. The anatomy of the domestic animals. Veterinary anatomy. 22 OSTEOLOGY layer, and an inner cellular osteogenic layer. During active growi:h the osteogenic layer is well developed, but later it becomes much reducecl. The fibrous layer varies much in thickness, being in general thickest in exposed situations. The adhesion of the periosteum to the bone also differs greatly in various places; it is usually very thin and easily detached where it is thickly covered with muscular tissue which has little or no attaclunent. The degree of vascularity conforms to the activity of the ]>eriosteum. The

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. The anatomy of the domestic animals. Veterinary anatomy. 22 OSTEOLOGY layer, and an inner cellular osteogenic layer. During active growi:h the osteogenic layer is well developed, but later it becomes much reducecl. The fibrous layer varies much in thickness, being in general thickest in exposed situations. The adhesion of the periosteum to the bone also differs greatly in various places; it is usually very thin and easily detached where it is thickly covered with muscular tissue which has little or no attaclunent. The degree of vascularity conforms to the activity of the ]>eriosteum. The endosteiun is a thin fittrous memlirane which lines the medullary cavity and the larger Haversian canals. The marrow (^Medulla ossium) occupies the interstices of the spongy bone and the medullar}' cavity of the long bones. There are two varieties in the adult— red and yellow. In the young subject there is only red marrow (Medulla ossium rubra), but later this is replaced in the medullary cavity by yellow marrow (JNIeduUa ossium flava). The red marrow contains several tjqDcs of characteristic cells and is a lilood-forming substance, while the yellow is practically ordinary adipose tissue.^ Vessels and Nerves.—It is customary to recognize two sets of arteries—the periosteal and the medullary. The former ramify in the periosteum and give off. ?CROSS-f Left Humerus ( passes through nutrient foramen and canal. innumerable small branches which enter minute openings (Volkmann's canals) on the siu'face and reach the Haversian canals of the compact substance. Other branches enter the extremities of the long bones and supply the spongy bone and marrow in them. In the case of the larger bones—and especially the long liones— the large nutrient ov medullary artery (Arteria nutricia) enters at the so-called nu- trient foramen (Foramen nutricium), passes in a canal (Canalis nutricius) through the compact substance, and ramifies in the marrow; its branches anastomose with the centra

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