12 THOMAS DWIGIIT ON THE 



the cup is seen better at another level. We see here the section of a median vertical 

 plate extending from the cup of compact tissue at the front, back to the head where it 

 breaks up into trabeculae. There is a band of spongy tissue running from the articular 

 suiface for the head of the rib obliquely backward and inward. It may be that elastic- 

 ity is gained by the constituent parts of the ball and socket joint at the ends of the 

 bone being made of spongy tissue, which transmits the pressure to deeper solid bone. 

 The articular processes that fit into one another are remarkably solid while the articular 

 tubercle for the head of the rib is cancellated. Fig. 35, showing a frontal section of a 

 dorsal vertebra of a bald eagle, makes a description superfluous. There is a general re- 

 semblance to the dorsal vertebi'a of an alligator that is very sti'iking. If we compare 

 this highly specialized type with the vertebra of a peacock that flies but little, at the 

 first glance the difference appears very slight, but with a little study the wonderful 

 lightness of the plates in the eagle becomes apj^arent. Let us now put together the 

 conclusions that seem justified by this series of studies of bones. 



We started by accepting as established, that the structure of bone both external and 

 internal, is, in general, correlated with the sti-ain it has to resist, but we found that its 

 external shape is modified so as to give room for the attachment of muscles. The in- 

 ternal structure is not modified to resist the pull of the muscles, except in some few 

 instances, and the fibres of tendon are not continuous with the fibi'es of decalcified bone. 



We find also that the strength of the osseous structure increases with the strain on 

 the bone. Thus the trabeculae of the human humerus are lighter than those of the fe- 

 mur, while in the uugulata they are nearly equal, and the lumbar vertebrae of the whale 

 are intermediate in density to the dorsal and caudal ones. 



We find a marked correspondence of structure between homologous parts of bones 

 having nearly the same function, as the proximal extremities of the humerus and femur 

 of the uugulata; and where the function is diverse there is a corresponding diffei-ence of 

 structure, as shown by the shorter neck of the humerus and the longer neck of the 

 femur in man and the apes, the traces of the common plan being recognizable. 



We find a similar correspondence between non-homologous bones having a nearly 

 similar function, as between the olecranon and the end of the calcaneum in many quad- 

 rupeds. 



The study of sections of diff'erent bones in the same animal, and my observations, are 

 by no means limited to the parts discussed in this paper, shows that certain classes, or- 

 ders and genera have a characteristic type. The main characteristics of the bones of 

 fishes and birds are too familiar to require mention, and I have not given enough study 

 to i-eptilian bones to discuss them in detail. Among mammals we find a characteristic 

 structure in the pinnipedia, and another in the cetacea. We find a considerable differ- 

 ence among the uugulata. These features are more marked in some bones of the same 

 skeleton than in others. Beside these peculiarities of the skeleton as a whole, there ai-e oc- 

 casional appearances that are charactei-istic of certain animals. Are these teleological? 



To consider first the outer shape of the bones, it is hard to believe that the needs fin- 

 support and motion should be so much alike in the horse and rhinoceros and so different 

 from those of the even-toed uugulata, that the former should require a third ti'ochanter, 



