celeration of the circulation. The quantity of air consumed varies, therefore, with the proportion of the blood which is sent to the lungs.

256. The proper temperature of an animal, or what is termed ANIMAL HEAT, depends on the combined activity of the respiratory and circulating systems, and is in direct proportion to it. In many animals the heat is maintained at a uniform temperature, whatever may be the variations of the surrounding medium. Thus, birds maintain a temperature of about 108° Fahrenheit; and in a large proportion of mamnals it is generally from 95° to 105°. These bear the general designation of warm-blooded animals.

257. Reptiles, fishes, and most of the still lower animals, have not this power of maintaining a uniform temperature. The heat of their body is always as low as from 35° to 50°, but varies perceptibly with the surrounding medium, being often, however, a little above it when the external temperature is very low, though some may be frozen without the loss of life. For this reason, they are denominated cold-blooded animals; and all animals which have such a structure of the heart that only a part of the blood which enters it is sent to the respiratory organs, are among them, (243.)

258. The production of animal heat is obviously connected with the respiratory process. The oxygen of the respired air is diminished, and carbonic acid takes its place. The carbonic acid is formed in the body by the combination of the oxygen of the air with the carbon of the blood. The chemical combination attending this function is, therefore, essentially the same as that of combustion. It is thus easy to understand how the natural heat of an animal is greater, in proportion as respiration is more active. How far nutrition in general, and more particularly assimilation, by which the liquid parts are fixed and solidif.ed, is connected with the maintenance of the proper temperature of animals, and the

uniform distribution of heat through the body, has not yet been satisfactorily ascertained.

259. Some of the higher warm-blooded animals do not maintain their elevated temperature during the whole year; but

pass the winter in a sort of lethargy called HIBERNATION, or the hibernating sleep. The marmot, the bear, the bat, the crocodile, and most reptiles, furnish examples. During this state the animal takes no food; and as it respires only after very prolonged intervals, its heat is diminished, and its vital functions generally are much reduced. The structural cause of hibernation is not ascertained; but the phenomena attending it fully illustrate the laws already stated, (254-8.)

260. There is another point of view in which respiration should be considered, namely, with reference to the buoyancy of animals, or their power of rising in the atmosphere, and their ability to live at different depths in the water, under a diminished or increased pressure. The organs of res piration of birds and insects are remarkably adapted for the purpose of admitting at will a greater quantity of air into their body, the birds being provided with large pouches extending from the lungs into the abdominal cavity and into the bones of the wing. In insects the whole body is penetrated by air tubes, the ramifications of their trachea, which are enlarged at intervals into wider cells; whilst most of the aquatic animals are provided with minute, almost microscopic tubes, penetrating from the surface into the substance, or the cavities of the body, admitting water into the interior, by which they thus adapt their whole system to pressures which would otherwise crush them. These tubes may with propriety be called water-tubes. In fishes, they penetrate through the bones of the head and shoulder, through skin and scales, and communicate with the blood vessels and heart, into which they pour water; in mollusks they are more numerous in the fleshy parts, as, for example, in the

foot, which they help to distend, and communicate with the main cavity of the body, supplying it also with liquid, in echinoderms they pass through the skin, and even through

260 a. In order fully o appreciate the homologies between the various respiratory apparatus observed in different animals, it is necessary to resort to a strict comparison of the fundamental connections of these organs with the whole system of organization, rather than to the consideration of their special adaptation to the elements in which they live. In Vertebrates, for instance, there are two sets of distinct respiratory organs, more or less developed at different periods of life, or in different groups. All Vertebrates, at first, have gills arising from the sides of the head, and directly supplied with blood from the heart; but these gills are the essential organs of respiration only in fishes and some reptiles, and gradually disappear in the higher reptiles, as well as in birds and Mammalia, towards the close of their embryonic growth. Again, all Vertebrates have lungs, opening in or near the head; but the lungs are fully developed only in Mammalia, birds, and the higher reptiles, in proportion as the branchial respiration is reduced; whilst in fishes the airbladder constitutes a rudimentary lung.

260 b. In Articulates, there are also two sorts of respiratory organs; aerial, called tracheæ in insects, and lungs in spiders; and aquatic, in crustacea and worms, called gills. But these trachea and lungs open separately upon the two sides of the body, (air never being admitted through the mouth or nostrils in Articulates ;) the gills are placed in pairs; those which are like the trachea occupying a similar position, so that there are nearly as many pairs of trachea and gills as there are segments in these animals, (Figs. 89 and 33.) The different respiratory organs in Articulates are in reality mere modifications of the same apparatus, as their mode of formation and successive metamorphoses distinctly show, and cannot be compared with either the lungs or gills of Vertebrates; they are special organs not found in other classes, though they perform the same functions. The same may be said of the gills and lungs of mollusks, which are essentially alike in structure, the lungs of snails and slugs being only a modification of the gills of aquatic mollusks; but these two kinds of organs differ again in their structure and relations from the trachea and gills of Articulates, as much as from the lungs and gills

the hard shell, whilst in polyps they perforate the walls of the general cavity of the body, which they constantly fill with water. O

of Vertebrates. In those Radiates which are provided with distinct respiratory organs, such as the Echinoderms, we find still another typical structure, their gills forming bunches of fringes around the mouth, or rows of minute vesicles along the radiating segments of the body.

CHAPTER NINTH.

OF THE SECRETIONS.

261. WHILE, by the process of digestion, a homogeneous fluid is prepared from the food, and supplies new material to the blood, another process is also going on, by which the blood is analyzed, as it were; some of its constituents being selected and so combined as to form products for useful purposes, while other portions of it which have become useless or injurious to the system are taken up by different organs, and expelled in different forms. This process is termed SECRETION.

262. The organs by which these operations are performed are much varied, consisting either of flat surfaces or membranes, of minute simple sacs, or of delicate elongated tubes, all lined with minute cells, called epithelium cells, which latter are the real agents in the process. Every surface of the body is covered by them, and they either discharge their products directly upon the surface, as on the mucous membrane, or they unite in clusters and empty into a common duct, and discharge by a single orifice, as is the case with some of the intestinal glands, and of those from which the perspiration issues upon the skin, (Fig. 94.)