334 SOLID PARTS OF ANIMALS. 



spicuous on the fore part of the neck. Below them are two pairs 

 of small cartilages ; namely, the ary taBnoid and cuneiform car- 

 tilages. 



The thyroid cartilage is deficient behind, its place being sup- 

 plied by a strong membrane. The cricoid cartilage makes a 

 complete circle round the tube. The trachea is a cylindrical 

 tube, which extends from the cricoid cartilage to the third dor- 

 sal vertebra. It is composed of fibro-cartilaginous rings, vary- 

 ing from sixteen to twenty in number, and of membranes which 

 connect them. The rings do not extend all round the tube ; 

 they are wanting behind where the trachea is contiguous to the 

 oasophagus. A thin elastic fibrous lamella forms the circumfe- 

 rence of the tube, serving to connect the cartilaginous rings, which 

 seem as if developed on its interior, and also to complete the cir- 

 cuit behind where the cartilages are wanting. Within, the 

 trachea is lined by a mucous membrane. Where the cartilages 

 are deficient, the mucous membrane is supported by some longi- 

 tudinal fibres, and beneath these we find a series of muscular 

 fibres, as in the intestinal canal. 



At the third dorsal vertebra the trachea divides into two 

 branches called bronchii, one of which proceeds to each lung. 

 They are composed of the same constituents as the trachea ; but 

 the rings, as we go downwards, gradually lose their annular form, 

 and become lamellae of irregular shape, placed in different parts of 

 the circumference of the canal. As the tubes pass down they 

 subdivide into more and more branches, and at the points of 

 subdivision they are still somewhat annular, so much so at least, 

 as to keep the orifices open. So far as recognizable by our sen- 

 ses the minute bronchii seem to be composed of the same mate- 

 rials as the larger tubes ; but reduced to the greatest degree of 

 tenuity. These minute tubes gradually terminate in small glo- 

 bular vesicles, a congeries of which constitute the body of the 

 lungs. 



The external surface of the lungs is smooth and convex. They 

 are divided into different lobes, and are covered by a thin serous 

 membrane, a continuation of the pleura. Upon the interior sur- 

 face of the small globular vesicles, in which the bronchiae termi- 

 nate, the pulmonary artery and vein ramify, so as to expose the 

 whole blood as it passes through the lungs to the action of the 

 air. These vesicles are doubtless coated by a continuation of 

 the mucous membrane which lines the trachea. 



