128 BIOLOGY OF THE LABORATORY MOUSE 



Small accumulations of lymphatic tissue are occasionally present near the 

 base of the epiglottis. Projecting into the glottis are the false vocal cords, 

 paired folds of the mucous membrane. Below these the true vocal cords 

 arise. Between the false and the true vocal cords are lateral pouches, the 

 ventricles of the larynx. The surface of the true vocal cords is covered by 

 stratified squamous epithelium. The lamina propria consists of dense 

 fibrous elastic tissue below which groups of striated muscle fibers are present. 

 There are no glands in the mucous membrane of the true vocal cords. With 

 the exception of these areas, glands are present throughout the entire length 

 of the larynx. They consist of small groups of branched alveolar glands 

 containing serous or mucous secreting acini and opening through short ducts 

 into the lumen. 



The cartilages of the larynx in old animals often show calcification. 



The trachea and the main bronchi. — The trachea is continuous with the 

 larynx. Its lumen is lined by pseudostratified ciliated columnar epithelium. 

 The fibrous lamina propria is rich in blood vessels. The rigidity of the wall 

 is due to the presence of hyaline cartilage rings. The rings are incomplete 

 and one end of each ring is connected by smooth muscle fibers to the opposite 

 end, forming the dorsal membranous wall of the trachea. The attachment 

 of the musculature is on the outer (dorsal) side of the cartilage. The mucous 

 membrane of the membranous wall is thrown into longitudinal folds. The 

 perichondrium which surrounds each cartilage is continuous with the lamina 

 propria. Only at the cephalic end of the trachea are there glands present 

 in the lamina propria between the cartilage rings. Loose connective tissue 

 forms the adventitia and attaches the trachea to the surrounding tissues. 



The trachea divides into two main bronchi which are similar to it in 

 structure. Here the cartilages form small irregular plates that completely 

 surround the tube. Circular smooth muscle fibers complete the wall. 

 Cartilage is not present in the walls of the bronchi beyond the point where 

 they enter the lungs. 



The lungs. — The thoracic cavity is lined by and divided into right 

 and left pleural sacs by a very thin membrane, the pleura. The membranes 

 of the two sacs meet in the median plane and form the mediastinal septum. 

 The lungs are covered by the visceral pleura, a thin serous membrane 

 composed of connective tissue containing collagenous and elastic fibers and 

 covered by mesothehum. The left lung has one, the right four lobes. The 

 inferior median lobe of the right lung is separated from the inferior lateral 

 lobe by the inferior vena cava, and is enclosed in a separate pleural sac 

 (Lauche in Jaffe, 56). 



