X RESPIRATORY ORGANS 501 



are supported by incomplete cartilaginous rings at their 

 anterior ends, but these gradually disappear after they have 

 entered the lungs. 



The elastic lungs (Figs. 125 and 128, Ing) are not hollow 

 sacs, like those of the frog, but are spongy bodies, of a light 

 pink colour, situated on either side of and above the 

 heart, and filling the greater part of the thoracic cavity but 

 collapsing as soon as the wall of the thorax is perforated. 

 Each is subdivided into two main lobes, and the right lung 

 has in addition two small accessory lobes, an anterior and a 

 posterior, the latter lying in the median line, behind the 

 heart, and being closely applied to the gullet. 



Each pulmonary artery (Fig. 125,^.0,) crosses the main bronchus 

 anteriorly to the point at which it branches into the various lobes, 

 except in the case of the anterior accessory lobe, the bronchus to which 

 comes off in front of the artery and may even arise from the trachea 

 before its bifurcation. Microscopic examination shows that the bronchi 

 divide and subdivide to form a ramifying system of tubes, each ultimate 

 branch of which opens into a minute chamber or infundibuhim, which 

 in structure closely resembles a frog's lung in miniature. 



The parietal layer of the pleura (p. 490) lines the cavity of 

 the thorax, and is reflected over each lung at the entrance of 

 the bronchus to form the visceral layer : in the median line 

 it forms a vertical partition, the mediastinum, with which it 

 is continuous ventrally to the vertebral column above and 

 beneath the pericardium below (Fig. 128). Thus each lung 

 (/. Ing, r. Ing) has its own separate pleural cavity (I. pi, r. pi), 

 separated from its fellow by the right and the left medias- 

 tinum, the space between which is called the mediastinal 

 space. The anterior and dorsal parts of this space are narrow 

 and enclose the posterior part of the trachea and the bronchi, 

 as well as the gullet (oes) and main blood-vessels (aort, 

 az.v, pt.cav) ; its middle part is wide, and encloses the heart 



