518 



THE RABBIT 



CHAP. 



trachea, are supported by incomplete cartilaginous rings 

 at their proximal ends, but these gradually disappear 

 after the bronchi have entered the lungs. 



The elastic lungs (Figs. 135 and 138, 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 rilling 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. 135, p. a) 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 

 infundibulum, which in structure closely resembles a frog's 

 lung in miniature. 



The parietal layer of the pleura (p. 506) 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 on the ventral 

 side of the vertebral column above, and beneath the 

 pericardium below (Fig. 138). Thus each lung (/. Ing, 

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

 separated from its fellow by the right and left media- 

 stinum, 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 (ces) and main blood- 



