chap. vii. j BONE. 51 



of a small amount of fibrous tissue as a matrix, and 

 in it are embedded numerous blood-vessels and cells. 

 The few afferent arterioles break up into a dense net- 

 work of capillaries, and these are continued as plexuses 

 of veins, characterised by their size and exceedingly 

 thin walls. The cells are of the same size, aspect, and 

 shape as the osteoblasts of the osteogenetic tissue, and 

 they are called marrow cells. 



In origin and structure, the tissue of the osteo- 

 genetic layer of the periosteum and the marrow are 

 identical. In the embryo, the marrow is derived 

 from an ingrowth of the osteogenetic layer of the 

 periosteum (see below), and also in the adult the two 

 tissues remain directly continuous. As will be shown 

 later, the marrow at the growing ends of the bones 

 is concerned in the new formation of osseous substance 

 in the same way as the osteogenetic layer of the peri- 

 osteum is in that of the surface ; and in both tissues 

 the highly vascular condition and the cells (osteoblasts 

 of the osteogenetic layer, and marrow cells of the 

 marrow) are the important elements in this bone 

 formation. Marrow is of two kinds, according to 

 the condition of the cells. If many or most of these 

 are transformed into fat cells, it has a yellowish aspect, 

 and is called yellow marrow ; if few or none of them 

 have undergone this change, it looks red, and is called 

 red marrow. In the central, or marrow, cavity of the 

 shaft of tubular bones, and in the spaces of some 

 spongy bones, the marrow is yellow ; at the ends of the 

 shaft, in the spongy bone substance in general, and in 

 young growing bones, it is red. 



The cells, especially those of red marrow, are the 

 elements from which normally vast numbers of red 

 blood-corpuscles are formed, as has been mentioned on 

 a former page. 



In marrow, particularly in red marrow, we meet 

 with huge multinucleated cells, called Myeloplaxes of 



