Page:Popular Science Monthly Volume 77.djvu/369

Rh organs are intermittently active. In the black bear, however, digestion is completely suspended and his intestines become plugged up with an indigestible mass composed chiefly of pine leaves. This mass is not discharged until the bear wakes in the spring.

It may be well to note here the important part played by the liver during hibernation in maintaining the animal's life; it acts as a storehouse, storing up energy in the form of glycogen, often called animal starch—a substance derived principally from starchy or carbohydrate food. This glycogen is converted in the liver into sugar and poured into the circulation, which carries it to the tissues, where it is consumed during hibernation, as well as in the waking state. The presence of glycogen in the liver-cells of the frog and other animals may actually be seen by the aid of the microscope, immediately before hibernation, and its absence, more or less complete, at the end of this period, demonstrated in the same manner. (This is disputed by Weinland and Richl—they claim that the amount of glycogen in the body remains constant during hibernation.

The excitability of the whole nervous system is greatly depressed. Physiologically, it resembles, as do the other tissues, that of coldblooded animals in general, in that all the tissues (muscular, particularly) retain their excitability for a long time after they are removed from the body.

A warm-blooded animal during hibernation loses all control (reflex) over its temperature-regulating mechanism, and acquires all the characteristics of a cold-blooded organism, that is, instead of its body having a regular, normal and steady temperature, its temperature becomes about the same as the surrounding media, and as this rises or falls, so does that of the animal. On the other hand, by arousing a dormant animal from its stupor, it is possible to make it exercise enough to bring its temperature up to normal, i. e., what it would be in its ordinary walking and active circumstances, or in other words to bring it back to warm-blooded conditions again.

On studying the changes in external temperature, we find that the output of carbon dioxide and the temperature vary with the activity of the animal. If the animal is very active, it responds to a fall in external temperature by more muscular activity, and by this way maintains the normal heat of its body. If, however, this animal is in a sleeping condition and there is a sudden fall in temperature, it somewhat arouses at first, becomes active, and this causes an increase in the output of carbon dioxide, but after a few minutes it coils itself up again and returns to its former somnolent condition, and from which it is not so readily aroused. It has also been observed that when the surrounding temperature has been raised, the temperature of the