l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



branching than in the plexiform type of circulation. They are 

 especially prominent in this section. 



TANGENTIAL SECTION OF THE OUTER TABLE OF A HUMAN 

 OCCIPITAL BONE, SHOWING THE CIRCULATION 



Pl. 5, Fig. 44 

 The section is taken a short distance above the foramen magnum. 

 The circulation is a dense branching type situated near the external 

 surface of the bone. There are many vascular expansions from 

 which branching vessels take their departure. Entering canals with 

 and without surrounding lamellae are present in the first type bone 

 substance. 



TANGENTIAL SECTION OF THE INNER TABLE OF THE SAME OCCIPITAL 

 BONE, SHOWING THE CIRCULATION 



Pl. 5, Fig. 45 

 The type of bone is first. Entering canals with and without sur- 

 rounding lamellae are numerous. The section is situated near the 

 cerebral surface. The circulation is branching in type and very 

 dense in character. Large branches extend in different directions 

 and from these, small branches are sent ofif into the bone substance. 

 The vascular expansions are not as numerous as those in the outer 

 table, figure 44. 



TWO VASCULAR EXPANSIONS AS THEY WERE SEEN IN THE OUTER 

 TABLE OF THE OCCIPITAL BONE 



Pl. 5, Fig. 46 

 The vascular expansions are observed in all branching and in 

 many sections of the plexiform type of circulation. They are nearly 

 always filled with fat globules with dark, wide contours which are 

 so closely faceted together that they completely occupy the expansion 

 cavities. This would, of course, block the circulation if it existed 

 in the living bone. The condition was misleading in significance 

 until it was noticed that the fat globules were found in ground sec- 

 tions and not in thin unground bone plates ; so that they are probably 

 produced by the melting heat of friction during the grinding process 

 and fill the expansions. The ex]:)ansions probably have a physical 

 value as distributing centers in the circulation or they may be for 

 the purpose of establishing and maintaining a circulatory equilibrium 

 in bone. 



