SIGNIFICANCE OF BONE STRUCTURE. 9 



At first sight it would seem that in man there is little in common between the olecra- 

 non and the calcaneum. It is possible, however, to recognize in the very delicate spongy 

 tissue of the human olecranon (fig. 22) two systems that pass upward crossing each other 

 in a series of graceful curves. The best marked comes from the posterior border, the 

 other from the articular surface of the great sigmoid cavity. In the gorilla we have the 

 same thing only the scale is larger and the plates stronger. The plan is such as we 

 find in many other prominences. 



Now in the lion we have an example of the arrangement common in qaadrn])cds in 

 which the olecranon, pointing upward just like the os calcis, has a strong tendon im- 

 planted at the extremity in the same way and transmits the weiglit to the shaft of the 

 boue as the os calcis transmits it to the tarsus. The teleological rehitions are strikingly 

 similar. The main difference is in the point last mentioned, that in the os calcis there is 

 a series of plates running downward and forward against the surface for the culjoid and 

 that in the ulna the pressure is transmitted through the shaft. In the lion (fig. 23), 

 which may do for a type, there are found in the elongated olecranon, two series of plates 

 crossing one another at the back strikingly resembling the calcaneum, and we find that the 

 compact substance of the back of the shaft is formed by the aggregation of many distinct 

 plates. In the hear the shape of the olecranon is peculiar. It is pointed and quite dif- 

 ferent from the rather human heel bone but the internal structure shows a decussation 

 of plates. In some of the ungulata the shape as well as the internal structure is very 

 similar. I think that in many cases it would be very difficult to tell whether a section, 

 which one could not compare with others, were through the end of the olecranon or the 

 end of the calcaneum. 



This series shows, therefore, a similarity of structure coincident with similarity of 

 function in non-homologous bones and the presence of the general plan throughout the 

 series. 



We shall next examine the vertebrae and more particularly their bodies. The essen- 

 tial features of the body of a mammalian vertebra are a series of longitudinal plates 

 running from one end to the other and a series of weaker plates crossing them at right 

 angles especially developed near the ends. The surfaces of the pedicles are almost 

 always thick. They give off" plates that in sagittal sections are sometimes seen joining 

 the transverse system and in other places describing curves and joining the longitudinal. 

 Sections parallel to the intervertebral discs show at times series of loops from the pedi- 

 cles across the body decussating with similar ones from the opposite side.' These are 

 probably formed in part by the cut edges of longitudinal plates. The chief system in 

 all mammals is the longitudinal one. 



In man (fig. 24) one is struck by the lightness of the whole structure. Tlie [ilates 

 are nearer together at the ends and wider apart at the centre, where they are thicker. 

 This may serve for a centre of su]iport and also form the walls of the vascular canals. 

 Something may be seen of the oblique systems, but they seem too weak to be of unich use. 



The same general description may apply to the lion (fig. 25) and the dog; but the 

 plates are nearer together and the formation is denser, there being little space in the 

 middle. In the liorse (fig. 26) again the essential system is a dense longitudinal one. 



■ Tin; reader will liiicl much that is iiiteresliiii^ miil valuable iu Bai-delebcu's mouo-raph on the spiuc alix-aily i-ulei-red to _ 



MEMOIKS IIOSTO.N SUC. NAT. HIST., Vol.. IV. 2 



