14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



THE CIRCULATION IN FLAT BONES 



CROSS, LONGITUDINAL, AND TANGENTIAL SECTIONS OF A 

 HUMAN FRONTAL BONE 



CROSS SECTION SHOWING THE STRUCTURE 



Pl. 4, Fig. 2^ 



The bone is composed of outer and inner tables united by a central 

 cancellous diploe. The two tables are composed of first type bone 

 enclosing a few Haversian systems in cross section and a few short 

 segments of blood vessels. The diploe is a coarse, cancellous bone 

 with large, small, and irregularly shaped cavities enclosed by first 

 type bone walls. The walls are composed of lamellae in which 

 Haversian systems are seen in cross section. This section was taken 

 from the vertical portion of the frontal bone. 



LONGITUDINAL SECTION OF THE SAME FRONTAL BONE, SHOWING THE 



STRUCTURE 



Vu 4, Fig. 37 

 This section has practically the same structure as the cross section, 

 as may be seen by comparing the drawings. This section is cut 

 at right angles to that seen in plate 4, figure 36. It is taken from the 

 same region, and Haversian systems, in cross section, are found in 

 both situations. The Haversian systems therefore run at right angles 

 to each other, which can hardly be accounted for on mechanical 

 grounds. 



TANGENTIAL SECTION OF THE OUTER TABLE OF THE SAME FRONTAL 



BONE TAKEN FROM THE SAME REGION AS FIGURE 36, PLATE 4, 



SHOWING THE CIRCULATION 



Pl. 4 Fig. 38 



The section is situated nearer the external surface of the bone than 

 the diploe. Numerous entering canals with and without enclosing 

 lamellae are found in the bone substance. The circulation is branch- 

 ing in type. The vascular expansions are large and numerous and 

 appear to form physical centers of distribution. The blood vessels 

 are relatively large and frequently branch. 



ENTERING VASCULAR CANALS OF THE OUTER TABLE OF THE 



FRONTAL BONE 



Pl. 5, Fig. 39 



There are two forms, the one at the left in the drawing without 

 enclosing lamellae (Volkmann's canals), and the one at the right 

 with enclosing lamellae. The Volkmann's canals are smaller than 



