THE LUNGS. 799 



shaped like the letter C. (Fig. 110, 5), with its open part turned back- 

 wards. The first cartilage, which is suspended to the cricoid cartilage 

 of the larynx, is the broadest; the last cartilage is V-shaped. The 

 fibrous coat not only connects the cartilages together, but covers both 

 their outer and inner surfaces ; behind, where the cartilages are absent, 

 this membrane is continuous. An external layer of longitudinal un- 

 striped muscular fibres is found connected with the fibrous membrane 

 and with the cartilages ; whilst an inner transverse set extends between 

 the ends of the cartilages. Bundles of elastic fibres are placed im- 

 mediately beneath the mucous membrane ; at the posterior flat mem- 

 branous part of the tube they are seen as yellow longitudinal bands. 

 The mucous membrane, continuous with that of tne larynx and bronchi, 

 has a columnar ciliated epithelium. Numerous tracheal mucous glands 

 are found embedded in the walls of the tube, especially at its back 

 part. Its nerves are derived from the pneumogastrics and the sym- 

 pathetic. 



The right bronchus measures about 1 inch in length ; it is wider, 

 and has a more horizontal direction than the left bronchus ; it enters 

 the root of the right lung, opposite the fourth dorsal vertebra. The 

 left bronchus is about twice as long as the right one ; it is, however, 

 narrower, and passes obliquely downward beneath the arch of the aorta, 

 and in front of the oesophagus, thoracic duct, and descending aorta, 

 to enter the root of the left Jung, opposite the fifth dorsal vertebra. 



The structure of the bronchi is similar to that of the trachea ; in 

 front and at the sides they are convex, and strengthened with incom- 

 plete hoops of cartilage, but behind, they are flat, membranous, and 

 muscular. The cartilages are narrower and shorter than those of the 

 trachea ; in the right bronchus, there are from six to eight, and in the 

 left, from nine to twelve. 



The Lungs. 



The lungs, Fig. 13, ?, I, are two in number, and occupy, completely 

 and accurately, the lateral or pleural chambers of the thorax, one on 

 each side of the pericardium and heart, h. Each lung is free in all 

 directions, except at a part of its inner surface, which is connected, 

 by means of the bronchi and the pulmonary arteries and veins, with 

 the trachea and the heart, Fig. 111. 



The lungs are porous, spongy organs, the tissue of which is so 

 elastic, that, although they fill the closed pleural chambers, they col- 

 lapse more or less, when the thorax is laid open. If squeezed, they 

 give rise to a peculiar sensation called crepitation, owing to the air 

 which is contained in them. Their size and weight present great va- 

 riations, depending on their state of inflation, and the quantity of 

 blood in their vessels, or of serum in their tissue. The average weight 

 of the two lungs together is, however, about 42 ounces, the right lung 

 being about 2 ounces heavier than the left ; they are larger in the 

 male than in the female ; they are, moreover, heavier, their proportion 

 to the body being as 1 to 37 in the former, and as 1 to 43 in the latter. 

 Owing to the air in the lungs, they float entirely, or even in portions, 

 in water ; the specific gravity of their substance ranges from 345 to 



