9 6 



HISTOLOGY 



with the Haversian canal, and their manner of branching may be readily 

 observed. Although these canals are all present in the decalcified bone, 

 they are usually inconspicuous and often invisible. It has been impossi- 

 ble to determine absolutely whether the bone-cells anastomose with one 

 another through these canals, but it is considered probable that their 

 processes do not extend very far into them. 



Vessels and Nerves in Bone. The blood vessels of the marrow, bone 

 and periosteum freely connect with one another. Small branches from 



the arteries and veins of the peri- 

 osteum enter the bone everywhere, 

 through the Volkmann's and Ha- 

 versian canals, and anastomose 

 with the vessels in the marrow. 

 The marrow receives its blood 

 from the nutrient artery, which 

 gives off branches on its way 

 through the compact bone and 

 forms a rich vascular network in 

 the marrow. Of the larger veins 

 which drain this network, one 

 passes out beside the nutrient 

 artery and others connect freely 

 with the vessels in the compact 

 bone. Lymphatic vessels are found 

 only in the outer layer of the 

 periosteum. Numerous medul- 

 lated and non-medullated nerves 

 present in the periosteum, 



FIG. 83. CROSS SECTION OF COMPACT BONE, FROM 

 THE SHAFT OF THE HUMERUS, SHOWING THREE- 

 HAVERSIAN SYSTEMS AND PART OF A FOURTH. 

 (Sharpey.) 



are 



where some of them end in lamellar 



corpuscles. Others enter the Haversian canals and marr6w, chiefly to 

 innervate the vessels. The nerves will be described in a later chapter. 



THE JOINTS. 



Bones may be joined in two ways, either by a synarthrosis which 

 allows little or no motion between them, or by a diarthrosis which permits 

 them to move freely upon one another. 



In a synarthrosis the mesenchymal tissue between the adjacent bones 

 may form dense connective tissue, such as passes from one bone to an- 

 other across the sutures of the skull (Fig. 79); or it may form cartilage, 

 in which case the joint is known as a synchondrosis. The cartilage may 

 be hyaline, as in the epiphyseal synchondroses, but often it is fibrous, as 

 in the intervertebral synchondroses. 



