THE RESPIRATORY SYSTEM OF MAMMALS 151 



bronchioles and CAeiitually reaches the microscopic end-pieces, the 

 respiratory alveoli, where o;aseous exchange takes place. 



The nose is an organ composed of bone, cartilage, mnscle, skin, 

 and epithelial membranes, so constructed as to provide open ways 

 for the passage of air. The olfactory area of the passage has epi- 

 thelium composed of three types of cells. There are small basal 

 cells and long, thin supporting cells; scattered between these cells 

 are long fusiform olfactory cells. 



The nasal cavity continues into many irregular spaces, or sinuses, 

 lined with a thin epithelial membrane. The nasal passage continues 

 posteriorly into the pharynx, where the epithelium consists of 

 pseudostratified ciliated epithelium. ]\Iany of the long superficial 

 cells are goblet cells, and the surface of the membrane is generally 

 well moistened with mucus. Under the epithelium is fibroelastic 

 connective tissue carrying lymphatics, blood vessels, nerves, and 

 small mixed mucoserous glands. 



The Larynx.— The larynx is posterior to the pharynx in quad- 

 ruped mammals and is sometimes called the voice box, for it 

 contains the vocal cords. It has a skeleton of a number of cartilag- 

 inous pieces, some of them single units, others occurring in pairs. 

 In general, these cartilages are of the hyaline variety. They are 

 covered with perichondrium which merges with the fibroelastic con- 

 nective tissue forming the external covering of the larynx. Many 

 small muscles of the voluntary striated type are present. The 

 cartilages are covered with loose fibroelastic connective tissue 

 which may be called a submucosa or tunica propria. It contains 

 much lymphoid tissue and possibly scattered single lymph nodules. 

 The epithelium of the mucosa is of the pseudostratified ciliated 

 variet,y, except over the vocal cords and under the epiglottis where 

 it is stratified squamous. 



The vocal cords are a pair of shelf-like tissue masses extending 

 from the lateral walls of the larynx into the laryngeal cavity. P^ach 

 vocal cord has an external skeletal muscle mass. Internal to this, 

 toward the lumen, is elastic tissue and the stratified squamous 

 epithelium covering the vocal cords. 



The Trachea.— The trachea is a dilated tube extending from the 

 larynx to the bronchi. It is just dorsal to the skin of the neck in 

 the mid-ventral line and dorsal to it is the esophagus. Laterally 

 the muscles and blood vessels of the neck are associated with 

 fibroelastic connective tissue which also forms the outer covering 

 of the trachea. The th\-roid gland lies against its ventral surface 



