BONE 85 



becoming more widely separated from each other, it is only to a slight 

 extent and in young bones ; they are therefore quite inactive. Each bone 

 cell occupies a space in the matrix, called as in cartilage, a lacuna, but 

 unlike the lacunae of cartilage those in bone are connected by numerous 

 delicate canals, the canaliculi. In ordinary specimens the canaliculi are 

 visible only as they enter the lacunae, which are thus made to appear stel- 

 late. The matrix around the lacunae resists strong hydrochloric acid 

 which destroys the ordinary matrix, and so may be isolated in the form 

 of "bone corpuscles." The "corpuscles" correspond with the capsules 

 of cartilage, which may be isolated in the same way. The bone cells 

 nearly fill the lacunae and send out very slender processes into the canal- 

 iculi. These may anastomose with the processes of neighboring cells, as 



Osteoblast becoming a bone cell. Bone cell. Osteoblast. 



It Uncalcified 



matrix. 



FIG. 72. PART OF A CROSS SECTION OF THE SHAFT OF THE HUMERUS, FROM A HUMAN EMBRYO OF THE 



FOURTH MONTH. X 675. 



can be seen in the embryo, but it is doubtful if this condition is retained 

 in the adult. The processes, moreover, are so fine that ordinarily they are 

 invisible. 



The spicules of bone, containing bone cells and beset with osteoblasts, 

 increase in size and unite with one another, so as to form a spongy net- 

 work enclosing areas of vascular connective tissue. These areas are 

 not entirely surrounded by bone, but retain connections with the exterior, 

 through which the vessels may enter and leave. It is evident that if the 

 spicules continued to thicken, while new ones were added at the periphery, 

 the bone would soon become quite solid and heavy. This is prevented 

 by the destruction or resorption of certain spicules, which begins at a 

 very early stage. It may be studied advantageously in the developing 

 mandible of a pig embryo, 10 cm. in length. At this stage the teeth 

 are growing rapidly, and around each tooth the spicules of bone are 

 being destroyed so as to produce a larger socket; at the same time the 

 jaw is increasing in thickness by the formation of new bone over its outer 



