novices in the operation, and who are desirous of perfecting themselves in the practice of the easier methods, before attempting the more costly. By M. Doyère, who first devised this method, it was simply recommended to throw in saturated solutions of the two salts, one after the other; but Dr. Goadby, who has had much experience in the use of it, advises that gelatine should be employed, in the proportion of 2 oz. dissolved in 8 oz. of water, to 8 oz. of the saturated solutions of each salt. This method answers very well for preparations that are to be mounted dry; but for such as are to be preserved in fluid, it is subject to the disadvantage of retaining in the vessels the solution of acetate of potash, which exerts a gradual corrosive action upon them. Dr. Goadby has met this objection, however, by suggesting the substitution of nitrate for acetate of lead; the resulting nitrate of potash having rather a preservative than a corrosive action on the vessels. When it is desired to inject two or more sets of vessels (as the arteries, veins, and gland-ducts) of the same preparation, different coloring substances should be employed. For a white injection, the carbonate of lead (prepared by mixing solutions of acetate of lead and carbonate of soda, and pouring off the supernatant liquid when the precipitate has fallen) is the best material. No blue injections can be much recommended, as they do not reflect light well, so that the vessels filled with them seem almost black; the best is freshly-precipitated Prussian blue (formed by mixing solutions of persulphate of iron and ferrocyanide of potassium), which, to avoid the alteration of its color by the free alkali of the blood, should be triturated with its own weight of oxalic acid and a little water, and the mixture should then be combined with size, in the proportion of 146 grains of the former to 4 oz. of the latter.

435. Injected preparations may be preserved either dry or in fluid. The former method is well suited to sections of many

solid organs, in which the disposition of the vessels does not sustain much alteration by drying; for the colors of the vessels are displayed with greater brilliancy than by any other method, when such slices, after being well dried, are moistened with turpentine and mounted in Canada balsam. But for such an injection as that shown in Fig. 328, in which the form and disposition of the intestinal villi would be completely altered by drying, it is indispensable that the preparation should be mounted in fluid, in a cell deep enough to prevent any pressure on its surface. Either Goadby's solution or weak spirit answers the purpose very well.

436. A well-injected preparation should have its vessels completely filled through every part; the particles of the coloring matter should be so closely compacted together, that they should not be distinguishable unless carefully looked for; and there should be no patches of pale uninjected tissue. Still, although the beauty of a specimen as a microscopic object is much impaired by a deficiency in the filling of its vessels, yet to the anatomist the disposition of the vessels will be as apparent when they are only filled in part, as it is when they are fully distended; and imperfectly injected capillaries are better seen, when thin sections are mounted as transparent objects, than are such as have been completely filled.

437. A relation may generally be traced between the disposition of the Capillary vessels, and the functions they are destined to subserve; but that relation is obviously (so to speak) of a mechanical kind; the arrangement of the vessels not in any way determining the function, but merely administering to it, like the arrangement of water or gas-pipes in a manufactory. Thus in Fig. 329, A, we see that the capillaries of fatty tissue are dis

posed in a network with rounded meshes, so as to distribute the blood among the fat-cells (§ 422); whilst at B we see the meshes enormously elongated, so as to permit muscular fibres to lie in them. Again, at c we observe the disposition of the capillaries around the orifices of the follicles of a mucous membrane; whilst at D we see the looped arrangement which exists in the papillary surface of the skin, and which is subservient to the nutrition of the epidermis and to the activity of the sensory nerves.

438. In no part of the circulating apparatus, however, does

FIG. 330.

the disposition of the capillaries present more points of interest, than it does in the Respiratory organs. In Fishes, the respiratory surface is formed by an outward extension into fringes of gills, each of which consists of an arch with straight laminæ hanging down from it; and every one of these laminæ (Fig. 330) is furnished with a double row of leaflets, which is most minutely supplied with bloodvessels, their network (as seen at A) being so close, that its meshes (indicated by the dots in the figure) cover less space than the vessels themselves. The gills of Fish are not ciliated on their surface like those of Mollusks and of the larva of the Water-newt; the necessity for such a mode of renewing the fluid in contact with them, Two branchial processes of the Gill of the being superseded by the muscular Eel. showing the branchial lamella. apparatus with which the gill champortion of one of these processes enlarged, ber is furnished. But in Reptiles, the respiratory surface is formed by the walls of an internal cavity, that of the lungs: these organs, however, are constructed on a plan very different from that which they present in higher Vertebrata, the great extension of surface which is effected in the latter by the minute subdivision of the cavity, not being here necessary. In the Frog (for example) the cavity of each lung is undivided;

showing the capillary network of the lamellæ.

FIG. 331.

its walls, which are thin and membranous at the lower part, there present a simple smooth expanse; and it is only at the upper part, where the extensions of the tracheal cartilage form a network over the interior, that its surface is depressed into sacculi, whose lining is crowded with bloodvessels (Fig. 331). In this manner, a set of air-cells is formed in the thickness of the upper wall of the lung, which communicates with the general cavity, and very much increases the surface over which the blood comes into relation with the air; but each air-cell has a capillary network of its own, which lies on

one side against its wall, so as only to be exposed to the air on its free surface. In the elongated lung of the Snake, the same general arrangement prevails; but the cartilaginous reticulation of its upper part projects much further into the cavity, and encloses in its meshes (which are usually square, or nearly so) several layers of air-cells, which communicate, one through another, with the general cavity. The structure of the lungs of Birds presents us with an arrangement of a very different kind, the purpose of which is to expose a very large amount of capillary surface to the influence of the air. The entire mass of each may be considered as subdivided into an immense number of "lobules" or "lunglets" (Fig. 332), each of which has its own bronchial tube (or

Interior structure of Lung of Fowl, as displayed by a section, A. passing in the direction of a bronchial tube, and by another section, B, cutting it across.

subdivision of the windpipe), and its own system of blood vessels, which have very little communication with those of other lobules. Each lobule has a central cavity, which closely resembles that of a Frog's lung in miniature; having its walls strengthened by a network of cartilage derived from the bronchial tube, in the interstices of which are openings leading to sacculi in their substance. But each of these cavities is surrounded by a solid plexus of bloodvessels, which does not seem to be covered by any limiting membrane, but which admits air from the central cavity freely between its meshes; and thus its capillaries are in immediate relation with air on all sides, a provision that is obviously very favorable to the complete and rapid aeration of the blood they contain. In the lung of Man and Mammals, again, the plan of structure differs from the foregoing, though the general effect of it is the same. For the whole interior is divided up into minute air-cells, which freely communicate with each other, and with the ultimate ramifications of the air-tubes into which the trachea (windpipe) subdivides; and the network of bloodvessels (Fig. 333) is so disposed in the partitions between these cavities, that the blood is exposed to the air on both sides. It has been calculated that the number of these air-cells grouped around the

termination of each air-tube in man, is not less than 18,000; and that the total number in the entire lungs is six hundred millions.

439. The following list of the

FIG. 333.

parts of the bodies of Vertebrata, of which Injected preparations are most interesting as Microscopic objects, may be of service to those who may be inclined to apply themselves to their production. Alimentary Canal; Stomach, showing the orifices of the gastric follicles, and the rudimentary villi near the pylorus; Small Intestine, showing the villi and the orifices of the follicles of Lieberkühn, and at its lower part the Peyerian glands; Large Intestine, showing the various glandular follicles:-Respiratory Organs; Lungs of Mammals, Birds, and Reptiles; Gills and Swimming-bladder of Fish:-Glandular Organs; Liver, Gall-bladder, Kidney, Parotid:-Generative Organs; Oviduct of Bird and Frog; Mammalian Placenta; Uterine and Foetal Cotyledons of Ruminants:Organs of Sense; Iris, Choroid, and Ciliary processes of Eye, Pupillary Membrane of foetus; Papillæ of Tongue; Mucous Membrane of Nose; Papillæ of Skin of finger:-Tegumentary Organs; Skin of different parts, hairy and smooth, with vertical sections showing the vessels of the Hair-follicles, Sebaceous glands, and Papillæ; Matrix of nails, hoofs, &c.:-Tissues; Fibrous, Muscular, Adipose, Sheath of Tendon.

440. Development.-The study of the Embryological development of Vertebrated animals has been pursued of late years with great zeal and success by the assistance of the Microscope; but as this is a department of inquiry which needs for its successful pursuit a thoroughly scientific culture, and is only likely to be taken up by a professed Physiologist, no good purpose seems likely to be served by here giving such an imperfect outline of the process, as could alone be introduced into a work like the present; and the reader who may desire information upon it, will find no difficulty in obtaining this through systematic treatises on Physiology.'

The Author takes the liberty of referring to his "Principles of Comparative Physiology," 4th Ed. chap. xi, as containing a general view of the whole subject, with references to the principal sources of more detailed information.