THE LUNGS 



551 



five U-shaped rings; the ends of the rings do not meet dorsally, so that here the 

 trachea has a membranous wall (Paries membranacea) , and is composed of a layer 

 of transverse smooth muscle-fibers outside of the rings, the fibrous membrane, and 

 the mucous membrane. 



The stem-bronchi diverge at an obtuse angle, and each divides into two 

 branches before entering the lung, but the ramification differs in the two lungs. 

 In the right lung the anterior bronchus goes to the apical lobe, and the stem- 

 bronchus gives off a branch to the cardiac lobe and another to the intermediate lobe. 

 In the left lung the anterior bronchus divides into two branches for the apical and 

 ardiac lobes. The anterior branch of the left bronchus crosses under the pul- 

 monary artery. 



THE LUNGS 



The lungs differ in shape from those of the horse and ox in conformity with 

 the shape of the thorax, which is relatively very wide in the dog; the lateral thoracic 

 walls are strongly curved, and the costal surface of the lungs is correspondingly 

 convex. 1 



The right lung is much. (ca. 25 per cent.) larger than the left. It is divided 



Dorsal border 



Cardiac notch 



Fig. 491. — Right Lung of Dog; Medial View. 

 L. a., Apical lobe; L. c, cardiac lobe; L. d., diaphragmatic lobe; L. i., intermediate lobe; 

 bronchi of apical lobe; 3, J', divisions of pulmonary artery; 



.?, stem-bronchus; ^, ^', 

 pulmonary veins; S, groove for anterior vena cava; 

 S, groove for%ena azy'gos; iO, groove for oesophagus; J i, groove for trunk of vertebral, cervical, and dorsal veins; Ig, 

 groove for internal thoracic vessels. 



into four lobes by deep fissures which extend to the root. The lobes are the apical, 

 cardiac, diaphragmatic, and intermediate. The apical lobe extends considerably 

 over the median plane in front of the pericardium. The intermediate lobe has the 

 form of an irregular, three-sided pyramid, with its base against the diaphragm and 

 its apex at the root; on its lateral face there is a deep groove which contains the 

 posterior vena cava and right phrenic nerve, enclosed in a special pleural fold. The 

 cardiac impression of the right lung is much deeper than that of the left. The 



'The costal attachment of the diaphragm is lower than in the other animals, thus further 

 increasing the capacity of the thorax (vide diaphragm). 



