CONNECTIVE TISSUE. 257 



nal, pericranium. Five or six years after birth, the latter separates 

 from the skull-bone and furnishes the dense investment of the 

 brain, the dura mater. The first trace of a bony formation is 

 invariably found in the middle field between two blood-vessels, 

 in those localities, therefore, where there is the least nutrition, in 

 the same manner in which the bony formation arises from carti- 

 lage. The further growth of the trabeculae always proceeds from 

 the medullary tissue, particularly from the medullary corpuscles, 

 usually of only one side, lying close to the border of the tra- 

 becula. This process is identical with that of the growth of 

 trabeculae which have arisen from original cartilage. 



For studying the details of the formation of bone from fibrous 

 connective tissue, with high amplifications of the microscope, the 

 best subject also is the skull of a human embryo. (See Fig. 

 106.) 



We observe at first that, from single medullary corpuscles, 

 often grouped together, basis-substance originates, which is at 

 once infiltrated with lime-salts. The embryonal condition is next 

 reestablished in these corpuscles by liquefaction of the basis- 

 substance, and they are again plastids, which may afterward 

 coalesce to form a larger mass, the first indication of a territory, 

 exhibiting a central bone-corpuscle. Still later, a number of 

 such masses, always through the intervening condition of uncal- 

 cified medullary tissue, coalesce, and a trabecula arises, with a 

 number of large and irregular bone-corpuscles, with their stellate 

 offshoots. It makes no difference whether a blood-vessel is pres- 

 ent from the beginning of the process, or whether it is formed 

 afterward; it will always occupy the center of a medullary 

 space. Upon close observation of the distances between the 

 single bone-corpuscles, we are satisfied that at first from only a 

 single corpuscle, and later from a limited number of medullary 

 corpuscles, calcified basis- substance arises in the same manner, 

 therefore, in w^hich the myxomatous basis-substance is formed 

 (see page 148). At first, no distinct territories are present ; these 

 are only recognized later, when the lamellated structure begins 

 to appear. 



Obviously, the originally formed bone-tissue is not perma- 

 nent. The bony trabeculae in turn are repeatedly reduced by 

 liquefaction of their basis-substance to the embryonal or medul- 

 lary condition, and new bone-tissue continues to form up to the 

 time of full development of the body. A continuous absorption 

 and reformation of bone is probably going on throughout life. 

 17 



