98 EBEN J. CAREY 



the heart is beating. All capillaries are then found peripherad 

 to the perichondrium. 



The last change in the above sequence of events on the convex 

 aspect of the bent femur is the initial step in the formation of 

 bone. The perichondrial fibrosis which effectively strangled the 

 cartilage cells becomes modified into a periosteal membrane. 

 Where the cartilage cells are degenerating a proliferation of the 

 osteoblasts from the deep aspect of the periosteum is seen. 

 These cells replace the dead cartilage cells. The osteoblasts 

 form a matrix which is mechanically situated to serve effectively 

 as a cellular reaction to the great strain to which the bent femoral 

 beam is subjected. This appositional growth of bone serves to 

 strengthen the femoral beam at its weakest part. The direct 

 mechanical result of this cellular reaction is the progressive 

 formation of a more stable base for the application of muscular 

 forces. 



The various steps in the increase of skeletal density, first, 

 from the blastemal to the cartilage period, and, secondly, from 

 the cartilage to the osseous period, in skeletal condensation is 

 next to be considered simultaneously with those changes extrinsic 

 to the zone of skeletal formation. During the early stages of 

 development, the weight of the entire limb is supported by the 

 femur acting like a cantilever beam. The weight of the limb 

 rapidly increases (table 1 and chart A). In an 18-mm. pig 

 embryo the femur constitutes one-fifth of the volume of the 

 limb and supports a weight of 0.013 mg. ; in a 38-mm. embryo 

 the femur constitutes one-sixth the volume of the limb and 

 supports a weight of 0.125 mg., whereas, at the 50-mm. stage 

 of the developing embryo, the femur constitutes one-seventh 

 of the volume of the limb and supports a weight of 0.25 gram. 

 Later, at the 20-cm. stage, the femur constitutes only one- 

 sixteenth of the volume of the limb, yet it supports the greatly 

 increased weight of 30 grams. In addition to sustaining the 

 above weight, the femur is opposed in growth by the accelerated 

 growth centers located proximally and distally. Finally, as 

 development continues, the resistance presented to longitudinal 

 femoral growth by the contracting and elastic reacting muscula- 



