The _circulation of the blood_, on the other hand, continues without interruption, though its rapidity is greatly retarded. This fact may be observed by microscopic examination of the wings of bats in a state of winter sleep. Moreover, in the case of a hedgehog lethargic from hibernation, it was experimentally shown that when the spinal cord was severed behind the occipital foramen, the brain removed and the entire spinal cord gently destroyed, the heart continued to beat strongly and regularly for several hours, the contraction of the auricles and ventricles being quite perceptible, though feeble, even after the lapse of ten hours. After eleven hours the organ was motionless; but resumed its activity when stimulated by a knife-point. Even after twelve hours both auricles responded to the same stimulus, though the ventricles remained motionless. Shortly afterwards the auricles gave no response. On the other hand, when the spinal cord of a hedgehog in a normal state of activity was severed at the occiput, the left ventricle ceased to beat almost at once, and the left auricle in less than fifteen minutes; the right auricle was the next to cease, whereas the right ventricle continued its contraction for about two hours. Experiments upon marmots have yielded very similar results. The heart of a marmot decapitated in a state of lethargy continued to beat for over three hours. The pulsations, at first strong and frequent and varying from 16 to 18 per minute, became gradually weaker and less frequent, until at the end of the third hour only 3 were recorded in the same length of time. Excised pieces of voluntary muscular tissue contracted vigorously three hours after death under electric stimulus. Only at the end of four hours did they cease to respond. The heart of an active marmot killed in the same way contracted about 28 times a minute at first, the number of pulsations falling to about 12 at the end of fifteen minutes, to 8 at the end of thirty minutes, and ceasing altogether at the end of fifty minutes. Similarly the response of the muscles to galvanic shock failed at a correspondingly rapid rate. It is evident, therefore, that during hibernation the irritability of the heart is augmented in a marked degree, and that the irritability of the left side of the organ is scarcely less pronounced than that of the right side. Similar reduction in the rate of the circulation has been demonstrated in certain hibernating mollusca, Mr C. Ashford having proved experimentally that the number of pulsations of the heart per minute gradually lessens with a falling temperature. At a temperature of 52 deg. F. the number was 22 in the common garden snail (_Helix hortensis_), and 21 in the cellar slug (_Hyalinia cellaria_). At a temperature of 30 deg. F. the pulsation fell to 4 in the former and to 3 in the latter animal.