In an investigation of this sort, it was natural to attempt to trace this singularity of habit in torpid animals to some peculiar conformation in the structure of the organs. Ac cordingly we find many anatomists assigning a peculiarity of organization as a yeason why these animals become tor pid, or at least pointing out a structure in torpid animals different from that which is observable in animals that are not subject to this bruma) lethargy.
Pallas observed the thymus gland unusually large in torpid quadrupeds, and also perceived two glandular bo dies under the throat aid upper part of the thorax, which appear particularly /loll() and vascular during their tor pidity.
Mangili is of opinion that the veins are larger in size, in proportion to the arteries, in those animals which be• conic torpid, than in others. lie supposes that, in conse quence of this arrangement, there is only as much blood transmitted to the brain during summer as is necessary to excite that organ to action. In winter, when the circula tion is slow, the small quantity of blood transmitted to the brain is inadequate to produce the effect. This circum stance, acting along with a reduced temperature and an empty stomach, he considers as the cause of torpidity. By analogy he infers, that the same cause operates in produc ing torpidity with all the other hybernating animals of the other classes.
Mr Carlisle, in his Croonian Lecture on Muscular Mo tion, asserts, that " animals of the class ruammalia, which hybernate and become torpid in the winter, have at all times a power of subsisting under a confined respiration, which would destroy other animals not having this peculiar habit. In all the hybcrnating mammalia there is a peculiar struc ture of the heart and its principal veins: the superior cava divides into two trunks, the left passing over the left auri cle of the heart into the inferior part of the tight auricle, near to the entrance of the vena cava inferior. The veins usually called azygos accumulate into two trunks, which open into the branch of the vena cava superior, on its own side of the thorax. The intercostal arteries and veins in these animals are unusually large." Phil. Trans. 1805.
We cannot refrain from observing, that these general views do not appear to be the result of a patient investiga tion of a number of different kinds of torpid animals, but a premature attempt to theorise from a few insulated parti culars. Passing, therefore, from these attempts of the anatomist to illustrate the phenomena in question, let us at tend to those other causes which are concerned in the pro duction or tot pidity.
From the consideration, that this state of torpidity com mences with the cold of winter, and terminates with the heat of spring, naturalists in general have been disposed to consider a reduced temperature as one of the principal causes of this lethargy. Nor are circumstances wanting to
give ample support to the conclusion.
When the temperature of the atmosphere is reduced, as we have already seen, below 50°, and towards the freezing point, these animals occupy their torpid posi tion, and by degrees relapse into their winter slumbers. 'When in this situation, an increase 'of temperature, the action of the sun, or a fire, rouse them to their former activity. This experiment may he repeated several times, and with the same result, and demonstrates the great share which a diminished temperature has in the pro duction of torpidity. if marmots are frequently disturbed in this manner during their lethargy, they die violently agitated, and a hemorrhage takes place from the mouth and nostrils.
The circumstance of torpid animals being chiefly found in the colder regions is another proof, that a diminished temperature promotes torpidity. And, in confirmation of this, Dr Barton informs us, that, in the United States of America, many species of animals, which become torpid in Pennsylvania, and other more northern parts of the coun try,do not become torpid in the Carolinas, and other south ern parts of the continent.
But while a certain degree of cold is productive of this lethargy, a greater reduction of temperature produces re viviscence as speedily as an increase of heat. Mangili placed a torpid marmot, which had been kept in a ture of 45°, in a jar surrounded with ice and muriate of lime, so that the thermometer sunk to 16'. In about half an hour a quickened respiration indicated returning anirna tion. in sixteen hours it was completely re' IVCd. It was trembling with cold, and made many to escape. tic also placed a torpid hat under a bell glass, where the tem perature was 29•, and where it had free air. Respiration soon became painful, and it attempted to escape. It then folded its wings, and its head shook With convulsive trem blings. In an hour no other motions were perceptible than those of respiration, which increased in strength and frequency until the fifth hour. From this period, the signs of respiration became less distinct ; and, by the sixth hour, the animal was found dead. He also exposed a torpid dor mouse (from a temperature of to a cold of 27", pro duced by a freezing mixture. Respiration increased from ten to thirty-two times in a minute, and without any inter vals of repose. There were no symptoms of uneasiness, and the respirations seemed like those in natural sleep. As the temperature rose, respiration became slower. He then placed it in the sun, when it awoke. Two hours afterwards. having exposed it to the wind, respiration became frequent and painful; it turned its back to the current, without, how ever, becoming torpid.