THE ORGANS OF THE THORAX 69 



That which surrounds the heart is called the pericardium (Gr. 

 peri, about ; and kardia, the heart), and that which invests each 

 lung the pleura. In both cases one layer of the membrane is so 

 closely attached to the organ that it can be separated only with 

 difficulty, while the other is reflected back from this so as tc form 

 a double bag or sac. The outer layer of the pericardium en- 

 velops the heart loosely, and is generally torn or cut when that 

 organ is removed from the thorax. The outer layer of the pleura 

 is firmly attached to the walls of the chest (See fig. 78.) 



TEACHEA or 

 WIND-PIPE 



LUNGS 

 or LIGHTS. 



SUMMARY. 



Leads from the mouth to the lungs. 



Composed of rings of cartilage connected by soft fibrous 

 tissue. 



I The upper, enlarged portion of the trachea. 

 Larynx \ Sometimes called the voice-box. 



{ Contains the vocal cords. 

 Bronchi -I ^ e two S reat divisions of the trachea. 



1 One enters each lung. 



Bronchial tubes the subdivisions of the bronchi. 

 Light, spongy bags. 



Contain air-cells at the terminations of the bronchial tubes. 

 Almost fill the thorax. 



( A membrane surrounding the lungs. 

 Pleurae -) ( One covers the lungs. 



( Composed of two layers \ The other lines the chest 



( walls. 



A hollow muscle (involuntary). 

 Weighs about 9 ozs. 



Occupies lower, front, middle portion of the chest. 

 Rests on the diaphragm. 



Its apex or point turns downward towards the left. 

 Has four cavities. 

 Forces the blood to all parts of the body. 



(A membrane surrounding the heart. 

 \ Pericardium ] ( One attached to the 



, , . heart. 



-Composed ottwo layers^ ^ Qther envelops 



( it loosely. 



I Leads from the mouth to the stomach. 

 A soft, fleshy tube. 

 Lies behind the trachea. Then passes between the lungs, 

 and through the diaphragm. 



QUESTIONS ON LESSON XII. 



1. Give the names of the chief organs of the thorax, and describe briefly the 



position of each. 



2. Where is the trachea ? Where does it begin, and where does it end ? 



3. What are the bronchi and the bronchial tubes ? Where are they to be 



found ? 



HEAET 



(ESOPHAGUS 



