WESLEY D. ANDERSON 
771 
Intrai'iilmonafy bponolil 
Figure 4. — Trachea, bronchi and associated structures 
of man. 
Reproduced by permission from: J. C. B. Grant, AN ATLAS OF 
ANATOMY (ed. 5). Copyright (1962). The Williams & Wilkins Co., 
Baltimore. 
in the young extends from the neck into the 
cranial mediastinum. The right lung of sheep 
and calf is larger than the left lung and is com- 
posed of four lobes. They are : the large cranial 
(formerly the apical lobe) lobe with its cranial 
portion and caudal portions, the middle lobe 
(formerly cardiac lobe), the caudal lobe (for- 
merly the diaphragmatic), and the accessory 
lobe (formerly called the azygos lobe). 
Aorta 
The ascending aorta of sheep weighing be- 
tween 120 and 220 lbs., is approximately 2.4 cm 
in diameter and 1.5-2.5 cm long,i while the 
ascending aorta in man is generally 5-5.5 cm 
in length 2 (Figures 7 and 8). After the ascend- 
ing aorta gives origin to the right and left 
coronary arteries, it ascends cranially and dor- 
sally in the middle mediastinum between the 
pulmonary trunk and right atrium. The ascend- 
ing aorta is covered by fibrous pericardium and 
shares a serous reflection with the pulmonary 
trunk. It is continued in the cranial mediastinum 
as the aortic arch and passes upward and 
slightly to the left. The aortic arch in the sheep 
and calf is characterized by giving off a single 
large branch, the brachiocephalic trunk, while 
in man it most commonly gives origin to the 
brachiocephalic trunk, left common carotid and 
left subclavian arteries. Occasionally in sheep, 
calf and man a small artery (or arteries) origi- 
nates from the left surface of the aortic arch at 
the level of the brachiocephalic trunk and de- 
scends to ramify over the pericardial sac and 
atria and is called a "middle coronary" artery. 
Variations in the Aortic Arch 
Figure 8 shows some variations in the 
branches of the human aorta. In contrast to the 
many anatomic variations in the aortic arch in 
man as previously mentioned, the aortic arch 
of sheep usually has only a single large branch, 
the brachiocephalic trunk. The aortic arch of 
the dog, cat, horse and pig on the other hand 
most commonly has two branches namely the 
brachiocephalic trunk and left subclavian artery 
arising from it. 
The Bronchial Arteries 
In quadruped mammals the bronchial and 
esophageal arteries commonly arise from the 
descending thoracic aorta as a trunk, the bron- 
choesophageal artery. Occasionally bronchial 
and esophageal branches will arise individually 
(Figure 9) which is the case most commonly 
in man. When bronchial and esophageal 
branches do not arise from the descending 
thoracic aorta, they are branches of the dorsal 
intercostal arteries IV-VI. The bronchial arter- 
ies are important nutritive arteries and accom- 
pany the bronchi distally as far as the alveolar 
ducts and supply the bronchioles, the connective 
tissue septa, vasa vasorum to the walls of the 
large pulmonary vessels, and the bronchiolar 
epithelium. In recent years with the advent of 
heart and lung transplantation and research 
into the development of cardiac assist devices, 
the significance of bronchial arteries has been 
emphasized, and thus they should be preserved 
in surgical procedures of the thorax. In sheep 
the bronchial arteries are generally 2-3 mm in 
diameter and in Figure 9 one is seen arising 
from the lower surface of the descending tho- 
racic aorta and passing with pulmonary arter- 
ies and veins deep into the substance of the 
