NO. 3 COMPAEATIVE HISTOLOGY OF FEMUR FOOTE 13 



among the Haversian systems— tlie lamellae are fragmentary-like and short, and 

 may be straight or curved. On cross-section the individual lamellaj are seen 

 to be, generally, of uniform thickness. Their ventral and dorsal surfaces, on 

 the whole, are regularly shaped, and the edges of the cross-sections are finely 

 serrated. The separate lamelte are joined by cement. They may constitute 

 the whole bone structure in a given specimen (pi. A, fig. E), or only a part of it; 

 they may surround the Haversian canal and form the basis of the Haversian 

 system, being then known as Haversian lamellae (pi. A, fig. F) ; they may en- 

 close the whole medullary canal, in which case they are known as internal cir- 

 cumferential lamellae ; or, finally, they may form the external boundary of the 

 bone, where they are known as external circumferential lamellae. The lamella, 

 therefore, may be considered as the primary differentiated unit of bone struc- 

 ture — the first to appear in progressive development. Bone cells, represented 

 by lacuna, may occur within the lamellae or between them (pi. A, fig. F ; pi. B, 

 figs. 1,4). 



It is interesting to notice that the first type bone unit (lamella) is found, 

 in some form, in all femora from amphibians to man, and, as already pointed 

 out, it may vary in its degree of dift'erentiation. According to such differentia- 

 tion, the lamellae may become the foundation of the second and third type bones. 

 The differentiation gives rise to three subtypes of lamellated bone: The uni- 

 formly lamellated, found in many of the amphibians, reptiles, birds, and bats, 

 but not in the higher mammals or man' (pi. 2, fig. 7) ; the tivofold, which was 

 seen in amphibians, reptiles, birds, bats, and in a few mammals, but not in adult 

 man (pi. 2, fig. 9) ; and the threefold, which occurs in amphibians, reptiles, birds, 

 and bats, but not in the higher mammals or man (pi. 2, fig. 27). 



The principal interest in these secondary differentiations of a first type 

 bone lies in the fact that they indicate the manner of origin of divisions which 

 are so frequently present in the higher and later third type or Haversian sys- 

 tem bones. In these the outer and inner lamellar rings are known as the 

 external circumferential and internal circumferential lamelte, while the central 

 broad ring has differentiated into Haversian systems. In some stage of dif- 

 ferentiation, the first type bone is found in all femora as an important structure, 

 and, therefore, may be considered as universal. 



THE SECOND TYPE 



This is composed of laminae in some stage of differentiation, arranged 

 concentrically around the medullary canal. While Haversian systems may be 

 present here and there, the lamiuai dominate and characterize the bone struc- 

 ture. The degree of differentiation present varies. In some cases it is only a 



' Of the total 440 femora examined, 19 per cent were composed entirely of uniformly lamellated 

 bone and were limited to amphibians, reptiles, and bats. (See table, p. 17.) 



