10 
The Bulletin 
the ends and a delayed formation of callus, islands or masses of cartil¬ 
age may appear . 8 
The osteoblasts from the periosteum give rise to the ensheathing 
callus and to the definitive callus, especially in the case of long bones. 
The callus is found replacing the periosteum, and extending for some 
distance around the bone both above and below the line of fracture, 
forming a spindle-shaped tumor, by which the ends of the fragments are 
surrounded. It also replaces the structures within the canal by similar 
substance forming the provisional plug or internal or endosteal callus; 
and between the ends of the fragments the permanent intermediate or 
definitive callus. 
The ensheathing callus and the internal callus are gradually organized 
into fibrous tissue, becoming harder and firmer. The outermost layers 
of the fibrous tissue into which the ensheathing callus is thus converted 
form a new periosteum. Ossification of the ensheathing callus is given 
by Spencer and Gask as beginning on the twelfth to the fifteenth day 
in the human , 9 but in fowls it is much earlier. This process usually 
begins in the angle between the periosteum and the hone, and extends 
along the surface of the bone, and also along the surface of the en¬ 
sheathing callus beneath a new periosteum, till the upper and lower 
layers of the ossifying callus meet opposite the line of fracture. 
Ossification of the internal callus goes on in a similar way, but be¬ 
gins a little later. The permanent callus, as soon as the ends of the 
bone are thus fixed by the ensheathing and internal callus, also under¬ 
goes ossification. The ossified callus is at first very vascular and por¬ 
ous, and can easily be stripped off the old bone, but later it becomes 
hard and dense, through formation of new bone around its blood spaces, 
and is then intimately connected with the old bone beneath it. Finally 
the ensheathing callus and internal callus, having completed their func¬ 
tion, are gradually absorbed, and if the fragments have been held in 
good apposition no sign of the fracture may ultimately remain. Spen¬ 
cer and Gask give the time for such processes as being complete in the 
human at six or eight weeks, and that many months elapse before repair 
can be spoken of as complete . 5 In the fowl these processes are much 
more rapid. 
The process of absorption of the temporary callus consists in the 
removal of the lime salts, leaving fibrous tissue, which in turn undergoes 
absorption, so that muscles, tendons, and nerves involved in the callus 
become freed and regain their function. 
The Healing of I?nproperly Set Fractures 
Where the bones are not in exact apposition a modeling process oc¬ 
curs which may transform much of the internal structure of the bone. 
