H4 PHYSIOLOGY FOR BEGINNERS CHAP. 



wards over the glottis when food is being swallowed, and so the 

 food is prevented from passing into the larynx. 



Structure of the Trachea and Lungs. Obtain from 

 a butcher the lungs and trachea of a sheep. The trachea is a 

 tube which left to itself remains widely open and indeed cannot 

 be closed by pressure. It is kept open by bands of cartilage 

 in its wall. These bands form rings of cartilage which, however, 

 can be felt to be incomplete down one side of the trachea. 

 This side is the back of the trachea, and in the body was in 

 contact with the oesophagus. Cut open the trachea longitudin- 

 ally with scissors. Notice that the internal surface of the 

 trachea is smooth, the cartilages lying in the thickness of the 

 wall. Each cartilage is a flattened band, the ends of which 

 are connected together along the back of the trachea by plain 

 muscular tissue. The external coat of the trachea consists of 

 connective tissue containing a little fat. The internal coat of 

 the trachea is a mucous membrane, consisting of an internal 

 epithelium resting on a layer of connective tissue. The 

 epithelium consists of cells, the surface layer of which are 

 columnar and bear a large number of short hair-like processes. 

 These processes are called cilia. The cilia are during life 

 constantly in a wavy motion, each bending sharply and then 

 slowly straightening again. Since all those in one area act 

 together, they drive any fluid on them in the direction in which 

 they bend, and this is, in the trachea, outwards towards the 

 mouth. Ciliated cells also line the larynx and parts of the 

 chambers of the nose. 



Close to the lungs thetrachea divides intotwo tubes, the bron- 

 chi, one going to each lung. Cut open one of the two bronchi 

 lengthways. Notice that it gives off branches and gradually 

 divides into a large number of smaller tubes, the bronchial 

 tubes. The bronchi and bronchial tubes have a structure 

 similar to that of the trachea, having a ciliated epithelium and 

 containing also cartilage in their walls, but the cartilage bands 

 are even less complete than in the trachea, and as the bronchi 

 become smaller became less regular until, in the smaller 

 bronchial tubes, only scattered pieces of cartilage are present 

 and in the smallest tubes none at all. Go on cutting open, 

 with fine scissors, the bronchial tubes as far as you can into 

 the substance of the lung. As they divide into smaller tubes 



