306 PHYSIOLOGY.

for other beings different from man, which are in immense num- |bers on the globe, but which like him and the nobler animals, arenot formed to breathe the empyreal air, must notwithstanding beof some important and essential use to all living bodies. It has ac-cordingly been found by experiment, that pure and unmixedoxygen gas cannot be breathed for any very considerable timewithout danger; that some azote is contained in the blood, and hasbeen extracted from the muscular fibre, when properly treatedwith the nitric acid. According to Berlhollet, five of its parts withone of hydrogen form ammonia or volatile alkali ; which dispelsthe glandular tumours of the body, and prevents the coagulationof blood, and the thickening of mucus which arise from acids.The azotic gas may therefore in part unite with hydrogen, mayprevent the coagulation of serum, the catarrhous formation of vis-cid mucus, and many combinations that oxygen might lorm, in-jurious to the system. But all living bodies’ are not supported bythe same kind of aerial food. Oxygen gas has indeed been ho-noured with the flattering appellation ot vital air; and nitrogengas been usually distinguished by that degrading e.pithet azotic ; aword which signifies destructive of life. I?ut though man, and allthe warm-blooded animals that have yet been examined, may diein respiring the nitrogen gas, this gas however, which constitutesmore than two-thirds of the whole atmosphere, may in general hecalled the vital air of the vegetable tribes, and of not a few of theorders of insects which thrive and live in it. For while man, andothers which respire as he does, emit both the hydrogen and car-bon, and return the nitrogen not sensibly diminished ; most vege-tables and many insects eagerly inhale them, and emit oxygen asnoxious or useless. These effects are the indications of a radicaldifference in constitution. Even the fibres of those living bodieswhich exhale oxygen, will, after death, attract it so powerfully,as to decompose the nitric acid ; but those bodies which inhale ni-trogen, have so very weak an affinity to oxygen, and so strong a oneto some of the bodies with which it is combined, that they caneasily decompose water and carbonated air. What fishes respireis not ascertained. Neither the change of the air, nor of the waterwhich they occasion when in close vessc is, have, so far as we know,been fully examined. Chaptal is assured, that, like other animals,they are sensible of the action of all gases. Fourcroy says, thatthey do not generate the carbonic acid, and that the air whichPriestley and he found in the air-vesicles of carp was nitrogen gas.We have now to inquire, what are the kinds of respiratory organs,and in what manner their functions are performed ? The preced-ing table has in some measure made us acquainted with this sub-ject. Some animals breathe by a trachea and lungs ; insects, byeither stigmata, or tracheae, opening into air-vessels ; plants, by air-vessels and leaves ; fishes, and numbers of the watery element, ifthey do not breathe, at least receive air, by their gills: the foetusin ovo, the polypus tribe, and many more organized bodies, bythe same organs which convey their food. The absorbents appearto be the first and most general way by which living bodies aresupplied with air : the mouths of these vessels are like small tuber-cles ; scattered over the body of the insect while wrapt in its mem-brane. In the horse and the bird they are blood-vessels spreadingon a membrane, and deriving nourishment from the uterus or egg,that had been itself nourished by absorbents. In a cow, they arevessels which, spreading on a membrane, terminate in glands;these glands being opposite to others which adhere to the uterus;and the membranous and uterine glands, when in contact, inclos-ing a third gland like a kernel. In man, they are vessels spread-ing on a membrane, and entering a large glandular body calledthe placenta. In the mouse and the hare, they are likewise ves-sels branching on a membrane and entering a placenta ; this pla-centa, when fixed, receives large veins from the parent, and whichmay be either inflated or injected from the cavity of the uterus.What are properiv respiratory organs exercise not their functiontill circulation and nutrition are begun. Not only are the respi-ratory organs thus late in exercising their functions ; in many ve-getables a great part of them is annually renewed, and laid asidein the torpid state. In those insects which undergo the most re-markable kinds of transformation they suffer a change ; and in allthose animals which spend their earlier days in the water, and after-wards come to live in the air, they are altered in kind. In allliving bodies the proper function of one part of the respiratory or-gans is, to secrete from the water or air tiiat particular aeriform

fluid which mingles with their juices, and which is necessary tolife and nutrition. In many places these 1 organs are placed exter-nally, and are always in contact with the air or water from whicBthey secrete. In other cases they are lodged internally; and airor water are then alternately admitted and expelled by varieties oforgans which serve .as auxiliaries. Vegetables secrete their aeri-form fluid from water and air. They receive air along with theliquids of their absorbents, which open on the roots, the trunk, andthe branches, and upon the inferior surfaces of leaves ; or, if na-ture has plunged these leaves under water, the absorbents open andimbibe their fluids on both sides. In many, however, the uppersurface of the leaf is intended to inhale air. As it is proved byIngenhousz and others, that the respiration of many leaves is as-sisted by light, we see a reason why plants growing in a dark roomturn to the place where light is admitted ; why the flowers and theleaves of many plants follow the diurnal course of the sun ; whythe brandies of trees, which require much light, die when placedin a thick shade ; why moonshine in autumn contributes so muchto the ripening of grain ; and why leaves and branches are arrang-ed in such a maimer as least to intercept that quantity of lightwhich nature has allotted to the genus of each. The air-vessels inthe body of plants are those vessels which contain juices but atcertain times, and which during the greatest part of the season arefilled with air. This air is collected irom the sap of the roots as itpasses along the diametral insertions, and from those vessels whichopen upon the trunk and upon the leaves. Like pulmonarytubes, which are seen branching through the bodies of insects,they perform an office similar to tiiat of the trachea and bronchia ;and are those general receptacles of air from which the neighbour-ing parts of the plant secrete what is needed.

Ok Digestion.

The function of digestion succeeds respiration by cither con-tinuing or supporting the growth of the living body. It dependson respiration for a portion of heat, and is that function by whichthe liquid and solid food undergoes its first preparation in the sys-tem. Though gaseous fluids, including the principles of heat andlight, may nourish and compose the substances of all living bodies,yet a part only can enter the system in a gaseous stale. This partis changed by the lungs, or by those fluids, which they contain.The organs of digestion, before they can act on aerial bodies, musthave them reduced to some new form. For the food of vegeta-bles, this form requires to be water, whose 100 parts consist of 844of oxygen and 15£ of hydrogen. See Water. When the gaseshave passed through both the watery and vegetable states, they, asjuices or solids, become the food of many animals. These ani-mals produce new changes, and by their preparation the gases be-come the food of others which are called carnivorous; and thenthe carnivorous and all living bodies, when the vivifying principlehas ceased within them, and when they are hastening to a state ofdissolution, are devoured by others who feed on corruption, arepartly converted into water and gas, and become in their turn thefood of the kinds on which they had fed. It has long been ob-served, that those animals which are not carnivorous feed uponplants; and, since the days of Van Helmont and Boyle, it hasbeen suspected that plants live upon water and air. This suspi-cion has now been confirmed by numerous experiments. Plantshave been raised from distilled water without earth ; and, insteadof requiring a vegetable mould, have spread their roots in moss, inpaper, in cotton, in pieces of cloth, in pounded glass, and powderof quartz. From these facts, the ingenious Chaptal supposes thatsoils act, but as so many sponges, affording water in different pro-portions, and in different ways, and that all that the plant wantsfrom the soil is a firm support, a permission to extend its rootswhere it chooses, and that proportioned supply of humidity whichwill secure it against the alternatives of being inundated or driedup. The late Dr. John Brown was of the same opinion. To an-swer, how'ever, these several condition.; M. Chaptal says it is ne-cessary in many cases "to make a proper mixture of the primitiveearths, as no one in particular possesses them.” On these ac-counts a single earth cannot constitute manure, and the characterof the earth intended to he meliorated ought to be studied beforethe choice of any addition is decided on. The best proportionsof a fertile earth for corn are three-eighths of clay, two-eighths ofsand, and three of the fragment:, of hard stone, “ The advantages

of