252 THE VISCERA. 



(Fig. 119, a, page 284), the vagus and sympathetic nerves 

 (Fig. 156, z), and the internal jugular vein (Fig. 119, b]. 

 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 (/;), 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, 1-4) is slightly larger than the 



