Fig. 94—Cross section of bone showing minute structure. Magnified. 1. Surface layer of bone. 2. Deeper portion. 3. Haversian canals from which pass the canaliculi. 4. A lacuna. Observe arrangement of lacunæ at surface and in deeper portion.
Minute Structure of Bone.—A microscopic examination of a thin slice of bone taken from the compact substance shows this to be porous as well as the spongy substance. Two kinds of small channels are found running through it in different directions, known as the Haversian canals and the canaliculi (Fig. 94). These serve the general purpose of distributing nourishment through the bone. The Haversian canals are larger [pg 220]than the canaliculi and contain small nerves and blood vessels, chiefly capillaries (Fig. 95). They extend lengthwise through the bone. The canaliculi are channels for conveying lymph. They pass out from the Haversian canals at right angles, going to all portions of the compact substance except a thin layer at the surface. In the surface layer of the bone the canaliculi are in communication with the periosteum.
Fig. 95—Section showing Haversian canal and contents, highly magnified (after Schäfer). 1. Arterial capillary. 2. Venous capillary. 3. Nerve fibers. 4. Lymph vessel.
The Bone Cells.—Surrounding the Haversian canals are thin layers of bone substance called the laminæ, and within these are great numbers of irregular bodies, known as the lacunæ. The walls of the lacunæ are hard and dense, but within each is an open space. In this lies a flattened body, having a nucleus, which is recognized as the bone cell, or the bone corpuscle (Fig. 96). It appears to be the work of the bone cells to deposit mineral matter in the walls surrounding them and in this way to supply the properties of hardness and stiffness to the bones. The canaliculi connect with the lacunæ in all parts of the bone, causing them to appear under the microscope like so many burs fastened together by their projecting spines (Fig. 94).
Fig. 96—Bone cell removed from the lacuna and very highly magnified. (From Quain's Anatomy.)
How the Bone Cells are Nourished.—The bone cells, like all the other cells of the body, are nourished by the lymph that escapes from the blood. This passes through the canaliculi to the cells in the different parts of the bone, as follows:
[pg 221]1. The cells in the surface layer of the bone receive lymph from the capillaries in the periosteum.[79] It gets to them through the short canaliculi that run out to the surface.