202 ON THE COMPOSITION, STRUCTURE 



It may therefore be looked upon as the last stage in the process of 

 bony development. Bones at the earliest period of ossification have 

 s]o medullary canal ; but, as condensation of the osseous tissue takes 

 place, so the medullary canal becomes apparent. Marrow is thus a 

 secondary product in the evolution of osseous tissue. 



The account given of marrow by the authors of works upon phy- 

 siology is generally exceedingly brief and sometimes altogether 

 wanting. 



By Havers it was held that marrow serves to preserve the tempera- 

 ture of bones — that it lubricates the articular extremities, &c., 

 &c. ; of late, however, some have maintained that it possesses 

 osteogenetic power ; but the glance we have given to its anatomy 

 favours no such idea, and numerous experiments fully establish the 

 fact that marrow does not develop bone unless under peculiar path- 

 ological conditions, and even then the ossific matter is due either to 

 the fibrous element of the areolar tissue which enters to a very limited 

 extent into its composition, or is derived bv a process of growth from 

 the bony walls of the medullary canal. 



The bones of birds have their canals filled with air, in order to 

 diminish the weight of the body, and in other animals these same 

 canals are filled with a substance whose specific gravity is less than 

 that of any other organized tissue. When, for the purpose of ex- 

 periment, we fracture the long bone of a bird, marrow is formed in its 

 interior, subsequently osseous union by growth, from the internal sur- 

 face of the bone, takes place, and after a time absorption of the 

 medulla above the point of fracture, finally, after a prolonged period 

 absorption of the medulla below the seat of fracture, and the bone is 

 thus restored to its original condition. 



Having thus learnt that the increase of the diameter of a long bone 

 is dependent upon the osteogenetic power of the sub-periosteal layer, 

 we have now to enquire by what means a bone increases in length. 

 Itmight be supposed that this could be effected by interstitial growth, 

 but further experiments prove conclusively that bone increases in 

 length by addition to its extremities, and that the shaft or diaphysis 

 once ossified increases in diameter only, without any corresponding 

 growth in the direction of the axis. 



Before entering upon the. consideration of this portion of our sub- 

 ject, permit me to give a very general account of cartilage. Cartilage or 

 gristle is the term applied to two structures which, in composition, are 



