under the tongue, or in the armpit, and holding it there for a few minutes, is a little over 98 deg.

F.( 98.4 deg.

to be exact); and this we call "body heat," or "blood heat," or "normal temperature." Our body cells are, in one way, a very delicate and sensitive sort of hot-house plants, though tough enough in other respects.