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SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE 



VOL. 35 



Structure. — The section is composed of two concentric ring's of bone sub- 

 stance, external and internal. The external — much the wider — forms most of 

 the wall of the bone, and is composed of channelled bone substance with round 

 lacuna? and relatively few canaliculi. In some portions the elongated meshes 

 assume the character of laminar formation. 



The internal circumferential lamellae with long lacunae and bushy canaliculi 

 form a narrow ring around the medullary canal. 



Type I. 



RIGHT FEMUR OF A WHITE FETUS, THREE AND ONE-HALF MONTHS OLD. 



NO. 89, CR. MED. COLL. 



Pl. 21, Pig. 300. Syn. Tab. VIII 



Antero-posterior diameter of bone, 2.5 mm.; lateral, '2 mm. 

 Antero-posterior diameter of medullary canal, 0.5 mm.; lateral, 0.5 mm. 

 The medullary canal is full. Medullary index, 5%. 



Structure. — The anterior wall is much thinner than the posterior. The 

 section is composed of bone substance within which are irregularly shaped 

 canals. Between the canals the bone substance, with oval lacuna" and short, 

 bushy canaliculi, is arranged concentrically around the medullary canal. In 

 some situations the canals with their adjacent lacuna 1 have the formations of 

 Haversian systems of the (la) differentiation. The posterior ridge forms the 

 whole posterior wall, and is composed of bone substance with wide canals hav- 

 ing a direction from the external to the medullary surface and presenting the 

 appearance of long canals divided into shorter ones until crude Haversian 

 canals are formed. The bone substance has oval lacuna 1 and bushy canaliculi. 

 The medullary canal is small and concentrically situated. 



Type II III, la. 



RIGHT FEMUR OF A WHITE FETUS, FOUR MONTHS. NO. 90, CR. MED. COLL. 



Pl. 21. Fig. 301. Syn. Tab. VIII 



Antero-posterior diameter of bone, 3.5 mm. ; lateral, 2.5 mm. 

 Antero-posterior diameter of medullary canal, 0.6 mm.; lateral, 0.5 mm. 

 The medullary canal is full. Medullary index, 3%. 



Structure. — The anterior wall is much thinner than the posterior. The 

 section is composed of bone substance with oval lacuna? and bushy canaliculi, 

 interrupted by wide, irregular canals bent around the medullary canal in the 

 shape of a horseshoe. A few crude, elongated Haversian systems are found in 

 the posterior wall. The posterior ridge is composed of a few undeveloped Ha- 

 versian systems of the (lb) differentiation and inter-Haversian bone substance 

 with wide canals, extending from the external to the medullary surface. The 



