sitive to atmospheric changes, more particularly as regards moisture and dryness. It is found that in a dry atmosphere the capillary circulation is quickened, and in a moist atmosphere retarded. The influence of heat and cold probably extends to the circulation and the nutritive functions.

Feelings of Respiration.

11. 'Respiration is that function by which an interchange of gases takes place between the interior of an organized being and the external medium; and, in the animal kingdom, oxygen is the gas received, and carbonic acid the gas given out.' The aeration of the animal fluids or juices is an essentia of their vitality; if this is put an end to, death ensues instantaneously; if insufficiently performed, the vigour of the animal is lowered, and a peculiar painful sensation experienced. In man and in air-breathing animals, there is a wind-apparatus, the lungs, inflated and contracted by muscles, so as to suck in and force out the air by turns.

In this action we have all the particulars necessary to constitute a Sense; an external object-the air of the atmosphere-which operates by physical contact upon the lining membrane of the tubes and cells of the lungs; an organ of sense, and a resulting state of feeling, or consciousness. The peculiarity of the case lies in its being almost entirely an emotional sense; generating feeling rather than yielding knowledge, or providing forms for the intellect; ranking, therefore, among the lower, and not among the higher, senses.

As respects the object of this sense, the external air, it need only be remarked, that the air differs considerably in its quality for breathing purposes, the chief point of difference being expressed by the term 'purity.' The purity is affected first by the loss of oxygen, which happens when the same air is repeatedly breathed, or otherwise consumed; secondly, by the accumulation of carbonic acid, from the same circumstance; and, thirdly, by the presence of foreign gases and effluvia arising from animal life, vegetation, or other causes. Closeness or confinement is the chief aggra

vation of all those impurities. Of the three evils-the loss of oxygen, the accumulation of carbonic acid, and the generation of effluvia of animal and other substances-the second is the least injurious; for, although the production of a carbonic acid atmosphere, by burning charcoal in a close room, is fatal to life, yet the quantity ordinarily occurring in rooms is not found to do any harm, if mixed with air otherwise pure. The loss of oxygen, and the diffusion of the gases of decay, are the main iufluences that deteriorate the atmosphere.

Of the organ acted upon, the lungs, a minute description is not necessary for our present purpose. The structure is so arranged by ramifications and doublings as to present a very extensive surface to the air; the surface consisting of a fine membrane, with capillary blood-vessels, thickly distributed on its inner surface. The exchange of gases takes place through the double medium of membrane and capillary tube. The muscular apparatus for sustaining the bellows-action, is the diaphragm and abdominal muscles, and the muscles of the chest or ribs. The integrity and vigour of these muscles, and of the centres that sustain and time their action, must be reckoned as a condition of healthy respiration.

The respiratory nerve centres are stimulated from all parts of the body, but chiefly from those that, like the muscies, are large consumers of oxygen. The portion of the eighth pair of nerves named the nervus vagus, is iustrumental in keeping up the rhythm of the lungs, and is also necessary to the feeling of suffocation.

The feelings of Respiration, both pleasurable and painful, are well marked. They include the gratification from pure air, enhanced by the increased action due to muscular exercise; the various shades of oppression from over-crowded rooms and unwholesome gases; the distressing experience of suffocation, or want of breath; and the pains attendant on disease of the lungs.

12. The influence of pure and stimulating air abundantly inhaled, spreads far and wide over the system, elevating all

the other functions by the improved quality imparted to the blood. The indirect consequences do not altogether hide the grateful sensibility arising from the lungs themselves, and referred by us to the region of the chest; a sensation not very acute or prominent, but possessing that choice and well known quality, expressed by the term 'freshness,' or 'refreshing.' This quality manifestly implies a contrast; for it is felt only when we pass from a lower to a higher degree of aeration. We may experience it at any time, by holding in the breath for a little, and then allowing it full play. No technical nomenclature can increase the conception possessed by every one of this remarkable sensibility; but for the sake of comparison with the other parts of our mental constitution, an attempt at verbal description is necessary. As just remarked, the sensation turns upon the contrast of the greater activity of the lungs with an immediately preceding activity of an inferior degree. It may be affirmed that no feeling arises from the lungs, after a given pace has been established for a length of time; but any acceleration of the rate of exchange of the two gases (by no means depending altogether on the rate of breathing) does for a time yield that delightful freshening sensation, which tells so immediately on the mental system as a contribution to our enjoyment, and as a stimulus to our activity and to our desire for rural recreation and bodily exercise.

The

13. The feelings of insufficient and impure air are manifested in the forms of faintness, sense of exhaustion and weariness, and are doubtless due, not to the lung-sense alone, but to the lowered condition of the body at large. characteristic sensibility of the lungs is shown in the state termed suffocation, arising from the want of air, as in drowning, in an atmosphere deteriorated by poisonous gases-such as chlorine or sulphurous acid, in attacks of asthma, and in voluntarily holding in the breath. After holding the breath for fifteen or twenty seconds during ordinary respiration, or forty seconds after a deep respiration, there arises an insupportable sensation over the whole chest, concentrated under

the sternum, and no effort can maintain the interruption of the respiratory acts. This urgent sensation of want of breath, when carried to its full extent by any mechanical impediment to the aeration of the blood, is one of the most painful and oppressive kind, and is referable to the pulmonary plexuses (of nerves) distributed to the bronchia, and perhaps on the walls of the lobular passages and cells. The impression made on these peripheral nerves by the absence of oxygen, and the undue presence of carbonic acid in the air in contact with them, is propagated to the spinal cord and medulla oblongata by the sympathetic and vagus, and there excites those combined actions of the muscles of inspiration which lead to the renewal of the air.'-(TODD and BOWMAN, II., 403.) The sensation is of the class 'racking pains,' and may be, in part, muscular.

Feclings of Heat and Cold.

14. The description of these important feelings comes properly under Organic Sensations, in so far as change of temperature affects all the organs of the body. Warmth, while abating the activity of the organic processes generally, induces in the skin a richer circulation, and a greater activity in the sweat, and in the oil glands. The various parts of the cuticle, the nails and the hair, are more abundantly produced. The sensory powers of the organ are greater, and the texture is softer and more polished.

Inasmuch as cold (not in excess) increases the activity of the muscles, the nerves, the respiration, and the digestion, the animal powers attain their maximum in cold climates, and in the winter season, allowance being made for constitutions unfitted to endure extreme depression of temperature.

Sudden changes of temperature derange the functions. A sudden increase will cause a slight feeling of suffocation, beating of the heart, and increased pulsation and respiration. A sudden chill makes breathing difficult, quick, and irregular, and increases the pulsations. The nerves lose their excit

SENSATION OF COLD.

121

ability both under a great depression, and under a great increase of temperature.

The feelings of heat and cold are very notable. Let us commence with Cold. The outward cause of this feeling is some influence tending to lower the temperature of the body. The natural heat of the blood is about 98°, and any contact below this point feels cold; any contact above it feels warm. There is a certain surplus heat generated in the human system, which enables us to live in a medium below 98', without feeling cold; and if this heat be husbanded by clothing, a very great depression of external temperature may be endured. A room is warm at 60°. The outer air can be en lured at freezing and far below, either by means of exercise, which evolves heat, or of clothing, which retains it.

An acute cold acts like a cut or a bruise, injuring the part affected, and causing painful sensations of the class arising from violent local injuries. The temperature of freezing mercury would destroy the skin like boiling water or a sharp

cut.

The proper sensation of Cold arises from a general cooling of the body, or any considerable part of it, below blood heat. The term chillness' expresses the state of feeling, which is of the painful class. The degree is not acute but massive. In the worst forms, it is wretchedness in the extreme. To a person suffering from excessive chillness, some powerful stimulant, such as the taking of food, alcohol, or tobacco, is necessary to restore equanimity. The volition and the memory are proportionally impressed by the pains of cold, aud they take a high rank in the reckonings of forethought and prudence.

It is a singular fact in our constitution, that an agency calculated to quicken the vitality of so many leading organs -muscles, nerves, lungs, stomach-should affect us so powerfully, by the depression of one organ. The fact is highly illustrative of the importance of the skin, whether from its organic functions or from its sensibility. Probably both circumstances enter into the case. It may be that the