DEVELOPMENT AND GROWTH OF BONE 23 
endings (Vater-Pacini corpuscles) in the periosteum are to be regarded as sensory, 
and probably are concerned in mediating the muscle sense (kinesthesia) 
DEVELOPMENT AND GROWTH OF BONE! 
The primitive embryonal skeleton consists of cartilage and fibrous tissue, in 
which the bones develop. The process is termed ossification or osteogenesis, and 
is effected essentially by bone-producing cells, called osteoblasts. It is customary, 
therefore, to designate as membrane bones those which are developed in fibrous 
tissue, and as cartilage bones those which are preformed in cartilage. The princi- 
pal membrane bones are those of the roof and sides of the cranium and most of the 
bones of the face. The cartilage bones comprise, 
therefore, most of the skeleton. Correspondingly 
we distinguish intramembranous and endochondral 
ossification. 
In intramembranous ossification the process 
begins at a definite center of ossification (Punctum 
ossificationis), where the osteoblasts surround them- 
selves with a deposit of bone. The process extends 
from this center to the periphery of the future bone, 
thus producing a network of bony trabecule. The 
trabecule rapidly thicken and coalesce, forming a 
bony plate which is separated from the adjacent 
bones by persistent fibrous tissue. The superficial 
part of the original tissue becomes periosteum, and 
on the deep face of this successive layers of perios- 
teal bone are formed by osteoblasts until the bone 
attains its definitive thickness. Increase in cireum- 
ference takes place by ossification of the surround- 
ing fibrous tissue, which continues to grow until 
the bone has reached its definitive size. 
In endochondral ossification the process is 
fundamentally the same, but not quite so simple. 
Osteoblasts emigrate from the deep face of the peri-  p,,. 4 Terr Fawr or Youne Pre, 
chondrium or primitive periosteum into the cartilage Lateran View, to SHow Dr 
and cause calcification of the matrix or ground- SNCs LONG BE CNEINTO 
i 5 SHarr (s) anD EXTREMITIES. 
substance of the latter. Vessels extend into the cal- si dh SS MR AT Ne eae 
cifying area, the cartilage cells shrink and disappear, two parts, head (h) and trochanter 
forming primary marrow cavities which are occupied = major (¢m.), which have _ separate 
by processes of the osteogenic tissue. There is thus "0"? saiaeerely aa ieee 
formed a sort of scaffolding of calcareous trabeculee (©; e1., epiphyseal cartilages; s.f,, 
on which the bone is constructed by the osteoblasts. — supracondyloid fossa. 
At the same time perichondral bone is formed by 
the osteoblasts of the primitive periosteum. The calcified cartilage is broken down 
and absorbed through the agency of large cells called osteoclasts, and is replaced 
by bone deposited by the osteoblasts. The osteoclasts also cause absorption of the 
primitive bone, producing the marrow cavities; thus in the case of the long bones 
the primitive central spongy bone is largely absorbed to form the medullary cavity 
of the shaft, and persists chiefly in the extremities. Destruction of the central 
part and formation of subperiosteal bone continue until the shaft of the bone has 
completed its growth. : 
A typical long bone is developed from three primary centers of ossification, 
’ 1 Only a brief general statement of osteogenesis can be made here; details must be sought 
in embryological literature. 
