Harvard system is mature dense bone with obvious characteristics of adult dense bone. It only appears in the middle femur of human bones at birth, and then gradually forms in all long bones. Although it runs vertically, there are often many branches that are widely consistent with each other. There are small blood vessels in Haval's canal, only a single one is mostly capillary, and sometimes two are arterioles or venules. The Zuckerman's duct and Haval's duct are perpendicular to each other and connected with each other, so the blood vessels inside are also connected with each other. Fine nerve fibers with blood vessels can also be seen in Haval canal. Most of them belong to unmyelinated nerve fibers, but occasionally myelinated nerve fibers can be seen. These nerves are mainly composed of nerve fibers distributed in periosteum, and their thickness shows that they contain both postganglionic sympathetic nerve fibers and slender unmyelinated pain afferent fibers.
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Osteon is a thick-walled cylindrical structure, which is arranged in parallel with the long axis of the spine, and there is a thin tube in the center, called Haval tube. Around Havel's canal, there are 5-20 layers of bone plates arranged in concentric circles, just like layers of sheaths. Haversian canal and the surrounding bone plates together form the osteon, also known as Harvard system. Clopton Havers first determined the structure of dense bone in 169 1, and named the basic structural unit of dense bone Harvard system. Because he didn't describe the bone plates arranged concentrically around the blood vessels at that time, some authors later named them osteon according to the concentric circles. Many small tubes radiate from Havel's canal to the cavity, connecting Havel's canal and the cavity. Its function is to make the bone cells in the fossa get nutrient solution through the bone tubules and excrete metabolites at the same time. The pit is a flat or oval structure, and there are numerous small cracks on its inner wall, which communicate with the bone tubule. The long and thin protrusions of many bone cells extend into the bone tubules through cracks. There is a layer of adhesive on the surface of each bone unit, which is strongly basophilic and contains a lot of bone salts, but little collagen. There is a strongly refracted contour line of bone unit on the cross-sectional bone grinding piece, which is called glue line. There are some irregular incomplete bone units filled between bone units, and most of them lack Haval tube, which is called intermittent lamina. It is a partially absorbed osteon, an ancient relic of osteon. The average diameter of Haversian tube is 300μm and its length is about 3 ~ 5 mm. The inner wall is lined with a layer of connective tissue, and the cell composition changes with the active state of each osteon. Most of the new bones are osteoblasts, while the destroyed osteon has osteoclasts. Generally, osteon is a spindle cell, and many early osteon, especially the cortical bone of newborns, lack concentric bone plates. Osteon formed by bone deposition on the surface of periosteum or endosteal sulcus, or osteon formed in cancellous bone lattice, is called primary osteon. Harvard's canal is surrounded by a variable slender concentric column of bone plates, with only a few layers of bone plates. Primary osteon is common in young bones, especially embryonic bones and baby bones. With the increase of age, the number of junior osteon decreased accordingly. The secondary osteon is similar to the primary osteon, but it was formed after the reconstruction of the primary osteon. The secondary osteon, also known as the secondary Harvard system, is easy to identify and separate from the adjacent mineralized tissues because of its adhesive line.