SIGNIFICANCE OF BONE STRUCTURE. 5 



for the support of the bone unclei- the changed conditions and that the strain of the 

 mnscles being diffused over its suriaccthe internal structure shows no special adaptation 

 to meet it. Finally, that, in some few eases, there are apparent exceptions to this last rule. 



We have so fiir considered an individnal bone as if it were simply a piece of a ma- 

 chine which, to fulfil its function, must have a certain size and shape and a certain power 

 of resistance. Othei- and broader questions now present themselves. 



"We know that certain classes and orders of animals have a characteristic structure of 

 bone of which the most strikiug instances are the slender bones of fishes and the ex- 

 panded bones of birds. The same is true of certain orders of mammals; but we do not 

 know just how far snch characteristic structui-es can be ascribed to genera or to species. 

 That these peculiarities are in general advantageous may be considered as accepted, but 

 are they always soV Do they extend to eveiy bone of the skeleton? Are useless or 

 rudimentary structures found in bone? Do i)articular bones show a similarity to corre- 

 sponding bones in other animals, and if so, is the structure in each case solely teleologi- 

 cal? In fine is there ground for Wolff's assertion that a bone is built on the only 

 possible plan ? 



It was the writer's intention originally to limit the discussion to mammalian bones, and 

 this plan has been followed in the main, but he has been led in one or two instances to 

 refer to those of other classes. 



Let us now pass in review, the proximal ends of the humerus and femur of a numl)er 

 of mammals, beginning with man, and having compared the bones of the two extremities 

 in the same species together, then compare each with the corresponding bones of other 

 animals. 



Man. These bones have been so often described, that a sunnnaiy sketch shall suffice. 

 In the humerus (fig. 2) the shaft is thin, the spongy tissue light and its plan veiy indis- 

 tinct. It is denser in the head Avhere it is of the round-meshed pattern. It is very light 

 in the greater tuberosity. Sometimes a system of plates I'unning up into the head from 

 the inner side of the shaft can be made out. The femur (fig. 3) is of a much heavier 

 make, the shaft is much thicker and the plan distinct. In brief, this consists of decussa- 

 ting plates from either side of the shaft, of a very well marked scries, running from the 

 lower side of the neck into the dense, round-meshed head, of a series of arches from 

 the outer shaft which meet and perhaps pass through this series, and finally of the light 

 tissue in the trochanter major with a few plates parallel to its surface. The important 

 differences between the bones ai'e the rudimentary neck of the humerus, its much lighter 

 structure and indistinct i)lan. The teleological significance is evident. 



Apes. The humerus of the chimpanzee (fig. 4) might easily be mistaken for a small 

 human one, its chief difference being a greater density of structure. The femur (fig. 5) 

 is on the same plan as in man. The tissue of the head is veiy dense. Its centre is al- 

 most solid bone. The bones are larger in the (jorllla. The humerus (fig. 6) is a little 

 broader and the series of plates running to the head from the inner side of the shaft is 

 more defined. The fenuir (fig. 7) is very like the human one, but below the trochanter 

 the shaft is convex. Tlie head is veiy dense. In both these animals the femur is much 

 denser and heavier than the humerus, though the difference is less than in man. It is 

 hard to say whether the difference between the two is greater in the chimpanzee or the 

 gorilla. In the mandrill the humei'us (fig. 8) is broader, the plates from the shaft show 



