His studies on sugar logically led Bernard to the investigation of heat production and heat regulation in the human body. Glycogen, the sugary substance produced by the liver, occurs abundantly in all the muscles of the body, and it was evident that muscular movement leads to its consumption and the consequent production of heat. Sugar is a carbon-containing substance, and its combustion always produces energy. The question of heat regulation was a much more complicated problem. Heat is always being produced in the human body and always being given off. Very different amounts of heat are required to keep up the temperature of the human body in the winter and summer seasons. Near the pole or at the equator man's temperature in health is always the same. To secure this identity of temperature some very delicately balanced mechanism is required. Without the most nicely adjusted equilibrium of heat production and dissemination human tissues would soon freeze up at a {282} temperature of 70° below zero, or the albumin of the body fluids and muscular tissue coagulate at a temperature above 110° F. While engaged in the investigation of this interesting problem Claude Bernard found that the cutting of the sympathetic nerves in the neck of a rabbit was followed by increased heat on the side of the head supplied by the nerve, and that this increased heat coincided with heightened sensibility and greater blood-supply in the parts affected. Here was an important factor in heat regulation laid bare. It was evident that the sympathetic nerve trunk supplied filaments to the small arteries, and that when these nerves no longer acted, as after the cutting of the nerve trunk, these arteries were no longer controlled by the nervous system and became dilated. The presence of more blood than usual in the tissues and its slower flow gave occasion to more chemical changes in the part than before, and consequently to the production of more heat.