Thorax 
thoracic wall and empties into the thoracic trunk 
(Fig. 4-24). 
4.4.2.2.2 The Esophagus 
The esophagus is a tube-like organ connecting 
the pharynx with the forestomach (Fig. 4-21). The 
cranial thoracic esophagus lies in the medial line. 
Dorsal to the left ventricle it is displaced to the left 
of the trachea and accompanies the aorta. It then 
turns somewhat ventrally and runs in relation to 
the venous branch from the intermediate accessory 
lobe, continuing in the dorsal mediastinum toward 
the esophageal hiatus {hiatus oesophageus) of the 
diaphragm. The thoracic esophagus is approxi- 
mately 25 to 30 mm long (Figs. 4-21 , 4-22). 
Lined with a keratinized stratified squamous epithelium, the entire 
musculature of the esophagus is striated (Fig. 4-23). 
The right and left vagus nerves descend through 
the dorsal mediastinum on the right and left sides, 
respectively, of the esophagus. The left phrenic 
nerve runs caudally on the ventral surface of the 
esophagus where it branches at the diaphragm. 
Paired sympathetic nerves (trunci sympathici) de- 
scend through the mediastinum to the right and left 
of the vertebral column. 
4.5 RESPIRATORY SYSTEM 
4.5.1 Trachea and Extrapulmonary 
Bronchi 
The trachea, occupying the median sagittal plane 
of the chest ventral to the esophagus in the cranial 
part of the dorsal mediastinum, runs for about 
10 mm (Tables 1, 5) from the cranial margin of the 
sternum to the level of the fourth or fifth rib where 
it divides into a smaller left and larger right main 
bronchus {bronchi principales) (Figs. 3-13, 3-14, 
3-15, 4-7, 4-17, 4-21). It is covered by the manu- 
brium, parts of the thymus, the brachiocephalic 
vein, the aortic arch, and the left subclavian and 
common carotid arteries. Dorsally, the trachea is in 
contact with, and remains ventral to, the esophagus, 
which runs above the trachea cranially but which 
now bends slightly to the left in the thorax. The 
right common carotid artery and brachiocephalic 
trunk lie to the right, and the left common carotid 
to the left, of the trachea, which crosses dorsal to the 
aortic arch. Parts of the thymus gland separate the 
vessels layer from the trachea. 
The caliber and structure of the thoracic trachea 
resemble those of the cervical part. The bifurcation 
of the trachea {bifurcatio tracheae) is at the level of 
the fourth thoracic vertebra where it divides into the 
two main bronchi {bronchi principales) (Fig. 4-7). 
The main bronchi are strengthened by C-shaped 
cartilages which run obliquely downward. At the 
margins of the main bronchi, a sagittal spur {ca- 
rina), 3-4 mm in length, runs upward into the 
lumen. The right bronchus continues closely in the 
original tracheal direction, but the left diverges 
more laterally. The bifurcation is displaced to the 
right by the aorta, and the left lung root is displaced 
to the left by the heart. The right main bronchus, 
about 9 mm in length, is about 1 mm longer than 
the left and has 6 or 7 cartilaginous rings while the 
left main bronchus consists of 5 or 6 cartilaginous 
rings. The left and right branches of the pulmonary 
artery run alongside their respective dorsal bron- 
chial surfaces. 
The left main bronchus, which does not divide 
into lobar bronchi (Eckel, et al, 1974a), runs cau- 
dolaterally from the trachea at an angle of 15° from 
the right main bronchus and then continues to the 
base of the left lobe {lobus sinister). Within the left 
lobe parenchyma the main bronchus gives rise to 
eight or ten segmental bronchi, from which several 
subsegmental bronchi and the terminal (lobular) 
bronchioles {bronchi terminales) originate (Fig. 
4-7). 
Before entering the right lung, the right main 
bronchus gives rise to four branches, the lobar 
bronchi, supplying five lobes of the right lung 
(Eckel, et al, 1974a). The first lobar bronchus 
runs dorsally to the right and ventilates the cranial 
lobe {lobus cranialis). Extending to the right, the 
second ventilates the middle lobe {lobus medius). 
Only the third lobar bronchus, which supplies the 
accessory lobe {lobus accessorius) and the interme- 
diate accessory lobe {lobus intermedius accessorius), 
branches to the left side of the lung. The fourth 
lobar bronchus extends caudally, supplies the dia- 
phragmatic lobe {lobus caudalis) and is the immedi- 
ate continuation of the main bronchus (Fig. 4-24). 
Although a single cartilaginous ring is occasion- 
ally found on a lobar bronchus in the lung paren- 
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