230 CONNECTIVE TISSUE. 



bone-tissue, while the innermost portion, closely attached to the 

 surface of the bone, is a dense ribboned layer, with a scanty 

 supply of blood-vessels, but rich in elastic substance. This layer 

 is often the seat of calcareous deposition, and owing to this fact 

 its plasticl! assume irregular, jagged contours similar to those of 

 bone-corpuscles. The calcined periosteum, however, is not true 

 lamellated bone. It is often described under the name of the 

 " osteoid layer" The periosteum is very thick at the points of 

 the attachment of tendons and ligaments. The inner surface 

 of the flat skull-bones, after the fifth or sixth year of life, is 

 destitute of a periosteal investment proper. This can be clearly 

 understood by the study of the development of these bones. 



Blood-vessels. The bone-tissue, its investment, the perios- 

 teum, and its contents, the medulla, are plentifully supplied 

 with blood-vessels. They enter the periosteum mainly at the 

 points of attachment of the large ligamentous formations men- 

 tioned above. Arteries and veins are most abundant in the 

 outermost 'portion of the periosteum, where extensive ramifica- 

 tion of these vessels takes place. The arterioles enter the larger 

 canals at the surface of the bones and branch into capillaries, 

 which unite to form the efferent veins accompanying the arteri- 

 oles. At the inner surface of the compact bone, also, there is a 

 free anastomosis with the capillaries of the medulla. The 

 medulla is supplied with numerous capillaries, arising from the 

 so-called nutrient arteries of the bone, which pierce the cortical 

 substance obliquely and split at acute angles, both within the 

 canal and after entering the medulla. The veins collect the 

 blood from tassel-like bundles of capillaries and accompany the 

 afferent arteries. The epiphyseal portions of shaft-bones, besides 

 the general medullary vessels, receive blood from the vessels 

 which supply the articulations. The capillaries terminate, in the 

 shape of loops, close below the articular cartilage. The veins in 

 the bone-tissue are without valves, but as soon as they reach the 

 surface of the bone, we find valves are present (C. Langer). 



Lymphatics are not yet known to exist in the bone-tissue. 



Nerves, both of the medullated and non-medullated variety, 

 accompany the larger blood-vessels, but they are not abundant 

 in the bone-tissue. In different places in the periosteum, Pacin- 

 ian corpuscles are found. 



The medulla of the bone is, in juvenile condition, a myx- 

 omatous tissue, at first of the medullary, later of the reticular 

 variety (see pages 147 and 148), and, being freely vascularized, is 



