438 



ZOOLOGY 



SECT. 



cent 



oes 



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 organs. The lung is attached only at its 

 root, where the pleural membrane is reflected over it. In this 



respect it differs widely 

 from the lung of the 

 bird. It differs also in 

 its minute structure. 



/ ^jy s'' \ T ne bronchus, entering 



/ _5^*^^L. \ a t the root, divides and 



/fr.pl : \ '' : A-a.*.tr subdivides to form a 



ramifying system of tubes 

 each of the ultimate 

 branches of which, or 

 terminal bronchioles, opens 

 into a minute chamber 

 or infundibulum, consist- 

 ing of a central, passage 

 and a number of thin- 

 walled air-vesicles or al- 

 veoli 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 Glands. 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 larynx. 



Nervous System. The neural cavity, as in the Pigeon, con- 

 tains the central organs of the cerebro-spinal nervous system 

 the brain and spinal cord. The brain (Figs. 1031-1033) of the 

 Rabbit contains the same principal parts as that of the Pigeon, 



Flo. 1030. Lepus cuniculus. Diagram of a trans- 

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

 tricles to show the relations of the pleurae, media- 

 stinum, etc. The lungs are contracted, aort. dorsal 

 aorta ; az. v. azygos vein ; ant. centrum of thoracic 

 vertebra ; I. Ing. left lung ; /. pi. left pleural sac ; 

 1. vent, left ventricle ; my. spinal cord ; (KS. oeso- 

 phagus ; pt.cav.. post-caval, close to its entrance 

 into right auricle ; r. Ing. right lung ; r. pL right 

 pleural cavity ; r. rent, right ventricle ; si. sternum ; 

 v. med. ventral mediastinum. 



