150 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 35 



Structure. — The section is composetl of two e-oiieeiitric rings of bone sub- 

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

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

 lacuns and relatively few canaliculi. In some portions the elongated meshes 

 assume the character of laminar formation. 



The internal circumferential lamelte with long lacunae and bushy canaliculi 

 form a narrow ring around the medullary canal. 



Type I. 



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



NO. 89, CR. MED. COLL. 



Pl. 21, Fiu. 300. SvN. Tail VIII 



Anteroposterior diameter of bone, li.5 mm.; lateral, 2 mm. 



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



The medullary canal is full. Medullary index, 57f . 



Structure. — The anterior wall is much tliinner tlian 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 tlie meduUary canal. In 

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

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

 whole posterior wall, and is composed of l)one substance with wide canals liav- 

 ing a direction from tlie 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 lacniue and bushy canaliculi. 

 The medullary canal is small and concentricallv situated. 



Type II-III, la. 



EIGHT FEMUR OF A WHITE FETUS, FOUR MONTHS. NO. i)0, CR. MED. COLL. 



Pl. 21, Fio. ;!01. Syn. Tab. VIII 



Antero-i)osterior diameter of bone, ;>.5 inm. ; lateral, L'.5 mm. 



Antero-posterior diameter of medullary canal, 0.6 mm.; latei'al, 0.5 nun. 



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



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

 section is composed of hone substance with oval lacuna' and bushy canaliculi, 

 interrupted hy wide, irregular canals lient around the medullary canal in the 

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

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

 versian systems of the (lb) diit'erentiation and inter-Haversian bone substance 

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



