224 THE CAT. [chap. viii. 



tlioii- cartilaginous rings arc shorter and narrower. The smaller 

 tubes, into which the bronchi subdivide within the lungs, arc called 

 bronchia. 



§ 4. The LUNGS (Fig. 89 and 112) are two in number, and are 

 placed one on each side of the heart in the thorax. Together with 

 the heart they fill up the main part of the whole thoracic cavity. 

 They are attached each by a small part of its inner surface (called 

 its root) to one of the two bronchi and to the great vessels con- 

 necting that lung with the heart — the blood-vessels and air-tubes 

 entering or leaving the lung, passing through one or other of its 

 " roots." From this attachment each lung hangs freely suspended, 

 being conical in shape, with a broad, concave base, which is apj^lied 

 to the front surface of the diaphragm. Each lung is enclosed in a 

 serous, shut sac, called a 2)lciira, and the two pleura) together may 

 be said to form the proper serous sac of the thorax, though each is 

 quite distinct from the other. 



The two pleura) line the right and left halves of the thorax, and 

 are reflected over the two lungs at their roots respectively. In this 

 way the two adjacent (inner) sides of the two pleura) traverse the 

 thorax from above downwards. They arc not, however, in contact, 

 but separated by two interspaces termed nmUastuut. 



The anterior mediastinum contains the heart in its pericardium. 



The 2)Osfrrior mediastinum contains the oesophagus, the aorta, the 

 vena azygos, and the thoracic duct, together with the two nerves 

 called pneumogastric. 



Each lung is divided by fissures into lobes as follows : 



The left lung is divided by a deep fissure in two large and 

 distinct lobes sub-equal in size (Fig. 104,^^ and ^). A less deep 

 fissure also separates off a small lower portion (»«) of the upper of 

 the two lobes of the left lung. The right lung is divided by three 

 deep fissures into four unequal and distinct lobes, the upj)ermost of 

 these (Fig. 104, ^) is large and triangular. The next (-) is narrow 

 and elongated, but much smaller. The third lobe (^) is the largest 

 of all, and has on its inner side the fourth lobe, wdiich is incompletely 

 divided by a fissure into a larger external part {*) and a much 

 smaller internal portion {^a), both of which arc narrow and pointed at 



the end. 



As to their minute structure, the lungs consist of a prodigious 

 number of small air-bags, called " lobules," attached to the finer 

 ramifications of the bronchi. These lobules are united together by 

 connective tissue vvith blood-vessels and muscular and elastic fibres, 

 and can veiy plainly be discerned at the surface of the lung. The 

 lungs may therefore be described as spongy and highly clastic 

 organs which (when once respiration has taken i)lace) will float if 

 thrown into water, and which, if artifically inflated after removal 

 from the body, will spontaneously contract and expel the air so 

 introduced, through the elastic nature of their substance. 



The smallest bronchial ramifications cease to be lined with ciliated 

 epithelium, and have* squamous epithelium instead. 



