1890-91.] Drs Symington & Thomson on Defective Ossification. 283 
present. The entire thickness is occupied by the periosteal bone, 
as described. This condition of affairs naturally presents an 
important obstacle to the development and ramification of the 
medullary blood-vessels ; their extension or projection towards the 
ossifying junction at the ends of the diaphysis would be specially 
interfered with. Probably this interference with the vessels has 
played an important part in the causation of the arrest in endo- 
chondral ossification, to be immediately described. 
The marrow itself is rich in small vessels, chiefly capillaries, while 
it is deficient in those of larger size. Further, very few giant cells 
are to be seen, and scarcely any Howships’ foveolse. The periosteal 
bone is almost continuously invested by osteoblasts. At the ends 
of the shaft the periosteal diaphysis is peripherally extended so as 
to form a cup, which embraces the cartilaginous extremity and a 
small wedge-shaped mass of endochondral bone. The apex and 
sides of this endochondral wedge occupy the concavity of the cup, 
while its base corresponds to the junction between it and the terminal 
cartilage (see Plate III.). 
Although the endochondral and periosteal bone are thus in imme- 
diate contact, they are readily differentiated from each other by their 
structure and connections. The ossifying junction between the 
endochondral bone and the terminal cartilage is in the form of an 
irregularly curved line, concave towards the diaphysis. The endo- 
chondral bone consists of a very irregular honeycomb, made up of 
branching masses, each of which contains a core of cartilage in the 
centre ; the bone is non-lamellated, and stains very intensely with 
carmine or eosine. It appears to have been formed by a direct 
conversion or metaplasia of the cartilage into bone, after the manner 
described by Kassowitz,* as occurring in the course of normal ossi- 
fication in cartilage, and similar to the process observed under 
pathological conditions in cartilage of new formation. By direct 
conversion of cartilage into bone, we further mean to convey that 
there is an entire absence of proliferative changes, or of any activity 
whatsoever in the cartilage itself, previous to its ossification ; there 
are no parallel rows of cells, no progressive formation of medullary 
spaces by the projection of medullary blood-vessels into the carti- 
* Die Normale Ossification , Wien, 1881. 
