cyanide of kakodyle, described in a previous chapter (p. 532). But all the disinfectants described and recommended in the preceding pages, are really poison-destructive as regards all natural evil smells and miasms with which we are yet acquainted.

NOTES TO ODOURS AND SMELLS.

1o. Chap. XXV.-In my concluding remarks upon sweet odours (p. 502), I have drawn the attention of the reader to the inconceivably minute quantities of odoriferous matter which make themselves sensible in the air. I have since found, in Dr. CARPENTER'S Comparative Physiology, that "a grain of musk has been kept freely exposed to the air of a room, of which the door and windows were constantly open, for a period of ten years; during all which time the air, though constantly changed, was completely impregnated with the odour of musk; and yet at the end of that time the particle was found not to have sensibly diminished in weight! Can anything illustrate more strikingly the very trifling quantities of foreign matter in the air by which sensible effects, whether for good or for evil, may be produced upon us?

20. Chap. XXVII.-Since this chapter was published, it has been announced that the substance propylamine, mentioned in p. 517 (note), as having the smell of stinking fish, has been found in the flowers of Cratagus oxyacantha (common hawthorn), Cratagus monogyna; also in those of Pyrus communis (the peartree), and Sorbus aucuparia. The odour of these flowers has often been thought to resemble that of decaying fish.

3o. In page 531 I have said that other Kakodyles might be formed besides the one there described; and this has in fact already been discovered-the combination of arsenic with Ethyle. Like the compound already known, it has a peculiarly insufferable smell, and takes fire in the air. It offers us another material for asphyxiating shells.

CHAPTER XXIX.

WHAT WE BREATHE AND BREATHE FOR.

What is it to breathe ?-Structure of the lungs.-Quantity of air inhaled.-Breathing by the skin.-Structure of the skin.-Effect of breathing on the composition of the air. It increases the proportion of moisture and carbonic acid, and diminishes that of oxygen.-To what extent it does so.-Quantity of carbonic acid given off from the lungs and the skin.-Purpose for which man breathes.-The oxygen absorbed helps to form the substance of the muscular and other tissues.-It converts the waste material of the body into urea and other soluble substances preparatory to its removal.-It converts the fat and starch of the food into carbonic acid and water.-Acts in a similar way upon alcohol.-Why the carbonic acid from the lungs varies in quantity.-Physiological effect of these chemical changes.-They are the chief source of animal heat.-Minor sources of this heat.-Careful provision for the constant disengagement of this heat.-Purposes served in external nature by the breathing of animals.

I. WHAT IS IT TO BREATHE?

1o. To breathe, in the usual acceptation of the term, is to draw in atmospheric air through the mouth and nose into the lungs, and after a brief interval to throw it out again.

The lungs into which the air is thus drawn, consist of two rounded oblong, somewhat flattened, masses of very cellular substance, situated in the cavity of the chest, and communicating with the atmosphere through the wind-pipe, or trachea. The general form of the human lung is represented in the annexed figure.

STRUCTURE OF THE LUNGS.

Fig. 98.

559

The air or wind pipe (a b fig. 98), as it descends from the throat, branches off into large (bronchial) tubes (cc); and these again and again into smaller, still smaller, and finally into hairlike vessels (d). Through these the air penetrates into the remot est parts of the cellular substance. Around each visible extremity nearly eighteen thousand cells are clustered (17,790, ROUCHOUX), each of which is connected through these minute tubes with the external air. The cells vary in size; they have a diameter of from one-seventieth to one two-hundredth, or, ou an average, of about one-hundredth of an inch. The total number of them is reckoned at six hundred millions! Their

Inman Lung.

a the larynx; b windpipe; ccc bronchial tubes or alt passages; e lung

walls are very thin; they are mere air-vesicles.

The lungs, as this structure implies, are very elastic, and consequently the volume of air they contain very varia ble. The average quantity which, by an effort, the lungs of au adult can be made to inhale, is from five to seven pints; and the quantity they draw in at an ordinary, natural, but full inspiration, may be as much as two pints and a half; an ordinary tranquil respiration, made without effort, takes in only about one pint.

At the easy average of eighteen inspirations a minute, this makes the bulk of air drawn in and thrown out again to amount—in common life-to about eighteen pints

a minute, a thousand pints an hour, or three thousand gallons a-day. Some estimate it as high as four thousand gal

Fig. 99.

Vertical section of the skin, greatly magnified,

a the cuticle, outer, or scarf skin; b d the true skin; c censory papillæ; e sweat glands and

their ducts, the outlets at the surface being the

pores; fhairs; g cellular substance.

lons a-day for an average man in average circumstances, and as high as five thousand seven hundred gallons a-day for an athletic man undergoing severe exer

tion.

20. But this lungwork forms only part of the operation of breathing; we breathe also through the skin. The cuticle or outer skin of most animals is perforated by numerous pores (fig. 99). These pores are the outlets of minute spiral vessels which penetrate through the skin into the cellular substance beneath (fig. 100, g). In the human cuticle, the pores are more numerous in some parts of the body than in others, but the outer skin of a full-grown man is sprinkled over with about seven

millions of them, while the united length of the spiral