24 OSTEOLOGY 
one, which appears first, for the diaphysis or shaft and one for each epiphysis or 
extremity. Many bones have secondary centers from which processes or apophyses 
develop. 
The foregoing outline accounts for the growth of bones except in regard to 
length. Increase in length may be explained briefly as follows: Provision for con- 
tinued ossification at either end of the diaphysis is made by a layer of actively 
growing cartilage—the epiphyseal cartilage—which intervenes between the diaph- 
ysis and the epiphysis. It is evident that so long as this cartilage persists and 
grows, new bone may continue to be formed at its expense, and increase of length 
is possible. When the epiphyseal cartilage ceases to grow, it undergoes ossification, 
the bone is consolidated, and no further increase in length is possible. This fusion 
takes place at fairly definite periods in the various bones, and it is of value to know 
the usual times at which it occurs in the larger bones of the limbs at least. In the 
case of membrane bones, increase in circumference is provided for by the ossification 
and new formation of the surrounding fibrous tissue. 
After the bones have reached their full size, the periosteum becomes relatively reduced and 
inactive so far as its osteogenic layer is concerned; the bone-forming function may be stimulated 
by various causes, as is well seen in the healing of fractures and the occurrence of bony enlarge- 
ments. 
Profound changes occur in the skeleton after birth, and during the period of growth the bones 
are much more plastic than might be supposed. In the new-born foal, for example, it is evident: 
that the metacarpal and metatarsal bones are relatively long and the scapula and humerus short; 
also that in general the shafts of the long bones are slender in comparison with the extremities. 
The various prominences are much less pronounced than in the adult, and most of the minor 
surface markings are absent, so that the bones have a relatively smooth appearance. The period 
of growth may be regarded as terminating with the union of the extremities and shafts of the long 
bones and the fusion of the parts of other bones. During adult life the skeletal changes proceed 
more slowly: they comprise accentuation of the larger prominences and depressions and the ap- 
pearance of smaller ones. These secondary markings are chiefly correlated with the attachments 
of muscles, tendons, and ligaments, or are produced by pressure exerted by various structures on 
the bones. Later in life ossification invades more or less extensively the cartilages and the at- 
tachments of tendons and ligaments. Senile changes in the bones, consisting of decrease of the 
organic matter and rarefaction of the bone tissue, render them brittle and liable to fracture. 
CHEMICAL COMPOSITION OF BONE 
Dried bone consists of organic and inorganic matter in the ratio of 1 : 2 ap- 
proximately. The animal matter gives toughness and elasticity, the mineral 
matter hardness, to the bone tissue. Removal of the organic matter by heat does 
not change the general form of a bone, but reduces the weight by about one-third, 
and makes it very fragile. Conversely, decalcification, while not affecting the form 
and size of the bone, renders it soft and pliable. The organic matter (ossein) when 
boiled yields gelatin. The following table represents the composition in 100 parts 
of ox bone of average quality: 
Gelatin...... Peis 
Phosphate of lime. 
Carbonate of lime. 
Phosphate of magnesia ; 
Carbonate and chlorid of sodium 
100.00 
PHYSICAL PROPERTIES OF BONE 
Fresh dead bone has a yellowish-white color; when macerated or boiled and 
bleached, it is white. The specific gravity of fresh compact bone is a little over 
1.93. It is very hard and resistant to pressure; a 5-millimeter cube of compact 
bone of the ox will resist pressure up to 852 pounds, if the pressure be applied in 
the line of the lamellze (Rauber). Its tensile strength is estimated to be nearly 
twice that of oak. 
