252 THE VISCERA. 
(Fig. 119, a, page 284), the vagus and sympathetic nerves 
(Fig. 156, z), and the internal jugular vein (Fig. 119, 8). 
In the thoracic cavity the great vessels coming from the 
heart lie against the ventral surface of the trachea (Fig. 
129, 2). At about the level of the sixth rib the trachea 
‘divides into the two main bronchi (Fig. 106). Each bronchus 
is supported by incomplete rings of cartilage like those of 
the trachea and has otherwise in general the structure of the 
trachea. In the lungs the bronchi become divided into many 
branches (Fig. 106), in the manner described in the account 
of the lungs. 
4. The Lungs. Pulmones.—Immediately after division 
of the trachea the two bronchi enter the lungs (Fig. 106). 
These are two large, much-lobed organs, which fill the greater 
part of the thoracic cavity. The main lobes of the lungs are 
completely separated from each other, except in so far as they 
are connected by the bronchi and connective tissue; the main 
lobes may also be partly subdivided into secondary lobes that 
are not thus completely separated. The two lungs are com- 
pletely separated from each other, except at the radix, where 
they are united by the bronchi; they lie in the right and left 
halves of the thoracic cavity, with the mediastinal septum 
between them. The bronchi on entering the lungs divide in 
the following manner. Each divides at first into two main 
branches. The cranial branch on the right side is known as 
the eparterial bronchus (4), because it lies craniad of the pul- 
monary artery. All the others are hyparterial; i.e., they lie 
caudad of the pulmonary artery. The right cranial bronchus 
does not further subdivide into large bronchi, but gives off 
numerous small branches. The right caudal bronchus divides 
into three main branches. There are thus four main branches 
of the right bronchus. The left cranial bronchus divides into 
two main branches; the left caudal bronchus continues caudad 
as a main trunk giving off small branchlets. Of the left 
bronchus there are thus but three main subdivisions. The 
main subdivisions of the bronchi on the two sides correspond, 
as will be seen, with the lobulation. of the lungs. 
The right lung (Fig. 106, 14) is slightly larger than the 
