OSTEOLOGY 



intersects the trabeculae of group (o) which rise from the opposite side. The trabeculae of this 

 group evidently carry small stresses, as is shown by their slenderness. 



"d. The Principal Tensile Group. This group springs from the outer part of the shaft imme- 

 diately below group c, and curves convexly upward and inward in nearly parallel lines across 1 he 

 neck of the femur and ends in the inferior portion of the head. These trabeculaa are somewhat 

 thinner and more widely spaced than those of the principal compressive group (6). All the trabec- 

 ulae of this group cross those of groups (a) and (bj at right angles. This group is the most impor- 

 tant of the lateral system (tensile) and, as will be shown later, the greatest tensile stresses of the 

 upper femur are carried by the trabeculse of this group. 



"e. The Secondary Tensile Group. This group consists of the trabeculse which spring from the 

 outer side of the shaft and lie below those of the preceding group. They curve upward and medially 

 across the axis of the femur and end more or less irregularly after crossing the midline, but a 

 number of these filaments end in the medial portion of the shaft and neck. They cross at right 

 angles the trabeculae of group (a). 



10 



= COMPRESSION 



= TENSION 



C=COMP. & TENSION 

 N'EUTRAL AXIS 



FIG. 251. Intensity of the maximum tensile and compressive stresses in the upper femur. Computed for the load of 

 100 pounds on the right femur. Corresponds to the upper part of Fig. 250. (After Koch.) 



"In general, the trabeculse of the tensile system are lighter in structure than those of the com- 

 pressive system in corresponding positions. The significance of the difference in thickness of these 

 two systems is that the thickness of the trabeculae varies with the intensity of the stresses at any 

 given point. Comparison of Fig. 247 with Fig. 251 will show that the trabeculae of the com- 

 pressive system carry heavier stresses than those of the tensile system in corresponding positions. 

 For example, the maximum tensile stress at section 8 (Fig. 251) in the outermost fiber is 771 

 pounds per square inch, and at the corresponding point on the compressive side the compressive 

 stress is 954 pounds per square inch. Similar comparisons may be made at other points, which 

 confirm the conclusion that the thickness and closeness of spacing of the trabeculse varies in 

 proportion to the intensity of the stresses carried by them. 



"It will be seen that the trabeculse lie exactly in the paths of the maximum tensile and com- 

 pressive stresses (compare Figs. 247, 248 and 251), and hence these trabeculse carry these stresses 

 in the most economical manner. This is in accordance with the well-recognized principle of 

 mechanics that the most direct manner of transmitting stress is in the direction in which the stress 

 acts. 



