1. A particle of the human blood as it appears when transparent and floating; 2. the same dry, seen as opaque, illuminated by a leiberkuhn; 3. the same as it appears when half the leiberkuhn is darkened; 4. a particle of the frog's blood floating; 5. the same seen on its edge. All the above objects are magnified 500 diameters[5].] 233. The red particle of the human blood is circular (fig. CXII. 1, 2, 3). It is circular also in all animals belonging to the class mammalia; but in the three lower classes of vertebrated animals, the bird, the reptile, and the fish, it is elliptical (fig. CXII. 4, 5). 234. The magnitude of the red particle of the human blood is variously estimated from the two-thousandth to the six-thousandth part of an inch in diameter. Bauer estimates it at the two-thousandth, Hodgkin and Lister at the three-thousandth, Kater at the four-thousandth, Wollaston at the five-thousandth, and Young at the six-thousandth part of an inch. Its magnitude is uniformly the same in all individuals of the same species, but differs exceedingly in the different classes. The elliptical particles are larger than the circular, but proportionally thinner; larger in fishes than in any other class of animals, and largest of all in the skate. 235. When perfect and entire, the red particles indicate a disposition to arrange themselves in a definite mode. They combine spontaneously into columns of variable length (fig. CXIII.). In order to observe this tendency, a small quantity of blood, the moment it is taken from its living vessel, should be placed between two strips of glass or covered with a bit of talc and placed under the microscope. When thus arranged, a considerable agitation at first takes place among the particles. As soon as this motion subsides, the particles apply themselves to each other by their broad surfaces, and thus form piles or columns of Considerable length (fig. CXIII.). The columns often again combine one among another, the end of one being attached to the side of another, sometimes producing very curious ramifications (fig. CXIII.). In like manner, the elliptical particles apply themselves to each other by their broad surfaces, but they are not so exactly matched as the circular, one particle partially overlapping another, so that they form less regular columns than the circular. [Illustration: Fig. CXIII. Columnar arrangement which the particles of the human blood assume immediately after it is drawn from its vessel.] 236. The red particles, as far as is known, constitute a distinct and peculiar form of animal matter: the red colour, according to some, depending on an impregnation of iron; according to others, on an animal substance of a gelatinous nature. 237. The exact proportion of the different substances contained in the blood, according to the most recent analysis of it, that by M. le Canu, is as follows, namely,