146 THE CONNECTIVE TISSUES, BY A. KOLLETT. 



vessels, and containing numerous fat cells, to which last its 

 yellow colour is due (yellow medulla). This fatty medulla can- 

 not in any way be compared with the above-described young 

 medulla of bones in process of formation ; it represents a stage 

 of development of the former which has progressed in another 

 direction, and no osteogenic activity can be attributed to it. 



The medullary spaces of the spongy substance, on the other 

 hand, contain a reddish mass traversed by numerous blood-ves- 

 sels (red marrow), and presenting, with but few fat cells, a large 

 number of granular cells similar to those that are found in the 

 embryonic medulla. Amoeboid movements occur in the cells of 

 the bone medulla analogous to those seen in the colourless blood 

 cells ;* in the latter localities the large many-nucleated masses of 

 protoplasm are found described by Robin-f under the name of 

 Myeloplaxes, and which are most abundant in the external layers 

 of the medullary masses occupying the bone cavities. BredichinJ 

 is of opinion that the colossal cells (Myeloplaxes) proceed from 

 the bone tissue itself, i.e. from the bone cells with coincident 

 absorption of the matrix, and that this conversion is continuous 

 with the formation of medullary canals during the growth of 

 the bone. As the different sized medullary spaces of the bone 

 are continuous with one another, so do also the yellow and red 

 medullse gradually pass into one another. In the skeleton of 

 birds many bone cavities morphologically comparable with the 

 medullary cavity of other animals are filled with air instead 

 of medulla. 



* Bizzozero. Rovida, Wiener Sitzungslerichte, Band Ivi., p. 608 ; and 

 Centralblatt fiir die medicin. Wissenschaften, 1868, p. 245. 



f Journal de V Anatomic et de la Physiologic, 1864, p. 88, plates 1, 2, 

 and 3. 



\ Centralblatt fiir die medicin. Wissenschaften, 1867, p. 563, provisional 

 communication. 



