350 



THE HUMAN BODY 



is a highly developed form of them which is employed in the 



Human Body. 



The Air-Passages and Lungs. In our own Bodies the es- 

 sential gaseous interchanges between 

 the Body and the atmosphere take 

 place in the lungs, two large sacs (lu, 

 Fig. 1) lying in the thoracic cavity, one 

 on each side of the heart. To these 

 sacs the air is conveyed through a series 

 of passages. Entering the pharynx 

 through the nostrils or mouth, it passes 

 out of this by the opening leading into 

 the larynx, or voice-box (a, Fig. 113), 

 lying in the upper part of the neck (the 

 communication of the two is seen in 

 Fig. 122); from the larynx passes back 

 the trachea or windpipe, b, which, after 

 entering the chest cavity, divides into 

 FiG."ii3.-The lungs and air- the right and left bronchi, d, e. Each 



passages seen f r m the front, bronchus divides up into smaller and 



pulmonary tissue has been dis- smaller branches, called bronchial tubes, 



sected away to show the rami- ,,,-, -, , ' -, 



fications of the bronchial tubes, within the lung on its own side; and 

 ronc y hus. ; Th^eft^onchuSs ^e smallest bronchial tubes end in 

 seen entering the root of its lung, sacculated dilatations, the infundibula of 

 the lungs, the sacculations (Fig. 115) being the alveoli. On the 

 walls of the alveoli the pulmonary capillaries ramify, and it is 

 in them that the interchanges of the 

 external respiration take place. 



Structure of the Trachea and Bronchi. 

 The windpipe may readily be felt in 

 the middle line of the neck, a little be- 

 low Adam's apple, as a rigid cylindrical 

 mass. It consists fundamentally of a 

 fibrous tube in which cartilages are ,-, 



FIG. 114. A small bronchial 



embedded, SO as to keep it from COl- tube, a, dividing into its terminal 

 , . , . ,. , . , n i branches, c; these have pouched 



lapsing; and is lined internally by a or saccuiated walls and end in 

 mucous membrane covered by several the sacculated infundibula, b. 

 layers of epithelium cells, of which the superficial is ciliated. 

 The elastic cartilages embedded in its walls are imperfect rings, 



