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LESSON XXV. 



THE TRACHEA AND LUNGS. 



1. IN sections of trachea, stained with logwood or borax-carmine, and mounted 

 in Canada balsam, notice the ciliated epithelium, the basement-membrane 

 (of some thickness in the human trachea), the lymphoid tissue of the mucous 

 membrane, the elastic tissue external to this, and lastly the fibrous membrane 

 containing the cartilages. In the mucous membrane and submucous areolar 

 tissue look for sections of mucous glands, ducts of which may be seen opening 

 on the surface. At the back of the trachea notice the plain muscular fibres 

 transversely arranged ; there may be larger mucous glands external to these. 



2. In sections of lung similarly prepared, notice the sections of the alveoli 

 collected into groups (infundibula) . Find sections of bronchial tubes, 

 some cut longitudinally and passing at their extremities into the infundibula, 

 others cut across ; the latter show the structure of the tubes best. In each 

 tube notice the ciliated epithelium internally. Next to this the mucous 

 membrane containing numerous elastic fibres and often thrown into folds ; 

 then the layer of circular muscular fibres, and outside this, loose fibrous tissue 

 in which in larger bronchial tubes the pieces of cartilage may be seen embedded. 

 Small mucous glands may also be observed in the fibrous tissue sending their 

 ducts through the other layers to open on the inner surface. Notice always 

 accompanying a section of a bronchial tube the section of a branch of the 

 pulmonary artery. 



In the sections of the alveoli observe the capillary vessels passing from 

 one side to the other of the intervening septa ; and in places where the thin 

 wall of an alveolus is to be seen in the section, try and make out the net- 

 work of blood-capillaries upon it. Notice within the alveoli nucleated cor- 

 piiscles which very frequently contain dark particles in their protoplasm. 

 They appear to be amoeboid cells which have migrated from the blood-vessels 

 and have taken in inhaled particles of carbon. They seem to pass back into 

 the lung tissue, for similar cells may be seen in this. Make a sketch of part 

 of the wall of a bronchial tube and of one or two of the alveoli. 



3. Mount in Canada balsam a section of lung in which the pulmonary 

 vessels have been injected. Study the general arrangement of the vessels 

 with a low power, and the network of capillaries of the alveoli with a high 

 power. Observe that the veins run apart from the arteries. Sketch the 

 capillary network of one or two adjoining alveoli. 



The trachea or windpipe is a fibrous and muscular tube, the wall 

 of which is rendered somewhat rigid by C- sna P e( i hoops of cartilage 

 which are embedded in the fibrous tissue. The muscular tissue, which 

 is of the plain variety, forms a flat band, the fibres of which run trans- 

 versely at the back of the tube. The trachea is lined by a mucous 



