on (he Conformation of Bones 383 



Hence a "linea aspera," formed by approximation of the two 

 septal lines on the back of the femur cannot be formed. 



The rounded shaft with linea aspera, which some artiodactyles 

 possess {e.g. sheep), is rendered possible by the speciahsation of the 

 femorococcygeus, and by the downward displacement of adductors. 

 The shaft is rounded in Tarsius from considerations of strength ; but 

 the septal hues are on the lateral margins of the bone — a femoro- 

 coccygeus is attached. Since the " interseptal " space provides 

 attachment for adductors, this if broad, as in the elephant, tends 

 to flattening — merely to enable the adductors to pass to their 

 more lateral insertion. Thus the posterior surface of types with 

 septal hues remains flattened, even where the anterior surface may 

 become very convex. Heavy types, such as the elephant and 

 Toxodon, show this pecuharity to advantage. 



The obhquity of the posterior surface in Carnivora has a similar 

 origin; the adductors here have a presumably important lateral 

 attachment; the backward external projection permits of the 

 accommodation of the large adductors. 



' Actual production of a narrow interseptal space, and finally of 

 a "linea aspera," as in man, is associated with expansion of the 

 knee extensors, and downward migration* of the bulk of the ad- 

 ductor musculature : these are characteristic of cursorials and salta- 

 torials. Considerations of accommodation would make it necessary, 

 when vasti muscles are attached behind the femur, that flattening 

 of the inner and outer margins of the bone should occur. For the 

 vasti ultimately gain attachment to the patella. It is a similar 

 problem to the accommodation of the enormous vastus externus 

 of Tarsius. The surface below the great trochanter is hollowed out 

 for its accommodation. 



Why is it that carnivora, and some Artiodactyla {e.g. Cervus), 

 show such a tendency to a wider interseptal space and a prominent 

 lateral position for the external septal line? It is not merely due to 

 the bulk of the adductors attached to the shaft, though large for 

 cursorials; for these are large also in catarrhine monkeys, and still 

 larger in man, but in them there is a "hnea aspera." There is 

 probably some functional significance in an extreme lateral attach- 

 ment for the adductors in the cursorial Carnivora; their external 

 rotatory effect will counterbalance the internal rotatory effect of 

 the synergic gluteus medius. This attachment is not found in the 

 catarrhine monkeys and man; it is sufficient in them for the ad- 

 ductors to be attached largely to intermuscular septa; and the 

 septal fines become placed close together. The structure of the 

 quadriceps extensor appears to be closely associated with the back- 

 ward displacement of the internal septal fine in man, and perhaps 

 monkeys too; a penniform arrangement of the fibres of the vastus 



* Except in monkeys and man. 



25—2 



