The teeth present, in their combinations, the greatest analogy to bone; the principal and organized substance of the teeth is indeed true bone, containing indeed less cartilage and more phosphate of lime than other bones. The enamel, which is an inorganic secretion from the surface of the long tooth, is almost destitute of any animal matter. _Of the Composition of the Blood._—Blood is, in the higher classes of animals, an opaque, thick, red fluid; it has a salty and nauseous taste, and a peculiar smell, resembling that of the animal whence it has been derived. When the blood of any red blooded animal is allowed to rest, it gradually forms a soft jelly, from which, after some time, a thin yellowish fluid (serum) separates, while the red jelly or coagulum contracts in volume and acquires great consistence. If this coagulation of the blood takes place slowly, the upper portion of the coagulum becomes white or pale yellow; forming thus, the buffy coat. There is no doubt that the blood, while in connection with the animal, participates in its life, and the phenomena of coagulation are to be referred to a new arrangement of its materials consequent on the loss of that vitality. The serum of the blood, when coagulation has been perfect, is of a yellowish, sometimes greenish, color; its taste is dull and salty; it is thick fluid, like olive oil; when heated to 140° it coagulates. If we examine under the microscope the appearance presented by blood, we find that it consists of a great number of red particles swimming in a nearly colorless liquor. These red particles are flattened disks; in man they are round. Their size is variable, being in man from one four-thousandth to one eight-thousandth of an inch in diameter, but larger in other animals. The blood contains a large quantity of _albumen_, partly dissolved and remaining in the serum after coagulation, partly in a solid state, forming the great mass of the globules. In the living body the blood also contains fibrine in solution, but this separates soon after extraction from the body; it assumes a solid form, and investing, as a sponge, the red globules, forms with them the coagulum. The fibrine is thus the element active in the coagulation of the blood, the globules being but passively engaged in it. In addition to this essential organic element, the blood contains a variety of salts, as common salt, phosphate of magnesia, ammonia and lime, lactates of soda and magnesia. The best analyses of the blood are those of Lecanu, and the results for blood and serum are that they contain, in the human subject of each sex: Water, 75.00 Albumen, 5.00 Globules, 7.14 Fibrine, .20 The fatty substance of the blood is a mixture of cholesterine with stearic and oleic acid and a peculiar fatty substance termed seroline, the history of which is yet incomplete. None of the fats of the brain, however, seem to exist in the blood.