STRUCTURE OF THE LUNGS. 355 



chial tubes, within the lung on its own side; and the smallest 

 bronchial tubes end in sacculated dilatations, the alveoli of 

 the lungs, the sacculations (Fig. 106) being the air-cells: 

 the word " cell" being here used 

 in its primitive sense of a small 

 cavity, and not in its later tech- 

 nical signification of a morpho- 

 logical unit of the Body. On 

 the walls of the air-cells the 

 pulmonary capillaries ramify, 

 and it is in them that the inter- 

 changes of the external respira- 



Fio. 105. A small bronchial tube, 



tlOn take place. dividing into its terminal branch- 



es, c ; these have pouched or saccu- 

 StrUCture Of the Trachea latea walls and end in the saccu- 



and Bronchi. The windpipe l 



may readily be felt in the middle line of the neck, a little 

 below Adam's apple, as a rigid cylindrical mass. It con- 

 sists fundamentally of a fibrous tube in which cartilages 

 are imbedded, so as to keep it from collapsing; and is lined 

 internally by a mucous membrane covered by several layers 

 of epithelium cells, of which the superficial is ciliated (Fig. 

 47)*. The cartilages imbedded in its walls are imperfect 

 rings, each somewhat the shape of a horseshoe and the 

 deficient part of each ring being turned backwards, it comes 

 to pass that the deeper or dorsal side of the windpipe has 

 no hard parts in it. Against this side the gullet lies, and 

 the absence thereof the cartilages no doubt facilitates swal- 

 lowing. 'The bronchi resemble the windpipe in structure. 



The Structure of the Lungs. These consist of the 

 bronchial tubes and their terminal dilatations; numerous 

 blood-vessels, nerves and lymphatics; and an abundance of 

 connective tissue, rich in elastic fibres, binding all together. 

 The bronchial tubes ramify in a tree-like manner (Fig. 104). 

 In structure the larger ones resemble the trachea, except 

 that the cartilage rings are not regularly arranged so as to 

 have their open parts all turned one way. As the tubes 

 become smaller their constituents thin away; the cartilages 

 become less frequent and finally disappear; the epithelium 

 is reduced to a single layer of cells which, though still cili- 



* P. 115. 



