2 4 PRACTICAL HISTOLOGY. [XXVII. 



LESSON XXVII. 

 TRACHEA LUNGS THYROID GLAND. 



THE trachea, a fibro-muscular tube, the wall of which contains 

 16-20 C-shaped pieces of hyaline cartilage held together by a 

 fibrous membrane. Behind, the rings are deficient, and the trachea 

 is membranous, and there it is strengthened by smooth muscle 

 tracJiealis muscle which stretches between the ends of the carti- 

 lages. It is lined by a mucous membrane, which is united to the 

 outer fibrous coat by a submucous coat. The mucous coat consists 

 from within outwards of (i.) Stratified, columnar, ciliated epithe- 

 lium; (2.) Basement membrane; (3.) A basis of connective tissue 

 with capillaries, and infiltrated with adenoid tissue ; (4.) A layer of 

 elastic fibres arranged longitudinally. Outside this is a loose sub- 

 mucous coat, in which lie the acini of the glands. 



The intra-pulmonary bronchi are lined by stratified ciliated epi- 

 thelium resting on a basement membrane. Outside this is a basis 

 of fibrous tissue, with numerous longitudinally-arranged elastic 

 fibres, and some adenoid tissue. Outside this, again, is a com- 

 pletely circular layer of smooth muscle bronchial muscle. Then 

 follows the submucous coat with its vessels, glands, and in some 

 animals (cat) masses of adenoid tissue. Most externally is the 

 fibrous coat, in which are embedded several pieces of hyaline 

 cartilage of irregular shape. As the bronchi pass into the lung, 

 they divide and form smaller and smaller tubes, until they end 

 in terminal bronchi or bronchioles. Each bronchiole, with thin 

 walls, no glands or cartilage, and the epithelium cubical and non- 

 ciliated, opens into several wider expanded parts infundibula or 

 alveolar passages which are beset with air-cells or alveoli. The 

 alveoli are spherical or polygonal vesicles, which open by a wide 

 opening into the infundibula ; the air-vesicles, however, do not open 

 into each other. The air-cells are lined by a layer of squames 

 large, flattened, irregular plates with small granular cells here and 

 there between them. 



Blood- Vessels. The branches of the pulmonary artery accom- 

 pany the bronchial tubes, and finally terminate in a rich capillary 

 plexus over and outside the basement membrane of the air-vesicles. 

 The blood is returned by the pulmonary veins. The bronchial 

 vessels seem to supply chiefly the connective tissue along the 

 bronchi and in the septa. Numerous ganglia exist in the intra- 

 pulmonary nerves. Lymphatics are numerous. 



