NO. lO THE CIRCULATORY SYSTEM IN BONE^ FOOTE 9 



CROSS AND TANGENTIAL SECTIONS OF A SECOND TYPE BONE 

 IN MAMMALS AS SEEN IN THE FEMUR OF A LAMB, OVIS 



CROSS SECTION SHOWING THE STRUCTURE 

 Pl. 2, Fig. i8 

 The section is composed of concentric laminae which are more com- 

 pletely differentiated than they are in birds. Here and there the 

 laminae are interrupted by Haversian systems of a late differentiation 

 and the concentric canals between the laminae are widened at inter- 

 vals and around them are incompletely differentiated lamellae form- 

 ing an early aberrant type of Haversian system. The posterior ridge 

 is composed of Haversian systems of a late differentiation. 



TANGENTIAL SECTION OF THE FEMUR OF THE SAME ANIMAL SHOWING 



THE CIRCULATION 



Pl. 2, Fig. 19 

 The circulatory plan of arrangement in this bone is not precisely 

 like that observed in the second type bone in birds. While it is plexi- 

 form in character there is about it more or less irregularity in distri- 

 bution approaching the branching type of circulation. Near the 

 sides of the drawing the plexus has a long mesh ; while in the central 

 portion it has more of a branching character. The vascular expan- 

 sion seen in the center of the drawing gives the impression of a 

 distributing point in the circulation. A few entering vascular canals 

 are seen. 



CROSS AND • TANGENTIAL SECTIONS OF ANOTHER SECOND 



TYPE BONE IN MAMMALS AS SEEN IN THE FEMUR OF 



THE MEXICAN BURRO 



CROSS SECTION SHOWING STRUCTURE 

 Pl. 2, Fig. 20 

 The section is composed of concentric laminae interrupted by 

 Haversian systems of different degrees of differentiation. The struc- 

 ture is similar to that seen in the femur of the lamb and a large 

 number of other mammals which have the same type bone. 



TANGENTIAL SECTION OF THE FEMUR OF THE SAME ANIMAL, 

 SHOWING THE CIRCULATION 



Pl. 2, Fig. 21 



The type is plexiform and the meshes of the plexus are much more 

 regular in shape than those seen in the femur of the lamb. There is 

 no evidence of a branching character. The vascular expansions are 

 not as prominent as they were in the femur of the lamb. 



