6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



cross section in the bone substance through which periosteal vessels 

 pass into the bone. The vascular plexus is very extensive and has 

 a general longitudinal direction. The meshes are irregular in size 

 and shape. In some portions of the section vascular expansions are 

 found. The blood vessels are round and occupy similarly shaped 

 channels in the bone substance. They are composed of very thin 

 connective tissue walls, without smooth muscle, and are striated 

 spirally and longitudinally. The exact purpose of the vascular ex- 

 pansions is not clearly understood. They are found too frequently 

 to be accidental, and, as will be noted later, are more prominent in 

 the branching than in the plexiform types of circulation. They may 

 have an important physical value. 



CROSS AND TANGENTIAL SECTIONS OF A SECOND TYPE BONE 



OF BIRDS AS SEEN IN THE FEMUR OF A DOMESTIC 



TURKEY, MELEAGRIS GALLOPAVO 



CROSS SECTION SHOWING THE STRUCTURE 



Pl. I, Fig. 8 



The section is composed of concentric laminae separated and 

 crossed by vascular canals. The wall of the bone is divided into 

 nearly equal segments by large radiating canals extending from the 

 medullary canal to the periosteal surface. From these canals are 

 sent off lateral, small, parallel canals which divide the wall of the 

 bone into laminae. The laminae are interrupted in the anterior wall 

 and posterior ridge by incompletely differentiated Haversian systems. 

 It was the femur of the turkey which first called attention to the 

 variations in structure and circulation. 



TANGENTIAL SECTION OF A FEMUR OF THE TURKEY, SHOWING 

 CIRCULATION 



Pl. I, Fig. 9 



This section consists of a very rich, small-meshed plexus of vessels 

 situated between the laminae and having a general longitudinal direc- 

 tion. In the central portion of the section two plexuses can be seen, 

 one above the other, with a lamina of bone between them. This is 

 one of the most extensive circulations observed in long bones and 

 with such a blood supply as is here shown, this becomes a vascular 

 organ of great importance. 



