538 PHYSIOLOGICAL DEVELOPMENT. 



borne by the outer layers of fibres. 'Now the shaft of a long 

 bone, subject to mechanical actions of this kind, similarly has 

 its outer layer most strained. In this layer, therefore, on the 

 mechanical hypothesis, ossification should commence, and here 

 it does commence — commences, too, midway between the ends 

 where the bends produce on the superficial parts their most 

 intense efl'ects. But we have not in this place simply 



to observe that ossification commences at the places of greatest 

 stress, but to ask what causes it to do this. Can we trace the 

 physical actions which set up this deposit of dense tissue ? It 

 is, I think, possible to indicate a " tr.ue cause " that is at work ; 

 though whether it is a sufficient cause may be questioned. 

 We concluded that in certain other cases, the formation of 

 dense tissue indirectly results from the alternate squeezing 

 and relaxation of the vessels running through the part ; and 

 the inquiry now to be made is, whether, in developing bone, 

 the same actions go on in such ways as to produce the ob- 

 served effects. At the outset we are met by what seems a 

 fatal difficulty — cartilage is a non-vascular tissue : this sub- 

 stance of which unossified bones consist is not permeated by 

 minute canals carrying nutritive liquid, and cannot, there- 

 fore, be a seat of actions such as those assigned. This ap- 

 parent difficulty, however, furnishes a confirmation. For 

 cartilage that is wholly without blood-vessels does not ossify : 

 ossification takes place only at those parts of it into 

 which the capillaries penetrate. Hence, we get additional 

 reason for suspecting that bone- formation is due to the alleged 

 cause ; since it occurs where mechanical strains can produce 

 the actions described, but does not occur where mechanical 

 strains cannot produce them. Let us consider more closely 

 what the factors are, and how they will cooperate under 

 the particular conditions. It seems possible that 



these canals that exist in the superficial layer of a cartilagin- 

 ous bone before it begins to ossify, are themselves produced 

 by the mechanical actions. For every time a mass of carti- 

 lage is strained and its superficial layers more especially 



