NO. 3 COMPARATIVE HISTOLOGY OF FEMUR FOOTE 21 



instances, though combinations of types are the most numerous and character- 

 istic of mammalian bone. The Haversian systems have differentiated into their 

 later stages, to reach their highest degree of perfection and prominence in man 

 (pis. 9-20, figs. 168-298). 



Man. — In human femora the bone structure, as already mentioned in part, 

 reaches, in many respects, the climax of its differentiation. The human femora 

 examined were the fetal and adult. The fetal bones belonged to the black and 

 white races, the adult to the black, yellow-brown, and white including the ancient 

 EgyiDtian. The number of fetal femora examined is small, but the results indi- 

 cate that the study of the comparative histology of a larger number of fetal 

 bones, not only of man but also of other animals, would clear up many interest- 

 ing points in the differentiations which are found in the adult (pis. 21-35, figs. 

 306-453). 



Fetal human femora. — These femora, representing the whole period of 

 ihtra-uterine life, showed basic bone substance in the early, and differentiated 

 lamellae and laminae in the later months ; also there was observed the horseshoe- 

 shaped band forming the anterior and lateral wall and the separate formation 

 of the posterior ridge (pi. 21, figs. 299-305). (For details see section III.) 



Advlt femora. — Generally speaking, the adult human femur is character- 

 ized by the predominance of completely differentiated third type units. An 

 exclusive first type was not found in the adult bone. The most prunitive form 

 was a first and third combination. The proportions of the structural units 

 (lamellae, lamiuse, and Haversian systems) vary greatly. In some femora first 

 type bone amounted to more than half of the sections ; in others it was reduced 

 to a small fraction; in still others there was a second and third instead of a 

 first and third; and, finally, some femora showed the first, second, and third type 

 bone in various combinations. 



When first type bone is present in the human femur it is found in the 

 form of a horseshoe-shaped band situated underneath the periosteum. The 

 inter-Haversian lamellae, frequently present in sections of human femora, are 

 apparently the remains of the disappearing horseshoe band just described. In 

 a fully differentiated human bone. Haversian systems form the whole structure. 



The three races, black, yellow-brown, Egyptian, and white, exhibit similar 

 types and combinations of types of bone structure. In each there is a first and 

 third, second and third, and complete third type, with or without senile changes. 

 As a race the white presents more third type femora than the black or yellow- 

 brown race. Early (primitive) and late (advanced) differentiations have been 

 found in the different femora of the negro, Pueblo, and Peruvian Indians, the 

 ancient Egyptian, and modern white. The linea aspera in the adult human 

 femur is always composed of Haversian systems. Senile changes, absent or 

 rare in other animals, are unexpectedly frequent in human femora. 



