Clinical Anatomy of the European Hamster 
The intermediate pleural cavity, which freely 
opens with the right pleural cavity, is bordered 
cranially by the wall of the pericardium, latero- 
ventrally and on the right by the plica venae cavae, 
on the left by the dorsal mediastinum, ventrally by 
the sternum and caudally by the diaphragm. The 
accessory lobe of the right lung, which the inter- 
mediate pleural sac invests, lies in a recess {recessus 
mediastini sive cavum pleurae intermedium) be- 
tween the dorsal mediastinum and the pleural fold 
{plica venae cavae) surrounding the vena cava at 
this site. Because of the large size of the accessory 
lobe, the ventral part of the dorsal mediastinum is 
displaced to the left and the capacity of the left pleu- 
ral sac is correspondingly reduced. 
The pericardial sac lies between the two pulmo- 
nary pleurae and is fixed to the sternum by two 
sternopericardial ligaments {ligamenta sternoperi- 
cardiaca). The pericardium extends between the 
second and seventh thoracic vertebrae. 
The pleura are completely in contact with the 
lungs only when the lungs are fully distended. In 
ordinary diaphragmatic breathing, when the lungs 
are not fully inflated, the ventral and caudal mar- 
gins of the lung do not extend as far as the medial 
and caudal limits, respectively, of the pleural reflec- 
tion. Since the costal pleura is also loosely attached 
to the endothoracic fascia and separates easily from 
the chest wall, it collapses on the mediastinal pleura 
forming a thin bursa-like slit, or costomediastinal 
sinus {recessus costomediastinalis). Further, the 
costal pleura, which is not approximated caudally 
by the inferior border of the lung, dips into the 
groove between the costal wall and diaphragm 
forming another slit-like narrowing, the costodia- 
phragmatic sinus {recessus costodiaphragmaticus). 
These two recesses are called reserve sinuses. 
The collapse of the edges of the two pleural folds 
along the lines of reflection prevents the formation 
of a cavity under physiological conditions. Just as 
the pleural space becomes a true cavity only in 
pathology, so the reserve sinuses become visible 
only under abnormal conditions. The reserve sin- 
uses are sensitive to effusions or adhesions, and 
conversion of the slits to true cavities can serve as 
yseful indices for regional tumors. For example, the 
presence of noninflammatory transudates in hydro- 
thorax, which will dilate the reserve sinuses, is often 
symptomatic of fluid accumulation from pulmonary 
compression. Moreover, the reserve sinuses are also 
useful in exploratory intervention. The costomedi- 
astinal sinus will allow transthoracic exploration of 
the mediastinal contents without opening the 
pleura. Further, the fatty areolar interval between 
the costal attachments to the diaphragm and the 
pleural reflection permits an extra-pleural approach 
to the diaphragm and subdiaphragmatic space. 
4.4.2 THE MEDIASTINUM 
The mediastinum, or central part of the thoracic 
cavity, is the space between the lungs bounded by 
the dorsal aspect of the sternum, the ventral surface 
of the vertebral column, the two pulmonary spaces 
and the diaphragm. Its dorsoventral depth at the 
level of the fifth thoracic vertebra is approximately 
40 mm (Fig. 4-1). It includes all of the thoracic 
viscera, except the lungs and pleurae, embedded 
in a thickened extension of the thoracic subserous 
fascia. The mediastinum can be artificially divided 
into a ventral and dorsal part by a frontal plane 
passing ventral to the trachea and its bifurcation 
and the dorsal surface of the heart. It is effectively 
insulated from the abdominal cavity except at the 
three following points: the aortic opening dorsally, 
the esophageal hiatus, and ventrally the narrow 
space between the sternal and costal attachments 
of the diaphragm filled with loose connective 
tissue. 
4.4.2.1 The Ventral Mediastinum 
The cranial part of the ventral mediastinum is 
bounded by the craniothoracic inlet, the cranial 
edge of the pericardium, the manubrium, the frontal 
plane passing ventral to the tracheal bifurcation 
and, laterally, the mediastinal pleura of the two 
lungs. It contains the aortic arch (at the level of the 
first rib pair), the brachiocephalic trunk with its 
right common carotid and right subclavian 
branches, the thoracic parts of the left common 
carotid and left subclavian arteries, the cranial half 
of the two cranial venae cavae (all of which are 
extrapericardial), the thymus gland, scattered 
lymph nodes and, between the venous and arterial 
layers, the vagus and phrenic nerves. 
The caudal part of the ventral mediastinum in- 
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