8 THOMAS DWIGIIT ON THE 



Reserving tilllater a discussion of several questions, Ave may mal^e from this series of 

 specimens at least the general conclusion that these homologous bones of the two ex- 

 tremities of the same animal correspond closely in structure when their function is nearly 

 the same, and that when their function is different their structure differs also but that 

 the main featui-es of the common plan can still be recognized. 



The study of the calcaneum and the olecranon is very instructive. A longitudinal sec- 

 tion of the human heel bone (fig. 17) may be described as follows: There is a thickening, 

 from which plates radiate, situated at the lower border of the upper articular surface. 

 Some go forward to the plate that rests against the cuboid, others downward, others back- 

 ward and downward, the latter being joined by other plates arising- above the point 

 mentioned. There is then a series of plates acting as ties passing from the posterior 

 towards the anterior surface describing a curve Avith a downward convexity. These 

 plates come very near to one another at the under surface of the bone. The Avhole 

 structure may very aptly be compared to a triangular rafter even to the appearance 

 of a vacant space in the middle at the neutral point. Besides these systems there is 

 often seen a series of plates parallel to the posterior surface held to be continuous 

 with the fibres of the tendon. 



This structure is so admirably adapted to the upright position that it seems very sur- 

 prising that a strikingly similar arrangement should be found in the os calcis of quad- 

 rupeds in which its position is utterly different, the hind end pointing upward in the air 

 instead of resting on the ground. Merkel uses this as an argument against the teleo- 

 loo-ical significance of the internal structure of bone, and it long seemed to me an al- 

 most insuperable difficulty; but in fact, rightly interpreted, it is a strong argument 

 on the other side. Let us suppose a man in the act of running Avith one foot in the air 

 and the whole weight of the body resting on the toes of the other. The heel is raised 

 and the foot is in the position of a quadruped's. Most of the Aveight is, of course, 

 transmitted through the leg to the os calcis and it may be divided into two components, 

 one running forAvard to the cuboid, the other backward to the tuberosities of the bone. 

 These no longer rest on the gi'ound and the pressure conveyed to that end of the bone 

 is resisted by the tense tendo Achillis, so that the conditions are essentially the same in 

 running and in standing; only of course there is a far greater strain in the former posi- 

 tion Avhich is the one normal to quadrupeds. 



The bone in the cliiinpanzee (fig. 18) is A^ery like the human one but the series of 

 plates that runs backward does not appear to reach the inferior surface of the bone. 



In the plantigrade bear (fig. 19) the bone is relatively longer, but the principal new 

 features it presents are the gi'eater thickness of the upper and lower surfaces. In the 

 lion (fig. 20), horse, deer and gazelle the bone is more elongated, but the same general 

 description Avill apply to all. In the seal (fig. 21) the bone is not used to bear the weight 

 of the animal but for purposes of pi-oj)ulsion in swimming. The contraction of the mus- 

 cles presses it against the astragalus and the line of pressure divides as in the other 

 animals. We find, therefore, an essentially similar plan of internal structure. The thick 

 loAver border is seen to be comj^osed of many of the laminae of the inferior system Avhich 

 meet at this ])lace. The structure of the bone is lighter than in most of the other ani- 

 mals mentioned, but the characters of the seal are less distinct than in the long bones 

 or the vertebi'ae. 



