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94 OSTEOLOGY. 
extremities of the first eight costal arches. In some rare cases only six pairs of ribs articulate 
by means of their costal cartilages with the sternum. ‘ 
Occasionally the presternum supports the first three ribs; in other words, the manubrium 
has absorbed the highest segment of the body. Keith has pointed out that this is the condition 
most commonly met with in the Gibbon, and regards its occurrence in man as a reversion to the 
simian type. As far as is at present known, its occurrence seems more common in the lower 
races. Through errors of development the sternum may be fissured throughout, due to failure 
of fusion of the cartilaginous hemisterna. The two ossified halves are usually widely separated 
above, but united together below by an arthrodial joint. The heart and pericardium are thus 
uncovered by the bone. Occasionally this condition is associated with ectopia cordis, under 
which circumstances life 
is rendered impossible. 
Through defects in ossi- 
fication the mesosternum 
may be pierced by a hole, 
usually in its lower part, 
or through failure of 
fusion of the lateral cen- 
tres one or more of the 
segments of the body may 
be divided longitudinally. 
Occasionally small ossicles are found in the ligaments of the 
sterno-clavicular articulation. These are the so-called episternal 
bones, the morphological signifiance of which, however, has not yet 
been satisfactorily determined. They are by some regarded as the 
homologues of the interelavicle or episternal bone of monotremes. 
Head Neck Tubercle 
Angle 
Articular part 
of tubercle 
THE RIBS. 
The ribs (costa) of which there are twelve pairs, form a 
series of curved osseous bands which support the thoracic 
wall; posteriorly they articulate with the dorsal or thoracic 
vertebrie, anteriorly each rib is provided with a costal 
cartilage. The first seven ribs articulate with the sternum 
by means of their cartilages, and are termed the true (costee 
vere) or vertebro-sternal ribs. The lower five ribs are not 
so supported, and are described as the false ribs (costie 
spurie). Of these the eighth, ninth, and tenth are united 
by their cartilages to the cartilage of the seventh rib, and 
are called the vertebro-chondral ribs, whilst the last two 
ribs are free at their anterior extremities, and are named 
the floating or vertebral ribs. 
A typical rib consists of a head (capitulum cost), 
a neck (collum cost), a tubercle (tuberculum coste), and a 
shaft (corpus cost), on which, near its posterior end, is the 
angle (angulus cost). 
The head, placed on the posterior or vertebral end of 
the bone, is somewhat expanded. Internally its articular 
surface is wedge-shaped and divided into two parts, an 
upper and lower, by a ridge or crest (crista capitull), to 
which the inter-articular ligament is attached. Of these 
two facets the lower is usually the larger, and articulates 
with the upper facet on the body of the vertebra in 
numerical correspondence with it, whilst the upper facet is 
for the corresponding area on the lower part of the body 
of the vertebra above. The head is supported by a more 
or less constricted bar of bone, the neck, This becomes 
continuous with the shaft externally, at which point there jy 79, #era Rent Rm 
is a well-marked tubercle on its posterior surface. The AS SEEN FROM BELOW. 
anterior surface of the neck is smooth; its posterior aspect 
is rough, and pierced by numerous small holes for vessels. Here is attached the 
middle costo-transverse hgament. Not uncommonly the upper border of the neck 
is lipped and ridged (crista colli cost), and affords attachment to the anterior 
costo-transverse ligament. 
Nutrient foramen 
Shaft————_ 
For costal cartilage 
