BLOOD-VESSELS OF THE MARROW. 267 



granules are, however, not uncommon. There further occur in the marrow, 

 especially in the neighbourhood of the osseous substance, large multi-nucleated 

 protoplasmic masses (myeloplaxes of Robin, fig. 307, a d), which, as pointed out by 

 Kolliker, appear to be more especially concerned with the process of absorption 

 of bone, under which head they will subsequently be further alluded to. The 

 myeloplaxes vary much in size, but are always larger than the proper marrow-cells. 

 Their nucleus is not always multiple, but when single it is usually enlarged, and 

 presents indications of division (fig. 307, a) ; it may even be so constricted as to 

 exhibit an irregularly moniliform appearance (fig. 307, &). 



Blood-vessels. The bones are well supplied with blood-vessels. A network of 

 periosteal vessels covers their outer surface ; fine vessels run from this through all 

 parts of the compact tissue in the Haversian canals ; others penetrate to the cavities 

 of the spongy part, in which they ramify ; and a considerable artery goes to the 

 marrow in the central part of the bone. In the long bones this medullary artery, 

 often, but improperly, called "the nutritious artery," passes into the medullary 

 canal, near the middle of the shaft, by a hole running obliquely through the 

 compact substance. The vessel, which is accompanied by one or two veins, then 

 sends branches upwards and downwards in the middle of the marrow ; from these 

 branches arterial capillaries pass radially towards the peripheiy. The comparatively 

 narrow arterial capillaries pass suddenly at the periphery of the marrow into the wide 

 venous ones, which form a close network of large thin-walled vessels throughout the 

 medullary tissue, so that the current of blood must be considerably retarded both in 

 these and in the large thin-walled veins. 



The ramifications of the medullary artery anastomose with the arteries of the 

 compact and cancellated structure ; indeed, there is a free communication between 

 the finest branches of all the vessels which proceed to the bone, and there is no 

 strictly defined limit between the parts supplied by each. In the thigh bone there 

 are frequently two medullary arteries entering at different points. 



The veins of the cancellated texture are peculiar and deserve special notice. 

 Their arrangement is best known in the bones of the skull, where, being lodged in 

 the diploe or spongy texture between the outer and inner compact tables, they have 

 received the name of the diploic veins. They are large and numerous, and run 

 separately from the arteries in canals formed in the cancellated structure, the sides 

 of which are constructed of a thin lamella of bone, perforated here and there for 

 the admission of branches from the adjoining cancelli. Being thus inclosed and 

 supported by the hard structure, the veins have exceedingly thin coats. They issue 

 from the bone by special apertures of large size. A similar arrangement is seen in 

 the bodies of the vertebras, whence the veins come out by large openings on the 

 posterior surface. In the long bones numerous apertures may be seen at the ends, 

 near the articular surfaces ; some of these give passage to arteries, but the greater 

 number, as well as the larger of them, are for the veins of the cancellated texture, 

 which run separately from the arteries. 



According to Hoyer and Rindfleisch the venous capillaries and veins of the red 

 marrow have incomplete walls, or rather are channels bounded only by the 

 medullary parenchyma, so that the blood-corpuscles which are being formed from 

 marrow-cells can readily get into the circulation. Langer, on the other hand, found 

 the vascular system of the marrow to be a closed one. In birds, this is certainly 

 the case according to the testimony of Bizzozero and of Denys, but in mammals it is 

 doubtful if the vascular walls are everywhere complete. 



The blood coming from the marrow possesses a large number of pale corpuscles, 

 and sometimes nucleated red corpuscles can be detected in it. 



Lymphatics. In addition to the lymphatics in the periosteum (which have 

 already been mentioned), there are others in the Haversian canals accompanying the 



VOL. I. T 



