LAWS OF BONE ARCHITECTURE 253 



neck of the feniiii'. Tlio ])rincipal group consists of much heavier 

 trabecuhie, all of Avhicli cross from the medial pnvt of the shaft 

 to end in the upper articular surface of the head. 



a. The secondary compressive group. This group of trabecu- 

 le leaves the inner border of the shaft beginning at about the 

 level of the lesser trochanter, and for a distance of almost 2 

 inches along the cur^'in,g shaft, with which the separate trabeculae 

 make an angle of about 45 degrees. They curve outwardly and 

 upwardly to cross in radiating, smooth curves to the opposite 

 side. The lower filaments end in the region of the great tro- 

 chanter; the adjacent filaments above these pursue a more 

 nearly vertical course and end in the upper portion of the neck 

 of the femur. 



The trabeculae of this group are thin and with wide spaces 

 between them. As they traverse the space between the medial 

 and lateral surfaces of the bone they cross at right angles the 

 system of curved trabeculae which rises from the lateral (outer) 

 portion of the shaft. 



It may not be amiss here to call attention to what may ap- 

 pear to be a trivial detail: the question as to whether these in- 

 tersections are always at 90 degrees, and if some of the angles 

 are not acute or obtuse angles. It has been strongly denied 

 by various authors (Albert '00) that these intersections of the 

 trabeculae of the head of the femur are at angles of 90 degrees. 

 For the purpose of illustrating how deceptive right-angle in- 

 tersections of curves may appear, figure 22 shows the inter- 

 sections at 90 degrees of arcs of circles having various radii. 

 The angle between the diverging curves appears greater than 90 

 degrees, while the angle between the converging curves appears 

 much less than 90 degrees. If there is added to this the further 

 apparent distortion of the angles by the curving axis of the 

 upper femur, it is clear that confusion could easily arise as to the 

 true angles between the systems of curved trabeculae. The cor- 

 rect method for the measurement of the angle between two curved 

 lines is to draw through their intersection straight lines per- 

 pendicular to the radius of each curve; the angle between these 

 two intersecting straight lines is the angle between the two 

 curves at their intersection. 



