CHAPTER FOUR 
THORAX 
The shape of the thorax resembles that of a trun- 
cated cone (Figs. 3-1, 4-1, 4-2). Its cranial apex 
at the level of the thoracic inlet {apertura thoracis 
cranialis) is a concave surface circumscribed dor- 
sally by the seventh cervical vertebra, laterally by 
the first rib pair and ventrally by the sternum at the 
level of the clavicular notch {incisura clauicularis) . 
The diaphragm serves as its much larger caudal 
base, curving upward from the eleventh and twelfth 
thoracic vertebrae. The ribs, with prominent curva- 
ture, are the lateral limits. 
The capacity of the thoracic cavity, which varies 
with the phase of respiration, is less than that of the 
bony thorax, because the lower part of the region 
enclosed by the ribs is encroached upon by the 
diaphragm. 
4.1 BONY THORAX 
The bony thoracic cage {ossa cavi thoracis) in- 
cludes the sternum, ribs and vertebrae (Figs. 2-1, 
4-1, 4-2), reinforced ventrally and laterally by a 
thin layer of soft tissue. The cavity is separated from 
the cervical region cranially by the first rib. The 
boundaries are formed by the ribs (costae), the 
spinal column {columna vertebralis) and the ster- 
num (Figs. 2-1, 4-1, 4-2, 4-3, 4-4). The eleventh 
rib, the xiphoid process {processus xiphoideus) and 
a dorsal line connecting the last rib and the inter- 
vertebral disc (discus intervertebralis) of the twelfth 
and thirteenth thoracic vertebrae are regarded as 
the caudal borders of the thoracic cavity, which is 
finally enclosed by the diaphragm (Figs. 4-2, 4-3). 
The length of the thorax from the midpoint of the 
cranial aperture to the midpoint of the caudal aper- 
ture {apertura thoracis caudalis) is 61 mm. The 
external depth of the thoracic space measured from 
the fifth thoracic vertebra to the sternum is 40 to 43 
mm in males and 35 to 40 mm in females (Figs. 
4-1, 4-4); the internal depth is 37 mm. The tho- 
racic inlet has an external depth of 18 to 20 mm and 
an internal depth of 8.5 to 9.5 mm in males and 
females. 
The sternum consists of the manubrium {manu- 
brium sterni), the body {corpus sterni) and the 
xiphoid process (Fig. 4-6). The sternum is subcu- 
taneous and readily palpable. A chondral plate 
{episternum) lies cranial to the manubrium while a 
thin, longitudinally oval xiphoid cartilage {cartilago 
xiphoidea), with an approximate dimension of 8 X 
10 mm, lies caudal to the xiphoid process. The 
xiphoid cartilage lies in the epigastric region, pro- 
jects into the ventral abdominal wall and, under 
extreme distension markedly protrudes, especially 
in young animals, outwards. Four bony sternebrae 
compose the body of the sternum (Figs. 4-2, 4-5, 
4-6), the length of each decreasing caudally. From 
the cartilaginous plate of the manubrium {cartilago 
manubrii) to the caudal end of the cartilaginous 
plate of the xiphoid process, the sternum has a 
length of between 45 and 50 mm in adult males and 
between 40 and 45 in adult females. 
The manubrium articulates with the clavicle and 
the first rib pair. The junction of the manubrium 
and the sternal body is opposite the second rib. At 
this point the cranial edge of the body is displaced 
ventrally, lying above the caudal edge of the manu- 
brium, to form a projection, or sternal angle {angu- 
lus sternae). The angle marks the level of the 
conventional separation of cranial and caudal 
mediastina. The sternal body represents a cranio- 
caudal line below which the pulmonary pleurae 
nearly contact. The cranial margin of the manu- 
brium is opposite the lower edge of the first thoracic 
vertebra, the caudal edge of the sternal body is 
opposite the fifth and the tip of the xiphoid cartilage 
is opposite the seventh. The line of junction between 
the sternal body and the xiphoid cartilage makes a 
palpable depression, to each side of which the 
cartilage of the seventh rib is felt. The tracheal 
bifurcation is at the level of the third to fourth 
sternebrae (Fig. 4-7). 
There are thirteen rib pairs, of which nine are 
sternal {costae verae) and four are asternal {costae 
spuriae). The ribs are not perpendicular but present 
a dorsoventral obliquity which is most pronounced 
in the first two rib pairs (Figs. 2-1, 4-1, 4-2, 4-3, 
4-4, 4-5). Accordingly, with a caudodorsal to cra- 
nioventral curvature, the first rib ventrally corre- 
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