18 smithsonian contributions to knowledge vol. 35 



Type Combinations ' 



In a large number of femora the three structural types — first, second, and 

 third — are seen represented simiiltaneouslj^ in various proportions and form 

 what may be called type combinations. As a rule, in these combinations, 

 the various types occupy similar positions in all the adult femora that present 

 such combinations. Usually first and second type constituents surround the 

 femur peripherally and encircle also the medullary canal, while the third type 

 bone forms the central portion of the sections. The bone units in these cases 

 show varying stages of development. 



While the single types occur as the sole structures in the femora of some 

 amphibians, reptiles, birds, mammals, and man, a large number, perhaps the 

 majority, are composed of various combinations of these types. There may be 

 a marked difference in the degrees of differentiation present in the several 

 structural units in the same bone. 



About GOyo of the whole number of femora examined presented combina- 

 tions of bone types. The first and third were found in association in the 

 largest number. 



Type of Bone Structxtee According to Classes of Animals 



amphibians 



In these animals tiie type of bone present in the various species showed 

 generally evidences of a progressive character in the structural units. There 

 was observed a change in the shape of the lacuna? from round to long and nar- 

 row, in the caualiculi from short and bushy to long and straight, and in the 

 arrangement of the lacuna> from diffuse to concentric. There was also noticed 



'The most common type combinations are as foUows: 



/-//. This combination is composed of lamellae and laminae. It is seen more especially in fetal 

 femora. In young bones the lamellae generally occupy the peripheral portion of the bone, and become 

 separated into lamins nearer the medullary canal. As differentiation advances, much of the first type 

 (lamellar) structure changes gradually into that of the second type (laminar) (pi. C, fig. 1). 



/-///. This is a combination of lamella; and Haversian systems. The lamellae form a sheath of 

 varying thickness around the bone and enclose either wholly or partly a ring of Haversian systems, 

 also of varying thickness. The Haversian systems may be of any grade of differentiation (pi. B, fig. 7; 

 pi. C, fig. 2). 



II-III. This is a combination of lamina and Haversian systems. The laminae, generally, form 

 thin, concentric sheaths around a stout ring of Haversian systems (pi. B, fig. 8; pi. C, fig. 3). Here 

 again the Haversian system may be of various grades of differentiation. If the laminar structure 

 is of advanced type, the Haversian system is also; but if it is not, the Haversian system appears 

 in some of its earlier forms. 



I-II-III. This is a combination of lamella, laminae, and Haversian systems. The simple lamellae 

 are generally located externally, the laminae more internally, and the Haversian systems form the 

 central portions of the bone. Tlie Haversian systems may be of any grade of development. As a rule, 

 the lamellae and laminae are well developed or fairly well developed in these forms of bone, while the 

 Haversian systems vary considerably in their degree of differentiation (pi. B, fig. 9; pi. C, fig. 4). 



