_Blood._--This blood consists of a yellowish liquid, in which very numerous red _blood-corpuscles_ or globules (Pl. IX. fig. 1) are suspended, and to which the red colour is owing. The blood-corpuscles are not globular, but discoidal, _i. e._ they are circular and flattened, the sides being slightly sunk in. Their form is best seen as they roll over on a slide, after the application of a glass cover. The coloured corpuscles are cells; they appear yellowish red under the microscope, the deep red colour of the blood depending upon the large number of them seen at once and crowded together. It need scarcely be stated that a drop of blood may easily be obtained, by puncturing the wrist with a clean needle. The blood is contained in the blood-vessels. These consist of the ar´teries, which convey the blood from the heart; the veins, which return it to the heart; and a very fine set of intermediate vessels, called the cap´illaries. If a little water be added to a drop of blood on a slide, _colourless corpuscles_, rather larger than the coloured disks, will be seen scattered among the latter. These are the colourless or lymph-corpuscles of the blood. They are truly spherical, and granular on the surface. _Bone._--In examining a transverse section of a bone, one or several very large cavities will be seen with the naked eye in the centre of the section; these contain the marrow, or medulla. In the long bones, the medullary cavity is single, and runs longitudinally down the bone; whilst in the flat bones the cavities are numerous, forming cancelli. Under the microscope, thin transverse sections of bone exhibit oval or rounded holes, or foramina (Pl. IX. fig. 15), which are sections of canals conveying blood-vessels through the bone; these are the _Haver´sian canals_. Around the sections of these canals are seen numerous concentric rings, indicating layers or lamellæ of bony matter. The substance of bone presents numerous black, somewhat elongated bodies (fig. 15 _a_), called the _lacúnæ_ (_lacúna_, a little hollow) or bone-corpuscles, which are however hollow, therefore not truly corpuscles, as they were formerly considered. Between the adjacent lacunæ run numerous fine, dark, branched lines, consisting of very minute canals, or _canalic´uli_. If the section of bone be viewed by reflected light, the lacunæ and canaliculi will appear white. In the dried bone they contain air. The structure of bone is best seen when viewed as a transparent object in the dry state; for when the section is immersed in liquid, the lacunæ and canaliculi become filled up. The size and form of the bone-corpuscles and canaliculi vary in different animals, so much so that the Class or Order to which an animal belongs may be determined by reference to these particulars.