Exhaust the brain by over-exercise of the muscles, or by fatigue, the brain requiring to be replenished with force or vital fluid; nervous congestion or ordinary sleep is induced, because the senses cannot any longer be supplied.

If the brain did not discharge at the direction of the will the nerve fluid, how could the muscles be moved?

There are two kinds of nervous congestion, as already stated, one from an excess, the other from a diminution of the natural supply. Natural sleep is an intermediate state; it is now that the functions of the lungs are especially called into requisition, in order to renew the supply to the brain of electricity vitalized, so as to subserve the purposes of volition, and rendering the body sentient to outward impressions. Or are there different fluids, one of motion, and another of sensation, and another adapted for organic life? It is more probable that the same fluid, by passing into special channels, is either motor, sensitive, or ganglionic. Or is there a special vibratory state? An imperfect supply is accompanied by a corresponding derangement of the function of the organ.

The natural sleep of the infant or of a person who has not exhausted the brain by fatigue, is marked by a gentle respiratory murmur, whereas the respiratory action of the man who has laboured all day, is much more active; the respiratory murmur is deep and sonorous.

The nervous matter of the brain has a variety of very distinct offices to perform. The senses have each their locality and nerves, which communicate with their outward organs. The olfactory nerves have a special portion of the brain, which takes cognizance of the various kinds of odour; the same with the eye and the ear. One of these may be destroyed without necessarily affecting the other. The power to recognise sounds may be wanting, still the organs of sight and smell remain perfect.

As we ascend to higher functions, these are found to be developed in different persons, so that one may be comparatively idiotic in one faculty, and in another possessed of remarkable mental capacities. What animal surpasses the eagle in visual powers? Does not the hearing of the antelope or the smelling of herbivorous animals surpass those senses which in man are comparatively feeble?

Each animal has a development of nervous matter in accordance with the habits and peculiarities of its existence. No one can confound the broad heads of the carnivora with the long, narrow heads of the graminivorous; so it is with their brains. To give the brain matter of the giraffe to the tiger would be an absurdity. The powerful jaws and claws of a tiger require a corresponding brain function. In fine, cerebral physiology demands a special study. No one who has devoted the least attention to the subject but must admit that the brain is a congeries of organs, each adapted to a distinct office.

In hybernating animals, during the state of torpor the lungs act sluggishly, supplying merely sufficient nervous force to sustain life. This torpid condition during the depth of winter is so great that the animal experiences no sensibility. This is particularly the case with various species of snakes in America. From the Yankee Nation, May, 1842. - "Dr. Collyer dwelt particularly on those animals which in the "dead of winter remain in a torpid state; some of these could be cut to "pieces without showing any signs of life, although their brains "contained the vital element."

Daily Ledger, April 18th, 1842. - "It is by means of the vital force "in the brain and the rest of the nervous system that hybernating "animals are enabled to pass the winter in an insensible state, or to "remain embedded in a tree or a stone for centuries without losing "vitality.

Fish are sometimes frozen so hard that they are broken like a "piece of glass; yet these identical fish, if gradually thawed in water "at from 35° to 40° Fahrenheit, swim about as if they had never been "in an unnatural condition.

In travelling from Hamilton, Canada, on a moonlight night, to the falls of Niagara, in sleigh, in the winter of 1842 and 1843, my companion, who was wrapped in buffalo robes, fell into a deep comatose state from breathing the cold atmosphere. He was rendered so insensible, that it required over an hour's friction before a large fire before consciousness was restored.

Captain Franklin, in describing the winter he passed in the Polar regions, says, "The fish froze as they were taken from the nets, and "became a solid mass of ice, and by the blow of a hatchet were split open, when the intestines were removed in a lump. If in this frozen "state they were thawed before a fire, they recovered animation. He "saw carp recover so far as to leap about with vigour, after having "been frozen for thirty-six hours."

Dr. B. W. Richardson says, "The benumbing influence of extreme cold may be accepted as a natural discovery coeval with the existence of mankind in the temperate and frigid zones. Physicians at an early date seem to have used cold for the relief of pain. Its systematic use probably came in later, after the revival of letters, so-called. It is related of Harvey, the discoverer of the circulation of the blood, that he was accustomed to seek relief from the pain of gout by going on the leads of his house and there immersing his painful foot in ice-cold water. But we have to name another man living in the same age, to see the same remedy employed in a direct manner, in order to remove sensation in a part of the living body before subjecting it to a surgical operation. This was Thomas Bartholinus, one of the most learned and industrious masters in physic. Bartholinus wrote a treatise of 232 pages on the medical use of snow (De Nivis usu Medico). This book, from beginning to end, is wonderfully suggestive, and in the 22nd chapter he advocates the practice of applying extreme cold to produce insensibility before the performance of surgical operations." The plan, he says, was taught him by Marcus Aurelius Severinus, of Naples. In our day the same application of cold has been brought into use by a man of singular originality and genius, Dr. James Arnott.

Dr. Richardson exhibited to the author in 1868 the freezing of the brain of a pigeon by the use of the ether spray; the insensibility was immediate, and the brain frozen in less than a minute. When, however, this same bird was first operated on, it took eight minutes to bring about a state of unconsciousness.

One of the most remarkable circumstances connected with the employment of anaesthetic agents, is the facility with which unconsciousness is produced after subjugation has been once or twice accomplished. The vital powers at first resist powerfully the action of a new state produced by foreign agents. This natural law is essential to the preservation and well-being of the individual. The principle of accommodation as exemplified in the pigeon also applies to man.

The nervous influence from a second person is hardly perceptible on the first, second, or third trial, except with remarkably susceptible persons, who under favourable circumstances are reduced to a state of nervous congestion, or unconsciousness, in a few minutes.