HIBERNATION 363 
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 store- 
house, 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, demon- 
strated 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. 
Nervous SysTEM 
The excitability of the whole nervous system is greatly depressed. 
Physiologically, it resembles, as do the other tissues, that of cold- 
blooded animals in general, in that all the tissues (muscular, particu- 
larly) retain their excitability for a long time after they are removed 
from the body. 
TEMPERATURE 
A warm-blooded animal during hibernation loses all control (re- 
flex) 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 be- 
comes 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, 7. 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 main- 
tains the normal heat of its body. If, however, this animal is in a 
sleeping condition and there is a sudden fall in temperature, it some- 
what 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 
