26 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 35 



appeared to a greater or less degree in the Haversian systems of such femora; 

 in one instance senile chang-es were manifest although the individual was not 

 over 35 years of age. 



5. FUNCTION 



The effect of function u]jon variation in bone structure can scarcely be 

 doubted in some instances, while in others there seems to be little or no evidence 

 of it. In amphibians the largest bones have the most Haversian canals. This, 

 however, is only true as a general rule. For example, the frog has no Ha- 

 versian canals or laminte, while the toad, which may be smaller than the frog, 

 shows both laminae and Haversian canals. In reptiles, the small lizards do not 

 have the Haversian canals, while the larger ones have many. On the other 

 hand, a large alligator has very incomplete Haversian systems, while a small 

 turtle has more advanced third type units. Again, small turtles have few, 

 while the large ones have many, Haversian systems. In birds, some of the 

 larger varieties, as the ostrich, have predominating second types and not the 

 third, while many small birds have predominating third type units. 



A turkey of 16 pounds weight has the second type of structure with a few 

 Haversian systems, while a turkey of 32 pounds has the same type with a 

 noticeable increase in the number of Haversian systems. That is, size or 

 weight seems to have a decided influence upon tliird type bone development in 

 some cases, and little, if any, in others. 



In mammals of the same species, provided the species has Haversian sys- 

 tems, there will be more of these systems in the larger than in the smaller 

 varieties. But in animals of dift'erent classes the larger species may have, on 

 the average, no more Haversian systems than those of the smaller species. For 

 example, a domestic pig, weighing 500 pounds, has a second type bone with 

 some Haversian systems, while a domestic turkey, weighing 32 pounds, has also 

 a second type bone and nearly as many Haversian systems. The two bones 

 of different classes differ from one another very materially in differentiation of 

 liotli ty])cs, ))nt not in the types themselves. In bipedal mammals of consider- 

 abh' weight like man, in which the weight of the animal is borne by two legs 

 instead of four, there is a greater tendency towards the third type bone, and 

 yet there are many exceptions. Furthermore, the os penis of the raccoon is an 

 Haversian system bone, and conforms, generally, to the femoral type of that 

 animal. In this case it is evident that function has had no effect on bone 

 structure (pi. 20, fig. 288). 



A cessation of function in an adult bone favors the appearance of marks 

 of senility. There is a difference between a rudimentary femur without func- 

 tion and a normal bone which has lost its function by accident. The rudi- 

 mentary femora of the python are first type bones like those of the lizards. 



