HIBERNATION AND ALLIED STATES IN ANIMALS 107 
natural consequence, stamped themselves on living 
things, plants as well as animals. Sleep, then, is essen- 
tially a rhythmic diminution of the activities of all the 
tissues, but especially of that one which controls all 
others, the nervous. Rhythm seems to be at the basis 
of all things organic and inorganic, but has not been 
enough considered in our explanation of living cells. 
It was long the custom to explain sleep by anemia of 
the brain, whereas the very anemia was due to a con- 
traction of the blood-vessels of the part, accompanied 
by the diminution of the heart-beat, both of which are 
periodical and dependent on the rhythm of the nervous 
system itself. Of course, temporary anemia of the 
brain favours sleep, though it is not the essential 
cause. As a natural consequence of the decline in the 
activity of the great controller of the cell activities 
(metabolism), z.e. the nervous centres, it is found that 
all the functions of the body, without exception perhaps, 
are diminished during sleep. Marshall Hall and others 
since his time have shown that the gaseous interchange 
in a hibernating animal is greatly lessened. This 
diminished metabolism explains why the animal does 
not require to eat, or but little. It explains the dimin- 
ished excretions, ete.; ete. 
This being understood, it is not surprising that 
hibernating animals may be kept under water for long 
periods as is the case with newly-born mammals, as 
kittens and puppies, whose vital machinery as yet 
works very slowly, which are, in fact, in a condition but 
slightly more advanced physiologically than the uterine, 
which is a sort of reptilian pre-natal state, as regards 
the circulation, respiration, etc. 
As the metabolism of reptiles and amphibians is of a 
much slower kind than that of mammals, it is not 
surprising that their winter sleep is more profound ; 
but it is to be observed that the change from their 
