184 SMITHSONIAN CONTEIBUTIONS TO KNOWLEDGE VOL. 35 



Structure. — External circumferential lamellae appear only in scattered frag- 

 ments. They are most pronounced in the posterior inner wall. The centi-al ring 

 of well develoijed Haversian systems constitutes practically the whole bone. In 

 most parts they form the external border directly under the periosteum, and 

 here and there a half system bounds the section. The systems are fairly uni- 

 form in size and circular in cross-section. Many of these have relatively large 

 Haversian canals and show slight senile changes. The Haversian canals fre 

 quently communicate by cross canals. On the two sides of the posterior ridge, 

 near the circumference, they are a little elongated and show faint traces of a 

 former laminar condition. 



The internal circumferential lamellis form a narrow ring around the medul- 

 lary canal. For the most part the lamellae assume the form of cancellous bone. 

 All bone units are well developed. 



Type III, C, senile. 



RIGHT FEMUR OF A WHITE CHILD, LESS THAN ONE YEAR OLD. NO. 249588, 



U. S. NAT. MUS. 



Pl. 29, Fig. 374. Syn. Tab. X 



Antero-posterior diameter of bone, 6.5 mm. ; lateral, 7.5 mm. 



Antero-posterior diameter of medullary canal, 4 mm. ; lateral, 5 mm. 



Tlie medullary canal is full. Medullary index, 70%. 



Structure. — Extending around the circumference of the section — posterior 

 ridge excepted — is a narrow ring of lamellfE with oval lacunfe and bushy cana- 

 liculi. The lamellae are frequently interrupted by canals running longitudinally. 



Beginning on both sides of the posterior ridge and extending around the 

 section underneath tlie external lamellfe is a horseshoe band of laminae and 

 Haversian systems forming the remainder of the bone. The inner wall is almost 

 entirelj" lamina?. As the laminae reach the anterior wall they are mostly dis- 

 placed by rather crude, elongated Haversian systems, which, however, generally 

 maintain concentric arrangements. In the outer wall the Haversian systems 

 occupy the medullary half and the laminas the circumferential half of the sec- 

 tion. The lacuna^ are oval. The Haversian canals are irregular in shape and 

 extend in various directions. The internal circumferential lamellae are frag- 

 mentary. 



The posterior ridge is incompletely formed and is composed of large canals, 

 surrounded by oval lacuna? with infrequent bordering canaliculi. Here and 

 there an Haversian system appears. The ridge is obviously a later formation 

 than the remaining walls. 



Type II-III, la, lb. 



