LEPUS CUNICULUS 



307 



heart about half-way along which it divides into two branches, the 

 bronchi, one going to each lung. These divide up into smaller and 

 smaller tubes, the bronchioles, also supported by small cartilaginous 

 rings, thus constituting a system of vessels through which the air 

 is conveyed to all parts of the lungs. The lungs themselves are 

 soft spongy pink bodies, and not hollowed sacs as in Rana, lying 

 freely in the thoracic cavity unattached save where the bronchi 

 and blood-vessels enter them. Each is divided into two main 

 lobes, but there are on the right side two small accessory lobes of 

 which the posterior and smallest lies in the middle line closely 

 applied to the oesophagus just behind the heart. The ultimate 

 branches of the bronchioles finally open into saccular chambers, 

 the alveoli, where the actual respiratory exchanges take place. 



The peritoneum lines the thoracic cavity and is reflected round 

 each lung separately and is termed the pleura, being divided into 

 the parietal pleura lining the wall of the cavity and the visceral 

 pleura around the lungs. Thus we can see that each lung has its 

 own pleural cavity, and further the two lungs are separated in the 

 middle line by the two layers of the pleura, constituting the media- 

 stinum and enclosing between them the mediastinal space. Within 

 this lie the oesophagus, heart, main blood-vessels, the end of the 

 trachea and the beginnings of the bronchi, and the space is shaped 

 so as to accommodate them. 



The mechanism of breathing differs much from that in 

 the frog owing to the presence of the ribs and sternum making an 

 airtight compartment for the lungs. The ribs lie somewhat 

 obliquely and by means of the intercostal muscles they can be raised 

 in such a manner as to enlarge the thoracic cavity, and the relaxation 

 of these muscles allows the chest to return to its original size. 

 This operation brings about what is termed costal breathing. 

 Further, the floor of the cavity is formed by the arched diaphragm 

 which, when its muscles contract, flattens, and so increases the 

 capacity of the thorax, the return is accomplished by the relaxing 

 of these muscles again and this brings about diaphragmal breathing. 

 In normal respiration both types play their part, and so air is drawn 

 into the lungs by the enlargement of the cavity. Their emptying 

 is brought about in part by the elasticity of the lungs themselves, 

 but also by a return to their original positions of the ribs and 

 diaphragm. 



Circulatory System. 



The circulatory system consists of the same main divisions 

 as in the frog, but it diifers considerably in points of detail and 

 represents a higher stage of development. 



