798 



SPECIAL PHYSIOLOGY. 



closed by the musculo-tendinous, movable structure, named the dia- 

 phragm, which is itself arched, and reaches higher up on the right side ; 

 this opening transmits the oesophagus, the great vessels, the thoracic 

 duct, and the vagi and certain sympathetic nerves. 



The Air- Passages. 



The nose, pharynx, and larynx, have already been described. The 

 trachea, or windpipe, placed in the middle line, descends from the 

 larynx, on a level with the fifth cervical vertebra, to opposite the third 

 dorsal vertebra, where it divides into two smaller air-tubes, named the 

 bronchi, one for each lung. The trachea (Fig. 110, 1) is about 4J 

 inches in length, and from f to 1 inch in width ; it is wider in the 

 male than in the female. Its anterior surface and sides are convex; 

 its posterior surface is flattened. It is overlapped at its upper part, 

 in front and at the sides, by the isthmus and lobes of the thyroid body ; 



Fig. 110. 



Fig. 110. Air-tubes of the human lung?, dissected out, and seen from the front. 1, trachea or wind-pipe. 

 2, the right and left bronchi, the right being the wider, the left the longer of the two. 3. outline of the 

 left lung collapsed. 4, bronchia, or bronchial tubes in the lung, cut short. 5, transverse section of a por- 

 tion of the trachea, showing the incomplete C-shaped rings, with the flat membranous part behind. 



it is also covered by certain muscles and vessels of the neck, and is 

 concealed, lower down, by the upper part of the sternum and by the 

 remains of the thymus gland. In the thorax, the large bloodvessels 

 are in front of it, but in the neck, it is placed between them. Behind, 

 its flattened surface rests on the oesophagus, by which it is separated 

 from the spinal column. Its lower thoracic portion is placed in the 

 space known as the posterior mediastinum, situated between the lungs. 



The trachea is composed of from sixteen to twenty independent, 

 transverse, incomplete hoops or rings of cartilage, held together by an 

 intermediate fibrous coat, within which are muscular and elastic fibres 

 and a mucous lining membrane. 



The cartilages are flattened bands, incomplete behind, each being 



