572 MANUAL OF HISTOLOGY. 



tium, into which an arterial twig is sent which makes its way eventually 

 into the central medullary canal as the arteria nutritia. The latter then 

 divides into an ascending and descending branch, which again break up 

 into a capillary network, including the fat-cells of the medulla in its 

 loops (see below), and giving off a series of vessels which enter the 

 internal opening of the Haversian canals to anastomose with those coining 

 from the periosteum. In the epiphyses, also, the supply of blood is 

 partly from without, through small vessels derived from the periosteum, or 

 larger twigs entering through the more numerous nutritious foramina of 

 these portions ; and partly from within, through close connection with the 

 vessels of the medullary canals of the diaphysis. These vessels, then, are 

 situated, in the first place, within the Haversian canals, and again distri- 

 buted through the medullary cavities. 



The course of the veins is analogous to that of the arteries. One set of 

 venous vessels convey the blood out of the part through the larger and 

 smaller nutrient canals ; another set of branches return to the periosteum 

 by the peripheral openings of the smaller medullary canals. 



Turning now to the other kinds of bones to the short and tabular, 

 namely, we find that they present the same arrangement of vascular 

 supply as the epiphyses, with the exception of the flat bones of the head. 

 Through the many openings, namely, on their surfaces, small arteries and 

 veins make their entrance and exit : their terminal branches are found, 

 however, more in the medullary cavities than the scanty Haversian 

 canals. The flat bones of the cranium, on the other hand, are supplied 

 by numerous fine arterial twigs, which enter through holes in the two 

 vitreous plates, and break up in the cavities of the diploe into capillaries 

 interlacing amongst themselves. The veins, however, present themselves, 

 as was discovered by Breschet, in quickly-branching wide bony canals, in 

 the form of very thin-walled tubes traversing the diploe in various direc- 

 tions, and emptying themselves partly into the external veins of the 

 head, and partly into those of the dura mater. The cartilage covering 

 the ends of bones is quite destitute of vessels. 



The existence of lymphatics in osseous substance has not been demon- 

 strated to. a certainty. 



The nerves with which bones are supplied present the same arrange- 

 ment as the blood-vessels. The periosteum is very richly supplied with 

 them ; but the greater proportion simply pierce this membrane to reach 

 the osseous tissue beneath, so that, in fact, but a' small number properly 

 belong to it itself. In this respect, however, the periosteum varies greatly, 

 according to locality : in some spots it appears to be quite without nerves, 

 while in others it is richly supplied. The nerves consist of broad and 

 medium-sked fibres which split up before their termination. 



One set of nerves enter the bone with the blood-vessels which pass 

 through the periosteum, by means of the Haversian canals : these are 

 very fine. Other stronger twigs find their way into the interior through the 

 foramina nutritia. From thence they are distributed to the larger medul- 

 lary cavities. Their ultimate termination is still a matter of doubt. Many of 

 the short and flat bones are, according to Koelliker, very highly innervated. 

 Most of the nerves are derived from the cerebro-spinal system. 



The capsules of the joints are also very rich in nervous supply, while 

 the ligaments are but scantily furnished with sentient elements. 



The cavities of the bones are filled up with a substance known as the 

 marrow. This presents itself under two forms, with intermediate varie- 



