172 STRUCTURE AND ENDOWMENTS OP ANIMAL TISSUES. 



tical section shows the tubuli running nearly parallel, but with occasional 

 undulations, from the internal surface towards the external; but no 

 traces of the original calcigerous cells can be detected in the fully-formed 

 shell, the process of fusion having gone so far as to obliterate them. 

 The manner in which these tubuli are formed, will be presently con- 

 sidered, under the head of Dental substance. 



28T. Now the condition of the osseous skeleton of Vertebrated animals 

 is altogether different. Its purpose is still only mechanical ; its sole use 

 being, to afford support and protection to the softer textures, and to 

 form inflexible levers by the action of the muscles, upon which motion 

 may be given to the different parts of the fabric. But it forms a part 

 of the internal substance of their bodies ; and as these grow in every 

 part, and not merely by addition to this or that portion, so must the 

 Bones also, in order to keep pace with the rest of the structure. Hence 

 we find them so formed, that the processes of interstitial deposition may 

 be continually going on in their fabric, as in that of the softer tissues ; 

 and the changes in their substance do not cease, even when they have 

 acquired their full size. The subsequent continuance of these changes 

 appears destined, not so much to repair any waste occasioned by decom- 

 position, for this must be very trifling in a tissue of such solidity, as 

 to keep the fabric in a condition, in which it may repair the injuries in 

 its substance occasioned by accident or disease. The degree of this 

 reparative power is proportional, as we shall presently see, to the activity 

 of the normal changes, which are continually taking place in the bone ; 

 and is thus much greater in youth than in middle life, and greater in 

 the vigour of manhood than in old age. 



288. The structure of Bones. is well adapted to demonstrate the dis- 

 tinction between the tissues themselves, and those subsidiary parts, by 

 which they are connected with the rest of the fabric. We have seen 

 that Cartilage is essentially non-vascular ; that is, even when it exists 

 in a considerable mass, it is not traversed by vessels, but is nourished 

 by absorption from the fluids contained in the vessels distributed on its 

 exterior. Now every mass of Bone is penetrated by vessels ; nevertheless 

 these do not penetrate its ultimate substance, and may be easily sepa- 

 rated from it, leaving the bone itself as it was. In fact, as Prof. Goodsir 

 observes, " a well-macerated bone is one of the most easily made, and, 

 at the same time one of the most curious anatomical preparations. It 

 is a perfect example of a texture completely isolated ; the vessels, nerves 

 membranes, and fat, are all separated, and nothing is left but the non- 

 vascular osseous substance." Precisely the same may be said of the 

 substance of a Tooth, from which the vascular lining of the pulp-cavity 

 has been removed ; for it then possesses neither vessels, nerves, nor 

 lymphatics ; and yet, as we shall presently see, it has a highly-organizec 

 structure, peculiar to itself. 



289. The general characters of Osseous texture vary according to the 

 shape of the Bone, and the part of it examined. Thus in the long bones 

 we find the shaft pierced by a central canal, which runs continuously 

 from one extremity to the other ; and the hollow cylinder which sur 

 rounds this is very compact in its structure. On the other hand, th( 

 dilated ends of the bone are not penetrated by the large central canal 



