THE DIAPHRAGM. 311 



812. Bronchi and Bronchioli. The trachea should be fol- 

 lowed from the neck into the thorax, cutting or tearing away any 

 blood vessels or nerves that cover its ventral surface, and also the 

 thymus (Fig. 77, 808). Near the intermediate lobe of the lung, 

 the trachea divides into the two bronchial tubes, one of which goes 

 to each lung. Tear away the substance of the lung sufficiently to 

 follow one bronchus. It will be seen to continually divide and so 

 form the bronchioli. 



813. Obvious Structure of a Lung. On entering the lung the bronchus divides like 

 a tree into branches (bronchioli). Near their termination the bronchioli dilate somewhat, 

 thus forming the so called infundibula or ultimate lobules. From the wall of the infun- 

 dibulutn project sac-like recesses singly or in groups. Each recess is called an alveolus, 

 and may be considered as the blind ampulliform termination of the smallest division of an 

 air tube. The alveoli may be readily seen by looking at the edge of an inflated lung as 

 directed above ( 811). 



814. Microscopic Structure. This is very complicated, its main features being as fol- 

 lows : (A) The air tubes are composed of an ectal dense, largely elastic, connective tissue 

 layer, in which are found plates of cartilage. (B) Unstriated muscular layer. (C) An 

 elastic connective tissue layer. (D) The mucous membrane with its ciliated epithelium. 



In the smallest air tubes the cartilage disappears. In the larger ones are small race- 

 mose glands as in the trachea. As an air tube enters a lobule, its ciliated epithelium is 

 supplanted by a stratum of cubical, non-ciliated cells. The unstriped muscle also disap- 

 pears, and finally the alveoli are lined with a single layer of pavement or scaly epithelium. 

 See Strieker, A, 437 ; Quain, A, II, 273. 



DIAPHRAGMA, as. DIAPHRAGM. 



References. Straus-Durckheim, A, II, 309 ; Quain, A, I, 308 ; Gray, A, 394 ; Milne- 

 Edwards, II, 406 ; Chauveau (Fleming), A, 245 ; Chauveau, A, 260 ; Gegenbaur (Lankes- 

 ter), A, 574 ; Hyrtl, A, 316 ; Cuvier, A, VII, 198 ; Owen, A, III, 1. 



815. The Diaphragm (Fig. 77, 90) is a musculo-tendinous cur- 

 tain completely separating the thoracic and abdominal cavities. 

 Many structures transverse it, but they are joined to their respective 

 apertures in such a manner that the partition is absolutely air-tight, 

 and yet no hindrance is put upon the free movement of the dia- 

 phragm in respiration. 



The tendinous part (Fig. 90) is near the middle. Its form is 

 somewhat crescent shaped, the horns of the crescent pointing dor- 

 sad. From the tendon radiate the muscular libers. Those of the 

 dorso-mesal third converge and form two thickened masses called 

 the crura or pillars of the diaphragm. 



The diaphragm is attached to the xiphisternum, to the last jme 

 ribs, and somewhat loosely to the thick muscles which lie ventrad 

 of the vertebrae. The crura unite, forming a single dense tendon, 



