OSTEOLOGY. 5 



They are covered with periosteum, and their cavities are 

 lined with endosteum and filled with medulla, or marrow. 



Periosteum is a fibre-vascular membrane, composed of two 

 layers, the outer formed chiefly of connective tissue, the inner 

 composed of several layers of elastic fibres, the deepest of which 

 is the "osteogenic," or bone-producing layer. The tendons and 

 ligaments are attached to the periosteum by a mutual interlace- 

 ment of the fibres. 



Endosteum, medullary membrane or internal periosteum, is 

 a delicate, highly vascular membrane lining the cylindrical cavity 

 of long bones. 



Medulla, or bone-marrow, is of two kinds, the red in the 

 flat and irregular bones, and the yellow in the shafts of adult 

 long bones. The red marrow has few blood-vessels, but many 

 corpuscular elements. These may be divided into three groups : 

 1, red nucleated true "marrow-cells;" 2, large pinkish cells, 

 "myelo-plaques" of Eobin, supposed to be the source of the red 

 blood-corpuscles; and 3, giant cells, or "osteoclasts." 



The yellow marrow contains numerous blood-vessels, a few 

 "marrow-cells," and a large quantity of fat which imparts to 

 it its color. 



Blood-vessels of Bone. The arteries consist of three sets: 

 The arteries of the (1) compact and (2) cancellous tissue are 

 derived chiefly from the periosteum, the latter being larger and 

 less numerous. The medullary canal of long bones has (3) one 

 or more nutrient arteries, which penetrate the compact tissue 

 obliquely and divide into two branches, one passing upward, the 

 other downward, in the canal. All the arteries anastomose 

 freely with each other. 



The veins consist of three sets also: 1. The veins of the 

 compact substance are small and join the periosteum. 2. The 

 veins of the cancellous tissue do not accompany the arteries, but 

 in certain localities form large, tortuous channels, as in the 

 diploic veins of the cranium. In the long bones they emerge 

 at the ends near the articular surfaces. 3. The nutrient arteries 

 are accompanied by one or more large veins, which emerge at 

 the nutrient foramen. 



Lymphatics and nerves are numerous in the periosteum and 

 are also found in the substance of the bone. 



Microscopical Appearance. Microscopically, bone-tissue con- 

 sists of concentric layers, or lamellae, arranged about the course 

 of a vascular or Haversian canal 1 / 16000 inch in diameter. In 

 and between these plates of bone-tissue are minute cavities, or 

 lnrinnf. oach containing a bone-coll, or "osteoblnst," and from 

 which diverge in every direction minute canals, or 



