NERVOUS TISSUES. 37 



it is required, and a further important change is 

 necessary. The soft transparent tissue, of which it 

 is in great part composed, becomes infiltrated with 

 earthy matter, consisting principally of phosphate of 

 lime and magnesia, and this process continues in 

 progress even until after the animal has attained its 

 perfect form. But the bone, which is first produced, 

 is only a temporaiy structure, and far too weak and 

 brittle for the requirements ; for, besides hardness, 

 bone must possess elasticity in some degree, so that 

 it may stand a sudden blow without breaking. The 

 whole of the bone first formed is, in fact, removed, 

 and gradually replaced by a firmer, harder, much 

 stronger, and more elastic and more permanent 

 tissue, very different in structure from that which 

 preceded it. But this more perfect type of bone 

 tissue could not have been developed at the first. 

 Its production involved a number of preliminary 

 changes useless to the economy until the whole series 

 was fully completed. Nor fiom the structural cha- 

 racters of the early tissue would it have been possible 

 to premise the structure assumed by the permanent 

 bone. Here, as in so many other cases, we see highly 

 elaborate and complex structure anticipated, as it 

 were, at a time when its actual production would 

 have been impossible. It is therefore "certain, that 

 the most thorough knowledge of the properties of the 

 matter of a living being would not enable us to form 

 any conception of the form the tissues were ultimately 

 to take, or the office they were to discharge. 



60. Xervous tissues. — The nervous tissue is de- 

 velopedjja?-/ 2jia.ss« with many other textures of the 

 body. And of all the tissues it undergoes the most 

 remarkable progressive changes. The relation of 

 some of the most important parts of the nervous 

 .ssystem is being continually altered, and yet without 

 any derangement of function in any poi'tion of it. 

 Nerve tissue comes into very close relationship with 



