NO. 3 



COMPARATIVE HISTOLOGY OF FEMUR FOOTE 



115 



The systems are well developed. In several places a half system borders 

 the medullary canal. The lacunae are long and narrow and their canaliculi are 

 long and branching. The Haversian systems are frequently united by short 

 inter-Haversian lamellae. A second crescent of Haversian systems borders the 

 medullary canal extending from the posterior prominence around the posterior 

 and outer to about the middle portion of the anterior wall. The systems of 

 the two crescents are the only fully developed systems present. The second 

 crescent is narrower than the first. Its widest part is in the posterior region 

 of the outer wall. The lacunae are long and narrow and the canaliculi are 

 straight. 



The main structure of the inner, anterior, and outer wall is lamellar. It 

 is composed of irregularly concentric lamellae, interrupted by rudimentary Ha- 

 versian systems of the (lb) differentiation. This lamellar structure makes up 

 practically the whole bone. The lacunae are generally long, and have long, 

 branching, and numerous canaliculi. In some places the lacunae are curved 

 and quite irregular in shape. The posterior wall and ridge is composed of 

 Haversian systems, poorly developed. They are separated by lamellae. Their 

 outlines are not sharply denned, but appear to merge into the surrounding 

 lamellae. 



Type I-III, lb, C. 



RIGHT FEMUR OF SCIURUS SP. (LARGE RED SQUIRREL.) CR. MED. COLL. 



Pl. 14, Fig. 230. Stn. Tab. VI 



Antero-posterior diameter of bone, 4.5 mm.; lateral, 6 mm. 

 Antero-posterior diameter of the medullary canal, 3.5 mm. ; lateral, 5 mm. 

 The medullary canal is full. Medullary index, 18%. 



Structure. — A ring of external circumferential lamellae of varying widths 

 surrounds the bone. Their lacunae are mostly long and narrow and their cana- 

 liculi are numerous, long, and branching. A central, irregularly shaped ring 

 of complete and incomplete Haversian systems is situated under the external 

 lamellae. It increases in thickness around the inner, posterior, and outer wall, 

 and reaches the surface in the outer wall. In many places in the inner wall the 

 systems are composed of oval lacunae with short, bushy canaliculi arranged in 

 a circular manner. 



Internal circumferential lamellae form an uneven, thick ring around the 

 medullary canal. Their lacunae are long and narrow and their canaliculi are 

 long, numerous, and branching. The bone, therefore, is composed of three very 

 uneven and irregularly shaped rings of structural units. 



Type I-III, lb, C. 



