STRUCTURE OF THE BONES. 337 



1. The True Osseous Tissue. 



This peculiar element of the bones has been described at length 

 (pages 321335). 



2. The Bloodvessels of the Bones. 



The bloodvessels of the bones are first sent to the periosteum, 

 which, besides the branches it transmits directly into the substance 

 of the bone, presents a pretty close network of capillaries, ^^^ of 

 an inch in diameter in its outer layer. The vessels entering the 

 substance of the bone are very numerous. 



On the long bones are distinct vessels for the nutrition of (1) the 

 cancellated structure of the extremities, (2) of the compact sub- 

 stance of the shaft, and (3) of the marrow. The latter, called the 

 vasa nutritia, enter the medullary cavity of the bones, one or two 

 to the shaft, and several to the epiphyses, through large openings 

 and canals ; and, except a few twigs given off to the innermost Ha- 

 versian canals of the compact substance, ramify in the marrow, 

 where they form a capillary plexus whose vessels are from 3^00 to 

 2 sVo of an i nca i n diameter. 



(2.) The vessels of the compact substance rise principally from 

 those of the periosteum. They very soon lose their muscular coat, 

 and form in the Haversian canals (which they fill either alone or 

 in connection with some medullary substance), a network of wide 

 vessels. The latter can hardly be regarded as capillaries, how- 

 ever, since they show a layer of areolar tissue and an epithelium ; 

 and fine capillaries co-exist with the main vessel only in the larger 

 canals. 



(3.) The cancellated structure of the extremities of the long 

 bones is supplied by the vessels transmitted by the numerous canals 

 seen by the unaided eye upon their external surface. 



The venous blood is returned from all the long bones in three 

 ways : (1), by a large vein accompanying the nutrient artery, and 

 whose ramifications it follows; (2), by numerous large and small 

 veins at the articular extremities; and, (3), by many small veins in- 

 dependent of each other in the compact tissue of the shaft, in which 

 their roots occupy the wider spaces and sinuses, or pouch-like ex- 

 cavations, which are very evident in sections of bone. (Fig. 214, 3.) 



All the vessels of bone just mentioned freely communicate, so 



99 



