570 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



secretions or of any foreign substance which may lodge upon it. When 

 the trachea reaches the second or third dorsal vertebra it bifurcates into 

 two principal bronchi, passing one to each lung, and subsequently these 

 again divide and subdivide in various directions until they have attained 

 the size of the most minute bronchial tubes. 



The larynx, which is placed at the laryngeal extremity of the 

 trachea, may be considered as corresponding to the base of a tree, the 

 trachea to its trunk, and the bronchi to its different branches ; while the 

 ultimate bronchi terminating in the air-vesicles of the lung may be 

 regarded as representing the leaves of the tree. The bronchi have the 

 same anatomical constituents as the trachea, and are composed of carti- 

 laginous rings, musculo-fibrous membrane, 

 and a lining mucous membrane (Pig. 245). 

 The cartilaginous rings are also imperfect, 

 but the imperfect spaces are irregularly dis- 

 tributed, sometimes in front and sometimes 

 at the side. The object of the rings is, of 

 course, the same as those of the trachea. 

 The tubes are thus mere gaseous conduits 

 kept patulous by their cartilaginous constit- 

 uents. In the bronchi a fibrous basement 

 membrane is found, as well as unstriped 

 muscular fibres, and in the bronchi the 

 muscular fibres do not merely connect the 

 ends of the rings, but completely encircle the 

 tubes in the form of annular fibres. These 

 muscular fibres are unstriped and involun- 

 tary, and therefore possess the same char- 

 acteristics as the unstriped muscular fibre 

 found elsewhere, and serve to regulate the 

 calibre of the tubes. The lining mucous 

 membrane of the bronchi is also a ciliated membrane and extends down 

 to the commencement of the finest bronchi. In the mucous membrane 

 are found tubular glands forming a mucous secretion. In the minute 

 bronchi the cartilaginous rings disappear and the bronchioles are then 

 constituted of a layer of circular muscular fibres with an inner epithelial 

 membrane, the cartilaginous rings disappearing when the bronchi have 

 been reduced to about one-thirtieth to one-fiftieth of an inch in diameter. 

 The bronchi ultimately terminate in a dilated portion, termed lobules orin- 

 fundibula, which consist simply of a homogeneous membrane abundantly 

 supplied with blood-vessels. This dilatation is formed by the same 

 material as constitutes the fibrous wall of the tube thrown up into folds, 

 between which ramify the blood-vessels. The contained blood is, there- 



Fig. 245.— Branchial Tree op 

 a Mammal (Horse), after 

 Aeby. (Jeffrey Bell.) 



A, eparterial, E, hyparterial ventral 

 (V), and. D, hyparterial dorsal bronchi; 

 PA. pulmonary artery ; P V, pulmonary 

 vein. 



