THE SKELETON AND ITS JOINTS 365 
reparative processes being more active in tliat the lamel- 
lae are wide and the vascular spaces narrower. As a 
rule the compact bone is absorbed, the Haversian canals 
are more or less confluent and there is generally a marked 
increase of newly formed osseoid tissue. The ossifying 
periosteitis obliterates the depressions for the cranial 
arteries and the sutures. The skull cap becomes finely 
porous, cancellous and even cavernous. The spaces are 
filled with a soft, red marrow-like material. The bones at 
the base of the skull are much less involved; the peri- 
cranium, dura and brain are normal. 
'' The long bones show interlacing narrow strands 
which are in some regions mde, in others narrow or thin 
as in spongy bone. Large irregular cavities are present 
and there is a disappearance of the compact bone and an 
encroachment on the medullary canal of a relatively dense 
new bone with small irregular trabeculae surrounded by 
osteoblasts and a fibrous connective tissue which fills the 
outer trabecular spaces. The new bone is often both 
subperiosteal and subendosteal, the latter often gaining 
on the former. It is always soft and irregularly calcified. 
The general arrangement of the strands in the deep layers 
is longitudinal but in the subperiosteal bone they are very 
irregular and almost at right angles with the central 
strands. In this new osseoid tissue cysts are frequent, 
varying in size from very small to rather large cavities 
filled with a cloudy gelatinous material. The picture here 
is very like osteitis fibrosa, cystica. Endosteal cells pro- 
liferate and may fill up the marrow spaces so that solid 
masses of fibrous tissue result. Frequently the osteoid 
material shows fibrillae. The compact bone may be irreg- 
ular with well marked Haversian systems. Toward the 
periosteum the cells may become scanty, the bone dense in 
structure while toward the interior the cells are more 
numerous and in the more cancellous portions, the 
trabeculae become slender and far apart; here they may 
