40 ANIMAL BIOLOGY. [Parti. 



ened with cartilaginous rings, strong ventrally, but incomplete 

 dorsally. Posteriorly the trachea bifurcates to form the two 

 bronchi, one for each lung, which break up within the spongy 

 substance of the lungs into a multitude of bronchial tubes. 



The mechanism of respiration differs from that in the frog. 

 Each lung lies in a separate pleural cavity, lined with a delicate 

 membrane, the pleura, a visceral layer of which is reflected on to 

 the lung. This cavity the lung completely fills. The pressure 

 of the air in the lung forces it to fill this cavity, which itself 

 contains no air or other gas. Between the lungs lies the heart 

 in its pericardial cavity. When the thorax is complete, there- 

 fore, it is completely filled by the distended lungs on either side, 

 and the heart in the middle. Any alteration in the size of this 

 thoracic box will cause a corresponding alteration of size in the 

 distensible lungs. In two ways can the size of the thoracic box 

 be altered, (1) By a slight change in the position of the ribs; 

 (2) By a change in the position of the diaphragm. When the 

 size is increased air is sucked in, when it is diminished air is 

 forced out. In the frog we had a buccal force-pump. In the 

 rabbit we have a costal and diaphragmatic suction pump. 



3. The Heart and Circulatory System. The pericardial cavity 

 in which the heart lies is between the pleural cavities in which 

 lie the lungs. The student should tear away a little of the 

 parietal layer of the pleura of one side, and satisfy himself that 

 the pericardium is a separate membrane. As he does so he may 

 notice between the two a white thread, the phrenic nerve which 

 passes to the diaphragm. 



The heart itself is slightly conical, with its blunt apex lying 

 somewhat to the left of the median line. Posteriorly are the ven- 

 tricles (16 and 17, 1. v., r. v.), there being two distinct cavities, and 

 not only one as in the frog and the fish. They are thick-walled 

 and fleshy. Anteriorly are the two auricles (r. au., I. au.), thin- 

 walled, and distended with blood, and, especially if the rabbit is 

 young, partially hidden by a soft fatty mass (the thymus gland), 

 which should be removed. There is no truncus arteriosus or 

 distinct sinus venosus, such as we saw in the frog. 



A large thick-walled artery crosses over the anterior end of 



