RESPIRATORY APPARATUS 299 



followed by a capillary plexus just beneath the epithelium. The lym- 

 phatic vessels similarly form two communicating networks, of which the 

 more superficial consists of smaller vessels and is situated beneath the 

 capillary plexus. The nerves form a deep and a superficial plexus which 

 are associated with microscopic ganglia. Non-medullated fibers end either 

 beneath the epithelium in bulbs and free endings with terminal knobs, or 

 within the epithelium in free ramifications and in taste buds. Below the 

 vocal folds, subepithelial nerve endings and buds are absent, but many 

 intraepithelial fibers occur, which surround individual taste cells. The 

 nerves and vessels of the larynx are numerous, except in the dense elastic 

 tissue of the vocal folds. 



TRACHEA AND BRONCHI. 



The trachea consists of a mucosa, submucosa, and a fibrous outer 

 layer containing the tracheal cartilages. The general arrangement of 

 the layers is the same as that found in the large bronchi (Fig. 295). 



The mucosa consists of pseudo-stratified columnar epithelium with 

 cilia proceeding from distinct basal bodies (Fig. 38, p. 49). Exception- 

 ally, the lining of the trachea, toward the oesophagus, has been found to 

 consist of stratified squamous epithelium resting on connective tissue 

 papillae. Beneath the epithelium there is a broad basement membrane, 

 followed by a layer of reticular tissue containing many lymphocytes, 

 forming a tunica propria. Beneath the reticular tissue there is a layer of 

 coarse longitudinal elastic fibers, which may readily be seen in haema- 

 toxylin and eosin preparations. This layer may be compared with the 

 muscularis mucosae of the intestine. 



The submucosa is a layer of loose fatty connective tissue extending to 

 the perichondrium of the tracheal cartilages. It contains the bodies of 

 the tracheal glands, which include both serous and mucous cells, and are 

 beautiful objects for the study of serous crescents. 



The outer layer of the trachea is continuous with the tissue of the 

 mediastinum. It contains abundant blood and lymphatic vessels, and 

 nerves, both medullated and non-medullated. Internally it forms the 

 perichondrium around the succession of C-shaped hyaline cartilages, the 

 free ends of which are toward the oesophagus. In the intervals between 

 these ends there is a layer of transverse smooth muscle fibers, usually 

 accompanied by outer longitudinal fibers. As in the intestine, elastic 

 fibers are abundant among the. muscle cells. In old age, the hyaline 

 cartilages show fibrous degenerative changes, and may become partly 

 calcified. 



The primary bronchi have the same structure as the trachea, but in 

 their subdivisions changes occur, and the C-shaped rings of cartilage are 



