470 RELATION OF THE RED PARTICLES TO NUTRITION. metry has arrived, and the use of good instruments, have made us now so well acquainted with the physical properties of the organic tissues, that we are enabled to refute the above theory, by merely comparing with precision the size of different parts. Microscopic examinations, if intended to serve as the basis for scientific researches and comparisons, must not ponsist merely in the direct measurement of each object; a much more important and essential mode of investigation is the comparison of the object to be measured with some other body that can be taken as a standard. Thus, for instance, in measuring the size of the muscular or nervous fibrils, they should be placed under the microscope, together with red particles of human blood, and both should then be observed at the same time. The admeasurements of the red particles of human blood, as stated by Kater, Wollaston, Prévost and Dumas, Weber, Wagner, and myself, agree so nearly, that their diameter may with great certainty be stated at ^qW of an inch English. We have thus a certain standard of measurement. As standards of comparison, I employ the red particles of human blood, easily obtained by slightly scratching the skin, those of frog’s blood which are about four times larger in diameter, and the nuclei of these latter bodies, obtained by the action of acetic acid, which measure about ^th or èd the long diameter of the entire red particle. We have already seen, (at pages 20-21,) that the ultimate fibres of muscles and nerves do not correspond in size either with the red particles or their nuclei. Moreover, the nuclei of the red particles of the frog’s blood are not globular but elliptic in form, and those of the red particles of the salamander’s blood are even flattened. How, then, can they compose the ultimate fibres of nerves and muscles? We shall see that the form of these tissues affords no grounds for such an opinion. And the recent observations of Schwann and others, on the mode of growth of all the elementary parts of the body, prove it to be incorrect. The most minute capillary vessels do not ramify upon the primi¬ tive fibrils of nerves and muscles; these fibrils are too fine to receive vessels, and are, in fact, more minute than the capillary vessels them¬ selves, which measure from jg^g to T-gi-g of an English inch in dia¬ meter. Nutrition, therefore, must be effected through the coats of the capillary vessels, and the process consists in the fluid parts of the blood permeating the parietes of the capillaries, while the solid par¬ ticles are visibly carried onwards into the veins. The most important materials for nutrition are the albumen and fibrin dissolved in the liquor sanguinis. A portion of these matters permeate the parietes of the capillaries, and are imbibed by the tissues; and what is effused over and above the quantity required for their nutrition, is taken up by the absorbent vessels, and carried again into the blood. It is here of importance to know that the capillaries really have solid parietes; the proofs of which were seated at page 224. Nothing can pass from the blood to the tissues, or from the tissue to the blood, without permeating, in the fluid state, the walls of these vessels. The hypothesis of the blood flowing in simple canals excavated in