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SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE 



VOL. 35 



Structure. — The bone is extended laterally and outwardly by a very prom- 

 inent, wide, curved process, occupying the middle portion of the femur. Only 

 a few femora have such a process. For convenience in description the section 

 may be divided into a body containing the medullary canal and adjoining middle 

 portion and a curved cancellous process. 



The body is composed of Haversian systems which form the entire inner 

 wall and of Haversian systems and laminae which form the anterior and poster- 

 ior wall. The systems vary in size, communicate by cross canals, and have oval 

 lacunae with straight canaliculi. Around the medullary region they show senile 

 changes and greater irregularity. Lamina? begin to appear in the anterior 

 and posterior wall and rapidly displace the systems as they reach the middle 

 portion. The middle portion is composed of long lamina} separated by wide 

 canals. In the center, crude Haversian systems and short lamina? are found. 

 The canals communicate with the medullary canal and with the cancellous 

 spaces of the curved process. The lacuna? are oval and the canaliculi are bushy. 

 The curved process is composed of narrow, bordering lamella? with small Ha- 

 versian systems, enclosing a wide central portion of cancellous bone, the spaces 

 of which are relatively large. The spaces communicate with the long canals of 

 the middle portion and also with the medullary canal. Internal circumferential 

 lamella? form an irregularly shaped ring around the medullary canal. Their 

 lacuna? are oval. The bone exhibits predominating lamina? and Haversian sys- 

 tems with oval lacuna? and busby and straight canaliculi. Haversian systems 

 are found only in the inner half of the body. 



Type TI-III, C, senile. 



RIGHT FEMTJE OF EQUUS BURCHELLI GRANTL ZEBRA. NO. 27749, AMER. MUS. NAT. HIST. 



Pl. 17, Fig. 264. Syn. Tab. VII 



Antero-posterior diameter of bone, 43 mm.; lateral, 37 mm. 

 Antero-posterior diameter of medullary canal, 20 mm.; lateral, 22 mm. 

 The medullary canal is full. Medullary index, 38%. 



Structure. — Three divisions are present. External circumferential lamella?, 

 frequently interrupted by Haversian systems, extend from the lateral bound- 

 aries of the posterior wall around the section. The lacuna? are oval and long. 

 The systems are well developed. 



The central ring is composed of a wide, horseshoe-shaped band of lamina?, 

 interrupted in the anterior wall by groups of well developed Haversian systems. 

 The lacuna? are oval and long and the canaliculi are straight. 



The internal circumferential lamella? form an enclosing ring around the 

 medullary canal. In the anterior and posterior wall it takes the form of can- 

 cellous bone. The lacuna? are oval and long. 



