104 EBEN J. CAREY 



factors, the muscles, joints, and various degrees in the condensa- 

 tion of the skeleton are mechanical resultants of the interaction 

 of the forces of differential growth. 



The intensity of the force of the accelerated growth segment 

 of the femur, together with the resisting reactions of the elongated 

 mesenchyme and the restrictions offered to femoral longitudinal 

 extension by the acetabular and tibial centers at the ends, 

 interact to cause the following definite results: First, the defini- 

 tive muscles tend to split opposite the joints from the pre- 

 muscle masses along lines of specific optimum tension. Second, 

 the traction to which the primitive muscles are subjected causes 

 an increase in the volume of this tissue. This is manifested by 

 cytoplasmic differentiation of myofibrils and the mitotic reaction 

 of the myoblasts. Third, the definitive muscles now present a 

 definite point of application in reacting to the traction to which 

 they are subjected by the rapid longitudinal growth of the femur. 

 This tends to outline definitely a more stable base or framework 

 on which the muscles act. Fourth, in addition to the force of 

 elasticity of traction presented by the embryonal muscles, the 

 femoral growth center meets resistance at the proximal and 

 distal ends by the acetabular and tibial growth centers, respec- 

 tively. This tends more and more to outline, then elaborate, 

 the opposed surfaces entering into the formation of the hip- 

 and knee-joints. Fifth, all the above factors converge to cause 

 the femur to assume a characteristic bend. At the center of this 

 bend the mechanical stimulus is first applied, causing the cells 

 to react by forming, first, cartilage, then bone, in the initial 

 differentiation of a progressively more rigid skeleton. Sixth, 

 the differentiation, first, of cartilage at the center of the femoral 

 beam; next, of degeneration of the cartilage, and, finally, periosteal 

 (former perichondrium) bone formation at this center shifts the 

 growing points of the femoral rod to the extremities. 



The differentiation of cartilage and its subsequent degenera- 

 tion at the center of the femoral beam is a critical stage in joint 

 formation. Once a skeletal segment begins to show alteration 

 of its cellular components at the center, they cease to proliferate 

 in the active elongation of the segment. The growth of the 



