PHYLUM CHORDATA 



467 



~~N cent 



aort 



T^lnc 



-pt.cav 



rvent 



flS.pi 



par.ver- 



cavity of the thorax. Each hing lies ia a cavity, the pleural sac, 

 hned by & pleural memlrane. The right and left pleural sacs are 

 separated by a considerable interval owing to the development in 

 the partition between them of a space, the mediastinum, in which, 

 as already explained, lie 



the heart and other m^ 



organs. The lung is 

 attached only at its root, 

 where the pleural mem- 

 brane is reflected over it. 

 In this respect it differs 

 widely from the lung of 

 the Bird. It differs also 

 in its minute structure. 

 The bronchus, entering 

 at the root, divides and 

 subdivides to form a 

 ramifying system of 

 tubes, each of the ulti- 

 mate branches of which, 

 or terminal bronchioles, 

 opens into a minute 

 chamber or infundi- 

 bulum, consisting of a 

 central passage and a 

 number of thin- walled 

 air-vesicles or alveoli 

 given off from it. A 

 group of these infundi- 

 bula, supplied by a single 

 bronchiole, which divides within it to form the terminal bronchioles, 

 is termed a lobule of the lung. 



In shape the lung may be roughly described as conical with the 

 apex directed forwards. The base, which is concave, lies, when 

 the lung is distended, in contact with the convex anterior surface 

 of the diaphragm. The outer or costal surface is convex in 

 adaptation to the form of the side-wall of the thorax ; the 

 internal surface is concave. 



Ductless Grlands. — The spleen is an elongated, compressed, 

 dark-red body situated in the abdominal cavity in close contact 

 with the stomach, to which it is bound by a fold of the peritoneum. 

 The thymus, much larger in the young rabbit than in the adult, is 

 a soft mass, resembling fat in appearance, situated in the 

 ventral division of the mediastinal space below the base of 

 the heart. The thyroid is a small, brownish, bilobed, glandular 

 body situated in close contact with the ventral surface of the 



Fig. 1093.— IiCpus cuniculus. Diagram of a trans- 

 verse section of the thorax in the region of the ven- 

 tricles to show the relations of the pleurfe (dotted 

 line), mediastinum, etc. The lungs are contracted. 

 aort. dorsal aoita ; as. v. azygos vein ; cent, centrum 

 of thoracic vertebra ; I. Ing. left lung ; I. pi. left 

 pleural sac ; (, vent, left ventricle ; mi/, spinal cord ; 

 (xs. oesophagus ; par. per. parietal layer of pericardium ; 

 pt. cav. post-caval, close to its entrance into right 

 auricle ;^ r. Ing. right lung ; r. pi. right pleural cavity; 

 r. vent, right ventricle ; st. sternum ; vise. per. visceral 

 layer of pericardium ; v. med. ventral mediastinum. 



